TRAPPIST-1g
TRAPPIST-1g is an exoplanet orbiting around the ultra-cool dwarf star TRAPPIST-1, located away from Earth in the constellation Aquarius. It was one of four new exoplanets to be discovered orbiting the star in 2017 using observations from the Spitzer Space Telescope. 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 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 ultracool dwarf star named TRAPPIST-1. The star has a mass of 0.08 M☉ and a radius of 0.11 R☉. It has a temperature of 2,550 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 5,778 K. The star is metal-rich, with a metallicity 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 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.