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TRAPPIST-1d

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TRAPPIST-1d is a small exoplanet (about 40 percent the mass of the Earth), which orbits on the inner edge of the habitable zone of the ultracool dwarf star TRAPPIST-1, located 40.7 light-years (12.5 parsecs) away from Earth in the constellation of Aquarius. The exoplanet was found by using the transit method. The first signs of the planet were announced in 2016, but it was not until the following years that more information concerning the probable nature of the planet was obtained. TRAPPIST-1d is the second-least massive planet of the system.[7] It receives just 4.3% more sunlight than Earth, placing it on the inner edge of the habitable zone.[8] A 2025 study, based on a James Webb Space Telescope observation, found that the data showed a "satisfactory fit" the planet has no atmosphere, it is a bare rock. However, there is still a "marginal possibility" the planet has an atmosphere, but determining this would require a different detection technique.[6]

TRAPPIST-1d
Artist's impression of TRAPPIST-1d (August 2025)
Discovery[1]
Discovered byMichaël Gillon et al.
Discovery siteTRAPPIST
Discovery dateMay 2, 2016
Transit
Orbital characteristics[2]
0.02227±0.00019 AU
Eccentricity0.00837±0.00093[3]
4.049219±0.000026 d
Inclination89.896°±0.077°
−8.73°±6.17°[3]
StarTRAPPIST-1[4]
Physical characteristics[2]
0.788+0.011
−0.010 R🜨
Mass0.388±0.012 M🜨
Mean density
4.354+0.156
−0.163 g/cm3
0.624±0.019 g
6.11±0.19 m/s2
TemperatureTeq: 286.2±2.8 K (13.1 °C; 55.5 °F)[5]
Atmosphere
Composition by volumeNone or extremely thin[6]

Physical characteristics

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Radius, mass, and temperature

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TRAPPIST-1d was detected with the transit method, allowing scientists to accurately determine its radius. The planet is about 0.788 R🜨 with a small error margin of about 70 km. Transit timing variations and complex computer simulations helped accurately determine the mass of the planet, which led to scientists being able to calculate its density, surface gravity, and composition. TRAPPIST-1d is a mere 0.388 M🜨, making it one of the least massive exoplanets yet found.[2] Initial estimates suggested that it has 61.6% the density of Earth (3.39 g/cm3) and just under half the gravity.[3] Compared to Mars, it has nearly three times that planet's mass but was thought to still be significantly less dense, which would indicate the presence of a significant atmosphere; models of the low density of TRAPPIST-1d indicated a mainly rocky composition, but with about ≤5% of its mass in the form of a volatile layer. The volatile layer of TRAPPIST-1d may consist of atmosphere, ocean, and/or ice layers.[3] However, refined estimates show that the planet is more dense, closer to 79.2% of Earth's bulk density (4.35 g/cm3).[2] TRAPPIST-1d has an equilibrium temperature of 282.1 K (9.0 °C; 48.1 °F), assuming an albedo of 0.[8] For an Earth-like albedo of 0.3, the planet's equilibrium temperature is around 258 K (−15 °C; 5 °F), very similar to Earth's at 255 K (−18 °C; −1 °F).[9]

Orbit

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TRAPPIST-1d is a closely orbiting planet, with one full orbit taking just 4.05 days (about 97 hours) to complete.[8] It orbits at a distance of just 0.02228 AU from the host star, or about 2.2% the distance between Earth and the Sun.[3] For comparison, Mercury, the Solar System's innermost planet, takes 88 days to orbit at a distance of about 0.38 AU. The size of TRAPPIST-1 and the close orbit of TRAPPIST-1d around it means that the star as seen from the planet appears 5.5 times as large as the Sun from the Earth. While a planet at TRAPPIST-1d's distance from the Sun would be a scorched world, the low luminosity of TRAPPIST-1 means that the planet gets only 1.043 times the sunlight that Earth receives, placing it within the inner part of the conservative habitable zone.[8]

Host star

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Main article: TRAPPIST-1

The planet orbits an (M-type) ultracool dwarf star named TRAPPIST-1. The star has a mass of 0.089 M☉ (close to the boundary between brown dwarfs and hydrogen-fusing stars) and a radius of 0.121 R☉. It has a temperature of 2,516 K (2,243 °C; 4,069 °F), and is between 3 and 8 billion years old. In comparison, the Sun is 4.6 billion years old[10] and has a temperature of 5778 K (5504.85 °C, 9940.73 °F).[11] The star is metal-rich, with a metallicity ([Fe/H]) of 0.04, or 109% the solar amount. This is particularly odd as such low-mass stars near the boundary between brown dwarfs and hydrogen-fusing stars should be expected to have considerably less metals than the Sun. Its luminosity (L☉) is 0.05% of that of the Sun.

Stars like TRAPPIST-1 have the ability to live up to 4–5 trillion years, 400–500 times longer than the Sun will live (the Sun only has about 8 billion years of lifespan left, slightly more than half of its lifetime).[12] Because of this ability to live for long periods of time, it is likely TRAPPIST-1 will be one of the last remaining stars when the Universe is much older than it is now, when the gas needed to form new stars will be exhausted, and the remaining ones begin to die off.

The star's apparent magnitude, or how bright it appears from Earth's perspective, is 18.8. Therefore, it is too dim to be seen with the naked eye (the limit for that is 6.5).

The star is not just very small and far away, it also emits comparatively little visible light, mainly shining in the invisible infrared. Even from the close in proximity of TRAPPIST-1d, about 50 times closer than Earth is from the Sun, the planet receives less than 1% the visible light Earth sees from the Sun. This would probably make the days on TRAPPIST-1d never brighter than twilight is on Earth. However, that still means that TRAPPIST-1 could easily shine at least 3000 times brighter in the sky of TRAPPIST-1d than the full moon does in Earth's night sky.

Atmosphere

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See also: Habitability of red dwarf systems

In 2025, the planet was observed by the James Webb Space Telescope. Study results determined with greater than 95% confidence that thick and cloud-free atmospheres could be ruled out (analogous to a clear Venus, early Mars, Titan, and Archean Earth). This leaves two main possibilities consistent with the data: the planet has no atmosphere, it is a bare rock. This scenario is the "satisfactory fit" for the data, according to the study. Or, there is a "marginal possibility" that TRAPPIST-1d has an atmosphere with high-altitude clouds or aerosols that block starlight from passing through, thus muting any molecular signatures from detection.[6] In this scenario, there could be "an extremely thin Mars-like CO2 atmosphere, a Venus-like [greenhouse] scenario with high-altitude clouds, or the CO2 signature of a modern Earth composition when accounting for Earth's water clouds."[6] This scenario lies at the very edge of the 2-sigma boundary of statistical significance, it cannot be confidently ruled out, however, it is also not a good fit for the data.[6] Lead scientist Piaulet-Ghorayeb said the detection method they used, transmission spectroscopy, is only one of multiple approaches, and while the study result "doesn’t kill our prospects" for finding some kind of atmosphere, admittedly it will be "a bit harder" after these results.[13]

Because TRAPPIST-1d is only ~30% the Earth's mass, it, like Venus and Mars, may have no magnetic field, which would allow the parent star's solar wind to strip away the more volatile components of its atmosphere (including water), leaving it hydrogen-poor like those planets.[14]

Discovery

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A team of astronomers headed by Michaël Gillon of the Institut d’Astrophysique et Géophysique at the University of Liège[15] in Belgium used the TRAPPIST (Transiting Planets and Planetesimals Small Telescope) telescope at the La Silla Observatory in the Atacama Desert, Chile,[16] to observe TRAPPIST-1 and search for orbiting planets. By utilising transit photometry, they discovered three Earth-sized planets orbiting the dwarf star; the innermost two are tidally locked to their host star while the outermost appears to lie either within the system's habitable zone or just outside of it.[17][18] The team made their observations from September to December 2015 and published its findings in the May 2016 issue of the journal Nature.[16][19]

The original claim and presumed size of the planet was revised when the full seven-planet system was revealed in 2017:

 
Artist's impression of the TRAPPIST-1 planetary system.
"We already knew that TRAPPIST-1, a small, faint star some 40 light years away, was special. In May 2016, a team led by Michaël Gillon at Belgium’s University of Liege announced it was closely orbited by three planets that are probably rocky: TRAPPIST-1b, c and d...
"As the team kept watching shadow after shadow cross the star, three planets no longer seemed like enough to explain the pattern. “At some point we could not make sense of all these transits,” Gillon says.
"Now, after using the space-based Spitzer telescope to stare at the system for almost three weeks straight, Gillon and his team have solved the problem: TRAPPIST-1 has four more planets.
"The planets closest to the star, TRAPPIST-1b and c, are unchanged. But there’s a new third planet, which has taken the d moniker, and what had looked like d before turned out to be glimpses of e, f and g. There’s a planet h, too, drifting farthest away and only spotted once."[20]

See also

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References

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  1. ^ Gillon, Michaël; Jehin, Emmanuël; Lederer, Susan M.; Delrez, Laetitia; et al. (May 2016). "Temperate Earth-sized planets transiting a nearby ultracool dwarf star". Nature. 533 (7602): 221–224. arXiv:1605.07211. Bibcode:2016Natur.533..221G. doi:10.1038/nature17448. ISSN 1476-4687. PMC 5321506. PMID 27135924.
  2. ^ a b c d Agol, Eric; Dorn, Caroline; Grimm, Simon L.; Turbet, Martin; et al. (1 February 2021). "Refining the Transit-timing and Photometric Analysis of TRAPPIST-1: Masses, Radii, Densities, Dynamics, and Ephemerides". The Planetary Science Journal. 2 (1): 1. arXiv:2010.01074. Bibcode:2021PSJ.....2....1A. doi:10.3847/psj/abd022. S2CID 222125312.
  3. ^ a b c d e Grimm, Simon L.; Demory, Brice-Olivier; Gillon, Michael; Dorn, Caroline; Agol, Eric; Burdanov, Artem; Delrez, Laetitia; Sestovic, Marko; Triaud, Amaury H.M.J.; Turbet, Martin; Bolmont, Emeline; Caldas, Anthony; de Wit, Julien; Jehin, Emmanuel; Leconte, Jeremy; Raymond, Sean N.; Van Grootel, Valerie; Burgasser, Adam J.; Carey, Sean; Fabrycky, Daniel; Heng, Kevin; Hernandez, David M.; Ingalls, James G.; Lederer, Susan; Selsis, Franck; Queloz, Didier (5 February 2018). "The nature of the TRAPPIST-1 exoplanets". Astronomy & Astrophysics. 613: A68. arXiv:1802.01377. Bibcode:2018A&A...613A..68G. doi:10.1051/0004-6361/201732233. S2CID 3441829.
  4. ^ Van Grootel, Valerie; Fernandes, Catarina S.; Gillon, Michaël; Jehin, Emmanuel; Scuflaire, Richard; et al. (5 December 2017). "Stellar Parameters for Trappist-1". The Astrophysical Journal. 853 (1): 30. arXiv:1712.01911. Bibcode:2018ApJ...853...30V. doi:10.3847/1538-4357/aaa023. S2CID 54034373.
  5. ^ Ducrot, E.; Gillon, M.; Delrez, L.; Agol, E.; et al. (1 August 2020). "TRAPPIST-1: Global results of the Spitzer Exploration Science Program Red Worlds". Astronomy & Astrophysics. 640: A112. arXiv:2006.13826. Bibcode:2020A&A...640A.112D. doi:10.1051/0004-6361/201937392. ISSN 0004-6361. S2CID 220041987.
  6. ^ a b c d e Piaulet-Ghorayeb, Caroline; et al. (11 August 2025). "Strict limits on potential secondary atmospheres on the temperate rocky exo-Earth TRAPPIST-1 d". arXiv:2508.08416 [astro-ph.EP].
  7. ^ Howell, Elizabeth (7 February 2018). "Rocky-Planet-Like Atmospheres Are Possible on 3 TRAPPIST-1 Planets". Space.com. Retrieved 10 February 2021.
  8. ^ a b c d Delrez, Laetitia; Gillon, Michael; H.M.J, Amaury; Brice-Oliver Demory, Triaud; de Wit, Julien; Ingalls, James; Agol, Eric; Bolmont, Emeline; Burdanov, Artem; Burgasser, Adam J.; Carey, Sean J.; Jehin, Emmanuel; Leconte, Jeremy; Lederer, Susan; Queloz, Didier; Selsis, Franck; Grootel, Valerie Van (9 January 2018). "Early 2017 observations of TRAPPIST-1 with Spitzer". Monthly Notices of the Royal Astronomical Society. 475 (3): 3577–3597. arXiv:1801.02554. Bibcode:2018MNRAS.475.3577D. doi:10.1093/mnras/sty051.
  9. ^ "HEC: Exoplanets Calculator - Planetary Habitability Laboratory @ UPR Arecibo". Archived from the original on 5 September 2019. Retrieved 8 February 2018.
  10. ^ Fraser Cain (16 September 2008). "How Old is the Sun?". Universe Today. Retrieved 19 February 2011.
  11. ^ Fraser Cain (15 September 2008). "Temperature of the Sun". Universe Today. Retrieved 19 February 2011.
  12. ^ Adams, Fred C.; Laughlin, Gregory; Graves, Genevieve J. M. "Red Dwarfs and the End of the Main Sequence". Gravitational Collapse: From Massive Stars to Planets. Revista Mexicana de Astronomía y Astrofísica. pp. 46–49. Bibcode:2004RMxAC..22...46A.
  13. ^ Fortin, Jayce (14 August 2025). "The Third Rock From Another Sun Shows No Signs of Life So Far". The New York Times. Retrieved 14 August 2024.
  14. ^ "Study brings new climate models of small star TRAPPIST 1's seven intriguing worlds". Archived from the original on 8 November 2020. Retrieved 10 December 2018.
  15. ^ "AGO - Department of Astrophysics, Geophysics and Oceanography".
  16. ^ a b "Could these newly-discovered planets orbiting an ultracool dwarf host life?". The Guardian. 2 May 2016.
  17. ^ "Three Potentially Habitable Worlds Found Around Nearby Ultracool Dwarf Star - Currently the best place to search for life beyond the Solar System". eso.org. Retrieved 2 May 2016.
  18. ^ "Three New Planets Are the Best Bets for Life". Popular Mechanics. 2 May 2016. Retrieved 2 May 2016.
  19. ^ Gillon, Michaël; Jehin, Emmanuël; et al. (2016). "Temperate Earth-sized planets transiting a nearby ultracool dwarf star". Nature. 533 (7602): 221–224. arXiv:1605.07211. Bibcode:2016Natur.533..221G. doi:10.1038/nature17448. PMC 5321506. PMID 27135924.
  20. ^ New Scientist. Exoplanet discovery
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