View source for Methods of detecting exoplanets

{{Short description|none}}
{{Use dmy dates|date=April 2019}}
[[File:Confirmed exoplanets by methods EPE.svg|thumb|400px|Number of extrasolar planet discoveries per year through 2022, with colors indicating method of detection:
{{div col|colwidth=22em}}
{{legend|#1f77b4|[[#Direct imaging|Direct imaging]]}}
{{legend|#ff7f0e|[[Gravitational microlensing|Microlensing]]}}
{{legend|#2ca02c|[[#Transit photometry|Transit]]}}
{{legend|#d62728|[[Doppler spectroscopy|Radial velocity]]}}
{{legend|#9467bd|[[Transit-timing variation|Timing]]}}
{{div col end}}]]

Any [[planet]] is an extremely faint light source compared to its parent [[star]]. For example, a [[star]] like the [[Sun]] is about a billion times as bright as the reflected light from any of the planets orbiting it. In addition to the intrinsic difficulty of detecting such a faint light source, the light from the parent star causes a glare that washes it out. For those reasons, very few of the [[exoplanet]]s reported {{as of|2024|01|lc=y}} have been observed directly, with even fewer being resolved from their host star.

Instead, [[astronomer]]s have generally had to resort to indirect methods to detect extrasolar planets. As of 2016, several different indirect methods have yielded success.

== Established detection methods ==
The following methods have at least once proved successful for discovering a new planet or detecting an already discovered planet:

=== Radial velocity ===
{{Main|Doppler spectroscopy|List of exoplanets detected by radial velocity}}
[[File:18_Del_b_rv.pdf|thumb|right|Radial velocity graph of [[18 Delphini b]].]]
A star with a planet will move in its own small orbit in response to the planet's gravity. This leads to variations in the speed with which the star moves toward or away from Earth, i.e. the variations are in the [[radial velocity]] of the star with respect to Earth. The radial velocity can be deduced from the displacement in the parent star's [[spectral line]]s due to the [[Doppler effect]].<ref>{{cite journal | title = The fundamental definition of "radial velocity" | last1 = Lindegren | first1 = Lennart | last2 = Dravins | first2 = Dainis | date = 31 January 2003 | journal = Astronomy & Astrophysics | volume = 401 | issue = 3 | pages = 1185–1201 | doi = 10.1051/0004-6361:20030181| arxiv=astro-ph/0302522| bibcode = 2003A&A...401.1185L | doi-access = free }}</ref> The radial-velocity method measures these variations in order to confirm the presence of the planet using the [[binary mass function]].

The speed of the star around the system's [[center of mass]] is much smaller than that of the planet, because the radius of its orbit around the center of mass is so small. (For example, the Sun moves by about 13&nbsp;m/s due to Jupiter, but only about 9&nbsp;cm/s due to Earth). However, velocity variations down to 3&nbsp;m/s or even somewhat less can be detected with modern [[spectrometer]]s, such as the HARPS ([[High Accuracy Radial Velocity Planet Searcher]]) spectrometer at the [[ESO]] 3.6 meter telescope in [[La Silla Observatory]], Chile, the [[W. M. Keck Observatory#Instruments|HIRES]] spectrometer at the [[Keck telescopes]] or [[EXPRES]] at the [[Lowell Discovery Telescope]].
An especially simple and inexpensive method for measuring radial velocity is "externally dispersed interferometry".<ref>{{cite web
|date=June 2006
|title=Externally Dispersed Interferometry
|url=http://www.spectralfringe.org/EDI/
|work=SpectralFringe.org
|publisher=[[Lawrence Livermore National Laboratory|LLNL]]/[[Space Sciences Laboratory|SSL]]
|access-date=2009-12-06
}}
* {{cite book
|author=D.J. Erskine |author2=J. Edelstein |author3=D. Harbeck |author4=J. Lloyd
|name-list-style=amp |date=2005
|chapter=Externally Dispersed Interferometry for Planetary Studies
|chapter-url=https://e-reports-ext.llnl.gov/pdf/322047.pdf
|editor=Daniel R. Coulter
|title=Proceedings of SPIE: Techniques and Instrumentation for Detection of Exoplanets II
|volume=5905 |pages=249–260
|title-link=Proceedings of SPIE }}</ref>

Until around 2012, the radial-velocity method (also known as [[Doppler spectroscopy]]) was by far the most productive technique used by planet hunters. (After 2012, the transit method from the [[Kepler space telescope]] overtook it in number.) The radial velocity signal is distance independent, but requires high [[signal-to-noise ratio]] spectra to achieve high precision, and so is generally used only for relatively nearby stars, out to about 160 light-years from Earth, to find lower-mass planets. It is also not possible to simultaneously observe many target stars at a time with a single telescope. Planets of Jovian mass can be detectable around stars up to a few thousand [[Light-year|light years]] away. This method easily finds massive planets that are close to stars. Modern [[spectrograph]]s can also easily detect Jupiter-mass planets orbiting 10 [[astronomical unit]]s away from the parent star, but detection of those planets requires many years of observation. Earth-mass planets are currently detectable only in very small orbits around low-mass stars, e.g. [[Proxima b]].

It is easier to detect planets around low-mass stars, for two reasons: First, these stars are more affected by gravitational tug from planets. The second reason is that low-mass main-sequence stars generally rotate relatively slowly. Fast rotation makes spectral-line data less clear because half of the star quickly rotates away from observer's viewpoint while the other half approaches. Detecting planets around more massive stars is easier if the star has left the main sequence, because leaving the main sequence slows down the star's rotation.

Sometimes Doppler spectrography produces false signals, especially in multi-planet and multi-star systems. Magnetic fields and certain types of stellar activity can also give false signals. When the host star has multiple planets, false signals can also arise from having insufficient data, so that multiple solutions can fit the data, as stars are not generally observed continuously.<ref>{{Cite journal|arxiv=1310.6907|last1=Auriere|first1=Michel|title=Pollux: A stable weak dipolar magnetic field but no planet ?|journal=Proceedings of the International Astronomical Union|volume=9|pages=359–362|last2=Konstantinova-Antova|first2=Renada|last3=Espagnet|first3=Olivier|last4=Petit|first4=Pascal|last5=Roudier|first5=Thierry|last6=Charbonnel|first6=Corinne|last7=Donati|first7=Jean-Francois|last8= Wade|first8=Gregg A.|year=2013|doi=10.1017/S1743921314002476|bibcode=2014IAUS..302..359A|s2cid=85549247}}</ref> Some of the false signals can be eliminated by analyzing the stability of the planetary system, conducting [[Photometry (astronomy)|photometry]] analysis on the host star and knowing its rotation period and stellar activity cycle periods.

Planets with orbits highly inclined to the line of sight from Earth produce smaller visible wobbles, and are thus more difficult to detect. One of the advantages of the radial velocity method is that eccentricity of the planet's orbit can be measured directly. One of the main disadvantages of the radial-velocity method is that it can only estimate a planet's [[minimum mass]] (<math>M_\text{true} * {\sin i} \, </math>). The posterior distribution of the inclination angle ''i'' depends on the true mass distribution of the planets.<ref>{{cite journal | bibcode = 2013PASP..125..933S | title=A Posteriori Transit Probabilities | journal=Publications of the Astronomical Society of the Pacific | volume=125 | issue=930 | pages=933–950 | year=2013 |author=Stevens, Daniel J. |author2=Gaudi, B. Scott | doi=10.1086/672572 | arxiv=1305.1298| s2cid=118494470 }}</ref> However, when there are multiple planets in the system that orbit relatively close to each other and have sufficient mass, orbital stability analysis allows one to constrain the maximum mass of these planets. The radial-velocity method can be used to confirm findings made by the [[#Transit photometry|transit method]]. When both methods are used in combination, then the planet's true mass can be estimated.

Although radial velocity of the star only gives a planet's minimum mass, if the planet's [[spectral line]]s can be distinguished from the star's spectral lines then the radial velocity of the planet itself can be found, and this gives the inclination of the planet's orbit. This enables measurement of the planet's actual mass. This also rules out false positives, and also provides data about the composition of the planet. The main issue is that such detection is possible only if the planet orbits around a relatively bright star and if the planet reflects or emits a lot of light.<ref>{{Cite journal|arxiv=1206.6197|last1=Rodler|first1=Florian|title=Weighing the Non-Transiting Hot Jupiter Tau BOO b|journal=The Astrophysical Journal|volume=753|issue=1|pages=L25|last2=Lopez-Morales|first2=Mercedes|last3=Ribas|first3=Ignasi|year=2012|doi=10.1088/2041-8205/753/1/L25|bibcode = 2012ApJ...753L..25R |s2cid=119177983}}</ref>

=== Transit photometry ===
{{See also|List of transiting exoplanets|Astronomical transit}}
[[File:Transit method variable-size planet 4K.webm|thumb|upright=1.4|Visualization of transit method for planets of different sizes, showing different light-curves.]]

====Technique, advantages, and disadvantages====
While the [[Doppler spectroscopy|radial velocity method]] provides information about a planet's mass, the [[Photometry (astronomy)|photometric]] method can determine the planet's radius. If a planet crosses ([[Astronomical transit|transits]]) in front of its parent star's disk, then the observed visual brightness of the star drops by a small amount, depending on the relative sizes of the star and the planet.<ref>{{Cite web|url=https://exoplanets.nasa.gov/5-ways-to-find-a-planet/#/2|title=5 Ways to Find a Planet|website=exoplanets.nasa.gov|access-date=2018-11-20}}</ref> For example, in the case of [[HD 209458]], the star dims by 1.7%. However, most transit signals are considerably smaller; for example, an Earth-size planet transiting a Sun-like star produces a dimming of only 80 parts per million (0.008 percent).

[[File:Curvas_de_luz_de_los_siete_planetas_de_TRAPPIST-1_durante_su_tránsito.png|alt=Lengthening brightness dips from 1b to 1h. Shallowest to deepest dips: 1h, 1d, 1e, 1f, 1g, 1c, 1b.|thumb|Graph showing dips in brightness in TRAPPIST-1 star by the planet's [[Astronomical transit|transits]] or obstruction of starlight. Larger planets create deeper dips and further planets create longer dips.]]

A theoretical transiting exoplanet light curve model predicts the following characteristics of an observed planetary system: transit depth (δ), transit duration (T), the ingress/egress duration (τ), and period of the exoplanet (P). However, these observed quantities are based on several assumptions. For convenience in the calculations, we assume that the planet and star are spherical, the stellar disk is uniform, and the orbit is circular. Depending on the relative position that an observed transiting exoplanet is while transiting a star, the observed physical parameters of the light curve will change. The transit depth (δ) of a transiting light curve describes the decrease in the normalized flux of the star during a transit. This details the radius of an exoplanet compared to the radius of the star. For example, if an exoplanet transits a solar radius size star, a planet with a larger radius would increase the transit depth and a planet with a smaller radius would decrease the transit depth. The transit duration (T) of an exoplanet is the length of time that a planet spends transiting a star. This observed parameter changes relative to how fast or slow a planet is moving in its orbit as it transits the star. The ingress/egress duration (τ) of a transiting light curve describes the length of time the planet takes to fully cover the star (ingress) and fully uncover the star (egress).  If a planet transits from the one end of the diameter of the star to the other end, the ingress/egress duration is shorter because it takes less time for a planet to fully cover the star. If a planet transits a star relative to any other point other than the diameter, the ingress/egress duration lengthens as you move further away from the diameter because the planet spends a longer time partially covering the star during its transit.<ref>{{Cite book|title=How Do You Find an Exoplanet?|last=Johnson|first=John|publisher=Princeton University Press|year=2015|isbn=978-0-691-15681-1|location=Princeton, NJ |pages=60–68}}</ref> From these observable parameters, a number of different physical parameters (semi-major axis, star mass, star radius, planet radius, eccentricity, and inclination) are determined through calculations. With the combination of radial velocity measurements of the star, the mass of the planet is also determined.

This method has two major disadvantages. First, planetary transits are observable only when the planet's orbit happens to be perfectly aligned from the astronomers' vantage point. The probability of a planetary [[Orbital plane (astronomy)|orbital plane]] being directly on the line-of-sight to a star is the ratio of the diameter of the star to the diameter of the orbit (in small stars, the radius of the planet is also an important factor). About 10% of planets with small orbits have such an alignment, and the fraction decreases for planets with larger orbits. For a planet orbiting a Sun-sized star at 1 [[Astronomical unit|AU]], the probability of a random alignment producing a transit is 0.47%. Therefore, the method cannot guarantee that any particular star is not a host to planets. However, by scanning large areas of the sky containing thousands or even hundreds of thousands of stars at once, transit surveys can find more extrasolar planets than the radial-velocity method.<ref>{{cite journal |author=Hidas, M. G. |title=The University of New South Wales Extrasolar Planet Search: methods and first results from a field centred on NGC 6633 |journal=[[Monthly Notices of the Royal Astronomical Society]] |date=2005 |volume=360 |issue=2 |pages=703–717 |bibcode=2005MNRAS.360..703H |doi=10.1111/j.1365-2966.2005.09061.x |arxiv=astro-ph/0501269 |last2=Ashley |first2=M. C. B. |last3=Webb |first3=J. K. |display-authors=3 |last4=Irwin |first4=M. |last5=Phillips |first5=A. |last6=Toyozumi |first6=H. |last7=Derekas |first7=A. |last8=Christiansen |first8=J. L. |last9=Nutto |first9=C. |last10=Crothers |first10=S.|doi-access=free |s2cid=197527136 }}</ref> Several surveys have taken that approach, such as the ground-based [[MEarth Project]], [[Wide Angle Search for Planets|SuperWASP]], [[Kilodegree Extremely Little Telescope|KELT]], and [[HATNet]], as well as the space-based [[COROT]], [[Kepler (spacecraft)|Kepler]] and [[Transiting Exoplanet Survey Satellite|TESS]] missions. The transit method has also the advantage of detecting planets around stars that are located a few thousand light years away. The most distant planets detected by [[Sagittarius Window Eclipsing Extrasolar Planet Search]] are located near the galactic center. However, reliable follow-up observations of these stars are nearly impossible with current technology.

The second disadvantage of this method is a high rate of false detections. A 2012 study found that the rate of false positives for transits observed by the [[Kepler mission]] could be as high as 40% in single-planet systems.<ref name="Santerne2012">{{Cite journal | last1 = Santerne | first1 = A. | last2 = Díaz | first2 = R. F. | last3 = Moutou | first3 = C. | last4 = Bouchy | first4 = F. | last5 = Hébrard | first5 = G. | last6 = Almenara | first6 = J. -M. | last7 = Bonomo | first7 = A. S. | last8 = Deleuil | first8 = M. | last9 = Santos | first9 = N. C. | title = SOPHIE velocimetry of Kepler transit candidates | doi = 10.1051/0004-6361/201219608 | journal = Astronomy & Astrophysics | volume = 545 | pages = A76 | year = 2012 |arxiv = 1206.0601 |bibcode = 2012A&A...545A..76S | s2cid = 119117782 }}</ref>  For this reason, a star with a single transit detection requires additional confirmation, typically from the radial-velocity method or orbital brightness modulation method. The radial velocity method is especially necessary for Jupiter-sized or larger planets, as objects of that size encompass not only planets, but also brown dwarfs and even small stars. As the false positive rate is very low in stars with two or more planet candidates, such detections often can be validated without extensive follow-up observations. Some can also be confirmed through the transit timing variation method.<ref>{{cite journal | author=O'Donovan |
title=Rejecting Astrophysical False Positives from the TrES Transiting Planet Survey: The Example of GSC 03885-00829 | journal=The Astrophysical Journal | date=2006 | volume=644 | issue=2 | pages=1237–1245 |
doi=10.1086/503740 | last2=Charbonneau | first2=David | last3=Torres | first3=Guillermo | last4=Mandushev | first4=Georgi | last5=Dunham | first5=Edward W. | last6=Latham | first6=David W. | last7=Alonso | first7=Roi | last8=Brown | first8=Timothy M. | last9=Esquerdo | first9=Gilbert A. |
last10=Everett |
first10=Mark E. |
last11=Creevey |
first11=Orlagh L. | bibcode=2006ApJ...644.1237O|arxiv = astro-ph/0603005 |
s2cid=119428457 |
display-authors=1 }}</ref><ref>{{cite web|url=http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=226|archive-url=https://web.archive.org/web/20130128110114/http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=226|archive-date=2013-01-28|title=Kepler: The Transit Timing Variation (TTV) Planet-finding Technique Begins to Flower|author=[NULL]|date=2015-03-31}}</ref><ref>{{cite web|url=http://www.nasa.gov/ames/kepler/nasas-kepler-mission-announces-a-planet-bonanza/|title=NASA's Kepler Mission Announces a Planet Bonanza, 715 New Worlds|work=NASA|date=2015-04-13|access-date=28 February 2014|archive-date=26 February 2014|archive-url=https://web.archive.org/web/20140226202703/http://www.nasa.gov/ames/kepler/nasas-kepler-mission-announces-a-planet-bonanza/|url-status=dead}}</ref>

Many points of light in the sky have brightness variations that may appear as transiting planets by flux measurements.  False-positives in the transit photometry method arise in three common forms: blended eclipsing binary systems, grazing eclipsing binary systems, and transits by planet sized stars. Eclipsing binary systems usually produce deep eclipses that distinguish them from exoplanet transits, since planets are usually smaller than about 2R<sub>J,</sub><ref>{{Cite book|title=Transiting Exoplanets|last=Haswell|first=Carole|publisher=Cambridge University Press|year=2010|isbn=978-0-521-13938-0|location=Cambridge|page=79}}</ref> but eclipses are shallower for blended or grazing eclipsing binary systems.

Blended eclipsing binary systems consist of a normal eclipsing binary blended with a third (usually brighter) star along the same line of sight, usually at a different distance. The constant light of the third star dilutes the measured eclipse depth, so the light-curve may resemble that for a transiting exoplanet. In these cases, the target most often contains a large main sequence primary with a small main sequence secondary or a giant star with a main sequence secondary.<ref>{{Cite journal|last=Collins|first=Karen|date=20 September 2018|title=The KELT Follow-Up Network and Transit False Positive Catalog: Pre-vetted False Positives for TESS|journal=Astrophysical Journal|volume=156|issue=5|page=234|arxiv=1803.01869|doi=10.3847/1538-3881/aae582|bibcode=2018AJ....156..234C|s2cid=119217050 |doi-access=free }}</ref>

Grazing eclipsing binary systems are systems in which one object will just barely graze the limb of the other. In these cases, the maximum transit depth of the light curve will not be proportional to the ratio of the squares of the radii of the two stars, but will instead depend solely on the small fraction of the primary that is blocked by the secondary. The small measured dip in flux can mimic that of an exoplanet transit. Some of the false positive cases of this category can be easily found if the eclipsing binary system has a circular orbit, with the two companions having different masses. Due to the cyclic nature of the orbit, there would be two eclipsing events, one of the primary occulting the secondary and vice versa. If the two stars have significantly different masses, and this different radii and luminosities, then these two eclipses would have different depths. This repetition of a shallow and deep transit event can easily be detected and thus allow the system to be recognized as a grazing eclipsing binary system. However, if the two stellar companions are approximately the same mass, then these two eclipses would be indistinguishable, thus making it impossible to demonstrate that a grazing eclipsing binary system is being observed using only the transit photometry measurements.
[[File:BrownDwarfComparison-pia12462.jpg|thumb|This image shows the relative sizes of brown dwarfs and large planets.]]

Finally, there are two types of stars that are approximately the same size as gas giant planets, white dwarfs and brown dwarfs. This is due to the fact that gas giant planets, white dwarfs, and brown dwarfs, are all supported by degenerate electron pressure. The light curve does not discriminate between masses as it only depends on the size of the transiting object. When possible, radial velocity measurements are used to verify that the transiting or eclipsing body is of planetary mass, meaning less than 13M<sub>J</sub>. Transit Time Variations can also determine M<sub>P</sub>. Doppler Tomography with a known radial velocity orbit can obtain minimum M<sub>P</sub> and projected sing-orbit alignment.

[[Red giant]] branch stars have another issue for detecting planets around them: while planets around these stars are much more likely to transit due to the larger star size, these transit signals are hard to separate from the main star's brightness light curve as red giants have frequent pulsations in brightness with a period of a few hours to days. This is especially notable with [[subgiant]]s. In addition, these stars are much more luminous, and transiting planets block a much smaller percentage of light coming from these stars. In contrast, planets can completely occult a very small star such as a neutron star or white dwarf, an event which would be easily detectable from Earth. However, due to the small star sizes, the chance of a planet aligning with such a stellar remnant is extremely small.

[[File:Exoplanet Mass-Radius Scatter Super-Earth.png|thumb|400px|Properties (mass and radius) of planets discovered using the transit method, compared with the distribution, n (light gray bar chart), of minimum masses of transiting and non-transiting exoplanets. [[Super-Earth]]s are black.]]
The main advantage of the transit method is that the size of the planet can be determined from the light curve. When combined with the radial-velocity method (which determines the planet's mass), one can determine the density of the planet, and hence learn something about the planet's physical structure. The planets that have been studied by both methods are by far the best-characterized of all known exoplanets.<ref name="charbonneautransitreview">{{cite conference | first=D. | last=Charbonneau |author2=T. Brown |author3=A. Burrows |author4= G. Laughlin  | title=When Extrasolar Planets Transit Their Parent Stars | book-title=Protostars and Planets V | publisher=University of Arizona Press | date=2006 | arxiv=astro-ph/0603376|bibcode = 2007prpl.conf..701C }}</ref>

The transit method also makes it possible to study the atmosphere of the transiting planet. When the planet transits the star, light from the star passes through the upper atmosphere of the planet. By studying the high-resolution [[stellar spectrum]] carefully, one can detect elements present in the planet's atmosphere.  A planetary atmosphere, and planet for that matter, could also be detected by measuring the polarization of the starlight as it passed through or is reflected off the planet's atmosphere.<ref>{{Cite journal|last=Burrows|first=Adam S.|date=September 2014|title=Highlights in the study of exoplanet atmospheres|journal=Nature|volume=513|issue=7518|pages=345–352|doi=10.1038/nature13782|pmid=25230656|issn=0028-0836|arxiv=1409.7320|bibcode=2014Natur.513..345B|s2cid=4469063}}</ref>

Additionally, the secondary eclipse (when the planet is blocked by its star) allows direct measurement of the planet's radiation and helps to constrain the planet's orbital eccentricity without needing the presence of other planets. If the star's [[photometry (astronomy)|photometric]] intensity during the secondary eclipse is subtracted from its intensity before or after, only the signal caused by the planet remains. It is then possible to measure the planet's temperature and even to detect possible signs of cloud formations on it. In March 2005, two groups of scientists carried out measurements using this technique with the [[Spitzer Space Telescope]]. The two teams, from the [[Harvard-Smithsonian Center for Astrophysics]], led by [[David Charbonneau]], and the [[Goddard Space Flight Center]], led by L. D. Deming, studied the planets [[TrES-1]] and [[HD 209458b]] respectively.  The measurements revealed the planets' temperatures: 1,060 [[Kelvin|K]] (790°[[Celsius|C]]) for TrES-1 and about 1,130 K (860&nbsp;°C) for HD 209458b.<ref name=":0">{{cite journal | author=Charbonneau | title=Detection of Thermal Emission from an Extrasolar Planet | journal=The Astrophysical Journal | date=2005 | volume=626 | issue=1 | pages=523–529 | doi=10.1086/429991 | last2=Allen | first2=Lori E.|author2-link=Lori Allen (astronomer) | last3=Megeath | first3=S. Thomas | last4=Torres | first4=Guillermo | last5=Alonso | first5=Roi | last6=Brown | first6=Timothy M. | last7=Gilliland | first7=Ronald L. | last8=Latham | first8=David W. | last9=Mandushev | first9=Georgi | last10=O'Donovan | first10=Francis T. | last11=Sozzetti | first11=Alessandro | bibcode=2005ApJ...626..523C|arxiv = astro-ph/0503457 | s2cid=13296966 | display-authors=1 }}</ref><ref name="Deming">{{cite journal | author=Deming, D. | author2=Seager, S. | author3=Richardson, J. | author4=Harrington, J. | title=Infrared radiation from an extrasolar planet | journal=Nature | date=2005 | volume=434 | issue=7034 | pages=740–743 | url=http://www.obspm.fr/encycl/papers/nature03507.pdf | doi=10.1038/nature03507 | pmid=15785769 | arxiv=astro-ph/0503554 | bibcode=2005Natur.434..740D | s2cid=4404769 | archive-url=https://web.archive.org/web/20060927012051/http://www.obspm.fr/encycl/papers/nature03507.pdf | archive-date=27 September 2006 | df=dmy-all }}</ref> In addition, the [[hot Neptune]] [[Gliese 436 b]] is known to enter secondary eclipse. However, some transiting planets orbit such that they do not enter secondary eclipse relative to Earth; [[HD 17156 b]] is over 90% likely to be one of the latter.

====History====
The first exoplanet for which transits were observed for [[HD 209458 b]], which was discovered using radial velocity technique. These transits were observed in 1999 by two teams led [[David Charbonneau]] and [[Gregory W. Henry]].<ref>{{cite journal | title=Detection of Planetary Transits Across a Sun-like Star| url=https://ui.adsabs.harvard.edu/abs/2000ApJ...529L..45C/abstract| last1=Charbonneau | first1=David | last2=Brown | first2=Timothy M. | last3=Latham | first3=David W. | last4=Mayor | first4=Michel | display-authors=1 | journal=Astrophysical Journal Letters | volume=529 | issue=1 | pages=45–48 | year=2000 | doi=10.1086/312457| pmid=10615033| arxiv=astro-ph/9911436 | bibcode=2000ApJ...529L..45C| s2cid=16152423}}</ref><ref>{{cite journal | title=A Transiting ''51 Peg-like'' Planet| last1=Henry|first1=Gregory W. |last2=Marcy|first2=Geoffrey W.| last3=Butler|first3=R. Paul|last4= Vogt|first4= Steven S.| display-authors=1 | journal=Astrophysical Journal Letters | volume=529 | issue=1 | pages=41–44 | year=2000 |doi=10.1086/312458 |pmid=10615032 | bibcode=2000ApJ...529L..41H|s2cid=18193299 |doi-access=free}}</ref><ref>[https://exoplanets.nasa.gov/alien-worlds/historic-timeline/#first-transiting-exoplanet-observed "Historic timeline"]</ref> The first exoplanet to be discovered with the transit method was [[OGLE-TR-56b]] in 2002 by the [[Optical Gravitational Lensing Experiment|OGLE]] project.<ref>{{cite journal | title=The Optical Gravitational Lensing Experiment. Search for Planetary and Low-Luminosity Object Transits in the Galactic Disk. Results of 2001 Campaign - Supplement | url=http://acta.astrouw.edu.pl/Vol52/n2/a_52_2_1.html | last1=Udalski | first1=A. | last2=Żebruń | first2=K. | last3=Szymański | first3=M. | last4=Kubiak | first4=M. | last5=Soszyński | first5=I. | last6=Szewczyk | first6=O. | last7=Wyrzykowski | first7=L. | last8=Pietrzyński | first8=G. | display-authors=1 | journal=Acta Astronomica | volume=52 | issue=2 | pages=115–128 | year=2002 | arxiv=astro-ph/0207133 | bibcode=2002AcA....52..115U }}</ref><ref>{{cite journal |author=[[Harvard University]] and [[Smithsonian Institution]] |date=2003-01-08 |title=New World of Iron Rain |url=http://www.astrobio.net/pressrelease/352/new-world-of-iron-rain |url-status=usurped |journal=Astrobiology Magazine |archive-url=https://web.archive.org/web/20100110135215/http://www.astrobio.net/pressrelease/352/new-world-of-iron-rain |archive-date=2010-01-10 |access-date=2010-01-25}}</ref><ref name="harvardgazette2">{{cite news |last=Cromie |first=William J. |date=2003-01-16 |title=New, far-out planet is discovered |work=Harvard Gazette |publisher=Harvard University |url=http://www.news.harvard.edu/gazette/2003/01.16/01-ogle.html |access-date=21 July 2010 |archive-url=https://web.archive.org/web/20090827072608/http://www.news.harvard.edu/gazette/2003/01.16/01-ogle.html |archive-date=27 August 2009}}</ref>

A [[CNES|French Space Agency]] mission, [[COROT|CoRoT]], began in 2006 to search for planetary transits from orbit, where the absence of atmospheric [[Scintillation (astronomy)|scintillation]] allows improved accuracy.  This mission was designed to be able to detect planets "a few times to several times larger than Earth" and performed "better than expected", with two exoplanet discoveries<ref>"COROT surprises a year after launch", [http://www.esa.int/SPECIALS/COROT/SEMF0C2MDAF_0.html ESA press release 20 December 2007]</ref> (both of the "hot Jupiter" type) as of early 2008. In June 2013, CoRoT's exoplanet count was 32 with several still to be confirmed. The satellite unexpectedly stopped transmitting data in November 2012 (after its mission had twice been extended), and was retired in June 2013.<ref>"01/2014 – CoRoT: collision evading and decommissioning", [http://smsc.cnes.fr/COROT/GP_actualite.htm#112012 CNES CoRoT News]</ref>

In March 2009, [[NASA]] mission [[Kepler Mission|Kepler]] was launched to scan a large number of stars in the constellation [[Cygnus (constellation)|Cygnus]] with a measurement precision expected to detect and characterize Earth-sized planets. The NASA [[Kepler Mission]] uses the transit method to scan a hundred thousand stars for planets. It was hoped that by the end of its mission of 3.5 years, the satellite would have collected enough data to reveal planets even smaller than Earth. By scanning a hundred thousand stars simultaneously, it was not only able to detect Earth-sized planets, it was able to collect statistics on the numbers of such planets around Sun-like stars.<ref>[https://web.archive.org/web/20051118033324/http://kepler.nasa.gov/about/ Kepler Mission page]</ref>

On 2 February 2011, the Kepler team released a list of 1,235 extrasolar planet candidates, including 54 that may be in the [[habitable zone]]. On 5 December 2011, the Kepler team announced that they had discovered 2,326 planetary candidates, of which 207 are similar in size to Earth, 680 are super-Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter. Compared to the February 2011 figures, the number of Earth-size and super-Earth-size planets increased by 200% and 140% respectively. Moreover, 48 planet candidates were found in the habitable zones of surveyed stars, marking a decrease from the February figure; this was due to the more stringent criteria in use in the December data. By June 2013, the number of planet candidates was increased to 3,278 and some confirmed planets were smaller than Earth, some even Mars-sized (such as [[Kepler-62c]]) and one even smaller than Mercury ([[Kepler-37b]]).<ref>{{cite web|url=http://exoplanetarchive.ipac.caltech.edu/|title=NASA Exoplanet Archive}}</ref>

The [[Transiting Exoplanet Survey Satellite]] launched in April 2018.

=== Reflection and emission modulations ===
Short-period planets in close orbits around their stars will undergo reflected light variations because, like the [[Moon]], they will go through [[Planetary phase|phases]] from full to new and back again. In addition, as these planets receive a lot of starlight, it heats them, making thermal emissions potentially detectable. Since telescopes cannot resolve the planet from the star, they see only the combined light, and the brightness of the host star seems to change over each orbit in a periodic manner. Although the effect is small — the photometric precision required is about the same as to detect an Earth-sized planet in transit across a solar-type star – such Jupiter-sized planets with an orbital period of a few days are detectable by space telescopes such as the [[Kepler Space Observatory]]. Like with the transit method, it is easier to detect large planets orbiting close to their parent star than other planets as these planets catch more light from their parent star. When a planet has a high albedo and is situated around a relatively luminous star, its light variations are easier to detect in visible light while darker planets or planets around low-temperature stars are more easily detectable with infrared light with this method. In the long run, this method may find the most planets that will be discovered by that mission because the reflected light variation with orbital phase is largely independent of orbital inclination and does not require the planet to pass in front of the disk of the star. It still cannot detect planets with circular face-on orbits from Earth's viewpoint as the amount of reflected light does not change during its orbit.

The phase function of the giant planet is also a function of its thermal properties and atmosphere, if any. Therefore, the phase curve may constrain other planet properties, such as the size distribution of atmospheric particles. When a planet is found transiting and its size is known, the phase variations curve helps calculate or constrain the planet's [[albedo]]. It is more difficult with very hot planets as the glow of the planet can interfere when trying to calculate albedo. In theory, albedo can also be found in non-transiting planets when observing the light variations with multiple wavelengths. This allows scientists to find the size of the planet even if the planet is not transiting the star.<ref>{{cite journal|last=Jenkins|first=J.M.|author2=Laurance R. Doyle|date=20 September 2003|title=Detecting reflected light from close-in giant planets using space-based photometers|journal=Astrophysical Journal|volume=1|issue=595|pages=429–445|doi=10.1086/377165|bibcode=2003ApJ...595..429J|arxiv = astro-ph/0305473 |s2cid=17773111}}</ref>

The first-ever direct detection of the spectrum of visible light reflected from an exoplanet was made in 2015 by an international team of astronomers. The astronomers studied light from [[51 Pegasi b]] – the first exoplanet discovered orbiting a [[main-sequence]] star (a [[Sunlike star]]), using the High Accuracy Radial velocity Planet Searcher (HARPS) instrument at the European Southern Observatory's La Silla Observatory in Chile.<ref>[http://physicsworld.com/cws/article/news/2015/apr/22/first-visible-light-detected-directly-from-an-exoplanet physicsworld.com 2015-04-22 First visible light detected directly from an exoplanet]</ref><ref>{{Cite journal|title=Evidence for a spectroscopic direct detection of reflected light from 51 Pegasi b|journal=Astronomy & Astrophysics|volume=576|pages=A134|doi=10.1051/0004-6361/201425298|year=2015|last1=Martins|first1=J. H. C.|last2=Santos|first2=N. C.|last3=Figueira|first3=P.|last4=Faria|first4=J. P.|last5=Montalto|first5=M.|last6=Boisse|first6=I.|last7=Ehrenreich|first7=D.|last8=Lovis|first8=C.|last9=Mayor|first9=M.|last10=Melo|first10=C.|last11=Pepe|first11=F.|last12=Sousa|first12=S. G.|last13=Udry|first13=S.|last14=Cunha|first14=D. |display-authors=5|arxiv=1504.05962|bibcode=2015A&A...576A.134M|s2cid=119224213}}</ref>

Both CoRoT<ref>{{cite journal
 |title=The changing phases of extrasolar planet CoRoT-1b
 |author=Snellen, I.A.G. |author2=De Mooij, E.J.W. |author3=Albrecht, S. |name-list-style=amp
 |journal=Nature
 |volume=459
 |issue=7246
 |pages=543–545
 |date=2009
 |doi=10.1038/nature08045|bibcode = 2009Natur.459..543S
 |pmid=19478779|arxiv = 0904.1208 |s2cid=4347612 }}</ref> and Kepler<ref name="kepler_phases">{{cite journal  | display-authors = 1| last1 = Borucki | first1 = W.J. | last2 = Koch | first2 = D. | last3 = Jenkins | first3 = J. | last4 = Sasselov | first4 = D. | last5 = Gilliland | first5 = R. | last6 = Batalha | first6 = N. | last7 = Latham | first7 = D. W. | last8 = Caldwell | first8 = D. | last9 = Basri | first9 = G. | last10 = Brown | first10 = T. | last11 = Christensen-Dalsgaard | first11 = J. | last12 = Cochran | first12 = W. D. | last13 = Devore | first13 = E. | last14 = Dunham | first14 = E. | last15 = Dupree | first15 = A. K. | last16 = Gautier | first16 = T. | last17 = Geary | first17 = J. | last18 = Gould | first18 = A. | last19 = Howell | first19 = S. | last20 = Kjeldsen | first20 = H. | last21 = Lissauer | first21 = J. | last22 = Marcy | first22 = G. | last23 = Meibom | first23 = S. | last24 = Morrison | first24 = D. | last25 = Tarter | first25 = J. | title = Kepler's Optical Phase Curve of the Exoplanet HAT-P-7b | journal = Science | volume = 325 | issue = 5941 | page = 709 | date = 2009 | doi = 10.1126/science.1178312 | bibcode = 2009Sci...325..709B | pmid = 19661420| s2cid = 206522122 | url = https://zenodo.org/record/1230908 | type = Submitted manuscript }}</ref> have measured the reflected light from planets. However, these planets were already known since they transit their host star. The first planets discovered by this method are [[Kepler-70b]] and [[Kepler-70c]], found by Kepler.<ref>{{cite journal |title=A compact system of small planets around a former red-giant star
  |author=Charpinet, S. |display-authors=4 |author2=Fontaine, G. |author3=Brassard, P. |author4=Green, E.M. |author5=Van Grootel, V. |author6=Randall, S.K. |author7=Silvotti, R. |author8=Baran, A.S. |author9=Østensen, R.H. |author10=Kawaler, S.D. |name-list-style=amp
  |journal=Nature
  |volume=480
  |issue=7378
  |pages=496–499
  |date=2011
  |doi=10.1038/nature10631
  |pmid=22193103 |bibcode = 2011Natur.480..496C |s2cid=2213885 }}</ref>

=== Relativistic beaming ===
{{main|Relativistic beaming}}
A separate novel method to detect exoplanets from light variations uses relativistic beaming of the observed flux from the star due to its motion. It is also known as Doppler beaming or Doppler boosting. The method was first proposed by [[Abraham Loeb]] and Scott Gaudi in 2003.<ref>{{Cite journal|doi= 10.1086/375551|arxiv=astro-ph/0303212|title=Periodic Flux Variability of Stars due to the Reflex Doppler Effect Induced by Planetary Companions|journal=The Astrophysical Journal|volume=588|issue=2|pages=L117|year=2003|last1=Loeb|first1=Abraham|last2=Gaudi|first2=B. Scott|bibcode = 2003ApJ...588L.117L |s2cid=10066891}}</ref> As the planet tugs the star with its gravitation, the density of photons and therefore the apparent brightness of the star changes from observer's viewpoint. Like the radial velocity method, it can be used to determine the orbital eccentricity and the minimum mass of the planet. With this method, it is easier to detect massive planets close to their stars as these factors increase the star's motion. Unlike the radial velocity method, it does not require an accurate spectrum of a star, and therefore can be used more easily to find planets around fast-rotating stars and more distant stars.

One of the biggest disadvantages of this method is that the light variation effect is very small. A Jovian-mass planet orbiting 0.025 AU away from a Sun-like star is barely detectable even when the orbit is edge-on. This is not an ideal method for discovering new planets, as the amount of emitted and reflected starlight from the planet is usually much larger than light variations due to relativistic beaming. This method is still useful, however, as it allows for measurement of the planet's mass without the need for follow-up data collection from radial velocity observations.

The first discovery of a planet using this method ([[Kepler-76b]]) was announced in 2013.<ref>{{Cite journal|arxiv=1304.6841|last1=Faigler|first1=Simchon|title=BEER analysis of Kepler and CoRoT light curves: I. Discovery of Kepler-76b: A hot Jupiter with evidence for superrotation|journal=The Astrophysical Journal|volume=771|issue=1|page=26|last2=Tal-Or|first2=Lev|last3=Mazeh|first3=Tsevi|last4= Latham|first4=Dave W.|last5= Buchhave|first5=Lars A.|year=2013|doi=10.1088/0004-637X/771/1/26|bibcode = 2013ApJ...771...26F |s2cid=119247392}}</ref><ref>[http://phys.org/news/2013-05-method-planets-scores-discovery.html New method of finding planets scores its first discovery], phys.org, May 2013</ref>

=== Ellipsoidal variations ===
Massive planets can cause slight tidal distortions to their host stars. When a star has a slightly ellipsoidal shape, its apparent brightness varies, depending if the oblate part of the star is facing the observer's viewpoint. Like with the relativistic beaming method, it helps to determine the minimum mass of the planet, and its sensitivity depends on the planet's orbital inclination. The extent of the effect on a star's apparent brightness can be much larger than with the relativistic beaming method, but the brightness changing cycle is twice as fast. In addition, the planet distorts the shape of the star more if it has a low semi-major axis to stellar radius ratio and the density of the star is low. This makes this method suitable for finding planets around stars that have left the main sequence.<ref>{{cite web|url=http://www.universetoday.com/102112/using-the-theory-of-relativity-and-beer-to-find-exoplanets/|title=Using the Theory of Relativity and BEER to Find Exoplanets - Universe Today|work=Universe Today|date=2013-05-13}}</ref>

=== Pulsar timing ===
{{About|the exoplanet detection method|the timing device|Pulsar clock|section=yes}}
{{See also|List of exoplanets detected by timing|Pulsar planet}}
[[File:Artist's concept of PSR B1257+12 system.jpg|thumb|right|220px|Artist's impression of the pulsar [[PSR B1257+12|PSR 1257+12]]'s planetary system.]]
A [[pulsar]] is a neutron star:  the small, ultradense remnant of a star that has exploded as a [[supernova]].  Pulsars emit radio waves extremely regularly as they rotate.  Because the intrinsic rotation of a pulsar is so regular, slight anomalies in the timing of its observed radio pulses can be used to track the pulsar's motion. Like an ordinary star, a pulsar will move in its own small orbit if it has a planet. Calculations based on pulse-timing observations can then reveal the parameters of that orbit.<ref name="EXP3">{{cite web |author=Townsend, Rich |title=The Search for Extrasolar Planets |type=Lecture |website=University College London |date=27 January 2003 |url=http://www.star.ucl.ac.uk/~rhdt/diploma/lecture_2/ |access-date=2006-09-10 |archive-url=https://web.archive.org/web/20050915080856/http://www.star.ucl.ac.uk/~rhdt/diploma/lecture_2/ |archive-date=15 September 2005 }}</ref>

This method was not originally designed for the detection of planets, but is so sensitive that it is capable of detecting planets far smaller than any other method can, down to less than a tenth the mass of Earth. It is also capable of detecting mutual gravitational perturbations between the various members of a planetary system, thereby revealing further information about those planets and their orbital parameters. In addition, it can easily detect planets which are relatively far away from the pulsar.

There are two main drawbacks to the pulsar timing method: pulsars are relatively rare, and special circumstances are required for a planet to form around a pulsar. Therefore, it is unlikely that a large number of planets will be found this way.<ref>{{Cite journal|arxiv=1308.6308|last1= Sinukoff|first1= E.|title= Below One Earth Mass: The Detection, Formation, and Properties of Subterrestrial Worlds|journal= Space Science Reviews|volume= 180|issue= 1–4|page= 71|last2=  Fulton|first2= B.|last3=  Scuderi|first3= L.|last4=  Gaidos|first4= E.|year= 2013|doi= 10.1007/s11214-013-0019-1|bibcode = 2013SSRv..180...71S |s2cid= 118597064}}</ref> Additionally, life would likely not survive on planets orbiting pulsars due to the high intensity of ambient radiation.

In 1992, [[Aleksander Wolszczan]] and [[Dale Frail]] used this method to discover planets around the pulsar [[PSR 1257+12]].<ref name=wolszczan>{{cite journal | author = [[Aleksander Wolszczan|A. Wolszczan]] and [[Dale Frail|D. A. Frail]] | title = A planetary system around the millisecond pulsar PSR1257+12 | journal = Nature|volume= 355 | issue = 6356 |pages= 145–147 | date = 9 January 1992 | url = http://www.nature.com/physics/looking-back/wolszczan/index.html | access-date = 2007-04-30 |bibcode = 1992Natur.355..145W |doi = 10.1038/355145a0 | last2 = Frail | s2cid = 4260368 }}</ref> Their discovery was confirmed by 1994, making it the first confirmation of planets outside the [[Solar System]].<ref>{{cite journal | url=http://www.astro.psu.edu/~niel/astro550/week07-wolszczan-pulsar-planets.pdf | last=Wolszczan |first=A. | title=Confirmation of Earth Mass Planets Orbiting the Millisecond Pulsar PSR B1257+12 | journal=[[Science (journal)|Science]] | volume=264 | issue=5158 | pages=538–542 | date=1994 | doi=10.1126/science.264.5158.538 | pmid=17732735 | bibcode=1994Sci...264..538W | s2cid=19621191 | url-status=dead | archive-url=https://web.archive.org/web/20110720034147/http://www2.astro.psu.edu/~niel/astro550/week07-wolszczan-pulsar-planets.pdf | archive-date=2011-07-20 }}</ref>

=== Variable star timing ===
Like pulsars, some other types of [[Variable star#Pulsating variable stars|pulsating variable stars]] are regular enough that [[radial velocity]] could be determined purely [[photometry (astronomy)|photometrically]] from the [[Doppler shift]] of the pulsation frequency, without needing [[spectroscopy]].<ref>{{Cite journal|arxiv=1202.0105|last1=Shibahashi|first1=Hiromoto|title=FM stars: A Fourier view of pulsating binary stars, a new technique for measuring radial velocities photometrically|journal=Monthly Notices of the Royal Astronomical Society|volume=422|issue=1|page=738|last2= Kurtz|first2=Donald W.|year=2012|doi=10.1111/j.1365-2966.2012.20654.x|doi-access=free |bibcode = 2012MNRAS.422..738S |s2cid=54949889}}</ref><ref>{{cite web|url=http://www.nasa.gov/mission_pages/kepler/news/keplerm-20120710_prt.htm|title=NASA - Mission Manager Update|access-date=25 July 2012|archive-date=8 August 2020|archive-url=https://web.archive.org/web/20200808070113/https://www.nasa.gov/mission_pages/kepler/news/keplerm-20120710_prt.htm|url-status=dead}}</ref> This method is not as sensitive as the pulsar timing variation method, due to the periodic activity being longer and less regular. The ease of detecting planets around a variable star depends on the pulsation period of the star, the regularity of pulsations, the mass of the planet, and its distance from the host star.

The first success with this method came in 2007, when [[V391 Pegasi b]] was discovered around a pulsating subdwarf star.<ref>{{cite journal|title=A giant planet orbiting the /'extreme horizontal branch/' star V&thinsp;391 Pegasi | volume=449 |issue=7159 | doi=10.1038/nature06143|pmid=17851517 |journal=Nature|pages=189–191|bibcode = 2007Natur.449..189S|year=2007 | last1 = Silvotti | first1 = R.| s2cid=4342338 | url=https://authors.library.caltech.edu/17168/2/Silvotti2007p5743Nature_supp.pdf }}</ref>

=== Transit timing ===
{{Main|Transit-timing variation}}
[[File:201008-2a PlanetOrbits 16x9- Transit timing of 1-planet vs 2-planet systems.ogv|thumb|250px|Animation showing difference between planet transit timing of 1-planet and 2-planet systems. Credit: NASA/Kepler Mission.]]
[[File:Keplerspacecraft-20110215.jpg|250px|thumb|right|The [[Kepler Mission]], A NASA mission which is able to detect extrasolar planets]]
The transit timing variation method considers whether transits occur with strict periodicity, or if there is a variation. When multiple transiting planets are detected, they can often be confirmed with the transit timing variation method. This is useful in planetary systems far from the Sun, where radial velocity methods cannot detect them due to the low signal-to-noise ratio. If a planet has been detected by the transit method, then variations in the timing of the transit provide an extremely sensitive method of detecting additional non-transiting planets in the system with masses comparable to Earth's. It is easier to detect transit-timing variations if planets have relatively close orbits, and when at least one of the planets is more massive, causing the orbital period of a less massive planet to be more perturbed.<ref>{{cite journal|author1=Miralda-Escude|title=Orbital perturbations on transiting planets: A possible method to measure stellar quadrupoles and to detect Earth-mass planets|date=2001|doi=10.1086/324279|journal=The Astrophysical Journal|volume=564|issue=2|pages=1019–1023|arxiv=astro-ph/0104034|bibcode = 2002ApJ...564.1019M |s2cid=7536842}}</ref><ref>{{cite journal |author1=Holman |author2=Murray| title=The Use of Transit Timing to Detect Extrasolar Planets with Masses as Small as Earth|date=2005 |doi=10.1126/science.1107822 |pmid=15731449|volume=307 |issue=5713 | pages = 1288–1291 |journal=Science |arxiv=astro-ph/0412028 |bibcode=2005Sci...307.1288H|s2cid=41861725}}</ref><ref>{{cite journal|author1=Agol|author2=Sari|author3=Steffen|author4=Clarkson|title=On detecting terrestrial planets with timing of giant planet transits|year=2005|doi=10.1111/j.1365-2966.2005.08922.x|journal=Monthly Notices of the Royal Astronomical Society|volume=359|issue=2|pages=567–579 |doi-access=free |arxiv=astro-ph/0412032|bibcode = 2005MNRAS.359..567A |s2cid=16196696}}</ref>

The main drawback of the transit timing method is that usually not much can be learnt about the planet itself. Transit timing variation can help to determine the maximum mass of a planet. In most cases, it can confirm if an object has a planetary mass, but it does not put narrow constraints on its mass. There are exceptions though, as planets in the [[Kepler-36]] and [[Kepler-88]] systems orbit close enough to accurately determine their masses.

The first significant detection of a non-transiting planet using TTV was carried out with NASA's [[Kepler space telescope]].  The transiting planet [[Kepler-19b]] shows TTV with an amplitude of five minutes and a period of about 300 days, indicating the presence of a second planet, [[Kepler-19c]], which has a period which is a near-rational multiple of the period of the transiting planet.<ref>[https://web.archive.org/web/20111019083552/http://kepler.nasa.gov/news/nasakeplernews/index.cfm?FuseAction=ShowNews&NewsID=148 Invisible World Discovered], NASA Kepler News, 8 September 2011</ref><ref>{{Cite journal|arxiv=1109.1561|author1=Sarah Ballard |display-authors=4 |author2=Daniel Fabrycky |title=The Kepler-19 System: A Transiting 2.2 R_Earth Planet and a Second Planet Detected via Transit Timing Variations|journal=The Astrophysical Journal |volume=743 |issue=2 |page=200 |date=2011 |author3=Francois Fressin |author4=David Charbonneau |author5=Jean-Michel Desert |author6=Guillermo Torres |author7=Geoffrey Marcy |author8=Burke |author9=Howard Isaacson|last10=Henze|first10=Christopher|last11= Steffen|first11=Jason H.|last12= Ciardi|first12=David R.|last13= Howell|first13=Steven B.|last14= Cochran|first14=William D.|last15=Endl|first15=Michael|last16= Bryson|first16=Stephen T.|last17= Rowe|first17=Jason F.|last18= Holman|first18=Matthew J.|last19= Lissauer|first19=Jack J.|last20= Jenkins|first20=Jon M.|last21=Still|first21=Martin|last22= Ford|first22=Eric B.|last23= Christiansen|first23=Jessie L.|last24= Middour|first24=Christopher K.|last25= Haas|first25=Michael R.|last26=Li|first26=Jie|last27= Hall|first27=Jennifer R.|last28=McCauliff|first28=Sean|last29= Batalha|first29=Natalie M.|last30= Koch|first30=David G.|doi=10.1088/0004-637X/743/2/200 |bibcode = 2011ApJ...743..200B |s2cid=42698813 }}</ref>

In [[circumbinary planet]]s, variations of transit timing are mainly caused by the orbital motion of the stars, instead of gravitational perturbations by other planets. These variations make it harder to detect these planets through automated methods. However, it makes these planets easy to confirm once they are detected.{{citation needed|date=July 2015}}

=== Transit duration variation ===
"Duration variation" refers to changes in how long the transit takes. Duration variations may be caused by an [[exomoon]], [[apsidal precession]] for eccentric planets due to another planet in the same system, or [[Tests of general relativity#Perihelion precession of Mercury|general relativity]].<ref>{{cite arXiv |eprint=1009.5905 |author1=Nascimbeni |author2=Piotto |author3=Bedin |author4=Damasso |title=TASTE: The Asiago Survey for Timing transit variations of Exoplanets |class=astro-ph.EP |date=2008}}</ref><ref>{{Cite journal|arxiv=0806.0629 |author1=Pal |author2=Kocsis |title=Periastron Precession Measurements in Transiting Extrasolar Planetary Systems at the Level of General Relativity |journal=Monthly Notices of the Royal Astronomical Society |volume=389 |issue=2008 |pages=191–198 |date=2008|doi=10.1111/j.1365-2966.2008.13512.x |doi-access=free |bibcode = 2008MNRAS.389..191P |s2cid=15282437 }}</ref>

When a circumbinary planet is found through the transit method, it can be easily confirmed with the transit duration variation method.<ref name="ReferenceA">{{Cite journal|arxiv=1308.6328|last1= Welsh|first1= William F.|title= Recent Kepler Results on Circumbinary Planets|journal= Proceedings of the International Astronomical Union|volume= 8|pages= 125–132|last2=  Orosz|first2= Jerome A.|last3=  Carter|first3= Joshua A.|last4=  Fabrycky|first4= Daniel C.|year= 2013|doi= 10.1017/S1743921313012684|bibcode= 2014IAUS..293..125W|s2cid= 119230654}}</ref> In close binary systems, the stars significantly alter the motion of the companion, meaning that any transiting planet has significant variation in transit duration. The first such confirmation came from [[Kepler-16b]].<ref name="ReferenceA"/>

=== Eclipsing binary minima timing ===
When a [[binary star]] system is aligned such that – from the Earth's point of view – the stars pass in front of each other in their orbits, the system is called an "eclipsing binary" star system. The time of minimum light, when the star with the brighter surface is at least partially obscured by the disc of the other star, is called the primary [[eclipse]], and approximately half an orbit later, the secondary eclipse occurs when the brighter surface area star obscures some portion of the other star. These times of minimum light, or central eclipses, constitute a time stamp on the system, much like the pulses from a [[pulsar]] (except that rather than a flash, they are a dip in brightness). If there is a planet in circumbinary orbit around the binary stars, the stars will be offset around a binary-planet [[center of mass]]. As the stars in the binary are displaced back and forth by the planet, the times of the eclipse minima will vary. The periodicity of this offset may be the most reliable way to detect extrasolar planets around close binary systems.<ref>{{cite journal|last1=Doyle|first1=Laurance R. |first2=Hans-Jorg|last2=Deeg|date=2002|title=Timing detection of eclipsing binary planets and transiting extrasolar moons|page=80|journal=Bioastronomy|volume=7|bibcode=2004IAUS..213...80D|arxiv = astro-ph/0306087 }} "Bioastronomy 2002: Life Among the Stars" IAU Symposium 213, R.P Norris and F.H. Stootman (eds), A.S.P., San Francisco, California, 80–84.</ref><ref>{{cite journal
|last1=Deeg|first1=Hans-Jorg
|last2=Doyle|first2=Laurance R.
|last3=Kozhevnikov|first3=V. P.
|last4=Blue|first4=J. Ellen
|last5=Martín|first5=L.
|last6=Schneider|first6=J.
|date=2000|title=A search for Jovian-mass planets around CM Draconis using eclipse minima timing|journal=Astronomy & Astrophysics|volume=358|issue=358|pages=L5–L8|url=http://citeseer.ist.psu.edu/379779.html|bibcode=2000A&A...358L...5D|arxiv = astro-ph/0003391 }}</ref><ref>Doyle, Laurance R., Hans-Jorg Deeg, J.M. Jenkins, J. Schneider, Z. Ninkov, R. P.S. Stone, J.E. Blue, H. Götzger, B, Friedman, and M.F. Doyle (1998). [http://citeseer.ist.psu.edu/doyle97detectability.html "Detectability of Jupiter-to-brown-dwarf-mass companions around small eclipsing binary systems"]. Brown Dwarfs and Extrasolar Planets, A.S.P. Conference Proceedings, in Brown Dwarfs and Extrasolar Planets,  R. Rebolo, E. L. Martin, and M.R.Z. Osorio (eds.), A.S.P. Conference Series 134, San Francisco, California, 224–231.</ref> With this method, planets are more easily detectable if they are more massive, orbit relatively closely around the system, and if the stars have low masses.

The eclipsing timing method allows the detection of planets further away from the host star than the transit method. However, signals around [[cataclysmic variable]] stars hinting for planets tend to match with unstable orbits.{{clarify|date=July 2015}}<ref>{{Cite arXiv|eprint=1302.5247|last1=Horner|first1=Jonathan|title=Dynamical Constraints on Multi-Planet Exoplanetary Systems|last2= Wittenmyer|first2=Robert A.|last3= Tinney|first3=Chris G.|last4=Robertson|first4=Paul|last5= Hinse|first5=Tobias C.|last6= Marshall|first6=Jonathan P.|class=astro-ph.EP|year=2013}}</ref> In 2011, Kepler-16b became the first planet to be definitely characterized via eclipsing binary timing variations.<ref>{{cite journal | bibcode = 2011Sci...333.1602D | title=Kepler-16: A Transiting Circumbinary Planet | journal=Science | volume=333 | issue=6049 | pages=1602–1606 | year=2011 |author=Doyle, Laurance R. |author2=Carter, Joshua A. |author3=Fabrycky, Daniel C. |author4=Slawson, Robert W. |author5=Howell, Steve B. |author6=Winn, Joshua N. |author7=Orosz, Jerome A. |author8=Přsa, Andrej |author9=Welsh, William F. |author10=Quinn, Samuel N. |author11=Latham, David |author12=Torres, Guillermo |author13=Buchhave, Lars A. |author14=Marcy, Geoffrey W. |author15=Fortney, Jonathan J. |author16=Shporer, Avi |author17=Ford, Eric B. |author18=Lissauer, Jack J. |author19=Ragozzine, Darin |author20=Rucker, Michael |author21=Batalha, Natalie |author22=Jenkins, Jon M. |author23=Borucki, William J. |author24=Koch, David |author25=Middour, Christopher K. |author26=Hall, Jennifer R. |author27=McCauliff, Sean |author28=Fanelli, Michael N. |author29=Quintana, Elisa V. |author30=Holman, Matthew J. |author31=Caldwell, Douglas A. |author32=Still, Martin |author33=Stefanik, Robert P. |author34=Brown, Warren R. |author35=Esquerdo, Gilbert A. |author36=Tang, Sumin |author37=Furesz, Gabor |author38=Geary, John C. |author39=Berlind, Perry |author40=Calkins, Michael L. |author41=Short, Donald R. |author42=Steffen, Jason H. |author43=Sasselov, Dimitar |author44=Dunham, Edward W. |author45=Cochran, William D. |author46=Boss, Alan |author47=Haas, Michael R. |author48=Buzasi, Derek |author49=Fischer, Debra | doi=10.1126/science.1210923 | pmid=21921192|arxiv = 1109.3432 | s2cid=206536332 }}</ref>

=== Gravitational microlensing ===
{{Main|Gravitational microlensing|List of exoplanets detected by microlensing}}
[[File:Gravitational micro rev.svg|thumb|right|255px|Gravitational microlensing.]]
Gravitational microlensing occurs when the gravitational field of a star acts like a lens, magnifying the light of a distant background star. This effect occurs only when the two stars are almost exactly aligned. Lensing events are brief, lasting for weeks or days, as the two stars and Earth are all moving relative to each other.  More than a thousand such events have been observed over the past ten years.

If the foreground lensing star has a planet, then that planet's own gravitational field can make a detectable contribution to the lensing effect. Since that requires a highly improbable alignment, a very large number of distant stars must be continuously monitored in order to detect planetary microlensing contributions at a reasonable rate. This method is most fruitful for planets between Earth and the center of the galaxy, as the galactic center provides a large number of background stars.

In 1991, astronomers Shude Mao and [[Bohdan Paczyński]] proposed using gravitational microlensing to look for binary companions to stars, and their proposal was refined by Andy Gould and [[Abraham Loeb]] in 1992 as a method to detect exoplanets. Successes with the method date back to 2002, when a group of Polish astronomers ([[Andrzej Udalski]], [[Marcin Kubiak]] and Michał Szymański from [[Warsaw]], and [[Bohdan Paczyński]]) during project OGLE (the [[Optical Gravitational Lensing Experiment]]) developed a workable technique. During one month, they found several possible planets, though limitations in the observations prevented clear confirmation. Since then, several confirmed extrasolar planets have been detected using microlensing. This was the first method capable of detecting planets of Earth-like mass around ordinary [[main-sequence]] stars.<ref name="Beaulieulensplanet" />

Unlike most other methods, which have detection bias towards planets with small (or for resolved imaging, large) orbits, the microlensing method is most sensitive to detecting planets around 1-10 astronomical units away from Sun-like stars.

A notable disadvantage of the method is that the lensing cannot be repeated, because the chance alignment never occurs again. Also, the detected planets will tend to be several kiloparsecs away, so follow-up observations with other methods are usually impossible. In addition, the only physical characteristic that can be determined by microlensing is the mass of the planet, within loose constraints. Orbital properties also tend to be unclear, as the only orbital characteristic that can be directly determined is its current semi-major axis from the parent star, which can be misleading if the planet follows an eccentric orbit. When the planet is far away from its star, it spends only a tiny portion of its orbit in a state where it is detectable with this method, so the orbital period of the planet cannot be easily determined. It is also easier to detect planets around low-mass stars, as the gravitational microlensing effect increases with the planet-to-star mass ratio.

The main advantages of the gravitational microlensing method are that it can detect low-mass planets (in principle down to Mars mass with future space projects such as the [[Nancy Grace Roman Space Telescope]]); it can detect planets in wide orbits comparable to Saturn and Uranus, which have orbital periods too long for the radial velocity or transit methods; and it can detect planets around very distant stars. When enough background stars can be observed with enough accuracy, then the method should eventually reveal how common Earth-like planets are in the galaxy.{{citation needed|date=July 2015}}

Observations are usually performed using networks of [[robotic telescope]]s. In addition to the [[European Research Council]]-funded OGLE, the [[Microlensing Observations in Astrophysics]] (MOA) group is working to perfect this approach.

The PLANET ([[Probing Lensing Anomalies NETwork]])/RoboNet project is even more ambitious. It allows nearly continuous round-the-clock coverage by a world-spanning telescope network, providing the opportunity to pick up microlensing contributions from planets with masses as low as Earth's.  This strategy was successful in detecting the first low-mass planet on a wide orbit, designated [[OGLE-2005-BLG-390Lb]].<ref name="Beaulieulensplanet">{{cite journal | author=J.-P. Beaulieu |display-authors=4 |author2=D.P. Bennett |author3=P. Fouque |author4=A. Williams |author5=M. Dominik |author6=U.G. Jorgensen |author7=D. Kubas |author8=A. Cassan |author9=C. Coutures |author10=J. Greenhill |author11=K. Hill |author12=J. Menzies |author13=P.D. Sackett |author14=M. Albrow |author15=S. Brillant |author16=J.A.R. Caldwell |author17=J.J. Calitz |author18=K.H. Cook |author19=E. Corrales |author20=M. Desort |author21=S. Dieters |author22=D. Dominis |author23=J. Donatowicz |author24=M. Hoffman |author25=S. Kane |author26=J.-B. Marquette |author27=R. Martin |author28=P. Meintjes |author29=K. Pollard |author30=K. Sahu |author31=C. Vinter |author32=J. Wambsganss |author33=K. Woller |author34=K. Horne |author35=I. Steele |author36=D. Bramich |author37=M. Burgdorf |author38=C. Snodgrass |author39=M. Bode |author40=A. Udalski |author41=M. Szymanski |author42=M. Kubiak |author43=T. Wieckowski |author44=G. Pietrzynski |author45=I. Soszynski |author46=O. Szewczyk |author47=L. Wyrzykowski |author48=B. Paczynski | title=Discovery of a Cool Planet of 5.5 Earth Masses Through Gravitational Microlensing | journal=Nature | date=2006 | volume=439 | issue= 7075| pages=437–440 | doi=10.1038/nature04441 | pmid=16437108|arxiv = astro-ph/0601563 |bibcode = 2006Natur.439..437B |s2cid=4414076 }}</ref>

The NASA [[Nancy Grace Roman Space Telescope]] scheduled for launch in 2027 includes a microlensing planet survey as one of its three core projects.

===Direct imaging===<!-- This section is linked from [[Extrasolar planet]] and from [[Optical vortex]]-->
{{See also|List of directly imaged exoplanets}}
[[File:444226main exoplanet20100414-a-full.jpg|250px|thumb|Direct image of [[exoplanet]]s around the star [[HR 8799]] using a [[Vortex coronagraph]] on a 1.5m portion of the [[Hale Telescope]]]] [[File:Beta Pictoris system annotated.jpg|250px|thumb|right|[[European Southern Observatory|ESO]] image of a planet near Beta Pictoris]]
Planets are extremely faint light sources compared to stars, and what little light comes from them tends to be lost in the glare from their parent star. So in general, it is very difficult to detect and resolve them directly from their host star. Planets orbiting far enough from stars to be resolved reflect very little starlight, so planets are detected through their [[thermal emission]] instead. It is easier to obtain images when the planetary system is relatively near to the Sun, and when the planet is especially large (considerably larger than [[Jupiter]]), widely separated from its parent star, and hot so that it emits intense [[infrared radiation]]; images have then been made in the infrared, where the planet is brighter than it is at visible wavelengths. [[Coronagraph]]s are used to block light from the star, while leaving the planet visible. Direct imaging of an Earth-like exoplanet requires extreme [[optothermal stability]].<ref>{{Cite book|doi=10.1117/12.2188371|chapter=Advanced Mirror Technology Development (AMTD) thermal trade studies|title=Optical Modeling and Performance Predictions VII|volume=9577|page=957703|year=2015|last1=Brooks|first1=Thomas|last2=Stahl|first2=H. P.|last3=Arnold|first3=William R.|editor2-first=Marie B|editor2-last=Levine-West|editor1-first=Mark A|editor1-last=Kahan|hdl=2060/20150019495|s2cid=119544105}}</ref> During the accretion phase of planetary formation, the star-planet contrast may be even better in [[H alpha]] than it is in infrared – an H alpha survey is currently underway.<ref>{{Cite journal|arxiv=1401.1273|last1= Close|first1= L. M.|title= Discovery of H-alpha Emission from the Close Companion Inside the Gap of Transitional Disk HD142527|journal= The Astrophysical Journal|volume= 781|issue= 2|pages= L30|last2=  Follette|first2= K. B.|last3=  Males|first3= J. R.|last4=  Puglisi|first4= A.|last5=  Xompero|first5= M.|last6=  Apai|first6= D.|last7=  Najita|first7= J.|last8=  Weinberger|first8= A. J.|last9=  Morzinski|first9= K.|last10=  Rodigas|first10= T. J.|last11=  Hinz|first11= P.|last12=  Bailey|first12= V.|last13=  Briguglio|first13= R.|year= 2014|doi= 10.1088/2041-8205/781/2/L30|bibcode = 2014ApJ...781L..30C |s2cid= 118654984}}</ref>

[[File:The ExTrA telescopes at La Silla.jpg|left|thumb|The ExTrA telescopes at [[La Silla Observatory|La Silla]] observes at infrared wavelengths and adds spectral information to the usual photometric measurements.<ref>{{cite web|title=First Light for Planet Hunter ExTrA at La Silla|url=https://www.eso.org/public/news/eso1803/|website=www.eso.org|access-date=24 January 2018}}</ref>]]

Direct imaging can give only loose constraints of the planet's mass, which is derived from the age of the star and the temperature of the planet. Mass can vary considerably, as planets can form several million years after the star has formed. The cooler the planet is, the less the planet's mass needs to be. In some cases it is possible to give reasonable constraints to the radius of a planet based on planet's temperature, its apparent brightness, and its distance from Earth. The spectra emitted from planets do not have to be separated from the star, which eases determining the chemical composition of planets.

Sometimes observations at multiple wavelengths are needed to rule out the planet being a [[brown dwarf]]. Direct imaging can be used to accurately measure the planet's orbit around the star. Unlike the majority of other methods, direct imaging works better with planets with [[Orbital inclination#Exoplanets and multiple star systems|face-on orbits]] rather than edge-on orbits, as a planet in a face-on orbit is observable during the entirety of the planet's orbit, while planets with edge-on orbits are most easily observable during their period of largest apparent separation from the parent star.

The planets detected through direct imaging currently fall into two categories. First, planets are found around stars more massive than the Sun which are young enough to have protoplanetary disks. The second category consists of possible sub-brown dwarfs found around very dim stars, or brown dwarfs which are at least 100 AU away from their parent stars.

Planetary-mass objects [[rogue planet|not gravitationally bound to a star]] are found through direct imaging as well.

==== Early discoveries ====
[[File:VLT Snaps An Exotic Exoplanet First.jpg|thumb|The large central object is the star [[CVSO 30]]; the small dot up and to the left is exoplanet CVSO 30c. This image was made using astrometry data from [[Very Large Telescope|VLT]]'s NACO and SINFONI instruments.<ref>{{cite web|title=VLT Snaps An Exotic Exoplanet "First"|url=http://www.eso.org/public/images/potw1624a/|access-date=15 June 2016}}</ref>]]
In 2004, a group of astronomers used the [[European Southern Observatory]]'s [[Very Large Telescope]] array in Chile to produce an image of [[2M1207b]], a companion to the [[brown dwarf]] 2M1207.<ref>{{cite journal |author=G. Chauvin |display-authors=4 |author2=A.M. Lagrange |author3=C. Dumas |author4=B. Zuckerman |author5=D. Mouillet |author6=I. Song |author7=J.-L. Beuzit |author8=P. Lowrance |title=A giant planet candidate near a young brown dwarf |journal=Astronomy & Astrophysics |date=2004 |volume=425 |issue=2 |pages=L29–L32 |url=http://www.edpsciences.org/articles/aa/abs/2004/38/aagg222/aagg222.html |doi=10.1051/0004-6361:200400056 |bibcode=2004A&A...425L..29C |arxiv=astro-ph/0409323 |s2cid=15948759 |access-date=4 October 2006 |archive-date=8 September 2006 |archive-url=https://web.archive.org/web/20060908015928/http://www.edpsciences.org/articles/aa/abs/2004/38/aagg222/aagg222.html |url-status=dead }}</ref> In the following year, the planetary status of the companion was confirmed.<ref>{{cite web | title= Yes, it is the Image of an Exoplanet (Press Release) | work=ESO website | date=30 April 2005| url=http://www.eso.org/public/news/eso0515/ | access-date=2010-07-09 }}</ref> The planet is estimated to be several times more massive than [[Jupiter]], and to have an orbital radius greater than 40 AU.

On 6 November 2008<ref>{{Cite journal |last1=Lafrenière |first1=David |last2=Jayawardhana |first2=Ray |last3=van Kerkwijk |first3=Marten H. |date=2008-12-20 |title=Direct Imaging and Spectroscopy of a Planetary-Mass Candidate Companion to a Young Solar Analog |url=https://iopscience.iop.org/article/10.1086/595870 |journal=The Astrophysical Journal |language=en |volume=689 |issue=2 |pages=L153–L156 |arxiv=0809.1424 |bibcode=2008ApJ...689L.153L |doi=10.1086/595870 |issn=0004-637X}}</ref> an object was published that was imaged first in April 2008 at a separation of 330 AU from the star [[1RXS J160929.1−210524]], already announced on 8 September 2008.<ref>{{cite news |date=15 September 2008 |title=Exoplanet 'circles normal star' |publisher=[[BBC News]] |url=http://news.bbc.co.uk/1/hi/sci/tech/7617031.stm |url-status=live |access-date=17 September 2008 |archive-url=https://web.archive.org/web/20080916120154/http://news.bbc.co.uk/1/hi/sci/tech/7617031.stm |archive-date=16 September 2008}}</ref> But it was not until 2010, that it was confirmed to be a companion planet to the star and not just a chance alignment.<ref>[http://cs.astronomy.com/asycs/blogs/astronomy/2010/06/22/astronomers-verify-directly-imaged-planet.aspx Astronomers verify directly imaged planet] {{webarchive|url=https://web.archive.org/web/20100630110925/http://cs.astronomy.com/asycs/blogs/astronomy/2010/06/22/astronomers-verify-directly-imaged-planet.aspx |date=30 June 2010 }}</ref> It is not confirmed, yet, whether the mass of the companion is above or below the deuterium-burning limit.

The first multiplanet system, announced on 13 November 2008, first seen in images of October 2007, using telescopes at both the [[Keck Observatory]] and [[Gemini Observatory]]. Three planets were directly observed orbiting [[HR 8799]], whose masses are approximately ten, ten, and seven [[Jupiter mass|times that of Jupiter]].<ref name="Marois2008">{{cite journal |last1=Marois |first1=Christian |date=November 2008 |title=Direct Imaging of Multiple Planets Orbiting the Star HR 8799 |journal=[[Science (journal)|Science]] |volume=322 |issue=5906 |pages=1348–52 |doi=10.1126/science.1166585 |pmid=19008415 |bibcode=2008Sci...322.1348M |arxiv=0811.2606 |display-authors=2 |last2=MacIntosh |first2=B. |last3=Zuckerman |first3=B. |last4=Song |first4=I. |last5=Patience |first5=J. |last6=Lafreniere |first6=D. |last7=Doyon |first7=R.|s2cid=206516630 }} ([http://exoplanet.eu/papers/exo_science.pdf Preprint at exoplanet.eu] {{webarchive|url=https://web.archive.org/web/20081217133217/http://exoplanet.eu/papers/exo_science.pdf |date=17 December 2008 }})</ref><ref>{{cite press release|url=http://www.keckobservatory.org/article.php?id=231 |title=Astronomers capture first image of newly-discovered solar system |publisher=W. M. Keck Observatory |date=13 October 2008 |access-date=2008-10-13 |archive-url=https://web.archive.org/web/20131126160805/http://www.keckobservatory.org/article.php?id=231 |archive-date=26 November 2013 }}</ref> On the same day, 13 November 2008, it was announced that the Hubble Space Telescope directly observed [[Fomalhaut b|an exoplanet]] orbiting [[Fomalhaut]], with a mass no more than {{Jupiter mass|3}}.<ref>{{cite web| title=Hubble Directly Observes a Planet Orbiting Another Star | url=http://www.nasa.gov/mission_pages/hubble/science/fomalhaut.html | access-date=13 November 2008 }}</ref> Both systems are surrounded by disks not unlike the [[Kuiper belt]].

On 21 November 2008, three days after acceptance of a letter to the editor published online on 11 December 2008,<ref>{{Cite journal |last1=Lagrange |first1=A.-M. |last2=Gratadour |first2=D. |last3=Chauvin |first3=G. |last4=Fusco |first4=T. |last5=Ehrenreich |first5=D. |last6=Mouillet |first6=D. |last7=Rousset |first7=G. |last8=Rouan |first8=D. |last9=Allard |first9=F. |last10=Gendron |first10=é. |last11=Charton |first11=J. |last12=Mugnier |first12=L. |last13=Rabou |first13=P. |last14=Montri |first14=J. |last15=Lacombe |first15=F. |date=2009-01-02 |title=A probable giant planet imaged in the β Pictoris disk: VLT/NaCo deep L' -band imaging |url=http://www.aanda.org/10.1051/0004-6361:200811325 |journal=Astronomy & Astrophysics |volume=493 |issue=2 |pages=L21–L25 |arxiv=0811.3583 |bibcode=2009A&A...493L..21L |doi=10.1051/0004-6361:200811325 |issn=0004-6361}}</ref> it was announced that analysis of images dating back to 2003, revealed a planet orbiting [[Beta Pictoris]].<ref name="ESO2008">{{cite press release|url=http://www.eso.org/public/outreach/press-rel/pr-2008/pr-42-08.html|title=Beta Pictoris planet finally imaged?|publisher=ESO|date=2008-11-21|access-date=2008-11-22|archive-url=https://web.archive.org/web/20090208234953/http://eso.org/public/outreach/press-rel/pr-2008/pr-42-08.html|archive-date=2009-02-08|url-status=dead}}</ref>

In 2012, it was announced that a "[[Super-Jupiter]]" planet with a mass about {{Jupiter mass|12.8}} orbiting [[Kappa Andromedae]] was directly imaged using the [[Subaru Telescope]] in Hawaii.<ref>{{cite web|title=Direct Imaging of a Super-Jupiter Around a Massive Star|url=http://subarutelescope.org/Pressrelease/2012/11/19/index.html|access-date=2012-11-19}}</ref><ref>{{cite web|title=NASA – Astronomers Directly Image Massive Star's 'Super Jupiter'|url=http://www.nasa.gov/topics/universe/features/super-jupiter_prt.htm|publisher=NASA.com|access-date=2012-11-19|author=Francis Reddy|date=19 November 2012}}</ref>  It orbits its parent star at a distance of about 55 AU, or nearly twice the distance of [[Neptune]] from the sun.

An additional system, [[GJ 758]], was imaged in November 2009, by a team using the [[Subaru (telescope)#Instruments|HiCIAO]] instrument of the [[Subaru (telescope)|Subaru Telescope]], but it was a brown dwarf.<ref name="subaru-image">{{Cite journal|last=Thalmann |first=Christian |arxiv=0911.1127 |title=Discovery of the Coldest Imaged Companion of a Sun-Like Star |journal=The Astrophysical Journal |volume=707 |issue=2 |pages=L123–L127 |date=2009 |display-authors=4 |author2=Joseph Carson |author3=Markus Janson |author4=Miwa Goto |author5=Michael McElwain |author6=Sebastian Egner |author7=Markus Feldt |author8=Jun Hashimoto |author9=Yutaka Hayano |last10=Henning |first10=Thomas |last11= Hodapp |first11=Klaus W. |last12=Kandori |first12=Ryo |last13=Klahr |first13=Hubert |last14=Kudo |first14=Tomoyuki |last15=Kusakabe |first15=Nobuhiko |last16=Mordasini |first16=Christoph |last17=Morino |first17=Jun-Ichi |last18=Suto |first18=Hiroshi |last19=Suzuki |first19=Ryuji |last20=Tamura |first20=Motohide |doi=10.1088/0004-637X/707/2/L123 |bibcode=2009ApJ...707L.123T|s2cid=116823073 }}</ref>

Other possible exoplanets to have been directly imaged include [[GQ Lupi b]], [[AB Pictoris b]], and [[SCR 1845 b]].<ref>{{cite journal | author=R. Neuhauser | display-authors=4 | author2=E. W. Guenther | author3=G. Wuchterl | author4=M. Mugrauer | author5=A. Bedalov | author6=P.H. Hauschildt | title=Evidence for a co-moving sub-stellar companion of GQ Lup | journal=Astronomy & Astrophysics | date=2005 | volume=435 | issue=1 | pages=L13–L16 | url=http://www.edpsciences.org/articles/aa/abs/2005/19/aagj061/aagj061.html | doi=10.1051/0004-6361:200500104 | bibcode=2005A&A...435L..13N | arxiv=astro-ph/0503691 | s2cid=7444394 | access-date=4 October 2006 | archive-date=2 May 2006 | archive-url=https://web.archive.org/web/20060502132012/http://www.edpsciences.org/articles/aa/abs/2005/19/aagj061/aagj061.html | url-status=dead }}</ref> As of March 2006, none have been confirmed as planets; instead, they might themselves be small [[brown dwarfs]].<ref>{{cite web | title=Is this a Brown Dwarf or an Exoplanet? | date=7 April 2005 | work=ESO Website | url=http://www.eso.org/outreach/press-rel/pr-2005/pr-09-05.html | access-date=2006-07-04 | archive-url=https://web.archive.org/web/20120916061533/http://www.eso.org/public/news/eso0511/ | archive-date=16 September 2012 }}</ref><ref>{{cite journal | author=M. Janson | author2=W. Brandner | author3=T. Henning | author4=H. Zinnecker | title=Early ComeOn+ adaptive optics observation of GQ Lupi and its substellar companion | journal=Astronomy & Astrophysics | date=2005 | volume=453 | issue=2 | pages=609–614 | url=http://www.edpsciences.org/articles/aa/abs/2006/26/aa4475-05/aa4475-05.html | doi=10.1051/0004-6361:20054475 | bibcode=2006A&A...453..609J | arxiv=astro-ph/0603228 | s2cid=18024395 | access-date=4 October 2006 | archive-date=25 May 2012 | archive-url=https://archive.today/20120525125602/http://www.edpsciences.org/articles/aa/abs/2006/26/aa4475-05/aa4475-05.html | url-status=dead }}</ref>

==== Imaging instruments ====
[[File:Exoplanet-Missions-graphic-final-Dec2022.png|thumb|upright=1.3|NASA graphic of present and future exoplanet missions as of 2022.]]
[[File:VLT image of exoplanet HD 95086 b.jpg|thumb|[[ESO]] VLT image of exoplanet [[HD 95086 b]]<ref>{{cite news|title=Lightest Exoplanet Imaged So Far?|url=http://www.eso.org/public/news/eso1324/|access-date=5 June 2013|newspaper=ESO Press Release}}</ref>]]

Several planet-imaging-capable instruments are installed on large ground-based telescope, such as [[Gemini Planet Imager]], [[VLT-SPHERE]], the [[Subaru (telescope)#Instruments|Subaru Coronagraphic Extreme Adaptive Optics (SCExAO)]] instrument, or [[Palomar Observatory|Palomar Project 1640]]. In space, there are currently no dedicated exoplanet imaging instrument. Although the [[James Webb Space Telescope]] does have some exoplanet imaging capabilities, it has not specifically been designed and optimised for that purpose. The [[Nancy Grace Roman Space Telescope]] will be the first space observatory to include a dedicated exoplanet imaging instrument. This instrument is designed [[JPL]] as a demonstrator for a future large observatory in space that will have the imaging of Earth-like exoplanets as one of its primary science goals. Concepts such as the [[Large Ultraviolet Optical Infrared Surveyor|LUVOIR]] or the [[Habitable Exoplanets Observatory|HabEx]] have been proposed in preparation of the 2020 [[Astronomy and Astrophysics Decadal Survey]].

In 2010, a team from [[NASA|NASA']]s [[Jet Propulsion Laboratory]] demonstrated that a [[vortex coronagraph]] could enable small scopes to directly image planets.<ref>{{cite web|url=https://www.space.com/8216-method-photograph-earth-planets.html|title=New method could image Earth-like planets|first1=Andrea|last1=Thompson|work=space.com|date=14 April 2010 }}</ref> They did this by imaging the previously imaged [[HR 8799]] planets, using just a 1.5 meter-wide portion of the [[Hale Telescope]].

Another promising approach is [[nulling interferometry]].<ref>{{cite web|url=http://www.jpl.nasa.gov/news/news.cfm?release=2010-207|title=News - Earth-like Planets May Be Ready for Their Close-Up|work=NASA/JPL|access-date=23 June 2010|archive-date=21 October 2011|archive-url=https://web.archive.org/web/20111021034838/http://www.jpl.nasa.gov/news/news.cfm?release=2010-207|url-status=dead}}</ref>

It has also been proposed that space-telescopes that focus light using [[Fresnel imager|zone plates]] instead of mirrors would provide higher-contrast imaging, and be cheaper to launch into space due to being able to fold up the lightweight foil zone plate.<ref>[http://www.economist.com/node/21556552 Twinkle, twinkle, little planet], The Economist, 9 June 2012</ref> Another possibility would be to use a large occulter in space designed to block the light of nearby stars in order to observe their orbiting planets, such as the [[New Worlds Mission]].
{{clear}}

==== Data Reduction Techniques ====
Post-processing of observational data to enhance signal strength of off-axial bodies (i.e. exoplanets) can be accomplished in a variety of ways. All methods are based on the presence of diversity in the data between the central star and the exoplanet companions: this diversity can originate from differences in the spectrum, the angular position, the orbital motion, the polarisation, or the coherence of the light. The most popular technique is  Angular [[:fr:Imagerie différentielle|Differential Imaging]] (ADI), where exposures are acquired at different [[parallactic angle]] positions and the sky is left to rotate around the observed central star. The exposures are averaged, each exposure undergoes subtraction by the average, and then they are (de-)rotated to stack the faint planetary signal all in one place.

Specral Differential Imaging (SDI) performs an analogous procedure, but for radial changes in brightness (as a function of spectra or wavelength) instead of angular changes.

Combinations of the two are possible (ASDI, SADI, or Combined Differential Imaging "CODI").<ref>{{cite journal |last1=Keifer |first1=Sven |title=Spectral and angular differential imaging with SPHERE/IFS. Assessing the performance of various PCA-based approaches to PSF subtraction |journal=Astronomy & Astrophysics |date=August 2021 |volume=652 |page=10 |doi=10.1051/0004-6361/202140285 |arxiv=2106.05278 |bibcode=2021A&A...652A..33K |s2cid=235390892 |url=https://ui.adsabs.harvard.edu/abs/2021A%26A...652A..33K/abstract |access-date=9 Jan 2023}}</ref>

=== Polarimetry ===
{{Main|Polarimetry}}
Light given off by a star is un-polarized, i.e. the direction of oscillation of the light wave is random.  However, when the light is reflected off the atmosphere of a planet, the light waves interact with the molecules in the atmosphere and become polarized.<ref>{{cite journal |author=Schmid, H. M. |title=Search and investigation of extra-solar planets with polarimetry |journal=Direct Imaging of Exoplanets: Science & Techniques. Proceedings of the IAU Colloquium #200 |date=2006 |volume=1 |issue=C200 |pages=165–170 |bibcode=2006dies.conf..165S |doi=10.1017/S1743921306009252 |last2=Beuzit |first2=J.-L. |last3=Feldt |first3=M. |display-authors=3 |last4=Gratton |first4=R. |last5=Henning |first5=Th. |last6=Joos |first6=F. |last7=Kasper |first7=M. |last8=Lenzen |first8=R. |last9=Mouillet |first9=D. |last10=Moutou |first10=C. |last11=Quirrenbach |first11=A. |last12=Stam |first12=D. M. |last13=Thalmann |first13=C. |last14=Tinbergen |first14=J. |last15=Verinaud |first15=C. |last16=Waters |first16=R. |last17=Wolstencroft |first17=R.|doi-access=free }}</ref>

By analyzing the polarization in the combined light of the planet and star (about one part in a million), these measurements can in principle be made with very high sensitivity, as polarimetry is not limited by the stability of the Earth's atmosphere. Another main advantage is that polarimetry allows for determination of the composition of the planet's atmosphere. The main disadvantage is that it will not be able to detect planets without atmospheres. Larger planets and planets with higher albedo are easier to detect through polarimetry, as they reflect more light.

Astronomical devices used for polarimetry, called polarimeters, are capable of detecting polarized light and rejecting unpolarized beams. Groups such as [[ZIMPOL/CHEOPS]]<ref>{{cite journal | author=Schmid, H. M. | title=ZIMPOL/CHEOPS: a Polarimetric Imager for the Direct Detection of Extra-solar Planets | journal=Astronomical Polarimetry: Current Status and Future Directions ASP Conference Series | date=2004 | volume=343 | page=89 | bibcode=2005ASPC..343...89S | last2=Gisler | last3=Joos | last4=Povel | last5=Stenflo | last6=Feldt | last7=Lenzen | last8=Brandner | last9=Tinbergen
| display-authors=3 | last10=Quirrenbach | first10=A. | last11=Stuik | first11=R. | last12=Gratton | first12=R. | last13=Turatto | first13=M. | last14=Neuhäuser | first14=R. }}</ref> and [[PlanetPol]]<ref>{{cite journal | author=Hough, J. H.  |display-authors=4 |author2=Lucas, P. W. |author3=Bailey, J. A. |author4=Tamura, M. |author5=Hirst, E. |author6=Harrison, D. |author7=Bartholomew-Biggs, M. | title=PlanetPol: A Very High Sensitivity Polarimeter | journal=Publications of the Astronomical Society of the Pacific | date=2006 | volume=118 | issue=847 | pages=1302–1318 | bibcode=2006PASP..118.1302H | doi=10.1086/507955
| doi-access=free }}</ref> are currently using polarimeters to search for extrasolar planets. The first successful detection of an extrasolar planet using this method came in 2008, when [[HD 189733 b]], a planet discovered three years earlier, was detected using polarimetry.<ref>{{cite journal|journal=The Astrophysical Journal |volume=673 |issue=1 |date=20 January 2008 |title=First detection of polarized scattered light from an exoplanetary atmosphere |first=Svetlana V. |last=Berdyugina |author2=Andrei V. Berdyugin |author3=Dominique M. Fluri |author4=Vilppu Piirola |doi=10.1086/527320 |pages=L83 |bibcode=2008ApJ...673L..83B |arxiv=0712.0193 |s2cid=14366978 }}</ref> However, no new planets have yet been discovered using this method.

=== Astrometry ===
{{Main|Astrometry}}
[[File:orbit3.gif|thumb|right|200px|In this diagram a planet (smaller object) orbits a star, which itself moves in a small orbit. The system's center of mass is shown with a red plus sign. (In this case, it always lies within the star.)]]
This method consists of precisely measuring a star's position in the sky, and observing how that position changes over time. Originally, this was done visually, with hand-written records. By the end of the 19th century, this method used photographic plates, greatly improving the accuracy of the measurements as well as creating a data archive. If a star has a planet, then the gravitational influence of the planet will cause the star itself to move in a tiny circular or elliptical orbit. Effectively, star and planet each orbit around their mutual centre of mass ([[Barycentric coordinates (astronomy)|barycenter]]), as explained by solutions to the [[two-body problem]]. Since the star is much more massive, its orbit will be much smaller.<ref name="EXP2">{{cite web | first=Amir | last=Alexander | title=Space Topics: Extrasolar Planets Astrometry: The Past and Future of Planet Hunting | url=http://www.planetary.org/explore/topics/extrasolar_planets/extrasolar/astrometry.html | publisher=The Planetary Society | access-date=2006-09-10 | archive-date=8 March 2006 | archive-url=https://web.archive.org/web/20060308170147/http://www.planetary.org/explore/topics/extrasolar_planets/extrasolar/astrometry.html | url-status=dead }}</ref> Frequently, the mutual centre of mass will lie within the radius of the larger body. Consequently, it is easier to find planets around low-mass stars, especially brown dwarfs.

[[File:Solar system barycenter.svg|thumb|right|200px|Motion of the center of mass (barycenter) of the Solar System relative to the Sun]]
Astrometry is the oldest search method for [[extrasolar planets]], and was originally popular because of its success in characterizing [[Binary star#Astrometric binaries|astrometric binary star]] systems. It dates back at least to statements made by [[William Herschel]] in the late 18th century. He claimed that an ''unseen companion'' was affecting the position of the star he cataloged as ''[[70 Ophiuchi]]''. The first known formal astrometric calculation for an extrasolar planet was made by [[William Stephen Jacob]] in 1855 for this star.<ref>{{cite journal
| title=On certain Anomalies presented by the Binary Star 70 Ophiuchi
| author=Jacob, W. S.
| journal=Monthly Notices of the Royal Astronomical Society
| volume=15
| issue=9
| pages=228–230
|date=Jun 1855
| bibcode=1855MNRAS..15..228J|doi = 10.1093/mnras/15.9.228 | doi-access=free
}}</ref> Similar calculations were repeated by others for another half-century<ref name="See" /> until finally refuted in the early 20th century.<ref>{{cite journal |url=http://www.shpltd.co.uk/jha.pdf |journal=Journal for the History of Astronomy |title=A Career of controversy: the anomaly OF T. J. J. See |last=Sherrill |first=Thomas J. |date= 1999 |volume=30 |access-date=2007-08-27 |doi=10.1177/002182869903000102 |pages=25–50|bibcode=1999JHA....30...25S|s2cid=117727302 }}</ref><ref>{{cite journal |last=Heintz |first=W.D. |date=June 1988 |title=The Binary Star 70 Ophiuchi Revisited |page=140 |journal=[[Journal of the Royal Astronomical Society of Canada]] |volume=82 |issue=3 |bibcode=1988JRASC..82..140H}}</ref>
For two centuries claims circulated of the discovery of ''unseen companions'' in orbit around nearby star systems that all were reportedly found using this method,<ref name="See">{{cite journal |last=See |first=Thomas Jefferson Jackson |author-link=Thomas Jefferson Jackson See |date=1896 |title=Researches on the Orbit of F.70 Ophiuchi, and on a Periodic Perturbation in the Motion of the System Arising from the Action of an Unseen Body |journal=The Astronomical Journal |volume=16 |page=17 |doi=10.1086/102368 |bibcode=1896AJ.....16...17S}}</ref> culminating in the prominent 1996 announcement, of multiple planets orbiting the nearby star [[Lalande 21185]] by [[George G. Gatewood|George Gatewood]].<ref name="AAS">{{cite journal
| title= Lalande 21185
| author=Gatewood, G.
| journal=Bulletin of the American Astronomical Society
| volume=28
| page=885
|date=May 1996
| bibcode=1996AAS...188.4011G}}</ref><ref>{{cite news|title=Data Seem to Show a Solar System Nearly in the Neighborhood|work=The New York Times|author=John Wilford|date=12 June 1996|access-date=2009-05-29|page=1|url=https://www.nytimes.com/1996/06/12/us/data-seem-to-show-a-solar-system-nearly-in-the-neighborhood.html}}</ref> None of these claims survived scrutiny by other astronomers, and the technique fell into disrepute.<ref>{{cite book|author=Alan Boss|title=The Crowded Universe|publisher=Basic Books|date=2 February 2009|isbn= 978-0-465-00936-7}}</ref> Unfortunately, changes in stellar position are so small—and atmospheric and systematic distortions so large—that even the best ground-based telescopes cannot produce precise enough measurements. All claims of a ''planetary companion'' of less than 0.1 solar mass, as the mass of the planet, made before 1996 using this method are likely spurious. In 2002, the [[Hubble Space Telescope]] did succeed in using astrometry to characterize a previously discovered planet around the star [[Gliese 876]].<ref>{{cite journal | author=Benedict | title=A Mass for the Extrasolar Planet Gliese 876b Determined from Hubble Space Telescope Fine Guidance Sensor 3 Astrometry and High-Precision Radial Velocities | journal=The Astrophysical Journal Letters | volume=581 | issue=2 | date=2002 | pages=L115–L118 | doi=10.1086/346073 | last2=McArthur | first2=B. E. | last3=Forveille | first3=T. | last4=Delfosse | first4=X. | last5=Nelan | first5=E. | last6=Butler | first6=R. P. | last7=Spiesman | first7=W. | last8=Marcy | first8=G. | last9=Goldman | first9=B. | last10=Perrier | first10=C. | last11=Jefferys | first11=W. H. | last12=Mayor | first12=M. | bibcode=2002ApJ...581L.115B|arxiv = astro-ph/0212101 | s2cid=18430973 | display-authors=1 }}</ref>

The space-based observatory [[Gaia (spacecraft)|''Gaia'']], launched in 2013, is expected to find thousands of planets via astrometry, but prior to the launch of ''Gaia'', no planet detected by astrometry had been confirmed. [[SIM PlanetQuest]] was a US project (cancelled in 2010) that would have had similar exoplanet finding capabilities to [[Gaia (spacecraft)|Gaia]].

One potential advantage of the astrometric method is that it is most sensitive to planets with large orbits.  This makes it complementary to other methods that are most sensitive to planets with small orbits.  However, very long observation times will be required&nbsp;— years, and possibly decades, as planets far enough from their star to allow detection via astrometry also take a long time to complete an orbit. Planets orbiting around one of the stars in binary systems are more easily detectable, as they cause perturbations in the orbits of stars themselves. However, with this method, follow-up observations are needed to determine which star the planet orbits around.

In 2009, the discovery of [[VB 10b]] by astrometry was announced. This planetary object, orbiting the low mass [[red dwarf]] star [[VB 10]], was reported to have a mass seven times that of [[Jupiter]]. If confirmed, this would be the first exoplanet discovered by astrometry, of the many that have been claimed through the years.<ref>{{cite journal
 |title=An Ultracool Star's Candidate Planet 
 |url=http://steps.jpl.nasa.gov/links/docs/pravdoshaklan09vb10b.pdf 
 |date=2009 
 |pages=623–632 
 |volume=700 
 |issue=1 
 |journal=[[The Astrophysical Journal]] 
 |last1=Pravdo 
 |first1=Steven H. 
 |last2=Shaklan 
 |first2=Stuart B. 
 |access-date=2009-05-30 
 |doi=10.1088/0004-637X/700/1/623 
 |bibcode=2009ApJ...700..623P 
 |arxiv=0906.0544 
 |s2cid=119239022 
 |archive-url=https://web.archive.org/web/20090604100620/http://steps.jpl.nasa.gov/links/docs/pravdoshaklan09vb10b.pdf 
 |archive-date=4 June 2009 
}}</ref><ref>{{cite web|url=http://planetquest.jpl.nasa.gov/news/firstFind.cfm|title=First find Planet-hunting method succeeds at last|date=28 May 2009|publisher=NASA PlanetQuest|access-date=2009-05-29|archive-url=https://web.archive.org/web/20090904010715/http://planetquest.jpl.nasa.gov/news/firstFind.cfm|archive-date=4 September 2009}}</ref> However recent [[radial velocity]] independent studies rule out the existence of the claimed planet.<ref name="crires-no">{{cite journal |last=Bean |first=J. |arxiv=0912.0003 |title=The Proposed Giant Planet Orbiting VB 10 Does Not Exist |date=2009 |display-authors=4 |author2=Andreas Seifahrt |author3=Henrik Hartman |author4=Hampus Nilsson |author5=Ansgar Reiners |author6=Stefan Dreizler |author7=Henry |author8=Guenter Wiedemann |journal=The Astrophysical Journal |volume=711 |issue=1 |pages=L19 |doi=10.1088/2041-8205/711/1/L19 |bibcode=2010ApJ...711L..19B |s2cid=122135256 }}</ref>
<ref name="vb10orvs">{{Cite journal|last=Anglada-Escude |first=G. |arxiv=1001.0043 |title=Strong Constraints to the Putative Planet Candidate around VB 10 Using Doppler Spectroscopy |journal=The Astrophysical Journal |volume=711 |issue=1 |pages=L24 |date=2010 |display-authors=4 |author2=Shkolnik |author3=Weinberger |author4=Thompson |author5=Osip |author6=Debes |doi=10.1088/2041-8205/711/1/L24 |bibcode = 2010ApJ...711L..24A |s2cid=119210331 }}</ref>

In 2010, six binary stars were astrometrically measured. One of the star systems, called [[HD 176051]], was found with "high confidence" to have a planet.<ref>{{Cite journal|arxiv=1010.4048|last1= Muterspaugh|first1= Matthew W.|title= The PHASES Differential Astrometry Data Archive. V. Candidate Substellar Companions to Binary Systems |journal= The Astronomical Journal|volume= 140|issue= 6|page= 1657|last2=  Lane|first2= Benjamin F.|last3=  Kulkarni|first3= S. R.|last4= Konacki|first4= Maciej|last5=  Burke|first5= Bernard F.|last6=  Colavita|first6= M. M.|last7=  Shao|first7= M.|last8=  Hartkopf|first8= William I.|last9=  Boss|first9= Alan P.|last10=  Williamson|first10= M.|year= 2010|doi= 10.1088/0004-6256/140/6/1657|bibcode = 2010AJ....140.1657M |s2cid= 59585356}}</ref>

In 2018, a study comparing observations from the [[Gaia (spacecraft)|''Gaia'' spacecraft]] to [[Hipparcos]] data for the [[Beta Pictoris]] system was able to measure the mass of Beta Pictoris b, constraining it to {{val|11|2}} Jupiter masses.<ref>{{cite journal |last1=Snellen |first1=Ignas |last2=Brown |first2=Anthony |title=The mass of the young planet Pictoris b through the astrometric motion of its host star |journal=Nature Astronomy |volume=2 |issue=11 |pages=883–886 |date=20 August 2018 |doi=10.1038/s41550-018-0561-6 |arxiv=1808.06257 |issn=2397-3366 |bibcode=2018NatAs...2..883S|s2cid=118896628 }}</ref> This is in good agreement with previous mass estimations of roughly 13 Jupiter masses.

In 2019, data from the Gaia spacecraft and its predecessor Hipparcos was complemented with [[High Accuracy Radial Velocity Planet Searcher|HARPS]] data enabling a better description of [[Epsilon Indi Ab|ε Indi Ab]] as the second-closest Jupiter-like exoplanet with a mass of 3 Jupiters on a slightly eccentric orbit with an orbital period of 45 years.<ref name="Feng2019">{{Citation | title=Detection of the nearest Jupiter analog in radial velocity and astrometry data | journal=Monthly Notices of the Royal Astronomical Society | volume=490 | issue=4 | pages=5002–5016 | first1=Fabo | last1=Feng | first2=Guillem  | last2=Anglada-Escudé | first3=Mikko | last3=Tuomi | first4=Hugh R. A. | last4=Jones | first5=Julio | last5=Chanamé | first6=Paul R. | last6=Butler | first7=Markus | last7=Janson | arxiv=1910.06804 | date=14 October 2019 | doi=10.1093/mnras/stz2912 | doi-access=free | bibcode=2019MNRAS.490.5002F | s2cid=204575783 }}</ref>

{{As of|2022}}, especially thanks to Gaia, the combination of radial velocity and astrometry has been used to detect and characterize numerous [[Jovian planet]]s,<ref name="Feng2021">{{cite journal | doi=10.1093/mnras/stab2225 | bibcode=2021MNRAS.507.2856F | arxiv=2107.14056 | title=Optimized modelling of Gaia–Hipparcos astrometry for the detection of the smallest cold Jupiter and confirmation of seven low-mass companions | year=2021 | last1=Feng | first1=Fabo | last2=Butler | first2=R Paul | last3=Jones | first3=Hugh R A. | last4=Phillips | first4=Mark W. | last5=Vogt | first5=Steven S. | last6=Oppenheimer | first6=Rebecca | last7=Holden | first7=Bradford | last8=Burt | first8=Jennifer | last9=Boss | first9=Alan P. |display-authors=4 | journal=Monthly Notices of the Royal Astronomical Society | volume=507 | issue=2 | pages=2856–2868 | doi-access=free }}</ref><ref name="Li2021">{{cite journal|doi=10.3847/1538-3881/ac27ab|title=Precise Masses and Orbits for Nine Radial-velocity Exoplanets|year=2021|last1=Li|first1=Yiting|last2=Brandt|first2=Timothy D.|last3=Brandt|first3=G. Mirek|last4=Dupuy|first4=Trent J.|last5=Michalik|first5=Daniel|last6=Jensen-Clem|first6=Rebecca|last7=Zeng|first7=Yunlin|last8=Faherty|first8=Jacqueline|author8-link=Jackie Faherty|last9=Mitra|first9=Elena L.|journal=The Astronomical Journal|volume=162|issue=6|page=266|arxiv=2109.10422|bibcode=2021AJ....162..266L|s2cid=237592581 |doi-access=free }}</ref><ref name="Feng2022">{{cite journal |last1=Feng |first1=Fabo |last2=Butler |first2=R. Paul |display-authors=etal |date=August 2022 |title=3D Selection of 167 Substellar Companions to Nearby Stars |journal=[[The Astrophysical Journal Supplement Series]] |volume=262 |issue=21 |page=21 |doi=10.3847/1538-4365/ac7e57 |arxiv=2208.12720 |bibcode=2022ApJS..262...21F|s2cid=251864022 |doi-access=free }}</ref><ref name="Winn2022">{{cite journal |last=Winn |first=Joshua N. |arxiv=2209.05516 |title=Joint Constraints on Exoplanetary Orbits from Gaia DR3 and Doppler Data |journal=The Astronomical Journal |date=September 2022|volume=164 |issue=5 |page=196 |doi=10.3847/1538-3881/ac9126 |bibcode=2022AJ....164..196W |s2cid=252211643 |doi-access=free }}</ref> including the nearest Jupiter analogues [[Epsilon Eridani b|ε&nbsp;Eridani b]] and ε Indi Ab.<ref name="Llop-Sayson2021">{{cite journal|last1=Llop-Sayson|first1=Jorge|last2=Wang|first2=Jason J.|last3=Ruffio|first3=Jean-Baptiste|last4=Mawet|first4=Dimitri|last5=Blunt|first5=Sarah|last6=Absil|first6=Olivier|last7=Bond|first7=Charlotte|last8=Brinkman|first8=Casey|last9=Bowler|first9=Brendan P.|last10=Bottom|first10=Michael|last11=Chontos|first11=Ashley|last12=Dalba|first12=Paul A.|last13=Fulton|first13=B. J.|last14=Giacalone|first14=Steven|last15=Hill|first15=Michelle|last16=Hirsch|first16=Lea A.|last17=Howard|first17=Andrew W.|last18=Isaacson|first18=Howard|last19=Karlsson|first19=Mikael|last20=Lubin|first20=Jack|last21=Madurowicz|first21=Alex|last22=Matthews|first22=Keith|last23=Morris|first23=Evan|last24=Perrin|first24=Marshall|last25=Ren|first25=Bin|last26=Rice|first26=Malena|last27=Rosenthal|first27=Lee J.|last28=Ruane|first28=Garreth|last29=Rubenzahl|first29=Ryan|last30=Sun|first30=He|last31=Wallack|first31=Nicole|last32=Xuan|first32=Jerry W.|last33=Ygouf|first33=Marie|display-authors=4|title=Constraining the Orbit and Mass of epsilon Eridani b with Radial Velocities, Hipparcos IAD-Gaia DR2 Astrometry, and Multiepoch Vortex Coronagraphy Upper Limits|journal=The Astronomical Journal|date=6 October 2021|volume=162|issue=5|page=181|issn=0004-6256|eissn=1538-3881 |arxiv=2108.02305|doi=10.3847/1538-3881/ac134a|bibcode=2021AJ....162..181L|s2cid=236924533 |doi-access=free }}</ref><ref name="Feng2019"/> In addition, [[radio astronomy|radio]] astrometry using the [[Very Long Baseline Array|VLBA]] has been used to discover planets in orbit around [[TVLM 513-46546]] and [[EQ Pegasi]] A.<ref name="Curiel2020">{{Cite journal |arxiv = 2008.01595|doi = 10.3847/1538-3881/ab9e6e|title = An Astrometric Planetary Companion Candidate to the M9 Dwarf TVLM 513–46546|year = 2020|last1 = Curiel|first1 = Salvador|last2 = Ortiz-León|first2 = Gisela N.|last3 = Mioduszewski|first3 = Amy J.|last4 = Torres|first4 = Rosa M.|s2cid = 220961489|journal = The Astronomical Journal|volume = 160|issue = 3|page = 97|bibcode = 2020AJ....160...97C | doi-access=free }}</ref><ref name="Curiel2022">{{cite journal |arxiv=2208.14553 |last1=Curiel |first1=Salvador |last2=Ortiz-León |first2=Gisela N. |last3=Mioduszewski |first3=Amy J. |last4=Sanchez-Bermudez |first4=Joel |date=September 2022 |title=3D Orbital Architecture of a Dwarf Binary System and Its Planetary Companion |journal=[[The Astronomical Journal]] |volume=164 |issue=3 |page=93 |doi=10.3847/1538-3881/ac7c66 |bibcode=2022AJ....164...93C|s2cid=251953478 |doi-access=free }}</ref>

=== X-ray eclipse ===
In September 2020, the detection of a candidate planet orbiting the [[high-mass X-ray binary]] M51-ULS-1 in the [[Whirlpool Galaxy]] was announced. The planet was detected by [[Transit (astronomy)|eclipses]] of the X-ray source, which consists of a stellar remnant (either a [[neutron star]] or a [[black hole]]) and a massive star, likely a [[B-type star|B-type]] [[Blue supergiant star|supergiant]]. This is the only method capable of detecting a planet in another galaxy.<ref name="DiStefano2020">{{cite arXiv |last=Di Stefano |first=R. |display-authors=etal |eprint=2009.08987 |title=M51-ULS-1b: The First Candidate for a Planet in an External Galaxy |class=astro-ph.HE |date=18 September 2020}}</ref>

=== Disc kinematics ===
Planets in formation can be detected by the signatures they produce in their natal [[Protoplanetary disk|protoplanetary disks]]. The velocities of the gas in a protoplanetary disk can be observed, and their morphology can reveal the presence of planets. Planets perturb the gas velocities by imprinting strong variations from Keplerian motion.<ref>{{Cite journal |last1=Perez |first1=Sebastian |last2=Dunhill |first2=A. |last3=Casassus |first3=S. |last4=Roman |first4=P. |last5=Szulágyi |first5=J. |last6=Flores |first6=C. |last7=Marino |first7=S. |last8=Montesinos |first8=M. |date=2015-09-14 |title=Planet Formation Signposts: Observability of Circumplanetary Disks Via Gas Kinematics |url=https://iopscience.iop.org/article/10.1088/2041-8205/811/1/L5 |journal=The Astrophysical Journal |volume=811 |issue=1 |pages=L5 |doi=10.1088/2041-8205/811/1/L5 |arxiv=1505.06808 |bibcode=2015ApJ...811L...5P |issn=2041-8213}}</ref><ref>{{Cite journal |last1=Pérez |first1=Sebastián |last2=Casassus |first2=S |last3=Benítez-Llambay |first3=P |date=2018-10-11 |title=Observability of planet–disc interactions in CO kinematics |url=https://academic.oup.com/mnrasl/article/480/1/L12/5039650 |journal=Monthly Notices of the Royal Astronomical Society: Letters |language=en |volume=480 |issue=1 |pages=L12–L17 |doi=10.1093/mnrasl/sly109 |doi-access=free |issn=1745-3925|arxiv=1806.05125 }}</ref> This method is now referred to as "disk kinematics." Notable examples of protoplanetary disks around young stars with signatures of embedded planets include HD 97048,<ref>{{Cite journal |last1=Pinte |first1=C. |last2=van der Plas |first2=G. |last3=Ménard |first3=F. |last4=Price |first4=D. J. |last5=Christiaens |first5=V. |last6=Hill |first6=T. |last7=Mentiplay |first7=D. |last8=Ginski |first8=C. |last9=Choquet |first9=E. |last10=Boehler |first10=Y. |last11=Duchêne |first11=G. |last12=Perez |first12=S. |last13=Casassus |first13=S. |date=2019-08-12 |title=Kinematic detection of a planet carving a gap in a protoplanetary disk |url=https://www.nature.com/articles/s41550-019-0852-6 |journal=Nature Astronomy |language=en |volume=3 |issue=12 |pages=1109–1114 |arxiv=1907.02538 |bibcode=2019NatAs...3.1109P |doi=10.1038/s41550-019-0852-6 |issn=2397-3366}}</ref> [[HD 163296]]<ref>{{Cite journal |last1=Teague |first1=Richard |last2=Bae |first2=Jaehan |last3=Bergin |first3=Edwin A. |last4=Birnstiel |first4=Tilman |last5=Foreman-Mackey |first5=Daniel |date=2018-06-10 |title=A Kinematical Detection of Two Embedded Jupiter-mass Planets in HD 163296 |journal=The Astrophysical Journal Letters |volume=860 |issue=1 |pages=L12 |arxiv=1805.10290 |bibcode=2018ApJ...860L..12T |doi=10.3847/2041-8213/aac6d7 |doi-access=free |issn=2041-8205}}</ref><ref>{{Cite journal |last1=Pinte |first1=C. |last2=Price |first2=D. J. |last3=Ménard |first3=F. |last4=Duchêne |first4=G. |last5=Dent |first5=W. R. F. |last6=Hill |first6=T. |last7=Gregorio-Monsalvo |first7=I. de |last8=Hales |first8=A. |last9=Mentiplay |first9=D. |date=2018-06-10 |title=Kinematic Evidence for an Embedded Protoplanet in a Circumstellar Disk |journal=The Astrophysical Journal Letters |volume=860 |issue=1 |pages=L13 |arxiv=1907.02538 |bibcode=2019NatAs...3.1109P |doi=10.3847/2041-8213/aac6dc |doi-access=free |issn=2041-8205}}</ref> and HD 100546.<ref>{{Cite journal |last1=Casassus |first1=Simon |last2=Pérez |first2=Sebastián |date=2019-10-01 |title=Kinematic Detections of Protoplanets: A Doppler Flip in the Disk of HD 100546 |journal=The Astrophysical Journal Letters |volume=883 |issue=2 |pages=L41 |doi=10.3847/2041-8213/ab4425 |doi-access=free |arxiv=1906.06302 |bibcode=2019ApJ...883L..41C |issn=2041-8205}}</ref>

== Other possible methods ==

=== Flare and variability echo detection ===
Non-periodic variability events, such as flares, can produce extremely faint echoes in the light curve if they reflect off an exoplanet or other scattering medium in the star system.<ref>{{cite journal | last=Argyle | first=Edward | title=On the observability of extrasolar planetary systems | journal=Icarus | publisher=Elsevier BV | volume=21 | issue=2 | year=1974 | issn=0019-1035 | doi=10.1016/0019-1035(74)90138-9 | pages=199–201 | bibcode=1974Icar...21..199A }}</ref><ref name="Bromley 1992">{{cite journal | last=Bromley | first=Benjamin C. | title=Detecting faint echoes in stellar-flare light curves | journal=Publications of the Astronomical Society of the Pacific | publisher=IOP Publishing | volume=104 | year=1992 | issn=0004-6280 | doi=10.1086/133089 | page=1049| bibcode=1992PASP..104.1049B | s2cid=121521634 | doi-access=free }}</ref><ref name="Gaidos 1994">{{cite journal | last=Gaidos | first=Eric J. | title=Light Echo Detection of Circumstellar Disks around Flaring Stars | journal=Icarus | publisher=Elsevier BV | volume=109 | issue=2 | year=1994 | issn=0019-1035 | doi=10.1006/icar.1994.1101 | pages=382–392| bibcode=1994Icar..109..382G }}</ref><ref name="Sugerman 2003">{{cite journal | last=Sugerman | first=Ben E. K. | title=Observability of Scattered-Light Echoes around Variable Stars and Cataclysmic Events | journal=The Astronomical Journal | volume=126 | issue=4 | year=2003 | issn=0004-6256 | doi=10.1086/378358 | pages=1939–1959| arxiv=astro-ph/0307245 | bibcode=2003AJ....126.1939S | s2cid=9576707 }}</ref>  More recently, motivated by advances in instrumentation and signal processing technologies, echoes from exoplanets are predicted to be recoverable from high-cadence photometric and spectroscopic measurements of active star systems, such as M dwarfs.<ref>{{cite report |last1=Mann |first1=Chris |display-authors=etal |title=Stellar Echo Imaging of Exoplanets |date=January 2016 |website=NASA Technical Report Server |ref=HQ-E-DAA-TN39155 |hdl=2060/20170002797 |hdl-access=free |url=https://ntrs.nasa.gov/citations/20170002797}}</ref><ref name="Sparks White Lupu Ford">{{cite journal | last1=Sparks | first1=William B. | last2=White | first2=Richard L. | last3=Lupu | first3=Roxana E. | last4=Ford | first4=Holland C. | title=The Direct Detection and Characterization of M-dwarf Planets Using Light Echoes | journal=The Astrophysical Journal | publisher=American Astronomical Society | volume=854 | issue=2 | date=2018-02-20 | issn=1538-4357 | doi=10.3847/1538-4357/aaa549 | page=134| arxiv=1801.01144 | bibcode=2018ApJ...854..134S | s2cid=119397912 | doi-access=free }}</ref><ref name="Mann Tellesbo Bromley Kenyon">{{cite journal | last1=Mann | first1=Chris | last2=Tellesbo | first2=Christopher A. | last3=Bromley | first3=Benjamin C. | last4=Kenyon | first4=Scott J. | title=A Framework for Planet Detection with Faint Light-curve Echoes | journal=The Astronomical Journal | publisher=American Astronomical Society | volume=156 | issue=5 | date=2018-10-12 | issn=1538-3881 | doi=10.3847/1538-3881/aadc5e | page=200| arxiv=1808.07029 | bibcode=2018AJ....156..200M | s2cid=119016095 | doi-access=free }}</ref> These echoes are theoretically observable in all orbital inclinations.

=== Transit imaging ===
An optical/infrared [[interferometer]] array (e.g, a 16 interferometer-array of the Big Fringe Telescope<ref>{{Cite web |last=Tognetti |first=Laurence |date=2024-08-25 |title=The Big Fringe Telescope. A 2.2 KILOMETER Telescope on the Cheap. And it Can Make Exoplanet "Movies". |url=https://www.universetoday.com/168209/the-big-fringe-telescope-a-2-2-kilometer-telescope-on-the-cheap-and-it-can-make-exoplanet-movies/#more-168209 |access-date=2024-08-26 |website=Universe Today |language=en-US}}</ref>) doesn't collect as much light as a single telescope of equivalent size, but has the resolution of a single telescope the size of the array. For bright stars, this resolving power could be used to image a star's surface during a transit event and observe the shadow of the planet transiting. This could provide a direct measurement of the planet's angular radius and, via [[parallax]], its actual radius. This is more accurate than radius estimates based on [[transit photometry]], which are dependent on stellar radius estimates which in turn depend on models of star characteristics. Imaging also provides more accurate determination of the inclination than photometry does.<ref>{{Cite journal|arxiv=1405.1983 |last1= van Belle |first1= Gerard T. |title= Direct Imaging of Planet Transit Events |journal= Proceedings of the International Astronomical Union |volume= 8 |pages= 378–381 |author2= Kaspar von Braun |last3= Boyajian |first3= Tabetha |last4= Schaefer |first4= Gail |year= 2014 |doi= 10.1017/S1743921313013197 |bibcode= 2014IAUS..293..378V |s2cid= 118316923 }}</ref>

=== Magnetospheric (auroral) radio emissions ===
[[Aurora (astronomy)|Auroral]] [[radio]] emissions from exoplanet [[Magnetosphere|magnetospheres]] could be detected with radio telescopes.  The emission may be caused by the exoplanet's magnetic field interacting with a stellar wind, adjacent [[Plasma (physics)|plasma]] sources (such as [[Jupiter]]'s volcanic moon [[Io (moon)|Io]] travelling through its magnetosphere) or the interaction of the magnetic field with the [[interstellar medium]].  Although several discoveries have been claimed, thus far, none have been verified.  The most sensitive searches for direct radio emissions from exoplanet magnetic fields, or from exoplanet magnetic fields interacting with those from their host stars, have been conducted with the [[Arecibo_Telescope|Arecibo radio telescope]].<ref>{{cite journal|last1=Route|first1=Matthew|last2=Wolszczan|first2=Alex|title=ROME. III. The Arecibo Search for Star–Planet Interactions at 5 GHz|journal=The Astrophysical Journal|date=1 August 2023|volume=952|issue=2|page=118|doi=10.3847/1538-4357/acd9ad|arxiv=2202.08899|bibcode=2023ApJ...952..118R|doi-access=free }}</ref><ref>{{cite journal|last1=Route|first1=Matthew|title=ROME. IV. An Arecibo Search for Substellar Magnetospheric Radio Emissions in Purported Exoplanet-hosting Systems at 5 GHz|journal=The Astrophysical Journal|date=1 May 2024|volume=966|issue=1|page=55|doi=10.3847/1538-4357/ad30ff|arxiv=2403.02226|bibcode=2024ApJ...966...55R|doi-access=free }}</ref>

In addition to allowing for a study of exoplanet magnetic fields, radio emissions may be used to measure the interior rotation rate of an exoplanet.<ref>{{cite journal| url=https://www.faculty.ece.vt.edu/swe/lwa/memo/lwa0013.pdf | title=Radio Detection of Extrasolar Planets: Present and Future Prospects|journal= Bulletin of the American Astronomical Society|volume=36|access-date=2008-10-15|bibcode=2004AAS...20513508L|archive-url=https://web.archive.org/web/20231122230408/https://faculty.ece.vt.edu/swe/lwa/memo/lwa0013.pdf|archive-date=22 November 2023|url-status=live| last1=Lazio| first1=T. J. W.| last2=Farrell| first2=W. M.| date=2004}}</ref>

=== Optical interferometry ===
In March 2019, [[European Southern Observatory|ESO]] astronomers, employing the [[Very Large Telescope#Instruments|GRAVITY instrument]] on their [[Very Large Telescope|Very Large Telescope Interferometer]] (VLTI), announced the first direct detection of an [[exoplanet]], [[HR 8799 e]], using [[Interferometry|optical interferometry]].<ref name="EA-20190327">{{cite news |author=European Southern Observatory |title=GRAVITY instrument breaks new ground in exoplanet imaging - Cutting-edge VLTI instrument reveals details of a storm-wracked exoplanet using optical interferometry |url=https://www.eurekalert.org/pub_releases/2019-03/e-gib032619.php |date=27 March 2019 |work=[[EurekAlert!]] |access-date=27 March 2019 |author-link=European Southern Observatory }}</ref>

=== Modified interferometry ===
By looking at the wiggles of an interferogram using a Fourier-Transform-Spectrometer, enhanced sensitivity could be obtained in order to detect faint signals from Earth-like planets.<ref>{{cite journal|doi=10.1088/0004-6256/144/3/71|title=Enhanced Interferometric Identification of Spectra in Habitable Extrasolar Planets|journal=The Astronomical Journal|volume=144|issue=3|page=71|year=2012|last1=Schwartz|first1=Eyal|last2=Lipson|first2=Stephen G.|last3=Ribak|first3=Erez N.|s2cid=59493938|bibcode = 2012AJ....144...71S }}</ref>

=== Detection of dust trapping around Lagrangian points ===
Identification of dust clumps along a protoplanetary disk demonstrate trace accumulation around [[Lagrangian points]]. From the detection of this dust, it can be inferred that a planet exists such that it has created those accumulations.<ref name="ab8c10">{{cite journal |author=Feng Long |display-authors=etal |title=ALMA Detection of Dust Trapping around Lagrangian Points in the LkCa 15 Disk |date=14 September 2022 |journal=[[The Astrophysical Journal Letters]] |volume=937 |issue=1 |pages=L1 |article-number=L1 |doi=10.3847/2041-8213/ac8b10 |arxiv=2209.05535 |bibcode=2022ApJ...937L...1L |s2cid=252211794 |doi-access=free }}</ref>

=== Gravitational waves ===
The [[Laser Interferometer Space Antenna]] (LISA) for observing gravitational waves is expected to detect the presence of large planets and [[brown dwarf]]s orbiting white dwarf binaries. The number of such detections in the [[Milky Way]] is estimated to range from 17 in a pessimistic scenario to more than 2000 in an optimistic scenario, and even extragalactic detections in the [[Magellanic Clouds]] might be possible, far beyond the current capabilities of other detection methods.<ref>{{cite journal
|title=Circumbinary exoplanets and brown dwarfs with the Laser Interferometer Space Antenna
|url=https://www.aanda.org/articles/aa/full_html/2019/12/aa36729-19/aa36729-19.html
|journal=Astronomy & Astrophysics
|volume=632
|article-number=A113
|doi=10.1051/0004-6361/201936729
|arxiv=1910.05414
|year=2019
|first1=Camilla
|last1=Danielski
|first2=Valeriya
|last2=Korol
|first3=Nicola
|last3=Tamanini
|first4=Elena Maria
|last4=Rossi
|bibcode=2019A&A...632A.113D
}}</ref><ref>
{{cite journal
|title=The gravitational-wave detection of exoplanets orbiting white dwarf binaries using LISA
|url=https://www.nature.com/articles/s41550-019-0807-y
|journal=Nature Astronomy
|volume=3
|issue=9
|pages=858–866
|year=2019
|doi=10.1038/s41550-019-0807-y
|first1=Nicola
|last1=Tamanini
|first2=Camilla
|last2=Danielski
|arxiv=1812.04330
|bibcode=2019NatAs...3..858T
}}</ref><ref>{{cite journal
| author-link = 
| date = 2019
| title = The gravitational-wave detection of exoplanets orbiting white dwarf binaries using LISA
| arxiv = 1812.04330
|first1=Nicola
|last1=Tamanini
|first2=Camilla
|last2=Danielski
| journal = Nature Astronomy
| volume = 3
| issue = 9
| pages = 858–866
| doi = 10.1038/s41550-019-0807-y
| bibcode = 2019NatAs...3..858T
}}</ref>

== Detection of extrasolar asteroids and debris disks ==

=== Circumstellar disks ===
[[File:Massive Smash-Up at Vega.jpg|thumb|250px|right|An artist's conception of two [[Pluto]]-sized dwarf planets in a collision around [[Vega]] ]]
Disks of space dust ([[debris disk]]s) surround many stars.  The dust can be detected because it absorbs ordinary starlight and re-emits it as [[infrared]] radiation.  Even if the dust particles have a total mass well less than that of Earth, they can still have a large enough total surface area that they outshine their parent star in infrared wavelengths.<ref name="greaveswyatt04">{{cite journal | author=J.S. Greaves |author2=M.C. Wyatt |author3=W.S. Holland |author4=W.F.R. Dent | title=The debris disk around tau Ceti: a massive analogue to the Kuiper Belt | journal=Monthly Notices of the Royal Astronomical Society | volume=351 | date=2004 | issue=3 | pages=L54 – L58 | doi=10.1111/j.1365-2966.2004.07957.x | bibcode=2004MNRAS.351L..54G| doi-access=free }}</ref>

The [[Hubble Space Telescope]] is capable of observing dust disks with its NICMOS (Near Infrared Camera and Multi-Object Spectrometer) instrument. Even better images have now been taken by its sister instrument, the [[Spitzer Space Telescope]], and by the [[European Space Agency]]'s [[Herschel Space Observatory]], which can see far deeper into [[infrared]] wavelengths than the Hubble can.  Dust disks have now been found around more than 15% of nearby sunlike stars.<ref name="greaveswyatt03">{{cite conference | first=J.S. | last=Greaves |author2=M.C. Wyatt |author3=W.S. Holland |author4= W.F.R. Dent  | title=Submillimetre Images of the Closest Debris Disks | book-title=Scientific Frontiers in Research on Extrasolar Planets | publisher=Astronomical Society of the Pacific | date=2003 | pages=239–244|bibcode=2003ASPC..294..239G}}</ref>

The dust is thought to be generated by collisions among comets and asteroids. [[Radiation pressure]] from the star will push the dust particles away into interstellar space over a relatively short timescale. Therefore, the detection of dust indicates continual replenishment by new collisions, and provides strong indirect evidence of the presence of small bodies like comets and [[asteroid]]s that orbit the parent star.<ref name="greaveswyatt03" /> For example, the dust disk around the star [[Tau Ceti]] indicates that that star has a population of objects analogous to our own Solar System's [[Kuiper Belt]], but at least ten times thicker.<ref name="greaveswyatt04" />

More speculatively, features in dust disks sometimes suggest the presence of full-sized planets. Some disks have a central cavity, meaning that they are really ring-shaped. The central cavity may be caused by a planet "clearing out" the dust inside its orbit. Other disks contain clumps that may be caused by the gravitational influence of a planet. Both these kinds of features are present in the dust disk around [[Epsilon Eridani]], hinting at the presence of a planet with an orbital radius of around 40 [[Astronomical unit|AU]] (in addition to the inner planet detected through the radial-velocity method).<ref name="greavesepseri05">{{cite journal |
author=Greaves, J. S. | title=Structure in the Epsilon Eridani Debris Disk | journal=The Astrophysical Journal Letters | date=2005 | volume=619 | issue=2 | pages=L187–L190 | doi=10.1086/428348 |
last2=Holland |
first2=W. S. |
last3=Wyatt |
first3=M. C. |
last4=Dent |
first4=W. R. F. |
last5=Robson |
first5=E. I. |
last6=Coulson |
first6=I. M. |
last7=Jenness |
first7=T. |
last8=Moriarty-Schieven |
first8=G. H. |
last9=Davis |
first9=G. R. | last10=Butner | first10=H. M. | last11=Gear | first11=W. K. | last12=Dominik | first12=C. | last13=Walker | first13=H. J. | bibcode=2005ApJ...619L.187G| display-authors=1 |doi-access=free }}</ref>  These kinds of planet-disk interactions can be modeled numerically using [[collisional grooming]] techniques.<ref>{{Cite journal|title=A New Algorithm for Self-consistent Three-dimensional Modeling of Collisions in Dusty Debris Disks|date = 2009|last1=Stark|first1= C. C|last2=Kuchner|first2=M. J|
 journal = The Astrophysical Journal|arxiv = 0909.2227|bibcode = 2009ApJ...707..543S |doi = 10.1088/0004-637X/707/1/543|volume=707|issue = 1|pages=543–553 |s2cid = 11458583}}</ref>

=== Contamination of stellar atmospheres ===
Spectral analysis of [[white dwarf]]s' [[stellar atmosphere|atmospheres]] often finds contamination of heavier elements like [[magnesium]] and [[calcium]]. These elements cannot originate from the stars' core, and it is probable that the contamination comes from [[asteroid]]s that got too close (within the [[Roche limit]]) to these stars by gravitational interaction with larger planets and were torn apart by star's tidal forces. Up to 50% of young white dwarfs may be contaminated in this manner.<ref>{{Cite journal|last1=Koester|first1=D.|last2=Gänsicke|first2=B. T.|last3=Farihi|first3=J.|date=2014-06-01|title=The frequency of planetary debris around young white dwarfs|journal=Astronomy & Astrophysics|language=en|volume=566|pages=A34|doi=10.1051/0004-6361/201423691|issn=0004-6361|bibcode=2014A&A...566A..34K|arxiv = 1404.2617 |s2cid=119268896}}</ref>

Additionally, the dust responsible for the atmospheric pollution may be detected by infrared radiation if it exists in sufficient quantity, similar to the detection of debris discs around main sequence stars. Data from the [[Spitzer Space Telescope]] suggests that 1-3% of white dwarfs possess detectable circumstellar dust.<ref>{{cite web|url=http://www.space.com/scienceastronomy/090420-mm-solar-system.html|title=Dead Stars Once Hosted Solar Systems |last=Thompson|first=Andrea|date=20 April 2009|publisher=SPACE.com|access-date=2009-04-21}}</ref>

In 2015, minor planets were discovered transiting the white dwarf [[WD 1145+017]].<ref>{{Cite journal|last1=Vanderburg|first1=Andrew|last2=Johnson|first2=John Asher|last3=Rappaport|first3=Saul|last4=Bieryla|first4=Allyson|last5=Irwin|first5=Jonathan|last6=Lewis|first6=John Arban|last7=Kipping|first7=David|last8=Brown|first8=Warren R.|last9=Dufour|first9=Patrick|date=2015-10-22|title=A disintegrating minor planet transiting a white dwarf|journal=Nature|language=en|volume=526|issue=7574|pages=546–549|doi=10.1038/nature15527|pmid=26490620|issn=0028-0836|bibcode=2015Natur.526..546V|arxiv = 1510.06387 |s2cid=4451207}}</ref> This material orbits with a period of around 4.5 hours, and the shapes of the transit light curves suggest that the larger bodies are disintegrating, contributing to the contamination in the white dwarf's atmosphere.

== Space telescopes ==
Most confirmed extrasolar planets have been found using space-based telescopes (as of 01/2015).<ref>{{cite web|url=http://exoplanetarchive.ipac.caltech.edu/index.html|title=NASA Exoplanet Archive}}</ref>{{update needed|date=July 2024}} Many of the detection methods can work more effectively with space-based telescopes that avoid atmospheric haze and turbulence. [[COROT]] (2007-2012) and [[Kepler Mission|Kepler]] were space missions dedicated to searching for extrasolar planets using transits. COROT discovered about 30 new exoplanets. 
[[Kepler space telescope|Kepler]] (2009-2013) and K2 (2013- ) have discovered over 2000 verified exoplanets.<ref name="NASA-20160510a">{{cite web |url=http://www.nasa.gov/press-release/nasas-kepler-mission-announces-largest-collection-of-planets-ever-discovered |title=NASA's Kepler Mission Announces Largest Collection of Planets Ever Discovered |work=[[NASA]] |date=10 May 2016| access-date=10 May 2016}}</ref> [[Hubble Space Telescope]] and [[Microvariability and Oscillations of STars telescope|MOST]] have also found or confirmed a few planets. The infrared [[Spitzer Space Telescope]] has been used to detect transits of extrasolar planets, as well as [[occultation]]s of the planets by their host star and [[Phase curve (astronomy)|phase curves]].<ref name=":0" /><ref name="Deming" /><ref>{{Cite journal|last1=Knutson|first1=Heather A.|last2=Charbonneau|first2=David|last3=Allen|first3=Lori E.|author3-link=Lori Allen (astronomer)|last4=Fortney|first4=Jonathan J.|last5=Agol|first5=Eric|last6=Cowan|first6=Nicolas B.|last7=Showman|first7=Adam P.|last8=Cooper|first8=Curtis S.|last9=Megeath|first9=S. Thomas|date=2007-05-10|title=A map of the day–night contrast of the extrasolar planet HD 189733b|journal=Nature|language=en|volume=447|issue=7141|pages=183–186|doi=10.1038/nature05782|pmid=17495920|issn=0028-0836|bibcode=2007Natur.447..183K|arxiv = 0705.0993 |s2cid=4402268}}</ref>

The [[Gaia mission|''Gaia'' mission]], launched in December 2013,<ref name="rssd.esa.int">[http://www.cosmos.esa.int/web/gaia Gaia Science Homepage]</ref> will use astrometry to determine the true masses of 1000 nearby exoplanets.<ref name="ESA-20121119">{{cite web |author=Staff |title=Announcement of Opportunity for the Gaia Data Processing Archive Access Co-Ordination Unit |url=http://sci.esa.int/science-e/www/object/index.cfm?fobjectid=51116 |date=19 November 2012 |publisher=[[ESA]] |access-date=17 March 2013 }}</ref><ref name="ESA-20120130">{{cite web |author=Staff |title=DPAC Newsletter no. 15 |url=http://www.rssd.esa.int/SA/GAIA/docs/DPAC/web/DPAC_NL_015.pdf |date=30 January 2012|publisher=[[European Space Agency]] |access-date=16 March 2013}}</ref>
[[Transiting Exoplanet Survey Satellite|TESS]], launched in 2018, [[CHEOPS]] launched in 2019 and [[PLATO (spacecraft)|PLATO]] in 2026 will use the transit method.

== Primary and secondary detection ==
{| class="wikitable"
|-
! Method
! Primary
! Secondary
|-
| Transit
| Primary eclipse. Planet passes in front of star.
| Secondary eclipse. Star passes in front of planet.
|-
| Radial velocity
| Radial velocity of star
| Radial velocity of planet.<ref>{{Cite journal|arxiv=1404.5712 |last1=Kawahara |first1=Hajime |title=Spectroscopic Coronagraphy for Planetary Radial Velocimetry of Exoplanets |journal=The Astrophysical Journal Supplement Series |volume=212 |issue=2 |page=27 |last2=Murakami |first2=Naoshi |last3=Matsuo |first3=Taro |last4=Kotani |first4=Takayuki |year=2014 |doi=10.1088/0067-0049/212/2/27 |bibcode = 2014ApJS..212...27K |s2cid=118391661 }}</ref> This has been done for [[Tau Boötis b]].
|-
| Astrometry
| Astrometry of star. Position of star moves more for large planets with large orbits.
| Astrometry of planet. Color-differential astrometry.<ref>{{cite journal |last1=Abe |first1=L. |last2=Vannier |first2=M. |last3=Petrov |first3=R. |last4=Enya |first4=K. |last5=Kataza |first5=H. |title=Characterizing Extra-Solar Planets with Color Differential Astrometry on SPICA |journal=SPICA Workshop |date=2009 |page=02005 |doi=10.1051/spica/200902005 |bibcode=2009sitc.conf.2005A |url=https://spica.edpsciences.org/index.php?option=com_article&access=standard&Itemid=129&url=/articles/spica/pdf/2009/01/spica2009_02005.pdf}}</ref> Position of planet moves quicker for planets with small orbits. Theoretical method—has been proposed for use for the [[SPICA (spacecraft)|SPICA spacecraft]].
|}

== Verification and falsification methods ==
* Verification by multiplicity<ref name="kepler1700">{{cite web |last1=Johnson |first1=Michele |last2=Harrington |first2=J.D. |title=NASA's Kepler Mission Announces a Planet Bonanza, 715 New Worlds |url=http://www.nasa.gov/ames/kepler/nasas-kepler-mission-announces-a-planet-bonanza/ |date=26 February 2014 |work=[[NASA]] |access-date=26 February 2014 |archive-date=26 February 2014 |archive-url=https://web.archive.org/web/20140226202703/http://www.nasa.gov/ames/kepler/nasas-kepler-mission-announces-a-planet-bonanza/ |url-status=dead }}</ref>
* Transit color signature<ref>{{Cite journal|arxiv=1405.5354|last1= Tingley|first1= B.|title= Confirmation of an exoplanet using the transit color signature: Kepler-418b, a blended giant planet in a multiplanet system|journal= Astronomy & Astrophysics|volume= 567|pages= A14|last2=  Parviainen|first2= H.|last3=  Gandolfi|first3= D.|last4=  Deeg|first4= H. J.|last5=  Pallé|first5= E.|last6=  Montañés Rodriguez|first6= P.|last7=  Murgas|first7= F.|last8=  Alonso|first8= R.|last9=  Bruntt|first9= H.|last10=  Fridlund|first10= M.|year= 2014|doi= 10.1051/0004-6361/201323175|bibcode = 2014A&A...567A..14T |s2cid= 118668437}}</ref>
* [[Doppler tomography]]<ref>{{cite journal |last1=Johnson |first1=Marshall Caleb |title=Doppler tomographic observations of exoplanetary transits |journal=American Astronomical Society Meeting Abstracts #221 |url=https://repositories.lib.utexas.edu/server/api/core/bitstreams/61ec23d3-b6b5-481f-a137-d87c1f1ab9a9/content |archive-url=https://web.archive.org/web/20231129063215/https://repositories.lib.utexas.edu/server/api/core/bitstreams/61ec23d3-b6b5-481f-a137-d87c1f1ab9a9/content |archive-date=29 November 2023 |date=2013 |volume=221 |url-status=live|bibcode=2013AAS...22134307J}}</ref>
* Dynamical stability testing<ref>{{cite arXiv|eprint=1302.5247|last1=Horner|first1=Jonathan|title=Dynamical Constraints on Multi-Planet Exoplanetary Systems|last2= Wittenmyer|first2=Robert A.|last3= Tinney|first3=Chris G.|last4=Robertson|first4=Paul|last5= Hinse|first5=Tobias C.|last6= Marshall|first6=Jonathan P.|class=astro-ph.EP|year=2013}}</ref>
* Distinguishing between planets and stellar activity<ref>{{Cite journal|arxiv=1409.0021|last1=Robertson|first1=Paul|title=Disentangling Planets and Stellar Activity for Gliese 667C|journal=The Astrophysical Journal|volume=793|issue=2|pages=L24|author2-link=Suvrath Mahadevan|last2=Mahadevan|first2=Suvrath|year=2014|doi=10.1088/2041-8205/793/2/L24|bibcode = 2014ApJ...793L..24R |s2cid=118404871}}</ref>
* Transit offset<ref>{{Cite journal|arxiv=1303.0052 |last1= Bryson |first1= Stephen T. |title= Identification of Background False Positives from Kepler Data |journal= Publications of the Astronomical Society of the Pacific |volume= 125 |issue= 930 |page= 889 |last2=  Jenkins |first2= Jon M. |last3=  Gilliland |first3= Ronald L. |last4=  Twicken |first4= Joseph D. |last5= Clarke |first5= Bruce |last6= Rowe |first6= Jason |last7= Caldwell |first7= Douglas |last8= Batalha |first8= Natalie |last9= Mullally |first9= Fergal |last10=  Haas |first10= Michael R. |last11= Tenenbaum |first11= Peter |year= 2013 |doi= 10.1086/671767 |bibcode = 2013PASP..125..889B |s2cid= 119199796 }}</ref>

== Characterization methods ==
* [[Transmission spectroscopy]]
* Emission spectroscopy,<ref>{{Cite journal|arxiv=1410.1400 |last1= Todorov |first1= Kamen O. |title= Updated Spitzer Emission Spectroscopy of Bright Transiting Hot Jupiter HD189733b |journal= The Astrophysical Journal |volume= 796 |issue= 2 |page= 100 |last2= Deming |first2= Drake |last3=  Burrows |first3= Adam S. |last4=  Grillmair |first4= Carl J. |year= 2014 |doi= 10.1088/0004-637X/796/2/100 |bibcode = 2014ApJ...796..100T |s2cid= 118858441 }}</ref> phase-resolved<ref>{{Cite journal|arxiv=1410.2241|last1= Stevenson|first1= Kevin B.|title= Thermal structure of an exoplanet atmosphere from phase-resolved emission spectroscopy|journal= Science|volume= 346|issue= 6211|pages= 838–41|last2= Desert|first2= Jean-Michel|last3=  Line|first3= Michael R.|last4=  Bean|first4= Jacob L.|last5=  Fortney|first5= Jonathan J.|last6=  Showman|first6= Adam P.|last7= Kataria|first7= Tiffany|last8= Kreidberg|first8= Laura|last9=  McCullough|first9= Peter R.|last10=  Henry|first10= Gregory W.|last11= Charbonneau|first11= David|last12= Burrows|first12= Adam|last13= Seager|first13= Sara|last14= Madhusudhan|first14= Nikku|last15=  Williamson|first15= Michael H.|last16= Homeier|first16= Derek|year= 2014|doi= 10.1126/science.1256758|pmid= 25301972|bibcode = 2014Sci...346..838S |s2cid= 511895}}</ref>
* [[Speckle imaging]]<ref>{{Cite journal|arxiv=1407.1009|doi=10.1088/0004-6256/149/1/24|title=Hubble Space Telescopehigh-Resolution Imaging Ofkeplersmall and Cool Exoplanet Host Stars|journal=The Astronomical Journal|volume=149|issue=1|page=24|year=2014|last1=Gilliland|first1=Ronald L.|last2=Cartier|first2=Kimberly M. S.|last3=Adams|first3=Elisabeth R.|last4=Ciardi|first4=David R.|last5=Kalas|first5=Paul|last6=Wright|first6=Jason T.|bibcode = 2015AJ....149...24G |s2cid=55691820}}</ref> / [[Lucky imaging]]<ref>{{Cite journal|arxiv=1405.3120|doi=10.1051/0004-6361/201423497|last1= Lillo-Box|first1=J.|title=High-resolution imaging of $Kepler$ planet host candidates. A comprehensive comparison of different techniques|journal=Astronomy & Astrophysics|volume=566|pages=A103|last2= Barrado|first2=D.|last3= Bouy|first3=H.|year=2014|bibcode = 2014A&A...566A.103L |s2cid=55011927}}</ref> to detect companion stars that the planets could be orbiting instead of the primary star, which would alter planet parameters that are derived from stellar parameters.
* Photoeccentric Effect<ref>{{Cite journal|arxiv=1412.0014|last1= Price|first1= Ellen M.|title= How Low Can You Go? The Photoeccentric Effect for Planets of Various Sizes|journal= The Astrophysical Journal|volume= 799|issue= 1|page= 17|last2=  Rogers|first2= Leslie A.|author3= John Asher Johnson|last4=  Dawson|first4= Rebekah I.|year= 2014|doi= 10.1088/0004-637X/799/1/17|bibcode = 2015ApJ...799...17P |s2cid= 26780388}}</ref>
* [[Rossiter–McLaughlin effect]]

== See also ==
* [[List of exoplanets]]
* [[Exomoon]]

== References ==
{{Reflist|30em}}

== External links ==
* [https://web.archive.org/web/20020119150729/http://planetquest.jpl.nasa.gov/ NASA's PlanetQuest]
* {{cite journal|doi=10.1063/1.3141941|title=The detection and characterization of exoplanets|journal=Physics Today|volume=62|issue=5|page=46|year=2009|last1=Lunine|first1=Jonathan I.|last2=MacIntosh|first2=Bruce|last3=Peale|first3=Stanton|s2cid=12379824|bibcode = 2009PhT....62e..46L |doi-access=free}}
* [http://www.perseus.gr/Astro-Photometry.htm Transiting exoplanet light curves]
* {{cite web|last=Hardy|first=Liam|title=Exoplanet Transit|url=http://www.deepskyvideos.com/videos/other/exoplanet_transit.html|work=Deep Space Videos|publisher=[[Brady Haran]]}}
* [https://www.relativitycalculator.com/radial_velocity_equation.shtml The Radial Velocity Equation in the Search for Exoplanets ( The Doppler Spectroscopy or Wobble Method )] {{Webarchive|url=https://web.archive.org/web/20211202160824/https://www.relativitycalculator.com/radial_velocity_equation.shtml |date=2 December 2021 }}
* {{Cite journal|doi=10.4249/scholarpedia.3991|title=Microlensing exoplanets|journal=Scholarpedia|volume=5|issue=1|page=3991|year=2010|last1=Sackett|first1=Penny|bibcode = 2010SchpJ...5.3991S |doi-access=free}}
* {{cite journal |last1=Alhejress |first1=Omaymah A. |title=Extraplanetary system under modified gravity |journal=Europhysics Letters |date=1 December 2023 |volume=144 |issue=5 |pages=59001 |doi=10.1209/0295-5075/ad152d|bibcode=2023EL....14459001A |url=https://iopscience.iop.org/article/10.1209/0295-5075/ad152d}}
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