View source for TW Hydrae b

{{Short description|Likely extrasolar planet in the constellation Hydra}}
{{Infobox planet
| name                     = TW Hydrae b
<!-- DISCOVERY           -->
| discoverer               = First: [[Johny Setiawan|Setiawan]] ''et al.''<br>New study: [[Atacama Large Millimeter Array]]
| discovery_site           = First: {{flag|Germany}}<br>New study: {{flag|Chile}}
| discovered               = First: December 2007 (disproven)<br>New study: September 2016
<!-- DESIGNATIONS        -->
<!-- ORBITAL             --> 
| apsis                    = astron
| semimajor                = {{convert|22|AU|km|abbr=on}}
| star                     = [[TW Hydrae]]
<!-- PHYS CHARS          --> 
| mean_radius              = ~4.25 {{Earth radius|link=y}}
| mass                     = 23.72<ref name="Tsukagoshi"/> {{Earth mass|link=y}}
| single_temperature       = ≥{{convert|40|K|C F}}
<!-- ATMOSPHERE          --> 
<!-- NOTES               --> 
}}
'''TW Hydrae b''' is a likely [[extrasolar planet]] orbiting the young [[T Tauri star]] [[TW Hydrae]] approximately 176 [[light-year]]s (54 [[parsec]]s, or nearly {{val|1.665|e=16}} [[km]]) away in the [[constellation]] of [[Hydra (constellation)|Hydra]]. It is likely a Neptune-like planet orbiting at a distance of nearly 22 AU from its star.<ref name="Tsukagoshi">{{cite journal|arxiv=1605.00289|last1=Tsukagoshi|first1=Takashi|title=A Gap with a Deficit of Large Grains in the protoplanetary disk around TW Hya|last2=Nomura|first2=Hideko|last3=Muto|first3=Takayuki|last4=Kawabe|first4=Ryohei|last5=Ishimoto|first5=Daiki|last6= Kanagawa|first6=Kazuhiro D.|last7=Okuzumi|first7=Satoshi|last8=Ida|first8=Shigeru|last9=Walsh|first9=Catherine|last10= Millar|first10=Tom J.|year=2016|doi=10.3847/2041-8205/829/2/L35|volume=829|issue=2|journal=The Astrophysical Journal|page=L35|bibcode=2016ApJ...829L..35T|s2cid=41738556 |doi-access=free }}</ref>

==Characteristics==

===Mass, radius and temperature===
TW Hydrae b is an [[ice giant]], an exoplanet with a radius and mass close to that of the ice giants [[Neptune]] and [[Uranus]]. It may have an [[equilibrium temperature]] of around {{convert|40|K|C F}}. It has an estimated mass of around 22.72 {{Earth mass|link=y}} (or 1.5 ''M''<sub>Neptune</sub>) and a possible radius of 4.25 {{Earth radius|link=y}}.

===Host star===
The planet orbits a ([[Stellar classification#Class K|K-type]]) [[T Tauri star]] named [[TW Hydrae]]. The star has a mass of 0.8 {{solar mass|link=y}} and a radius of 1.1 {{solar radius}}. It has a temperature of 4000 [[Kelvin scale|K]] and is about 9 million years old. In comparison, the [[Sun]] is 4.6 billion years old<ref>{{cite web |url=http://www.universetoday.com/18237/how-old-is-the-sun/ |title=How Old is the Sun? |author=Fraser Cain |date=16 September 2008 |publisher=[[Universe Today]] |accessdate=19 February 2011}}</ref> and has a temperature of 5778 K.<ref>{{cite web |url=http://www.universetoday.com/18092/temperature-of-the-sun/ |title=Temperature of the Sun |author=Fraser Cain |date=15 September 2008 |publisher=Universe Today |accessdate=19 February 2011}}</ref> Its luminosity ({{solar luminosity|link=y}}) is 28% of that of the Sun.<ref group="note">From <math>\begin{smallmatrix}L=4 \pi R^2 \sigma T_{\rm eff}^4 \end{smallmatrix}</math>, where <math>\begin{smallmatrix}L \end{smallmatrix}</math> is the luminosity, <math>\begin{smallmatrix}R \end{smallmatrix}</math> is the radius, <math>\begin{smallmatrix}T_{\rm eff}\end{smallmatrix}</math> is the effective surface temperature and <math>\begin{smallmatrix}\sigma \end{smallmatrix}</math> is the [[Stefan–Boltzmann constant]].</ref>

The star's [[apparent magnitude]], or how bright it appears from Earth's perspective, is 11.27. Therefore, it is too dim to be seen with the naked eye.

===Orbit===
TW Hydrae b orbits its host star at a distance of 22 AU (somewhat less than the orbital distance of [[Neptune]] from the Sun, which is 30.11 [[Astronomical unit|AU]]). The orbital period is not known, although taken its similar orbital distance as Neptune, the orbital period may be around the same value.

== Discovery ==
===First claims===
In December 2007, a team led by Johny Setiawan of the [[Max Planck Institute for Astronomy]] in [[Heidelberg]], [[Germany]] announced discovery of a planet orbiting TW Hydrae, dubbed "TW Hydrae b" with a minimum mass around 1.2 [[Jupiter mass]]es, a period of 3.56 days, and an orbital radius of 0.04 [[astronomical unit]]s (inside the inner rim of the protoplanetary disk). Assuming it orbited in the same plane as the outer part of the dust disk ([[inclination]] 7±1°<ref name="setiawan08">{{cite journal|journal=Nature|volume= 451|pages= 38–41|date=3 January 2008| doi=10.1038/nature06426| title=A young massive planet in a star–disk system
| first1=J.| last1= Setiawan
| first2=Th.| last2= Henning
| first3=R.| last3= Launhardt
| first4=A.| last4= Müller
| first5=P.| last5= Weise
| first6=M.| last6= Kürster
| pmid=18172492 | issue=7174| bibcode=2008Natur.451...38S|s2cid= 4431370}}</ref>), it would have a true mass of 9.8±3.3 Jupiter masses.<ref name="setiawan08" /><ref name=newscientist>{{cite news| url=https://www.newscientist.com/article/dn13135-first-planet-discovered-around-a-youthful-star.html | title=First planet discovered around a youthful star| date=2 January 2008| publisher= NewScientist.com news service| first= Maggie| last= McKee| accessdate=2008-01-02}}</ref> However, if the inclination was similar to the inner part of the dust disk (4.3±1.0°<ref name="pontoppidan08">{{cite journal|title=Spectro-astrometric imaging of molecular gas within protoplanetary disk gaps|author=Pontoppidan, Klaus M.|display-authors=etal|date=2008|journal=[[The Astrophysical Journal]]|volume=684|issue=2|pages=1323–1329|doi=10.1086/590400|bibcode=2008ApJ...684.1323P|arxiv = 0805.3314 |s2cid=15445587 }}</ref>), the mass would be 16{{±|5|3}} Jupiter masses, making it a [[brown dwarf]].<ref name="pontoppidan08" /> Since the star itself is so young, it was presumed this was the youngest extrasolar planet yet discovered, and essentially still in formation.<ref name=mpia>{{cite web|url=http://www.mpia.de/Public/menu_q2e.php?Aktuelles/PR/2008/PR080103/PR_080103_en.html|title=A young extrasolar planet in its cosmic nursery: Astronomers from Heidelberg discover planet in a dusty disk around a newborn star|date=2008-01-02|publisher=[[Max Planck Institute for Astronomy]]|accessdate=2008-01-03}}</ref> (only surpassed by [[K2-33b]] and [[V830 Tau b]], both discovered nearly 9 years later).

===Disproven status===
In 2008 a team of Spanish researchers concluded that the planet did not exist: the radial velocity variations were not consistent when observed at different [[wavelength]]s, which would not occur if the origin of the radial velocity variations was caused by an orbiting planet. Instead, the data was better modelled by [[starspot]]s on TW Hydrae's surface passing in and out of view as the star rotates. "Results support the spot scenario rather than the presence of a hot Jupiter around TW Hya".<ref name=huelmo>{{cite journal|author=Huelamo, N.|display-authors=etal|title=TW Hydrae: evidence of stellar spots instead of a Hot Jupiter|journal=[[Astronomy and Astrophysics]]|volume=489|issue=2|pages=L9–L13|date=2008|doi=10.1051/0004-6361:200810596|bibcode=2008A&A...489L...9H|arxiv = 0808.2386 |s2cid=18775872 }}</ref> Similar wavelength-dependent radial velocity variations, also caused by starspots, have been detected on other T Tauri stars.<ref name=prato>{{cite journal|author=Prato, L.|display-authors=etal|title=A Young Planet Search in Visible and IR Light: DN Tau, V836 Tau, and V827 Tau|date=2008|journal=[[The Astrophysical Journal]]|volume=687|issue=2|pages=L103–L106|doi=10.1086/593201|bibcode=2008ApJ...687L.103P|arxiv = 0809.3599 |s2cid=14888302 }}</ref>

===New proposal===
In 2016, astronomers studying the protoplanetary disk of the star began to speculate why there was small dust grains in the gaps, including the one at 22 AU, but not large dust grains. Further investigations began to suggest that there may be a 1.5 ''M''<sub>Neptune</sub> ice giant orbiting within the gap at 22 AU, which would be responsible for the observed gaps.

The study was then published in the online journal archive [[arXiv]] on September 1, 2016, gaining wide interest from media outlets.<ref name="Tsukagoshi"/>

== Notes ==
{{reflist|group=note}}

== References ==
{{reflist}}

{{Sky|11|01|52|-|34|42|17}}
{{2016 in space|state=collapsed}}

[[Category:Hydra (constellation)]]