TRAPPIST-1 g is the sixth of seven Earth-sized planets around TRAPPIST-1, a small, cool red dwarf about 40 light-years away. It is the largest of the seven, a little over 1.1 times Earth's width, and orbits every 12.35 days. It receives about a quarter of the light Earth gets from the Sun.
It was one of four new planets announced in 2017 by a team led by Michaël Gillon, after the star was watched intensively from the ground and with NASA's Spitzer Space Telescope. That study found that the six inner planets form a near-resonant chain: their orbital periods are close to ratios of small whole numbers. The team argued that this arrangement suggests the planets formed farther from the star and then migrated inward.
In 2021 a team led by Eric Agol used four years of Spitzer transit times to measure every planet's mass to within a few percent. Those masses showed that all seven planets have similar densities, consistent with a rocky makeup slightly lower in iron than Earth, or with an Earth-like makeup that includes some lighter material such as a water layer. TRAPPIST-1 g has about 1.32 Earth masses. Agol's team also found the orbits to be nearly circular and lying almost exactly in one plane. Its estimated equilibrium temperature, set by starlight alone, is about 200 kelvin. Whether it has an atmosphere, and so whether it could hold liquid water, is not known.