TRAPPIST-1b, also designated as 2MASS J23062928-0502285 b, is a mainly rocky, Venus-like exoplanet orbiting around the ultra-cool dwarf star TRAPPIST-1, located approximately 40 light-years (12 parsecs) away from Earth in the constellation of 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 (1.7275188×106 ± 8.5 km)[1]
Eccentricity	0.00622 (± 0.000304)[1]
Orbital period (sidereal)	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] REarth
Mass	1.017 +0.154 −0.143[1] MEarth
Mean density	3.98 + 0.51–0.50 g/cm3 (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, has confirmed 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]

Physical characteristics

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_Earth, a mass of 1.02 M_Earth, 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 volatiles, likely in the form of a thick Venus-like atmosphere due to it receiving nearly four 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] In addition, the planet should be very geologically active due to tidal squeezing similar to Jupiter's moon Io (see TRAPPIST-1#Resonance for references).

Orbit

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 (1.72 million km; 1.07 million mi) 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 has a mass of 0.089 M_☉ and is only 0.121 R_☉, with a surface temperature of 2,511 K (2,238 °C; 4,060 °F) and an age between 3 and 8 billion years. The Sun, in comparison, has a surface temperature of 5,778 K (5,505 °C; 9,941 °F) 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.

Atmosphere

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. Also, no helium emission from TRAPPIST-1b has been detected.[5] Other atmospheres, from a cloud-free water-vapor atmosphere to a Venus-like atmosphere, remain consistent with the featureless spectra.[6]

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 kilometers (32 miles) (Earth's being 8 km (5.0 mi), and Venus' at 15.9 km (9.9 mi)) 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 (62 mi) 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, and would be vulnerable to photodissociation where CO₂ would not be. 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[7]
• 
  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 three of the planets (b, c, and d) orbiting TRAPPIST-1
• Artist’s impression video, near one of the three planets orbiting TRAPPIST-1. One of the inner planets is shown in transit across the disc of its tiny and dim parent star.

See also

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

References

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