TRAPPIST-1b, also designated as 2MASS J23062928-0502285 b, is a mainly rocky,

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never fa ...

-like

exoplanet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, init ...

orbiting around the

ultra-cool dwarf An ultra-cool dwarf is a stellar or sub-stellar object of spectral class M that has an effective temperature lower than . This category of dwarf stars was introduced in 1997 by J. Davy Kirkpatrick, Todd J. Henry, and Michael J. Irwin. It origi ...

star TRAPPIST-1, located approximately away from Earth in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Aquarius Aquarius may refer to: Astrology * Aquarius (astrology), an astrological sign * Age of Aquarius, a time period in the cycle of astrological ages Astronomy * Aquarius (constellation) * Aquarius in Chinese astronomy Arts and entertainment ...

. 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. The planet is about the same mass as

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

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

, as well as being very hot and potentially rich in CO₂. 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 the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

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 , a mass of 1.02 , 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 Volatiles are the group of chemical elements and chemical compounds that can be readily vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as refractory substances. On planet Earth, the term ' ...

, 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 and , potentially as high as . This is much hotter than the surface of Venus and may be hot enough that the surface is molten lava. In addition, the planet should be very geologically active due to tidal squeezing similar to Jupiter's moon Io, which happens to have a similar orbital period and eccentricity (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. It orbits about from its star, just 1.2% the distance between Earth and the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

. 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 and is only 0.121 , with a surface temperature of and an age between 3 and 8 billion years. The Sun, in comparison, has a surface temperature of 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. Other atmospheres, from a cloud-free water-vapor atmosphere to a Venus-like atmosphere, remain consistent with the featureless spectra. In 2018, the planet's atmosphere was better examined by the

Spitzer Space Telescope The Spitzer Space Telescope, formerly the Space Infrared Telescope Facility (SIRTF), was an infrared space telescope launched in 2003. Operations ended on 30 January 2020. Spitzer was the third space telescope dedicated to infrared astronomy, f ...

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 Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

, and one rich in

water vapor (99.9839 °C) , - , Boiling point , , - , specific gas constant , 461.5 J/( kg·K) , - , Heat of vaporization , 2.27 MJ/kg , - , Heat capacity , 1.864 kJ/(kg·K) Water vapor, water vapour or aqueous vapor is the gaseous pha ...

. The more likely CO₂ atmosphere would have a

scale height In atmospheric, earth, and planetary sciences, a scale height, usually denoted by the capital letter ''H'', is a distance (vertical or radial) over which a physical quantity decreases by a factor of e (the base of natural logarithms, approximate ...

of approximately (Earth's being , and Venus' at ) and an average temperature in excess of , far greater than its equilibrium temperature of . A water vapor atmosphere would need to have a scale height of > and a temperature > to produce the variations seen in the planet's transit depths and its transmission spectrum, and would be vulnerable to

photodissociation Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...

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.

Gallery

File:Artist's view of planets transiting red dwarf star in TRAPPIST-1 system.jpg, Artist's view of planets transiting red dwarf star in TRAPPIST-1 system File:Comparison between the Sun and the ultracool dwarf star TRAPPIST-1.jpg, 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. File:TRAPPIST-1 and its three planets.jpg, Artist's impression of three of the planets (b, c, and d) orbiting TRAPPIST-1 File:Artist’s impression of the ultracool dwarf star TRAPPIST-1 from close to one of its planets.ogv, 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.

References

{{DEFAULTSORT:TRAPPIST-1b Exoplanets discovered in 2016 Near-Earth-sized exoplanets Transiting exoplanets TRAPPIST-1 Aquarius (constellation)