TRAPPIST-1g, also designated as 2MASS J23062928-0502285 g and K2-112 g, is an

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

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 ...

. It was one of four new exoplanets to be discovered orbiting the star in 2017 using observations from 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 ...

. The exoplanet is within the optimistic habitable zone of its host star. It was found by using the transit method, in which the dimming effect that a planet causes as it crosses in front of its star is measured. The second most distant known planet in its system, TRAPPIST-1g is a planet somewhat larger than Earth and with a similar density, meaning it is likely a rocky planet.

Physical characteristics

Mass, radius, and temperature

TRAPPIST-1g has a radius of and a mass of , with a density only slightly less than Earth's, though initial estimates suggested its density was only 4.186 g/cm³, about 76% of Earth's. Based on mass-radius calculations and its distant location relative to its host star (0.047 AU) and the fact that the planet only receives 25.2% of the stellar flux that Earth does, the planet is likely covered by a thick ice envelope if an atmosphere does not exist.

Atmosphere

TRAPPIST-1g could have a global water ocean or an exceptionally thick steam atmosphere. According to a simulation of magma ocean-atmosphere interaction, TRAPPIST-1g is likely to retain a large fraction of primordial steam atmosphere during the initial stages of evolution, and therefore today is likely to possess a thick ocean covered by atmosphere containing hundreds of bars of abiotic oxygen. On 31 August 2017, astronomers at the Hubble Space Telescope reported the first evidence of possible water content on the TRAPPIST-1 exoplanets.

Host star

The planet orbits an ( M-type)

ultracool 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 A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

named TRAPPIST-1. The star has a mass of 0.08 ''M''_☉ and a radius of 0.11 ''R''_☉. It has a temperature of 2550 K. The age of the star is about billion years old. In comparison, the Sun is 4.6 billion years old and has a temperature of 5778 K. The star is metal-rich, with a

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...

( e/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 metal content than the Sun. Its luminosity ( ''L''_☉) is 0.05% of that of the Sun. The star's apparent magnitude, or how bright it appears from Earth's perspective, is 18.8, too dim to be seen with the naked eye.

Orbit

TRAPPIST-1g orbits its host star with an orbital period of about 12.354 days and an orbital radius of about 0.0451 times that of Earth's (compared to the distance of

Mercury Mercury commonly refers to: * Mercury (planet), the nearest planet to the Sun * Mercury (element), a metallic chemical element with the symbol Hg * Mercury (mythology), a Roman god Mercury or The Mercury may also refer to: Companies * Merc ...

from the Sun, which is about 0.38 AU). This is in the outer limit of TRAPPIST-1's theoretical habitable zone. The orbit of TRAPPIST-1g has an eccentricity of 0.00208, much lower than that of Earth and the lowest in its system. Its orbit varies by only about 41,000 kilometers (compared to about 5 million km for Earth), meaning the planet's climate is likely very stable. It is in a 3:2 orbital resonance with

TRAPPIST-1h TRAPPIST-1h, also designated as 2MASS J23062928-0502285 h, is an exoplanet orbiting around the ultra-cool dwarf star TRAPPIST-1 39 light-years (12 parsecs) away from Earth in the constellation Aquarius. It was one of four new exoplanets to be d ...

and a 3:4 resonance with

TRAPPIST-1f TRAPPIST-1f, also designated as 2MASS J23062928-0502285 f, is an exoplanet, likely rocky, orbiting within the habitable zone around the ultracool dwarf star TRAPPIST-1, located away from Earth in the constellation of Aquarius. The exoplanet ...

References