TRAPPIST-1

TRAPPIST-1 is a cool red dwarf star[lower-alpha 2] with seven known exoplanets. It lies in the constellation Aquarius about 40.66 light-years away from Earth, and has a surface temperature of about 2,566 kelvins (2,290 degrees Celsius; 4,160 degrees Fahrenheit). 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.

TRAPPIST-1
TRAPPIST-1 lies in the northwestern part of the constellation Aquarius, close to the ecliptic.
TRAPPIST-1 is within the red circle in the constellation Aquarius.
Observation data
Epoch J2000      Equinox J2000
Constellation Aquarius
Right ascension 23h 06m 29.368s[1]
Declination −05° 02′ 29.04″[1]
Apparent magnitude (V) 18.798±0.082[2]
Characteristics
Evolutionary stage Main sequence
Spectral type M8V[3]
Apparent magnitude (R) 16.466±0.065[2]
Apparent magnitude (I) 14.024±0.115[2]
Apparent magnitude (J) 11.354±0.022[4]
Apparent magnitude (H) 10.718±0.021[4]
Apparent magnitude (K) 10.296±0.023[4]
V−R color index 2.332
R−I color index 2.442
J−H color index 0.636
J−K color index 1.058
Astrometry
Proper motion (μ) RA: 930.788[1] mas/yr
Dec.: −479.038[1] mas/yr
Parallax (π)80.2123 ± 0.0716 mas[1]
Distance40.66 ± 0.04 ly
(12.47 ± 0.01 pc)
Details
Mass0.0898±0.0023[5] M☉
Radius0.1192±0.0013[5] R☉
Luminosity (bolometric)0.000553±0.000018[5] L☉
Surface gravity (log g)5.2396+0.0056
−0.0073[lower-alpha 1][5] cgs
Temperature2,566±26[5] K
Metallicity [Fe/H]0.04±0.08[6] dex
Rotation3.295±0.003 days[7]
Rotational velocity (v sin i)6[8] km/s
Age7.6±2.2[9] Gyr
Other designations
2MUDC 12171,[10] 2MASS J23062928–0502285, EPIC 246199087,[11] K2-112,[12] SPECULOOS-1, an internal name of the star used by the SPECULOOS project, as this star was its first discovery,[13] and TRAPPIST-1a.[14]
Database references
SIMBADdata
Exoplanet Archivedata
Extrasolar Planets
Encyclopaedia		data

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 planets, of unknown types. It takes the seven planets between about 1.5 and 19 days to orbit around the star in circular orbits. They are 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. 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 colleagues[16] during a survey of close-by ultra-cool dwarf stars.[17][18] It appeared in sample C[16][17] of the surveyed stars, which was obtained in June 1999. Publication of the discovery took place in 2000.[19] The name is a reference to the TRansiting Planets and PlanetesImals Small Telescope (TRAPPIST)[11][lower-alpha 3] project that discovered the first two exoplanets around the star.[23]

Its planetary system was discovered by a team led by Michaël Gillon, a Belgian astronomer[24] at the University of Liege,[25] in 2016[26] during observations made at the La Silla Observatory, Chile,[27][28] using the TRAPPIST telescope. The discovery was based on anomalies in the light curves[lower-alpha 4] 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 were[11] the Spitzer Space Telescope; the ground-based TRAPPIST and TRAPPIST-North in Oukaïmeden Observatory, Morocco; the South African Astronomical Observatory; and the Liverpool Telescopes and William Herschel Telescopes in Spain.[30]

The observations of TRAPPIST-1 are considered among the most important research findings of the Spitzer Space Telescope.[31] Complementing the findings were observations by the Himalayan Chandra Telescope, the United Kingdom Infrared Telescope, and the Very Large Telescope.[32] Since then, research has confirmed the existence of at least seven planets in the system,[33] the orbits of which have been calculated using measurements from the Spitzer and Kepler telescopes.[34] 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).[35]

Description
see caption
True-colour illustration of the Sun (left) next to TRAPPIST-1 (right). TRAPPIST-1 is darker, redder, and smaller than the Sun.

TRAPPIST-1 is in the constellation Aquarius,[25] five degrees south of the celestial equator.[lower-alpha 5][1][37] It is a relatively close star[38] located 40.66±0.04 light-years from Earth,[lower-alpha 6][1] with a large proper motion[lower-alpha 7][38] and no companion stars.[41]

It is a red dwarf of spectral class M8.0±0.5,[lower-alpha 8][32][44] meaning it is relatively small and cold.[45] With a radius 12% of that of the Sun, TRAPPIST-1 is only slightly larger than the planet Jupiter (though much more massive).[32] Its mass is approximately 9% of that of the Sun,[45] being just sufficient to allow nuclear fusion to take place.[46][47] TRAPPIST-1's density is unusually low for a red dwarf.[48] It has a low effective temperature[lower-alpha 9] of 2,566 K (2,293 °C) making it, as of 2022, the coldest-known star to host planets.[50] TRAPPIST-1 is cold enough for condensates to form in its photosphere;[lower-alpha 10] these have been detected through the polarisation they induce in its radiation during transits of its planets.[52]

There is no evidence that it has a stellar cycle.[lower-alpha 11][54] Its luminosity, emitted mostly as infrared radiation, is about 0.055% that of the Sun.[45][55] Low precision[56] measurements from the XMM-Newton satellite[57] and other facilities[58] show that the star emits faint radiation at short wavelengths such as x-rays and UV radiation.[lower-alpha 12][57] There are no detectable radio wave emissions.[60]

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.[61] Its rotational axis may be slightly offset from that of its planets.[62]

Using a combination of techniques, the age of TRAPPIST-1 has been estimated at about 7.6±2.2 billion years,[63] making it older than the Solar System, which is about 4.5 billion years old.[64] It is expected to shine for ten trillion years – about 700 times[65] longer than the present age of the Universe[66] – whereas the Sun will run out of hydrogen and leave the main sequence[lower-alpha 13] in a few billion years.[65]

Activity

Photospheric features have been detected on TRAPPIST-1.[68] The Kepler and Spitzer Space Telescopes have observed possible bright spots, which may be faculae,[lower-alpha 14][70][71] although some of these may be too large to qualify as such.[72] Bright spots are correlated to the occurrence of some stellar flares.[lower-alpha 15][73]

The star has a strong magnetic field[74] with a mean intensity of about 600 gauss.[lower-alpha 16][76] The magnetic field drives high chromospheric[lower-alpha 17][74] activity, and may be capable of trapping coronal mass ejections.[lower-alpha 18][69][77]

According to Garraffo et al. (2017), TRAPPIST-1 loses about 3×10−14 solar masses per year[78] to the stellar wind, a rate which is about 1.5 times that of the Sun. [79] Dong et al. (2018) simulated the observed properties of TRAPPIST-1 with a mass loss of 4.1×10−15 solar masses per year.[78] 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.[80]

Planetary system
The TRAPPIST-1 system is about as compact as Jupiter's moons and much more than the Solar System
Comparison of the orbits of the TRAPPIST-1 planets with the Solar System and Jupiter's moons

TRAPPIST-1 is orbited by seven planets, designated TRAPPIST-1b, 1c, 1d, 1e, 1f, 1g, and 1h[81] in alphabetic order going out from the star.[lower-alpha 19][84] These planets have orbital periods ranging from 1.5–19 days,[85][86][6] at distances of 0.011–0.059 astronomical units[lower-alpha 20] (1,700,000–8,900,000 km).[88]

All the planets are much closer to their star than Mercury is to the Sun,[26] making the TRAPPIST-1 system very compact.[89] Kral et al. (2018) did not detect any comets around TRAPPIST-1,[90] and Marino et al. (2020) found no evidence of a Kuiper belt,[91] although it is uncertain whether a Solar System-like belt around TRAPPIST-1 would be observable from Earth.[92] Observations with the Atacama Large Millimeter Array found no evidence of a circumstellar dust disk.[93]

The inclinations of planetary orbits relative to the system's ecliptic are less than 0.1 degrees[lower-alpha 21],[95] making TRAPPIST-1 the flattest planetary system in the NASA Exoplanet Archive.[96] The orbits are highly circular, with minimal eccentricities[lower-alpha 22][89] and are well-aligned with the spin axis of TRAPPIST-1.[98] The planets orbit in the same plane and, from the perspective of the Solar System, transit TRAPPIST-1 during their orbit[99] and frequently pass in front of each other.[100]

Size and composition

The radii of the planets are estimated to range between 77.5+1.4
−1.4 and 112.9+1.5
−1.3% of Earth's radius.[101] The planets:star mass ratio of the TRAPPIST-1 system resembles that of the moons:planet ratio of the Solar System's gas giants.[102]

The TRAPPIST-1 planets are expected to have compositions that resemble each other[103] as well as that of Earth.[104] The estimated densities of the planets are lower than Earth's[34] which may imply that they have large amounts of volatile chemicals.[lower-alpha 23] Alternatively, their cores may be smaller than that of Earth;[106] or include large amounts of elements other than iron;[107] or their iron may exist in an oxidised form rather than as a core;[106] or they may be rocky planets with less iron than that of Earth.[108] Their densities are too low for a pure magnesium silicate composition,[lower-alpha 24] requiring the presence of lower-density compounds such as water.[110][111] Planets b, d, f, g and h are expected to contain large quantities of volatile chemicals.[112] The planets may have deep atmospheres and oceans, and contain vast amounts of ice.[113] Several compositions are possible considering the large uncertainties in their densities.[114] The photospheric features of the star may introduce inaccuracies in measurements of the properties of TRAPPIST-1's planets,[68] including their densities being underestimated by 8+20
   -7 percent,[115] and incorrect estimates of their water content.[116]

Resonance and tides

The planets are in orbital resonances.[117] 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,[118] and each set of three is in a Laplace resonance.[lower-alpha 25][89] 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,[120] such as the masses of the planets, when other techniques are not available.[121] The resonances and the proximity to the host star have led to comparisons between the TRAPPIST-1 system and the Galilean moons of Jupiter.[99] Kepler-223 is another exoplanet system with a TRAPPIST-1-like long resonance.[122]

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.[123] 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.[124] Other processes that can prevent synchronous rotation are torques induced by stable triaxial deformation of the planets,[lower-alpha 26] which would allow them to enter 3:2 resonances.[126]

The closeness of the planets to TRAPPIST-1 results in tidal interactions[127] stronger than those on Earth.[128] All the planets have reached an equilibrium with slow planetary rotations and tidal locking,[127] which can lead to the synchronisation of a planet's rotation to its revolution around its star.[lower-alpha 27][130]

The planets are likely to undergo substantial tidal heating[131] due to deformations arising from their orbital eccentricities and gravitational interactions with one another.[132] Such heating would facilitate volcanism and degassing[lower-alpha 28] especially on the innermost planets, with degassing facilitating the establishment of atmospheres.[134] According to Luger et al. (2017), tidal heating of the four innermost planets is expected to be greater than Earth's inner heat flux.[135] 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.[136]

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 oceans[137] where volcanism and hydrothermal venting[lower-alpha 29] could occur.[139] It may further be sufficient to melt the mantles of the four innermost planets, in whole or in part,[140] potentially forming subsurface magma oceans.[141] This heat source is likely dominant over radioactive decay, both of which have substantial uncertainties and are considerably less than the stellar radiation received.[142] Intense tides could fracture the planets' crusts even if they are not sufficiently strong to trigger the onset of plate tectonics.[143] Tides can also occur in the planetary atmospheres.[144]

Skies and impact of stellar light
TRAPPIST-1 planets are of similar or smaller size than Earth and have similar or smaller densities
Relative sizes, densities,[lower-alpha 30] and illumination of the TRAPPIST-1 system compared to the inner planets of the Solar System

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 sunset[146] and only a little brighter than a night with a full moon. Ignoring atmospheric effects, illumination would be orange-red.[147] 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;[26] observers on TRAPPIST-1e, f and g, however, could never experience a total stellar eclipse.[lower-alpha 31][84] 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 atmosphere[148] and less reflected by ice,[149] although the development of highly reflective hydrohalite ice may negate this effect.[150] The same amount of radiation results in a warmer planet compared to Sun-like irradiation;[148] 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.[151]

Habitable zone
Further information: Habitability of red dwarf systems
1e, 1f and 1g is in the habitable zone
Habitable zones of TRAPPIST-1 and the Solar System. The displayed planetary surfaces are speculative.

For a dim star like TRAPPIST-1, the habitable zone[lower-alpha 32] is located closer to the star than for the Sun.[152] Three or four[57] planets might be located in the habitable zone; these include e, f, and g;[152] or d, e, and f.[74] As of 2017, this is the largest-known number of planets within the habitable zone of any known star or star system.[153] The presence of liquid water on any of the planets depends on several other factors, such as albedo (reflectivity),[154] the presence of an atmosphere[155] and any greenhouse effect.[156] Surface conditions are difficult to constrain without better knowledge of the planets' atmospheres.[155] 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.[157] Other factors for the occurrence of liquid water include the presence of oceans and vegetation;[158] the reflective properties of the land surface; the configuration of continents and oceans;[159] the presence of clouds;[160] and sea ice dynamics.[161] The effects of volcanic activity may extend the system's habitable zone to TRAPPIST-1h.[162]

Intense extreme ultraviolet (XUV) and X-ray radiation[163] 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.[149] This process is believed to have removed water from Venus.[164] 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 atmosphere[165] or tidal heating are intense enough to melt ice.[166]

Moons

No moons with a size comparable to Earth's have been detected in the TRAPPIST-1 system,[167] 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[lower-alpha 33] or stripped from the planet by leaving its Hill radius[lower-alpha 34][170] 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,[171] a timeframe slightly longer than the current age of the Universe.[172] Despite these factors, it is possible the planets could host moons.[173]

Magnetic effects

The TRAPPIST-1 planets are expected to be within the Alfvén surface of their host star,[174] 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.[175] Stellar energetic particles would not create a substantial radiation hazard for organisms on TRAPPIST-1 planets if atmospheres reached pressures of about 1 bar.[176] Estimates of radiation fluxes have considerable uncertainties due to the lack of knowledge about the structure of TRAPPIST-1's magnetic field.[177] Induction heating from the star's time-varying electrical and magnetic fields[140][178] may occur on its planets[179] but this would make no substantial contribution to their energy balance[142] and is vastly exceeded by tidal heating.[136]

Formation history

The TRAPPIST-1 planets most likely formed further from the star and migrated inwards,[180] although it is possible they formed in their current locations.[181] According to Ormel et al. (2017), the planets formed when a streaming instability[lower-alpha 35] at the water-ice line gave rise to precursor bodies, which accumulated additional fragments and migrated inwards, eventually giving rise to planets.[183] The migration may initially have been fast and later slowed,[184] and tidal effects may have further influenced the formation processes.[185] 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.[183] 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.[186] The resonance may have been either present from the system's formation and was preserved when the planets simultaneously moved inwards,[187] or it might have formed later when inward-migrating planets accumulated at the outer edge of the gas disk and interacted with each other.[181] 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.[188][189] 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,[190] and with the cavity in the protoplanetary disc created by TRAPPIST-1's magnetic field.[191] Alternatively, TRAPPIST-1h may have formed in or close to its current location.[192]

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.[193] Raymond et al. (2021) concluded the TRAPPIST-1 planets assembled in 1–2 million years, after which time little additional mass was accreted.[194] This would limit any late delivery of water to the planets[195] and also implies the planets cleared the neighbourhood[lower-alpha 36] of any additional material.[196] 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.[197]

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.[198]

List of planets
Distances between TRAPPIST-1 planets are roughly comparable with Earth-Moon distances
The TRAPPIST-1 system with distances to scale, compared with the Moon-Earth distance

TRAPPIST-1b
Main article: TRAPPIST-1b

TRAPPIST-1b has a semi-major axis of 0.0115 astronomical units (1,720,000 km)[199] and an orbital period of 1.51 Earth days. It is expected to be tidally locked to its star. The planet is outside the habitable zone;[200] its expected irradiation is more than four times that of Earth.[200] 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.[201] Owing to its black-body temperature of 124 °C (397 K), TRAPPIST-1b may have had a runaway greenhouse effect similar to that of Venus;[74] its atmosphere, if present, may be similarly deep, dense, and hot.[202] Based on several climate models, the planet would have been desiccated by TRAPPIST-1's stellar wind and radiation;[203][204] it could be quickly losing hydrogen and therefore any hydrogen-dominated atmosphere.[lower-alpha 37] Water, if any exists, could persist only in specific settings on the planet,[206] whose surface temperature could be as high as 1,200 °C (1,470 K), making TRAPPIST-1b a candidate magma ocean planet.[207]

TRAPPIST-1c
Infrared measurements by the NASA / ESA / Canadian Space Agency / James Webb Space Telescope of TRAPPIST-1 c indicate that it is likely not as Venus-like as once imagined.
Main article: TRAPPIST-1c

TRAPPIST-1c has a semi-major axis of 0.0158 AU (2,360,000 km)[199] and orbits its star every 2.42 Earth days. It is close enough to TRAPPIST-1 to be tidally locked.[200] Its atmosphere could either be absent or thick like that of Venus.[202] TRAPPIST-1c is outside the habitable zone[200] as it receives about twice as much stellar irradiation as Earth[208] and thus either is or has been a runaway greenhouse.[74] Based on several climate models, the planet would have been desiccated by TRAPPIST-1's stellar wind and radiation.[203] TRAPPIST-1c could harbour water only in specific settings on its surface.[206] Observations in 2017 showed no escaping hydrogen,[58] but observations by the Hubble Space Telescope (HST) in 2020 indicated that hydrogen may be escaping at a rate of 1.4×107 g/s.[205]

TRAPPIST-1d
Main article: TRAPPIST-1d

TRAPPIST-1d has a semi-major axis of 0.022 AU (3,300,000 km) and an orbital period of 4.05 Earth days. It is more massive but less dense than Mars.[209] Based on fluid dynamical arguments, TRAPPIST-1d is expected to have weak temperature gradients on its surface if it is tidally locked,[210] and may have significantly different stratospheric dynamics than that of Earth.[211] Several climate models suggest that the planet may[203] or may not have been desiccated by TRAPPIST-1's stellar wind and radiation;[203] density estimates, if confirmed, indicate it is not dense enough to consist solely of rock.[201] The current state of TRAPPIST-1d depends on its rotation and climatic factors like cloud feedback;[lower-alpha 38][202] 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.[213] Water could persist in specific settings on the planet.[206]

TRAPPIST-1e
Main article: TRAPPIST-1e

TRAPPIST-1e has a semi-major axis of 0.029 AU (4,300,000 km)[199] and orbits its star every 6.10 Earth days.[214] It has density similar that of Earth.[215] Based on several climate models, the planet is the most likely of the system to have retained its water,[203] 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.[216] Based on observations of its Lyman-alpha radiation emissions, TRAPPIST-1e may be losing hydrogen at a rate of 0.6×107 g/s.[205]

TRAPPIST-1e is in a comparable position within the habitable zone to that of Proxima Centauri b,[lower-alpha 39][218][219] which also has an Earth-like density.[215] TRAPPIST-1e could have retained masses of water equivalent to several of Earth's oceans.[74] Moderate quantities of carbon dioxide could warm TRAPPIST-1e to temperatures suitable for the presence of liquid water.[204]

TRAPPIST-1f
Main article: TRAPPIST-1f

TRAPPIST-1f has a semi-major axis of 0.038 AU (5,700,000 km)[199] and orbits its star every 9.21 Earth days.[214] It is likely too distant from its host star to sustain liquid water, being instead an entirely glaciated snowball planet.[203] Moderate quantities of CO2 could warm TRAPPIST-1f to temperatures suitable for the presence of liquid water.[206] TRAPPIST-1f may have retained masses of water equivalent to several of Earth's oceans[74] and which could comprise up to half of the planet's mass;[220] it could thus be an ocean planet.[lower-alpha 40][222]

TRAPPIST-1g
Main article: TRAPPIST-1g

TRAPPIST-1g has a semi-major axis of 0.047 AU (7,000,000 km)[199] and orbits its star every 12.4 Earth days.[214] It is likely too distant from its host star to sustain liquid water, being instead a snowball planet.[203] Moderate quantities of CO2[206] or internal heat from radioactive decay and tidal heating may warm its surface to above the melting point of water.[223] TRAPPIST-1g may have retained masses of water equivalent to several of Earth's oceans;[74] density estimates of the planet, if confirmed, indicate it is not dense enough to consist solely of rock.[201] Up to half of its mass may be water.[220]

TRAPPIST-1h
Main article: TRAPPIST-1h

TRAPPIST-1h has a semi-major axis of 0.062 astronomical units (9,300,000 km); it is the system's least massive known planet[199] and orbits its star every 18.9 Earth days.[214] It is likely too distant from its host star to sustain liquid water and may be a snowball planet,[203] or have a methane/nitrogen atmosphere resembling that of Titan.[224] Large quantities of CO2, hydrogen or methane,[225] or internal heat from radioactive decay and tidal heating,[223] would be needed to warm TRAPPIST-1h to the point where liquid water could exist.[225] TRAPPIST-1h could have retained masses of water equivalent to several of Earth's oceans.[74]

Data table
TRAPPIST-1 planets data table[226][86][6]
Planet M SX OP E I R RX T G ORb ORi
b 1.374
±0.069		 0.01154
±0.0001		 1.510826
±0.000006		 0.00622
±0.00304		 89.728
±0.165°		 1.116
+0.014
−0.012		 4.153
±0.160		 397.6±3.8K
(124.5 ± 3.8 °C; 256.0 ± 6.8 °F)[lower-alpha 41]		 1.102
±0.052		 — —
c 1.308
±0.056		 0.01580
±0.00013		 2.421937
±0.000018		 0.00654
±0.00188		 89.778
±0.118°		 1.097
+0.014
−0.012		 2.214
±0.085		 339.7±3.3K
(66.6 ± 3.3 °C; 151.8 ± 5.9 °F)		 1.086
±0.043		 5:8 5:8
d 0.388
±0.012		 0.02227
±0.00019		 4.049219
±0.000026		 0.00837
±0.00093		 89.896
±0.077°		 0.770
+0.011
−0.010		 1.115
±0.04		 286.2±2.8K
(13.1 ± 2.8 °C; 55.5 ± 5.0 °F)		 0.624
±0.019		 3:8 3:5
e 0.692
±0.022		 0.02925
±0.00025		 6.101013
±0.000035		 0.00510
±0.00058		 89.793
±0.048°		 0.920
+0.013
−0.012		 0.646
±0.025		 249.7±2.4K
(−23.5 ± 2.4 °C; −10.2 ± 4.3 °F)		 0.817
±0.024		 1:4 2:3
f 1.039
±0.031		 0.03849
±0.00033		 9.207540
±0.000032		 0.01007
±0.00068		 89.740
±0.019°		 1.045
+0.013
−0.012		 0.373
±0.014		 217.7±2.1K
(−55.5 ± 2.1 °C; −67.8 ± 3.8 °F)		 0.951
±0.024		 1:6 2:3
g 1.321
±0.038		 0.04683
±0.0004		 12.352446
±0.000054		 0.00208
±0.00058		 89.742
±0.012°		 1.129
+0.015
−0.013		 0.252
±0.010		 197.3±1.9K
(−75.8 ± 1.9 °C; −104.5 ± 3.4 °F)		 1.035
±0.026		 1:8 3:4
h 0.326
±0.020		 0.06189
±0.00053		 18.772866
±0.000214		 0.00567
±0.00121		 89.805
±0.013°		 0.775
+0.014
−0.014		 0.144
±0.006		 171.7±1.7K
(−101.5 ± 1.7 °C; −150.6 ± 3.1 °F)		 0.570
±0.038		 1:12 2:3

Potential planetary atmospheres
Lengthening brightness dips from 1b to 1h. Shallowest to deepest dips: 1h, 1d, 1e, 1f, 1g, 1c, 1b.
Graph showing dips in brightness in TRAPPIST-1 star by the planet's transits or obstruction of starlight. Larger planets create deeper dips and further planets create longer dips.

As of 2020, there is no definitive evidence that any of the TRAPPIST-1 planets have an atmosphere,[lower-alpha 42][228] but atmospheres could be detected in the future.[229] The outer planets are more likely to have atmospheres than the inner planets.[180] Several studies have simulated how different atmospheric scenarios would look to observers, and the chemical processes underpinning these atmospheric compositions.[230] The visibility of an exoplanet and of its atmosphere scale with the inverse square of the radius of its host star.[229] Detection of individual components of the atmospheres – in particular CO2, ozone, and water[231] – would also be possible, although different components would require different conditions and different numbers of transits.[232] A contamination of the atmospheric signals through patterns in the stellar photosphere is a further impediment to detection.[233]

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 impacts,[89] incoming material from a protoplanetary disk[lower-alpha 43],[235] and outgassing and volcanic activity.[236] 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.[237][238] Although the properties of TRAPPIST-1 are unfavourable to the continued existence of atmospheres around its planets,[239] the formation conditions of the planets would give them large initial quantities of volatile materials,[180] including oceans over 100 times larger than those of Earth.[240]

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.[241] 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.[242] 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[lower-alpha 44] can form. Further complications are a potential runaway feedback loop between melting ice and evaporation, and the greenhouse effect.[244]

Numerical modelling and observations constrain the properties of hypothetical atmospheres around TRAPPIST-1 planets:[180]

  • Theoretical calculations[245] and observations have ruled out the possibility the TRAPPIST-1 planets have hydrogen-rich[222][246] or helium-rich atmospheres.[247] Hydrogen-rich exospheres[lower-alpha 45] may be detectable[249] but have not been reliably detected,[250] except perhaps for TRAPPIST-1b and 1c by Bourrier et al. (2017).[192][14]
  • 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.[201] The spectral properties of the planets imply they do not have a cloud-free, water-rich atmosphere.[251]
  • 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.[252]
  • 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.[253] Ducrot et al. (2020) interpreted observational data as implying methane-dominated atmospheres are unlikely around TRAPPIST-1 planets.[254]
  • Nitrogen-dominated atmospheres are particularly unstable with respect to atmospheric escape, especially on the innermost planets, although the presence of CO2 may slow evaporation.[255] Unless the TRAPPIST-1 planets initially contained far more nitrogen than Earth, they are unlikely to have retained such atmospheres.[256]
  • 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),[204] and ozone.[257]

Theoretical modelling by Krissansen-Totton and Fortney (2022) suggests the inner planets most likely have oxygen-and-CO2-rich atmospheres, if any.[258] 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.[259] Planets in the habitable zone are expected to have an atmospheric circulation regime resembling Earth's tropical regions with largely uniform temperatures.[260] 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.[74] Carbon dioxide freezing-out can occur only on the outermost planets unless special conditions are met, and other volatiles do not freeze out.[261]

Stability
see caption
Observed brightness of the TRAPPIST-1 star, showing large variation in brightness. The graph displays dips, indicating the transit of exoplanets. The planet corresponding to the dips in brightness are plotted below with diamond markers.

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.[261] It can cause the ongoing removal of atmospheres from planets.[89] XUV radiation-induced atmospheric escape has been observed on gas giants.[262] M dwarfs emit large amounts of XUV radiation;[261] TRAPPIST-1 and the Sun emit about the same amount of XUV radiation[lower-alpha 46] and because TRAPPIST-1's planets are much closer to the star than the Sun's, they receive much more intense irradiation.[55] TRAPPIST-1 has been emitting radiation for much longer than the Sun.[264] 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.[246]

TRAPPIST-1 has moderate to high stellar activity[lower-alpha 47],[32] and this may be another difficulty for the persistence of atmospheres and water on the planets:[27]

  • Dwarfs of the spectral class M have intense flares;[261] TRAPPIST-1 averages about 0.38 flares per day[74] and four to six superflares[lower-alpha 48] per year.[267] Such flares would have only small impacts on atmospheric temperatures but would substantially affect the stability and chemistry of atmospheres.[89] According to Samara, Patsourakos and Georgoulis (2021), the TRAPPIST-1 planets are unlikely to be able to retain atmospheres against coronal mass ejections.[268]
  • 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 planets[269] up to planet f. The pressure would push the wind deep into the atmospheres,[203] facilitating loss of water and evaporation of the atmospheres.[89][224] Stellar wind-driven escape in the Solar System is largely independent from planetary properties such as mass;[270] stellar wind from TRAPPIST-1 could remove the atmospheres of its planets on a timescale of 100 million to 10 billion years.[271]
  • Ohmic heating[lower-alpha 49] of the atmosphere of TRAPPIST-1e, f, and g amounts to 5–15 times the heating from XUV radiation; if the heat is effectively absorbed, it could destabilise the atmospheres.[273]

The star's history also influences the atmospheres of its planets.[274] Immediately after its formation, TRAPPIST-1 would have been in a pre-main-sequence state, which may have lasted between hundreds of millions[261] and two billion years.[233] 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.[275] Thus, all of the system's planets would have been heated to a runaway greenhouse[lower-alpha 50] for at least part of their existence.[261] The XUV radiation would have been even higher during the pre-main-sequence stage.[89]

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.[27] 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:[276][277]

  • Due to multiple interactions, TRAPPIST-1 planets are expected to have intense tides.[278] If oceans are present,[lower-alpha 51] the tides could: lead to alternate flooding and drying of coastal landscapes triggering chemical reactions conducive to the development of life;[280] favour the evolution of biological rhythms such as the day-night cycle that otherwise would not develop in a synchronously rotating planet;[281] mix oceans, thus supplying and redistributing nutrients;[282] and stimulate periodic expansions of marine organisms similar to red tides on Earth.[283]
  • TRAPPIST-1 may not produce sufficient quantities of radiation for photosynthesis to support an Earth-like biosphere.[284][285][286] Mullan and Bais (2018) speculated that radiation from flares may increase the photosynthetic potential of TRAPPIST-1,[287] but according to Lingam and Loeb (2019), the potential would still be small.[288]
  • Due to the proximity of the TRAPPIST-1 planets, it is possible rock-encased microorganisms ripped[lower-alpha 52] 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.[289]
  • Too much UV radiation from a star can sterilise the surface of a planet[111][152] but too little may not allow the formation of chemical compounds that give rise to life.[14][290] 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.[291] 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 high[292] – to being sufficient to sterilise the planets if they do not have protective atmospheres.[293] As of 2020 it is unclear which effect would predominate around TRAPPIST-1,[233] 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. [267]
  • The outer planets in the TRAPPIST-1 system could host subsurface oceans similar to those of Enceladus and Europa in the Solar System.[294] Chemolithotrophy, the growth of organisms based on non-organic reduced compounds,[295] could sustain life in such oceans.[139] Very deep oceans may be inimical to the development of life.[296]
  • Some planets of the TRAPPIST-1 system may have enough water to completely submerge their surfaces.[297] If so, this would have important effects on the possibility of life developing on the planets, and on their climates,[298] 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.[299]

In 2017, a search for technosignatures that would indicate the existence of past or present technology in the TRAPPIST-1 system found only signals coming from Earth.[300] 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.[301]

Reception and scientific importance
GIF image of a pixellated star
Kepler image of TRAPPIST-1

Public reaction and cultural impact
Planet hop from TRAPPIST-1e – Voted best 'hab zone' vacation within 12 parsecs of Earth
Fictional TRAPPIST-1e tourism poster made by NASA

The discovery of the TRAPPIST-1 planets drew widespread attention in major world newspapers, social media, streaming television and websites.[302][303] As of 2017, the discovery of TRAPPIST-1 led to the largest single-day web traffic to the NASA website.[304] 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.[305] The dynamics of the TRAPPIST-1 planetary system have been represented as music, such as Tim Pyle's Trappist Transits,[306] in Isolation's single Trappist-1 (A Space Anthem)[307] and Leah Asher's piano work TRAPPIST-1.[308] 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.[309] In 2018, Aldo Spadon created a giclée (digital artwork) named "TRAPPIST-1 Planetary System as seen from Space".[310] A website was dedicated to the TRAPPIST-1 system.[311]

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.[312] At least one conference was organised to recognise works of fiction featuring TRAPPIST-1.[313] The planets have been used as the basis of science education competitions[314] and school projects.[315][316] Websites offering TRAPPIST-1-like planets as settings of virtual reality simulations exist,[317] such as the "Exoplanet Travel Bureau"[318] and the "Exoplanets Excursion" – both by NASA.[319] Scientific accuracy has been a point of discussion for such cultural depictions of TRAPPIST-1 planets.[320]

Scientific importance

TRAPPIST-1 has drawn intense scientific interest.[228] 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.[33] Future observations with space-based observatories and ground-based facilities may allow further insights into their properties such as density, atmospheres, and biosignatures.[lower-alpha 53] TRAPPIST-1 planets[322][323] are considered an important observation target for the James Webb Space Telescope (JWST)[lower-alpha 54][228] and other telescopes under construction.[158] Together with the discovery of Proxima Centauri b, the discovery of the TRAPPIST-1 planets and the fact that three of the planets are within the habitable zone has led to an increase in studies on planetary habitability.[326] The planets are considered prototypical for the research on habitability of M dwarfs.[327] The star has been the subject of detailed studies[104] of its various aspects[328] including the possible effects of vegetation on its planets; the possibility of detecting oceans on its planets using starlight reflected off their surfaces;[329] possible efforts to terraform its planets;[330] and difficulties any inhabitants of the planets would encounter with discovering the law of gravitation[331] and with interstellar travel.[332]

The role EU funding played in the discovery of TRAPPIST-1 has been cited as an example of the importance of EU projects,[35] and the involvement of a Moroccan observatory as an indication of the Arab world's role in science. The original discoverers were affiliated with universities spanning Africa, Europe, and North America,[333] and the discovery of TRAPPIST-1 is considered to be an example of the importance of co-operation between observatories.[334] It is also one of the major astronomical discoveries from Chilean observatories.[335]

Exploration

TRAPPIST-1 is too distant from Earth to be reached by humans with current or expected technology.[336] 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 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.[337]

See also

Notes
  1. A log(g) of 2.992 for the Earth indicates that TRAPPIST-1 has a surface gravity approximately 177 times stronger than Earth's.
  2. A red dwarf is a very small and cold star. They are the most common type of star in the Milky Way.[15]
  3. TRAPPIST is a 60-centimetre (24 in) telescope[11] intended to be a prototype for the "Search for habitable Planets EClipsing ULtra-cOOl Stars" project (SPECULOOS), which aims to identify planets around close, cold stars.[20][21] TRAPPIST is used to find exoplanets, and is preferentially employed on stars colder than 3,000 K (2,730 °C; 4,940 °F).[22]
  4. When a planet moves in front of its star, it absorbs part of the star's radiation, which may be observed via telescopes.[29]
  5. The celestial equator is the equator's projection into the sky.[36]
  6. Based on parallax measurements;[1] the parallax is the position of a celestial object with respect to other celestial objects for a given position of Earth. It can be used to infer the distance of the object from Earth.[39]
  7. The movement of the star in the sky, relative to background stars.[40]
  8. Red dwarfs include the spectral type M and K.[42] Spectral types are used to categorise stars by their temperature.[43]
  9. The effective temperature is the temperature a black body that emits the same amount of radiation would have.[49]
  10. The photosphere is a thin layer at the surface of a star, where most of its light is produced.[51]
  11. The solar cycle is the Sun's 11-year long period, during which solar output varies by about 0.1%.[53]
  12. Including Lyman-alpha radiation[59]
  13. The main sequence is the longest stage of a star's lifespan, when it is fusing hydrogen.[67]
  14. Faculae are bright spots on the photosphere.[69]
  15. Flares are presumably magnetic phenomena lasting for minutes or hours during which parts of the star emit more radiation than usual.[69]
  16. For comparison, a strong fridge magnet has a strength of about 100 gauss and Earth's magnetic field about 0.5 gauss.[75]
  17. The chromosphere is an outer layer of a star.[69]
  18. A coronal mass ejection is an eruption of coronal material to the outside of a star.[69][77]
  19. Exoplanets are named in order of discovery as "b", "c" and so on; if multiple planets are discovered at once they are named in order of increasing orbital period.[82] The term "TRAPPIST-1a" is used to refer to the star itself.[83]
  20. One astronomical unit (AU) is the mean distance between the Earth and the Sun.[87]
  21. For comparison, Earth's orbit around the Sun is inclined by about 1.578 degrees.[94]
  22. The inner two planets' orbits may be circular; the others could have a small eccentricity.[97]
  23. A volatile is an element or compound with a low boiling point, such as ammonia, carbon dioxide, methane, nitrogen, sulfur dioxide or water.[105]
  24. The composition of the mantle of rocky planets is typically approximated as a magnesium silicate.[109]
  25. A Laplace resonance is an orbital resonance that consists of three bodies, similar to the Galilean moons Europa, Ganymede and Io around Jupiter.[119]
  26. Where a planet, rather than being a symmetric sphere, has a different radius for each of the three main axes.[125]
  27. This causes one half of the planet to perpetually face the star in a permanent day and the other half perpetually face away from the star in a permanent night.[129]
  28. Degassing is the release of gases, which can end up forming an atmosphere, from the mantle or from magma.[133]
  29. Hydrothermal vents are hot springs that occur underwater, and are hypothesised to be places where life could originate.[138]
  30. Not accounting for gravitational compression.[145]
  31. That is, the inner planets could never cover the entire disk of TRAPPIST-1 from the vantage point of these planets.[84]
  32. The habitable zone is the region around a star where temperatures are neither too hot nor too cold for the existence of liquid water; it is also called the "Goldilocks zone".[29][74]
  33. The Roche limit is the distance at which a body is ripped apart by tides.[168]
  34. The Hill radius is the maximum distance at which a planet's gravity can hold a moon without the star's gravity ripping the moon off.[169]
  35. A streaming instability is a process where interactions between gas and solid particles cause the latter to clump together in filaments. These filaments can give rise to the precursor bodies of planets.[182]
  36. According to the International Astronomical Union criteria, a body has to clear its neighbourhood to qualify as a planet in the Solar System.[196]
  37. On the basis of the Lyman-alpha radiation emissions, TRAPPIST-1b may be losing hydrogen at a rate of 4.6×107 g/s.[205]
  38. Clouds on the day side reflecting starlight could cool TRAPPIST-1d down to temperatures that allow the presence of liquid water.[212]
  39. The exoplanet Proxima Centauri b resides in the habitable zone of the nearest star to the Solar System.[217]
  40. Ocean bodies can still be referred to as such when they are covered by ice.[221]
  41. Measured surface temperature of 503 K (230 °C; 446 °F).[227]
  42. Bourrier et al. (2017) interpreted UV absorption data from the Hubble Space Telescope as implying the outer TRAPPIST-1 planets still have an atmosphere.[14]
  43. A protoplanetary disk is a disk of matter surrounding a star. Planets are thought to form in such disks.[234]
  44. A clathrate is a chemical compound where one compound (or chemical element) e.g. carbon dioxide (or xenon), is trapped within a cage-like assembly of molecules from another compound.[243]
  45. The exosphere is the region of an atmosphere where density is so low that atoms or molecules no longer collide. It is formed by atmospheric escape and the presence of a hydrogen-rich exosphere implies the presence of water.[248]
  46. Different sources estimate that TRAPPIST-1 emits as much as the Sun at solar minimum,[14] the same amount[233] or more than the Sun.[263]
  47. Stellar activity is the occurrence of luminosity changes, mostly in the X-ray bands, caused by a star's magnetic field.[265]
  48. Flares with an energy of over 1×1034 ergs (1.0×1027 J).[266]
  49. Ohmic heating takes place when electrical currents excited by the stellar wind flow through parts of the atmosphere, heating it.[272]
  50. In a runaway greenhouse, all water on a planet is in the form of vapour.[275]
  51. Non-ocean bearing planets can also be subject to tidal heating (or flexing), resulting in structural deformation.[279]
  52. For example, meteorite impacts could break off rocks from planets at a sufficient speed that they escape its gravity.[289]
  53. Biosignatures are properties of a planet that can be detected from far away and which suggest the existence of life, such as atmospheric gases that are produced by biological processes.[321]
  54. As of 2017 they were among the smallest planets known where JWST would be able to detect atmospheres.[324] It is possible the JWST may not have time to reliably detect certain biosignatures such as methane and ozone.[325]

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