TRAPPIST-1b, also designated as 2MASS J23062928-0502285 b, is a mainly rocky, Venus-like exoplanet orbiting around the ultra-cold dwarf star TRAPPIST-1, located approximately 39 light-years (12 parsecs) away from Earth in the constellation Aquarius. The planet was detected using the transit method, where a planet dims the host star's light as it passes in front of it. It was first announced on May 2, 2016, and between 2017 and 2018, more studies were able to refine its physical parameters.

Artist's impression of TRAPPIST-1b (February 2018)
Discovery dateMay 2, 2016
Orbital characteristics
Apastron0.01162 + 0.0000031–0.000004 AU (1,738,330 + 460–600 km)
Periastron0.011476 + 0.0000034–0.000004 AU (1,716,790 + 510–600 km)
0.01154775 ± 5.7×10−08 AU (1.7275188×106 ± 8.5 km)[1]
Eccentricity0.00622 (± 0.000304)[1]
1.51087637 ± 0.00000039 d (36.2610329 ± 9.4×10−6 h; 130,539.718 ± 0.034 s)[2]
Inclination89.56 (± 0.23)[2]
Physical characteristics
Mean radius
1.121 +0.031
[1] R
Mass1.017 +0.154
[1] M
Mean density
3.98 + 0.51–0.50 g/cm3 (2.30 + 0.29–0.29 oz/cu in)
0.812 +0.104
[1] g
Temperature391.8 ± 5.5 K (118.65 ± 5.50 °C; 245.57 ± 9.90 °F) (equilibrium)[2]
≥1,400 K (1,130 °C; 2,060 °F) (atmosphere)[2]
750–1,500 K (477–1,227 °C; 890–2,240 °F) (surface)[1]

    The planet is about the same mass as Earth, but about 12% larger. Its relatively low density, along with spectroscopic observations, have confirmed that it has an extremely thick and hot atmosphere. Observations published in 2018 showed that the atmosphere of TRAPPIST-1b was much larger than that of Earth or Venus, as well as being very hot and potentially rich in CO2.[2] More recent modeling studies have suggested the planet is too hot to allow the formation of sulfuric acid clouds, such as are found on Venus, the hottest planet in our solar system.[4]


    Mass, radius, and temperature

    TRAPPIST-1b is very similar in both mass, radius, and gravity to Earth. It has a radius of 1.121 R, a mass of 1.02 M, and about 81% Earth's surface gravity. However, the density of the planet indicates that it is not entirely rocky. With a density of 3.98 g/cm3, about ≤5% of its mass must be water, likely in the form of a thick Venus-like atmosphere due to it receiving being nearly 4 times more energy than Earth does. The planet's surface temperature is estimated to be between 750 K (477 °C; 890 °F) and 1,500 K (1,230 °C; 2,240 °F), potentially as high as 2,000 K (1,730 °C; 3,140 °F). This is much hotter than the surface of Venus and may be hot enough that the surface is molten lava.[1]


    TRAPPIST-1b orbits very close to its parent star. One orbit requires only 36 hours, or about 1.51 Earth days.[2] It orbits about 0.0115 AU from its star, just 1.2% the distance between Earth and the Sun.[1] The close proximity to its host star means that TRAPPIST-1b is likely tidally locked. It also has a very circular orbit, with an eccentricity of 0.00622, significantly more circular than Earth's orbit.

    Host Star

    TRAPPIST-1b orbits the ultracool dwarf star TRAPPIST-1. It is has a mass of 0.089 M and is only 0.121 R, with a surface temperature of 2511 K and an age between 3 and 8 billion years. The Sun, in comparison, has a surface temperature of 5778 K and is about 4.5 billion years old. TRAPPIST-1 is also very dim, with a luminosity about 0.0005 times that of the Sun. It is too faint to be seen with the naked eye, having an apparent magnitude of 18.80.


    The combined transmission spectra of TRAPPIST-1 b and c rule out cloud-free hydrogen-dominated atmospheres for both planets, so they are unlikely to harbor extended gas envelopes. Other atmospheres, from a cloud-free water-vapor atmosphere to a Venus-like atmosphere, remain consistent with the featureless spectra.[5]

    In 2018, the planet's atmosphere was better examined by the Spitzer Space Telescope and found to be quite large and hot. The planet's transmission spectrum and refined density estimate suggest two main possibilities for the atmosphere: one rich in carbon dioxide, and one rich in water vapor. The more likely CO2 atmosphere would have a scale height of approximately 52 km (Earth's being 8 km, and Venus' at 15.9 km) and an average temperature in excess of 1,400 K (1,130 °C; 2,060 °F), far greater than its equilibrium temperature of 391.8 K (118.7 °C; 245.6 °F). A water vapor atmosphere would need to have a scale height of >100 km and a temperature >1,800 K (1,530 °C; 2,780 °F) to produce the variations seen in the planet's transit depths and its transmission spectrum. Other sources for the effects seen, such as hazes and thick clouds, would require an even larger atmosphere. TRAPPIST-1b will have to be studied further to confirm its potential large atmosphere.[2][1]

    See also

    • 55 Cancri e, another very hot planet with a confirmed atmosphere.


    1. 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 (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.
    2. 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 (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.
    3. Van Grootel, Valerie; Fernandes, Catarina S.; Gillon, Michaël; Jehin, Emmanuel; Scuflaire, Richard; et al. (2018). "Stellar parameters for TRAPPIST-1". The Astrophysical Journal. 853: 30. arXiv:1712.01911. Bibcode:2018ApJ...853...30V. doi:10.3847/1538-4357/aaa023.
    4. University of Washington (21 November 2018). "Study brings new climate models of small star TRAPPIST 1's seven intriguing worlds". EurekAlert!. Retrieved 22 November 2018.
    5. de Wit, Julien; et al. (2016). "A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c". Nature. 537 (7618): 69–72. arXiv:1606.01103. Bibcode:2016Natur.537...69D. doi:10.1038/nature18641. PMID 27437572.
    6. "Artist's view of planets transiting red dwarf star in TRAPPIST-1 system". Retrieved 21 July 2016.
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