TRAPPIST-1

Template:Short description Template:Featured article

TRAPPIST-1 is a cool red dwarf star Template:Efn with seven known exoplanets. It lies in the constellation Aquarius about Template:Val light-years away from Earth, and has a surface temperature of about Template:Cvt. Its radius is slightly larger than Jupiter and it has a mass of about 9% of the Sun. It is estimated to be 7.6 billion years old, making it older than the Solar System. The discovery of the star was first published in 2000.

Observations in 2016 from the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) at La Silla Observatory in Chile and other telescopes led to the discovery of two terrestrial planets in orbit around TRAPPIST-1. In 2017, further analysis of the original observations identified five more terrestrial planets. It takes the seven planets between about 1.5 and 19 days to orbit around the star in circular orbits. They are all likely tidally locked to TRAPPIST-1, such that one side of each planet always faces the star, leading to permanent day on one side and permanent night on the other. Their masses are comparable to that of Earth and they all lie in the same plane; from Earth they seem to move past the disk of the star.

Up to four of the planets—designated d, e, f and g—orbit at distances where temperatures are suitable for the existence of liquid water, and are thus potentially hospitable to life. There is no evidence of an atmosphere on any of the planets, and observations of TRAPPIST-1b have ruled out the existence of an atmosphere. It is unclear whether radiation emissions from TRAPPIST-1 would allow for such atmospheres. The planets have low densities; they may consist of large amounts of volatile materials. Due to the possibility of several of the planets being habitable, the system has drawn interest from researchers and has appeared in popular culture.

Discovery

The star now known as TRAPPIST-1 was discovered in 1999 by astronomer John Gizis and colleaguesTemplate:Sfn during a survey of close-by ultra-cool dwarf stars.Template:Sfn Template:Sfn It appeared in sample CTemplate:Sfn Template:Sfn of the surveyed stars, which was obtained in June 1999. Publication of the discovery took place in 2000.Template:Sfn The name is a reference to the TRAnsiting Planets and PlanetesImals Small Telescope (TRAPPIST)Template:Sfn Template:Efn project that discovered the first two exoplanets around the star.Template:Sfn

Its planetary system was discovered by a team led by Michaël Gillon, a Belgian astronomerTemplate:Sfn at the University of Liege,Template:Sfn in 2016Template:Sfn during observations made at the La Silla Observatory, Chile,Template:Sfn Template:Sfn using the TRAPPIST telescope. The discovery was based on anomalies in the light curves Template:Efn measured by the telescope in 2015. These were initially interpreted as indicating the existence of three planets. In 2016, separate discoveries revealed that the third planet was in fact multiple planets. The telescopes and observatories involved wereTemplate:Sfn the Spitzer Space Telescope and the ground-based TRAPPIST, TRAPPIST-North in Oukaïmeden Observatory, Morocco, the South African Astronomical Observatory, and the Liverpool Telescopes and William Herschel Telescopes in Spain.Template:Sfn

The observations of TRAPPIST-1 are considered among the most important research findings of the Spitzer Space Telescope.Template:Sfn Complementing the findings were observations by the Himalayan Chandra Telescope, the United Kingdom Infrared Telescope, and the Very Large Telescope.Template:Sfn Since then, research has confirmed the existence of at least seven planets in the system,Template:Sfn the orbits of which have been calculated using measurements from the Spitzer and Kepler telescopes.Template:Sfn Some news reports incorrectly attributed the discovery of the TRAPPIST-1 planets to NASA; in fact the TRAPPIST project that led to their discovery received funding from both NASA and the European Research Council of the European Union (EU).Template:Sfn

Description

TRAPPIST-1 is in the constellation Aquarius,Template:Sfn five degrees south of the celestial equator.Template:Efn Template:Sfn Template:Sfn It is a relatively close starTemplate:Sfn located Template:Val light-years from Earth,Template:Efn Template:Sfn with a large proper motion Template:Efn Template:Sfn and no companion stars.Template:Sfn

It is a red dwarf of spectral class MTemplate:Val,Template:Efn Template:Sfn Template:Sfn meaning it is relatively small and cold.Template:Sfn With a radius 12% of that of the Sun, TRAPPIST-1 is only slightly larger than the planet Jupiter (though much more massive).Template:Sfn Its mass is approximately 9% of that of the Sun,Template:Sfn being just sufficient to allow nuclear fusion to take place.Template:Sfn Template:Sfn TRAPPIST-1's density is unusually low for a red dwarf.Template:Sfn It has a low effective temperature Template:Efn of Template:Cvt making it, Template:As of, the coldest-known star to host planets.Template:Sfn TRAPPIST-1 is cold enough for condensates to form in its photosphere;Template:Efn these have been detected through the polarisation they induce in its radiation during transits of its planets.Template:Sfn Elements heavier than helium form compounds in its atmosphere, which display as absorption lines in TRAPPIST-1's spectrum.Template:Snf

There is no evidence that it has a stellar cycle.Template:Efn Template:Sfn Its luminosity, emitted mostly as infrared radiation, is about 0.055% that of the Sun.Template:Sfn Template:Sfn Low-precisionTemplate:Sfn measurements from the XMM-Newton satelliteTemplate:Sfn and other facilitiesTemplate:Sfn show that the star emits faint radiation at short wavelengths such as x-rays and UV radiation.Template:Efn Template:Sfn There are no detectable radio wave emissions.Template:Sfn

Rotation period and age

Measurements of TRAPPIST-1's rotation have yielded a period of 3.3 days; earlier measurements of 1.4 days appear to have been caused by changes in the distribution of its starspots.Template:Sfn Its rotational axis may be slightly offset from that of its planets.Template:Sfn

Using a combination of techniques, the age of TRAPPIST-1 has been estimated at about Template:Val billion years,Template:Sfn making it older than the Solar System, which is about Template:Val billion years old.Template:Sfn It is expected to shine for ten trillion years—about 700 timesTemplate:Sfn longer than the present age of the Universe Template:Sfn—whereas the Sun will run out of hydrogen and leave the main sequence Template:Efn in a few billion years.Template:Sfn

Activity

Photospheric features have been detected on TRAPPIST-1.Template:Sfn The Kepler and Spitzer Space Telescopes have observed possible bright spots, which may be faculae,Template:Efn Template:Sfn Template:Sfn although some of these may be too large to qualify as such.Template:Sfn Bright spots are correlated to the occurrence of some stellar flares.Template:Efn Template:Sfn Kepler K2 observations have shown that TRAPPIST-1 produces frequent flares (42 flares in 80 days), including large, complex flaresTemplate:Sfn that could alter nearby planetary atmospheres irreversibly and significantly, raising doubts of hosting life as we know it on Earth.Template:Sfn

The star has a strong magnetic field Template:Sfn with a mean intensity of about 600 gauss Template:Efn Template:Sfn which may be an underestimate.Template:Snf The magnetic field drives high chromospheric Template:Efn Template:Sfn activity, and may be capable of trapping coronal mass ejections.Template:Efn Template:Sfn Template:Sfn

According to Garraffo et al. (2017), TRAPPIST-1 loses about Template:Val solar masses per yearTemplate:Sfn to the stellar wind, a rate which is about 1.5 times that of the Sun.Template:Sfn Dong et al. (2018) simulated the observed properties of TRAPPIST-1 with a mass loss of Template:Val solar masses per year.Template:Sfn Simulations to estimate mass loss are complicated because, as of 2019, most of the parameters that govern TRAPPIST-1's stellar wind are not known from direct observation.Template:Sfn

Planetary system

TRAPPIST-1 is orbited by seven planets, designated TRAPPIST-1b, 1c, 1d, 1e, 1f, 1g and 1h Template:Sfn in alphabetic order going out from the star.Template:Efn Template:Sfn These planets have orbital periods ranging from 1.5 to 19 days,Template:Sfn Template:Sfn Template:Sfn at distances of 0.011–0.059 astronomical unitsTemplate:Efn (1.7–8.9 million km).Template:Sfn

All the planets are much closer to their star than Mercury is to the Sun,Template:Sfn making the TRAPPIST-1 system very compact.Template:Sfn Kral et al. (2018) did not detect any comets around TRAPPIST-1,Template:Sfn and Marino et al. (2020) found no evidence of a Kuiper belt,Template:Sfn although it is uncertain whether a Solar System-like belt around TRAPPIST-1 would be observable from Earth.Template:Sfn Observations with the Atacama Large Millimeter Array found no evidence of a circumstellar dust disk.Template:Sfn

The inclinations of planetary orbits relative to the system's ecliptic are less than 0.1 degrees,Template:Efn Template:Sfn making TRAPPIST-1 the flattest planetary system in the NASA Exoplanet Archive.Template:Sfn The orbits are highly circular, with minimal eccentricities Template:Efn Template:Sfn and are well-aligned with the spin axis of TRAPPIST-1.Template:Sfn The planets orbit in the same plane and, from the perspective of the Solar System, transit TRAPPIST-1 during their orbitTemplate:Sfn and frequently pass in front of each other.Template:Sfn

Size and composition

The radii of the planets are estimated to range between 77.5Template:±% and 112.9Template:±% of Earth's radius.Template:Sfn The planet/star mass ratio of the TRAPPIST-1 system resembles that of the moon/planet ratio of the Solar System's gas giants.Template:Sfn

The TRAPPIST-1 planets are expected to have compositions that resemble each otherTemplate:Sfn as well as that of Earth.Template:Sfn The estimated densities of the planets are lower than Earth'sTemplate:Sfn which may imply that they have large amounts of volatile chemicals.Template:Efn Alternatively, their cores may be smaller than that of Earth and therefore they may be rocky planets with less iron than that of Earth,Template:Sfn Template:Sfn include large amounts of elements other than iron,Template:Sfn or their iron may exist in an oxidised form rather than as a core.Template:Sfn Their densities are too low for a pure magnesium silicate composition,Template:Efn requiring the presence of lower-density compounds such as water.Template:Sfn Template:Sfn Planets b, d, f, g and h are expected to contain large quantities of volatile chemicals.Template:Sfn The planets may have deep atmospheres and oceans, and contain vast amounts of ice.Template:Sfn Subsurface oceans, buried under icy shells, would form in the colder planets.Template:Sfn Several compositions are possible considering the large uncertainties in their densities.Template:Sfn The photospheric features of the star may introduce inaccuracies in measurements of the properties of TRAPPIST-1's planets,Template:Sfn including their densities being underestimated by 8Template:Su percent,Template:Sfn and incorrect estimates of their water content.Template:Sfn

Resonance and tides

File:PIA21427 - TRAPPIST-1 Planetary Orbits and Transits.ogv

The planets are in orbital resonances.Template:Sfn The durations of their orbits have ratios of 8:5, 5:3, 3:2, 3:2, 4:3 and 3:2 between neighbouring planet pairs,Template:Sfn and each set of three is in a Laplace resonance.Template:Efn Template:Sfn Simulations have shown such resonances can remain stable over billions of years but that their stability is strongly dependent on initial conditions. Many configurations become unstable after less than a million years. The resonances enhance the exchange of angular momentum between the planets, resulting in measurable variations—earlier or later—in their transit times in front of TRAPPIST-1. These variations yield information on the planetary system,Template:Sfn such as the masses of the planets, when other techniques are not available.Template:Sfn The resonances and the proximity to the host star have led to comparisons between the TRAPPIST-1 system and the Galilean moons of Jupiter.Template:Sfn Kepler-223 is another exoplanet system with a TRAPPIST-1-like long resonance.Template:Sfn

The closeness of the planets to TRAPPIST-1 results in tidal interactionsTemplate:Sfn stronger than those on Earth.Template:Sfn All the planets have reached an equilibrium with slow planetary rotations and tidal locking,Template:Sfn which can lead to the synchronisation of a planet's rotation to its revolution around its star.Template:Efn Template:Sfn However, the mutual interactions of the planets could prevent them from reaching full synchronisation, which would have important implications for the planets' climates. These interactions could force periodic or episodic full rotations of the planets' surfaces with respect to the star on timescales of several Earth years.Template:Sfn Vinson, Tamayo and Hansen (2019) found the planets TRAPPIST-1d, e and f likely have chaotic rotations due to mutual interactions, preventing them from becoming synchronised to their star. Lack of synchronisation potentially makes the planets more habitable.Template:Sfn Other processes that can prevent synchronous rotation are torques induced by stable triaxial deformation of the planets,Template:Efn which would allow them to enter 3:2 resonances.Template:Sfn

The planets are likely to undergo substantial tidal heating Template:Sfn due to deformations arising from their orbital eccentricities and gravitational interactions with one another.Template:Sfn Such heating would facilitate volcanism and degassing Template:Efn especially on the innermost planets, with degassing facilitating the establishment of atmospheres.Template:Sfn According to Luger et al. (2017), tidal heating of the four innermost planets is expected to be greater than Earth's inner heat flux.Template:Sfn For the outer planets Quick et al. (2020) noted that their tidal heating could be comparable to that in the Solar System bodies Europa, Enceladus and Triton,Template:Sfn and may be sufficient to drive detectable cryovolcanic activity.Template:Sfn

Tidal heating could influence temperatures of the night sides and cold areas where volatiles may be trapped, and gases are expected to accumulate; it would also influence the properties of any subsurface oceansTemplate:Sfn where cryovolcanism,Template:Efn Template:Sfn volcanism and hydrothermal venting Template:Efn could occur.Template:Sfn It may further be sufficient to melt the mantles of the four innermost planets, in whole or in part,Template:Sfn potentially forming subsurface magma oceans.Template:Sfn This heat source is likely dominant over radioactive decay, both of which have substantial uncertainties and are considerably less than the stellar radiation received.Template:Sfn Intense tides could fracture the planets' crusts even if they are not sufficiently strong to trigger the onset of plate tectonics.Template:Sfn Tides can also occur in the planetary atmospheres.Template:Sfn

Skies and impact of stellar light

Because most of TRAPPIST-1's radiation is in the infrared region, there may be very little visible light on the planets' surfaces; Amaury Triaud, one of the system's co-discoverers, said the skies would never be brighter than Earth's sky at sunsetTemplate:Sfn and only a little brighter than a night with a full moon. Ignoring atmospheric effects, illumination would be orange-red.Template:Sfn For TRAPPIST-1e, the central star would be four times as wide in the sky as the Sun in Earth's.Template:Sfn All of the planets would be visible from each other and would, in many cases, appear larger than Earth's Moon in the sky of Earth,Template:Sfn and each would be recognizable as a planet rather than a star.Template:Sfn They would undergo noticeable retrograde motions in the sky.Template:Sfn Observers on TRAPPIST-1e, f and g, however, could never experience a total stellar eclipse.Template:Efn Template:Sfn Assuming the existence of atmospheres, the star's long-wavelength radiation would be absorbed to a greater degree by water and carbon dioxide than sunlight on Earth; it would also be scattered less by the atmosphereTemplate:Sfn and less reflected by ice,Template:Sfn although the development of highly reflective hydrohalite ice may negate this effect.Template:Sfn The same amount of radiation results in a warmer planet compared to Sun-like irradiation;Template:Sfn more radiation would be absorbed by the planets' upper atmosphere than by the lower layers, making the atmosphere more stable and less prone to convection.Template:Sfn

Habitable zone

Template:Further

For a dim star like TRAPPIST-1, the habitable zone Template:Efn is located closer to the star than for the Sun.Template:Sfn Three or fourTemplate:Sfn planets might be located in the habitable zone; these include Template:Em, Template:Em and Template:Em;Template:Sfn or Template:Em, Template:Em and Template:Em.Template:Sfn Template:As of, this is the largest-known number of planets within the habitable zone of any known star or star system.Template:Sfn The presence of liquid water on any of the planets depends on several other factors, such as albedo (reflectivity),Template:Sfn the presence of an atmosphereTemplate:Sfn and any greenhouse effect.Template:Sfn Surface conditions are difficult to constrain without better knowledge of the planets' atmospheres.Template:Sfn A synchronously rotating planet might not entirely freeze over if it receives too little radiation from its star because the day-side could be sufficiently heated to halt the progress of glaciation.Template:Sfn Other factors for the occurrence of liquid water include the presence of oceans and vegetation;Template:Sfn the reflective properties of the land surface; the configuration of continents and oceans;Template:Sfn the presence of clouds;Template:Sfn and sea ice dynamics.Template:Sfn The effects of volcanic activity may extend the system's habitable zone to TRAPPIST-1h.Template:Sfn Even if the outer planets are too cold to be habitable, they may have ice-covered subsurface oceansTemplate:Sfn that may harbour life.Template:Sfn

Intense extreme ultraviolet (XUV) and X-ray radiationTemplate:Sfn can split water into its component parts of hydrogen and oxygen, and heat the upper atmosphere until they escape from the planet. This was thought to have been particularly important early in the star's history, when radiation was more intense and could have heated every planet's water to its boiling point.Template:Sfn This process is believed to have removed water from Venus.Template:Sfn In the case of TRAPPIST-1, different studies with different assumptions on the kinetics, energetics and XUV emissions have come to different conclusions on whether any TRAPPIST-1 planet may retain substantial amounts of water. Because the planets are most likely synchronised to their host star, any water present could become trapped on the planets' night sides and would be unavailable to support life unless heat transport by the atmosphereTemplate:Sfn or tidal heating are intense enough to melt ice.Template:Sfn

Moons

No moons with a size comparable to Earth's have been detected in the TRAPPIST-1 system,Template:Sfn and they are unlikely in such a densely packed planetary system. This is because moons would likely be either destroyed by their planet's gravity after entering its Roche limit Template:Efn or stripped from the planet by leaving its Hill radius Template:Efn Template:Sfn Although the TRAPPIST-1 planets appear in an analysis of potential exomoon hosts, they do not appear in the list of habitable-zone exoplanets that could host a moon for at least one Hubble time,Template:Sfn a timeframe slightly longer than the current age of the Universe.Template:Sfn Despite these factors, it is possible the planets could host moons.Template:Sfn

Magnetic effects

The TRAPPIST-1 planets are expected to be within the Alfvén surface of their host star,Template:Sfn the area around the star within which any planet would directly magnetically interact with the corona of the star, possibly destabilising any atmosphere the planet has.Template:Sfn Stellar energetic particles would not create a substantial radiation hazard for organisms on TRAPPIST-1 planets if atmospheres reached pressures of about Template:Val.Template:Sfn Estimates of radiation fluxes have considerable uncertainties due to the lack of knowledge about the structure of TRAPPIST-1's magnetic field.Template:Sfn Induction heating from the star's time-varying electrical and magnetic fieldsTemplate:Sfn Template:Sfn may occur on its planetsTemplate:Sfn but this would make no substantial contribution to their energy balanceTemplate:Sfn and is vastly exceeded by tidal heating.Template:Sfn

Formation history

The TRAPPIST-1 planets most likely formed further from the star and migrated inwards,Template:Sfn although it is possible they formed in their current locations.Template:Sfn According to the most popular theory on the formation of the TRAPPIST-1 planets (Ormel et al. (2017)),Template:Sfn the planets formed when a streaming instability Template:Efn at the water-ice line gave rise to precursor bodies, which accumulated additional fragments and migrated inwards, eventually giving rise to planets.Template:Sfn The migration may initially have been fast and later slowed,Template:Sfn and tidal effects may have further influenced the formation processes.Template:Sfn The distribution of the fragments would have controlled the final mass of the planets, which would consist of approximately 10% water consistent with observational inference.Template:Sfn Resonant chains of planets like those of TRAPPIST-1 usually become unstable when the gas disk that gave rise to them dissipates, but in this case, the planets remained in resonance.Template:Sfn The resonance may have been either present from the system's formation and was preserved when the planets simultaneously moved inwards,Template:Sfn or it might have formed later when inward-migrating planets accumulated at the outer edge of the gas disk and interacted with each other.Template:Sfn Inward-migrating planets would contain substantial amounts of water—too much for it to entirely escape—whereas planets that formed in their current location would most likely lose all water.Template:Sfn Template:Sfn According to Flock et al. (2019), the orbital distance of the innermost planet TRAPPIST-1b is consistent with the expected radius of an inward-moving planet around a star that was one order of magnitude brighter in the past,Template:Sfn and with the cavity in the protoplanetary disc created by TRAPPIST-1's magnetic field.Template:Sfn Alternatively, TRAPPIST-1h may have formed in or close to its current location.Template:Sfn

The presence of other bodies and planetesimals early in the system's history would have destabilised the TRAPPIST-1 planets' resonance if the bodies were massive enough.Template:Sfn Raymond et al. (2021) concluded the TRAPPIST-1 planets assembled in one to two million years, after which time little additional mass was accreted.Template:Sfn This would limit any late delivery of water to the planetsTemplate:Sfn and also implies the planets cleared the neighbourhood Template:Efn of any additional material.Template:Sfn The lack of giant impact events (the rapid formation of the planets would have quickly exhausted pre-planetary material) would help the planets preserve their volatile materials,Template:Sfn only once the planet formation process was complete.Template:Sfn

Due to a combination of high insolation, the greenhouse effect of water vapour atmospheres and remnant heat from the process of planet assembly, the TRAPPIST-1 planets would likely have initially had molten surfaces. Eventually the surfaces would cool until the magma oceans solidified, which in the case of TRAPPIST-1b may have taken between a few billions of years, or a few millions of years. The outer planets would then have become cold enough for water vapour to condense.Template:Sfn

List of planets

Template:Wide image

TRAPPIST-1b

Template:Main TRAPPIST-1b has a semi-major axis of 0.0115 astronomical units (Template:Convert)Template:Sfn and an orbital period of 1.51 Earth days. It is tidally locked to its star. The planet is outside the habitable zone;Template:Sfn its expected irradiation is more than four times that of EarthTemplate:Sfn and the James Webb Space Telescope (JWST) has measured a brightness temperature of Template:Val on the day side.Template:Sfn TRAPPIST-1b has a slightly larger measured radius and mass than Earth but estimates of its density imply it does not exclusively consist of rock.Template:Sfn Owing to its black-body temperature of Template:Cvt, TRAPPIST-1b may have had a runaway greenhouse effect similar to that of Venus;Template:Sfn JWST observations indicate that it has either no atmosphere at all or one nearly devoid of CO₂.Template:Sfn Based on several climate models, the planet would have been desiccated by TRAPPIST-1's stellar wind and radiation;Template:Sfn Template:Sfn it could be quickly losing hydrogen and therefore any hydrogen-dominated atmosphere.Template:Efn Water, if any exists, could persist only in specific settings on the planet,Template:Sfn whose surface temperature could be as high as Template:Cvt, making TRAPPIST-1b a candidate magma ocean planet.Template:Sfn According to JWST observations, the planet has an albedo of about zero.Template:Sfn

TRAPPIST-1c

Template:Main TRAPPIST-1c has a semi-major axis of Template:Convert Template:Sfn and orbits its star every 2.42 Earth days. It is close enough to TRAPPIST-1 to be tidally locked.Template:Sfn JWST observations have ruled out the existence of Venus-like atmospheres,Template:Sfn or CO₂-rich atmospheresTemplate:Sfn without a temperature inversion.Template:Sfn Airlessness is possible,Template:Sfn but only if the surface is subject to rapid volcanic overprinting which is expected given the amount of tidal heating.Template:Sfn However, water vapour- or oxygen-rich atmospheres or no-atmosphere scenarios are possible.Template:Sfn These data imply that relative to Earth or Venus, TRAPPIST-1 c has a lower carbon content.Template:Sfn TRAPPIST-1c is outside the habitable zoneTemplate:Sfn as it receives about twice as much stellar irradiation as EarthTemplate:Sfn and thus either is or has been a runaway greenhouse.Template:Sfn Based on several climate models, the planet would have been desiccated by TRAPPIST-1's stellar wind and radiation.Template:Sfn TRAPPIST-1c could harbour water only in specific settings on its surface.Template:Sfn Observations in 2017 showed no escaping hydrogen,Template:Sfn but observations by the Hubble Space Telescope (HST) in 2020 indicated that hydrogen may be escaping at a rate of Template:Val.Template:Sfn

TRAPPIST-1d

Template:Main TRAPPIST-1d has a semi-major axis of Template:Convert and an orbital period of 4.05 Earth days. It is more massive but less dense than Mars.Template:Sfn Based on fluid dynamical arguments, TRAPPIST-1d is expected to have weak temperature gradients on its surface if it is tidally locked,Template:Sfn and may have significantly different stratospheric dynamics than that of Earth.Template:Sfn Several climate models suggest that the planet mayTemplate:Sfn or may not have been desiccated by TRAPPIST-1's stellar wind and radiation;Template:Sfn density estimates, if confirmed, indicate it is not dense enough to consist solely of rock.Template:Sfn The current state of TRAPPIST-1d depends on its rotation and climatic factors like cloud feedback;Template:Efn Template:Sfn it is close to the inner edge of the habitable zone, but the existence of either liquid water or alternatively a runaway greenhouse effect (that would render it uninhabitable) are dependent on detailed atmospheric conditions.Template:Sfn Water could persist in specific settings on the planet.Template:Sfn

TRAPPIST-1e

Template:Main TRAPPIST-1e has a semi-major axis of Template:Convert Template:Sfn and orbits its star every 6.10 Earth days.Template:Sfn It has density similar that of Earth.Template:Sfn Based on several climate models, the planet is the most likely of the system to have retained its water,Template:Sfn and the most likely to have liquid water for many climate states. A dedicated climate model project called TRAPPIST-1 Habitable Atmosphere Intercomparison (THAI) has been launched to study its potential climate states.Template:Sfn Based on observations of its Lyman-alpha radiation emissions, TRAPPIST-1e may be losing hydrogen at a rate of Template:Val.Template:Sfn

TRAPPIST-1e is in a comparable position within the habitable zone to that of Proxima Centauri b,Template:Efn Template:Sfn Template:Sfn which also has an Earth-like density.Template:Sfn TRAPPIST-1e could have retained masses of water equivalent to several of Earth's oceans.Template:Sfn Moderate quantities of carbon dioxide could warm TRAPPIST-1e to temperatures suitable for the presence of liquid water.Template:Sfn

TRAPPIST-1f

Template:Main TRAPPIST-1f has a semi-major axis of Template:Convert Template:Sfn and orbits its star every 9.21 Earth days.Template:Sfn It is likely too distant from its host star to sustain liquid water, being instead an entirely glaciated snowball planet Template:Sfn that might host a subsurface ocean.Template:Sfn Moderate quantities of CO₂ could warm TRAPPIST-1f to temperatures suitable for the presence of liquid water.Template:Sfn Lakes or ponds with liquid water might form in places where tidal heating is concentrated.Template:Sfn TRAPPIST-1f may have retained masses of water equivalent to several of Earth's oceansTemplate:Sfn and which could comprise up to half of the planet's mass;Template:Sfn it could thus be an ocean planet.Template:Efn Template:Sfn

TRAPPIST-1g

Template:Main TRAPPIST-1g has a semi-major axis of Template:Convert Template:Sfn and orbits its star every 12.4 Earth days.Template:Sfn It is likely too distant from its host star to sustain liquid water, being instead a snowball planetTemplate:Sfn that might host a subsurface ocean.Template:Sfn Moderate quantities of CO₂Template:Sfn or internal heat from radioactive decay and tidal heating may warm its surface to above the melting point of water.Template:Sfn Template:Sfn TRAPPIST-1g may have retained masses of water equivalent to several of Earth's oceans;Template:Sfn density estimates of the planet, if confirmed, indicate it is not dense enough to consist solely of rock.Template:Sfn Up to half of its mass may be water.Template:Sfn

TRAPPIST-1h

Template:Main TRAPPIST-1h has a semi-major axis of Template:Convert; it is the system's least-massive-known planetTemplate:Sfn and orbits its star every 18.9 Earth days.Template:Sfn It is likely too distant from its host star to sustain liquid water and may be a snowball planet,Template:Sfn Template:Sfn or have a methane/nitrogen atmosphere resembling that of Titan.Template:Sfn It might host a subsurface ocean.Template:Sfn Large quantities of CO₂, hydrogen or methane,Template:Sfn or internal heat from radioactive decay and tidal heating,Template:Sfn would be needed to warm TRAPPIST-1h to the point where liquid water could exist.Template:Sfn TRAPPIST-1h could have retained masses of water equivalent to several of Earth's oceans.Template:Sfn

Data table

TRAPPIST-1 planets data tableTemplate:Sfn Template:Sfn Template:Sfn
Planet	Mass (Template:Earth mass)	[[Semi-major axis\|Template:Nowrap axis]] (au)	[[Semi-major axis\|Template:Nowrap axis]] (km)	Template:Nowrap (days)	Orbital eccentricity Template:Sfn	Orbital inclination Template:Sfn	Radius (Template:Earth radius)	Radiant flux Template:Sfn	Temperature Template:Sfn	Template:Nowrap (g)Template:Sfn	ORb Template:Efn	ORi Template:Efn
b	1.374 Template:±	0.01154 Template:±	1,726,000 Template:±	1.510826 Template:±	0.00622 Template:±	89.728 Template:±	1.116 Template:±	4.153 Template:±	397.6Template:±K (124.5 ± 3.8 °C; 256.0 ± 6.8 °F)Template:Efn	1.102 Template:±	—	—
c	1.308 Template:±	0.01580 Template:±	2,370,000 Template:±	2.421937 Template:±	0.00654 Template:±	89.778 Template:±	1.097 Template:±	2.214 Template:±	339.7Template:±K (66.6 ± 3.3 °C; 151.8 ± 5.9 °F)	1.086 Template:±	5:8	5:8
d	0.388 Template:±	0.02227 Template:±	3,340,500 Template:±	4.049219 Template:±	0.00837 Template:±	89.896 Template:±	0.770 Template:±	1.115 Template:±	286.2Template:±K (13.1 ± 2.8 °C; 55.5 ± 5.0 °F)	0.624 Template:±	3:8	3:5
e	0.692 Template:±	0.02925 Template:±	4,387,500 Template:±	6.101013 Template:±	0.00510 Template:±	89.793 Template:±	0.920 Template:±	0.646 Template:±	249.7Template:±K (−23.5 ± 2.4 °C; −10.2 ± 4.3 °F)	0.817 Template:±	1:4	2:3
f	1.039 Template:±	0.03849 Template:±	5,773,500 Template:±	9.207540 Template:±	0.01007 Template:±	89.740 Template:±	1.045 Template:±	0.373 Template:±	217.7Template:±K (−55.5 ± 2.1 °C; −67.8 ± 3.8 °F)	0.951 Template:±	1:6	2:3
g	1.321 Template:±	0.04683 Template:±	7,024,500 Template:±	12.352446 Template:±	0.00208 Template:±	89.742 Template:±	1.129 Template:±	0.252 Template:±	197.3Template:±K (−75.8 ± 1.9 °C; −104.5 ± 3.4 °F)	1.035 Template:±	1:8	3:4
h	0.326 Template:±	0.06189 Template:±	9,283,500 Template:±	18.772866 Template:±	0.00567 Template:±	89.805 Template:±	0.775 Template:±	0.144 Template:±	171.7Template:±K (−101.5 ± 1.7 °C; −150.6 ± 3.1 °F)	0.570 Template:±	1:12	2:3

Potential planetary atmospheres

Template:As of, the existence of an atmosphere around TRAPPIST-1b has been ruled out by James Webb Space Telescope observations, and there is no evidence for the other planets in the system,Template:Efn Template:Sfn but atmospheres are not ruled outTemplate:Sfn Template:Efn and could be detected in the future.Template:Sfn The outer planets are more likely to have atmospheres than the inner planets.Template:Sfn Several studies have simulated how different atmospheric scenarios would look to observers, and the chemical processes underpinning these atmospheric compositions.Template:Sfn The visibility of an exoplanet and of its atmosphere scale with the inverse square of the radius of its host star.Template:Sfn Detection of individual components of the atmospheres—in particular CO₂, ozone and waterTemplate:Sfn—would also be possible, although different components would require different conditions and different numbers of transits.Template:Sfn A contamination of the atmospheric signals through patterns in the stellar photosphere is a further impediment to detection.Template:Sfn Template:Sfn

The existence of atmospheres around TRAPPIST-1's planets depends on the balance between the amount of atmosphere initially present, its rate of evaporation, and the rate at which it is built back up by meteorite impactsTemplate:Efn,Template:Sfn incoming material from a protoplanetary diskTemplate:Efn,Template:Sfn and outgassing and volcanic activity.Template:Sfn Impact events may be particularly important in the outer planets because they can both add and remove volatiles; addition is likely dominant in the outermost planets where impact velocities are slower.Template:Sfn Template:Sfn The formation conditions of the planets would give them large initial quantities of volatile materials,Template:Sfn including oceans over 100 times larger than those of Earth.Template:Sfn

If the planets are tidally locked to TRAPPIST-1, surfaces that permanently face away from the star can cool sufficiently for any atmosphere to freeze out on the night side.Template:Sfn This frozen-out atmosphere could be recycled through glacier-like flows to the day side with assistance from tidal or geothermal heating from below, or could be stirred by impact events. These processes could allow an atmosphere to persist.Template:Sfn In a carbon dioxide (CO₂) atmosphere, carbon-dioxide ice is denser than water ice, under which it tends to be buried. CO₂–water compounds named clathrates Template:Efn can form. Further complications are a potential runaway feedback loop between melting ice and evaporation, and the greenhouse effect.Template:Sfn

Numerical modelling and observations constrain the properties of hypothetical atmospheres around TRAPPIST-1 planets:Template:Sfn

Theoretical calculationsTemplate:Sfn and observations have ruled out the possibility the TRAPPIST-1 planets have hydrogen-richTemplate:Sfn Template:Sfn or helium-rich atmospheres.Template:Sfn Hydrogen-rich exospheres Template:Efn may be detectableTemplate:Sfn but have not been reliably detected,Template:Sfn except perhaps for TRAPPIST-1b and 1c by Bourrier et al. (2017).Template:Sfn Template:Sfn
Water-dominated atmospheres, though suggested by some density estimates, are improbable for the planets because they are expected to be unstable under the conditions around TRAPPIST-1, especially early in the star's life.Template:Sfn The spectral properties of the planets imply they do not have a cloud-free, water-rich atmosphere.Template:Sfn
Oxygen-dominated atmospheres can form when radiation splits water into hydrogen and oxygen, and the hydrogen escapes due to its lighter mass. The existence of such an atmosphere and its mass depends on the initial water mass, on whether the oxygen is dragged out of the atmosphere by escaping hydrogen and of the state of the planet's surface; a partially molten surface could absorb sufficient quantities of oxygen to remove an atmosphere.Template:Sfn Template:Sfn
Atmospheres formed by ammonia and/or methane near TRAPPIST-1 would be destroyed by the star's radiation at a sufficient rate to quickly remove an atmosphere. The rate at which ammonia or methane are produced, possibly by organisms, would have to be considerably larger than that on Earth to sustain such an atmosphere. It is possible the development of organic hazes from ammonia or methane photolysis could shield the remaining molecules from degradation caused by radiation.Template:Sfn Ducrot et al. (2020) interpreted observational data as implying methane-dominated atmospheres are unlikely around TRAPPIST-1 planets.Template:Sfn
Nitrogen-dominated atmospheres are particularly unstable with respect to atmospheric escape, especially on the innermost planets, although the presence of CO₂ may slow evaporation.Template:Sfn Unless the TRAPPIST-1 planets initially contained far more nitrogen than Earth, they are unlikely to have retained such atmospheres.Template:Sfn
CO₂-dominated atmospheres escape slowly because CO₂ effectively radiates away energy and thus does not readily reach escape velocity; on a synchronously rotating planet, however, CO₂ can freeze out on the night side, especially if there are no other gases in the atmosphere. The decomposition of CO₂ caused by radiation could yield substantial amounts of oxygen, carbon monoxide (CO),Template:Sfn and ozone.Template:Sfn

Theoretical modelling by Krissansen-Totton and Fortney (2022) suggests the inner planets most likely have oxygen-and-CO₂-rich atmospheres, if any.Template:Sfn If the planets have an atmosphere, the amount of precipitation, its form and location would be determined by the presence and position of mountains and oceans, and the rotation period.Template:Sfn Planets in the habitable zone are expected to have an atmospheric circulation regime resembling Earth's tropical regions with largely uniform temperatures.Template:Sfn Whether greenhouse gases can accumulate on the outer TRAPPIST-1 planets in sufficient quantities to warm them to the melting point of water is controversial; on a synchronously rotating planet, CO₂ could freeze and precipitate on the night side, and ammonia and methane would be destroyed by XUV radiation from TRAPPIST-1.Template:Sfn Carbon dioxide freezing-out can occur only on the outermost planets unless special conditions are met, and other volatiles do not freeze out.Template:Sfn

Stability

The emission of extreme ultraviolet (XUV) radiation by a star has an important influence on the stability of its planets' atmospheres, their composition and the habitability of their surfaces.Template:Sfn It can cause the ongoing removal of atmospheres from planets.Template:Sfn XUV radiation-induced atmospheric escape has been observed on gas giants.Template:Sfn M dwarfs emit large amounts of XUV radiation;Template:Sfn TRAPPIST-1 and the Sun emit about the same amount of XUV radiationTemplate:Efn and because TRAPPIST-1's planets are much closer to the star than the Sun's, they receive much more intenseTemplate:Efn irradiation.Template:Sfn TRAPPIST-1 has been emitting radiation for much longer than the Sun.Template:Sfn The process of atmospheric escape has been modelled mainly in the context of hydrogen-rich atmospheres and little quantitative research has been done on those of other compositions such as water and CO₂.Template:Sfn

TRAPPIST-1 has moderate to high stellar activityTemplate:Efn,Template:Sfn and this may be another difficulty for the persistence of atmospheres and water on the planets:Template:Sfn

Dwarfs of the spectral class M have intense flares;Template:Sfn TRAPPIST-1 averages one flare every two daysTemplate:Sfn and about four to six superflares Template:Efn per year.Template:Sfn Such flares would have only small impacts on atmospheric temperatures but would substantially affect the stability and chemistry of atmospheres.Template:Sfn According to Samara, Patsourakos and Georgoulis (2021), the TRAPPIST-1 planets are unlikely to be able to retain atmospheres against coronal mass ejections.Template:Sfn
The stellar wind from TRAPPIST-1 may have a pressure 1,000 times larger than that of the Sun at Earth's orbit, which could destabilise atmospheres of the star's planetsTemplate:Sfn up to planet f. The pressure would push the wind deep into the atmospheres,Template:Sfn facilitating loss of water and evaporation of the atmospheres.Template:Sfn Template:Sfn Stellar wind-driven escape in the Solar System is largely independent from planetary properties such as mass,Template:Sfn scaling instead with the stellar wind mass flux impacting the planet.Template:Sfn Stellar wind from TRAPPIST-1 could remove the atmospheres of its planets on a timescale of 100 million to 10 billion years.Template:Sfn
Ohmic heating Template:Efn of the atmosphere of TRAPPIST-1e, f, and g amounts to five to fifteen times the heating from XUV radiation; if the heat is effectively absorbed, it could destabilise the atmospheres.Template:Sfn

The star's history also influences the atmospheres of its planets.Template:Sfn Immediately after its formation, TRAPPIST-1 would have been in a pre-main-sequence state, which may have lasted between hundreds of millionsTemplate:Sfn and two billion years.Template:Sfn While in this state, it would have been considerably brighter than it is today and the star's intense irradiation would have impacted the atmospheres of surrounding planets, vaporising all common volatiles such as ammonia, CO₂, sulfur dioxide and water.Template:Sfn Thus, all of the system's planets would have been heated to a runaway greenhouse Template:Efn for at least part of their existence.Template:Sfn The XUV radiation would have been even higher during the pre-main-sequence stage.Template:Sfn

Possible life

Life may be possible in the TRAPPIST-1 system, and some of the star's planets are considered promising targets for its detection.Template:Sfn On the basis of atmospheric stability, TRAPPIST-1e is theoretically the planet most likely to harbour life; the probability that it does is considerably less than that of Earth. There are an array of factors at play:Template:Sfn Template:Sfn

Due to multiple interactions, TRAPPIST-1 planets are expected to have intense tides.Template:Sfn If oceans are present,Template:Efn the tides could: lead to alternate flooding and drying of coastal landscapes triggering chemical reactions conducive to the development of life;Template:Sfn favour the evolution of biological rhythms such as the day-night cycle that otherwise would not develop in a synchronously rotating planet;Template:Sfn mix oceans, thus supplying and redistributing nutrients;Template:Sfn and stimulate periodic expansions of marine organisms similar to red tides on Earth.Template:Sfn
TRAPPIST-1 may not produce sufficient quantities of radiation for photosynthesis to support an Earth-like biosphere.Template:Sfn Template:Sfn Template:Sfn Mullan and Bais (2018) speculated that radiation from flares may increase the photosynthetic potential of TRAPPIST-1,Template:Sfn but according to Lingam and Loeb (2019), the potential would still be small.Template:Sfn
Due to the proximity of the TRAPPIST-1 planets, it is possible rock-encased microorganisms rippedTemplate:Efn from one planet may arrive at another planet while still viable inside the rock, allowing life to spread between the planets if it originates on one.Template:Sfn
Too much UV radiation from a star can sterilise the surface of a planetTemplate:Sfn Template:Sfn but too little may not allow the formation of chemical compounds that give rise to life.Template:Sfn Template:Sfn Inadequate production of hydroxyl radicals by low stellar-UV emission may allow gases such as carbon monoxide that are toxic to higher life to accumulate in the planets' atmospheres.Template:Sfn The possibilities range from UV fluxes from TRAPPIST-1 being unlikely to be much larger than these of early Earth—even in the event that TRAPPIST-1's emissions of UV radiation are highTemplate:Sfn—to being sufficient to sterilise the planets if they do not have protective atmospheres.Template:Sfn Template:As of it is unclear which effect would predominate around TRAPPIST-1,Template:Sfn although observations with the Kepler Space Telescope and the Evryscope telescopes indicate the UV flux may be insufficient for the formation of life or its sterilisation.Template:Sfn
Intense flaring activity of the host star—that could alter nearby planets' atmospheres irreversibly and significantly—raised doubts of the habitability of the system.Template:Sfn
Although initial water reservoirs could have been lost during the early life of the system due to the stellar activity, a potential subsequent water delivery event, like the late heavy bombardment in the Solar system, could replenish planetary water reservoirs.Template:Sfn
The outer planets in the TRAPPIST-1 system could host subsurface oceans similar to those of Enceladus and Europa in the Solar System.Template:Sfn Template:Sfn Chemolithotrophy, the growth of organisms based on non-organic reduced compounds,Template:Sfn could sustain life in such oceans.Template:Sfn Very deep oceans may be inimical to the development of life.Template:Sfn
Some planets of the TRAPPIST-1 system may have enough water to completely submerge their surfaces.Template:Sfn If so, this would have important effects on the possibility of life developing on the planets, and on their climates,Template:Sfn as weathering would decrease, starving the oceans of nutrients like phosphorus as well as potentially leading to the accumulation of carbon dioxide in their atmospheres.Template:Sfn

TRAPPIST-1 is well-suited to the search of technosignatures that would indicate the existence of past or present technology in the TRAPPIST-1 system.Template:Sfn Searches in 2017 found only signals coming from Earth,Template:Sfn others in 2024 found nothingTemplate:Sfn although their sensitivity is low.Template:Sfn In less than two millennia, Earth will be transiting in front of the Sun from the viewpoint of TRAPPIST-1, making the detection of life on Earth from TRAPPIST-1 possible.Template:Sfn

Reception and scientific importance

GIF image of a pixellated star — Kepler image of TRAPPIST-1

Public reaction and cultural impact

The discovery of the TRAPPIST-1 planets drew widespread attention in major world newspapers, social media, streaming television and websites.Template:Sfn Template:Sfn Template:As of, the discovery of TRAPPIST-1 led to the largest single-day web traffic to the NASA website.Template:Sfn NASA started a public campaign on Twitter to find names for the planets, which drew responses of varying seriousness, although the names of the planets will be decided by the International Astronomical Union.Template:Sfn The dynamics of the TRAPPIST-1 planetary system have been represented as music, such as Tim Pyle's Trappist Transits,Template:Sfn in Isolation's single Trappist-1 (A Space Anthem)Template:Sfn and Leah Asher's piano work TRAPPIST-1.Template:Sfn The alleged discovery of an SOS signal from TRAPPIST-1 was an April Fools prank by researchers at the High Energy Stereoscopic System in Namibia.Template:Sfn In 2018, Aldo Spadon created a giclée (digital artwork) named "TRAPPIST-1 Planetary System as seen from Space".Template:Sfn A website was dedicated to the TRAPPIST-1 system.Template:Sfn

Exoplanets are often featured in science-fiction works; books, comics and video games have featured the TRAPPIST-1 system, the earliest being The Terminator, a short story by Swiss author Laurence Suhner published in the academic journal that announced the system's discovery.Template:Sfn Template:Sfn At least one conference was organised to recognise works of fiction featuring TRAPPIST-1.Template:Sfn The planets have been used as the basis of science education competitionsTemplate:Sfn and school projects,Template:Sfn Template:Sfn their surfaces portrayed in artistic imagery.Template:Sfn Websites offering TRAPPIST-1-like planets as settings of virtual reality simulations exist,Template:Sfn such as the "Exoplanet Travel Bureau"Template:Sfn and the "Exoplanets Excursion"—both by NASA.Template:Sfn Scientific accuracy has been a point of discussion for such cultural depictions of TRAPPIST-1 planets.Template:Sfn

Scientific importance

TRAPPIST-1 has drawn intense scientific interest.Template:Sfn Its planets are the most easily studied exoplanets within their star's habitable zone owing to their relative closeness, the small size of their host star, and because from Earth's perspective they frequently pass in front of their host star.Template:Sfn Future observations with space-based observatories and ground-based facilities may allow further insights into their properties such as density, atmospheres and biosignatures.Template:Efn TRAPPIST-1 planetsTemplate:Sfn Template:Sfn are considered an important observation target for the James Webb Space TelescopeTemplate:Efn Template:Sfn and other telescopes under construction;Template:Sfn JWST began investigating the TRAPPIST-1 planets in 2023.Template:Sfn Together with the discovery of Proxima Centauri b, the discovery of the TRAPPIST-1 planets and the fact that at least three of the planets are within the habitable zone has led to an increase in studies on planetary habitability.Template:Sfn The planets are considered prototypical for the research on habitability of M dwarfs.Template:Sfn The star has been the subject of detailed studiesTemplate:Sfn of its various aspectsTemplate:Sfn including the possible effects of vegetation on its planets; the possibility of detecting oceans on its planets using starlight reflected off their surfaces;Template:Sfn possible efforts to terraform its planets;Template:Sfn and difficulties any inhabitants of the planets would encounter with discovering certain laws of physics (general relativity, Kepler's laws Template:Sfn and the law of gravitation Template:Sfn) and with interstellar travel.Template:Sfn

The role EU funding played in the discovery of TRAPPIST-1 has been cited as an example of the importance of EU projects,Template:Sfn and the involvement of a Moroccan observatory and a Saudi Arabian university as an indication of the Islamic and Arab world's role in science. The original discoverers were affiliated with universities spanning Africa, Europe, and North America,Template:Sfn Template:Sfn and the discovery of TRAPPIST-1 is considered to be an example of the importance of co-operation between observatories.Template:Sfn It is also one of the major astronomical discoveries from Chilean observatories.Template:Sfn

Exploration

TRAPPIST-1 is too distant from Earth to be reached by humans with current or expected technology.Template:Sfn Spacecraft mission designs using present-day rockets and gravity assists would need hundreds of millennia to reach TRAPPIST-1; even a theoretical interstellar probe travelling at near the speed of light would need decades to reach the star. The speculative Breakthrough Starshot proposal for sending small, laser-accelerated, uncrewed probes would require around two centuries to reach TRAPPIST-1.Template:Sfn

Notes

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References

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Sources

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External links

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