TRAPPIST-1b

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.

TRAPPIST-1b

Artist's impression of TRAPPIST-1b (February 2018)
Discovery
Discovery date	May 2, 2016
Detection method	Transit
Orbital characteristics
Apastron	0.01162 + 0.0000031–0.000004 AU (1,738,330 + 460–600 km)
Periastron	0.011476 + 0.0000034–0.000004 AU (1,716,790 + 510–600 km)
Semi-major axis	0.01154775 ± 5.7×10^−08 AU (1.7275188×10^6 ± 8.5 km)[1]
Eccentricity	0.00622 (± 0.000304)[1]
Orbital period	1.51087637 ± 0.00000039 d (36.2610329 ± 9.4×10^−6 h; 130,539.718 ± 0.034 s)[2]
Inclination	89.56 (± 0.23)[2]
Star	TRAPPIST-1[3]
Physical characteristics
Mean radius	1.121 ^+0.031 −0.032[1] R⊕
Mass	1.017 ^+0.154 −0.143[1] M⊕
Mean density	3.98 + 0.51–0.50 g/cm^3 (2.30 + 0.29–0.29 oz/cu in)
Surface gravity	0.812 ^+0.104 −0.102[1] g
Temperature	391.8 ± 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 on CO₂.[2] More recent modeling studies have suggested the planet is too hot even to allow the formation of sulfuric acid clouds, as found on Venus, the hottest planet in our solar system.[4]

Characteristics

Mass, radius, and temperature

TRAPPIST-1b is very close 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 its high stellar flux being nearly 4 times higher than Earth's. The planet's extremely thick atmosphere has likely risen its temperature far above its equilibrium temperature of 391.8 K (118.7 °C; 245.6 °F). Its 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 can melt certain metals.[1]

Orbit

TRAPPIST-1b orbits very close to its parent star. One orbit on this planet lasts about 1.51 Earth days, or about 36 hours.[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 0.121 R_☉ and 0.089 M_☉, with a temperature of 2511 K and an age between 3 and 8 billion years. For comparison, the Sun has a temperature of 5778 K and is about 4.5 billion years old. TRAPPIST-1 is also very dim, with about 0.0005 times the luminosity of the Sun. It is too faint to be see with the naked eye, having an apparent magnitude of 18.80.

Atmosphere

The combined transmission spectrum of TRAPPIST-1 b and c rules out a cloud-free hydrogen-dominated atmosphere for each planet, so they are unlikely to harbor an extended gas envelope. Other atmospheres, from a cloud-free water-vapor atmosphere to a Venus-like atmosphere, remain consistent with the featureless spectrum.[5]

In 2018, the planet's atmosphere was better examined with the Spitzer Space Telescope and found to be quite large and hot. The planet's transmission spectrum and refined density suggested two main possibilities for the atmosphere: one rich in carbon dioxide, and one with water vapor. The more likely CO₂ atmosphere would have a scale height of approximately 52 km (with Earth's being at 8 km, and Venus's at 15.9 km) and an average temperature in excess of 1,400 K (1,130 °C; 2,060 °F), far higher than its equilibrium temperature. A water vapor atmosphere would need to have a scale height of >100 km and a temperature of >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]

Gallery

• 
  Artist's view of planets transiting red dwarf star in TRAPPIST-1 system[6]
• 
  The Sun and the ultracool dwarf star TRAPPIST-1 to scale. The faint star has only 11% of the diameter of the Sun and is much redder in colour.
• 
  Artist's impression of the three planets (b, c, and d) orbiting TRAPPIST-1

See also

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

References

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". www.spacetelescope.org. Retrieved 21 July 2016.