TRAPPIST-1 b is the innermost of the seven Earth-sized planets around TRAPPIST-1, a small, cool red dwarf about 40 light-years away. It orbits every 1.51 days and receives about four times the light Earth gets from the Sun.
It was found in 2016 by a team led by Michaël Gillon, together with its neighbour TRAPPIST-1 c. The team caught the small dips in starlight as the planets crossed the star. In 2017 they announced that TRAPPIST-1 has at least seven planets. In 2021 a team led by Eric Agol used years of transit timing, in which the planets' tugs on each other shift when each crossing happens, to measure all seven masses to within a few percent. TRAPPIST-1 b has about 1.37 Earth masses and is about 1.12 times Earth's width.
In 2023 a team led by Thomas Greene measured its heat with Webb by watching it pass behind its star, and found that its day side re-radiates the starlight it receives without spreading heat to the night side. The simplest explanation was little or no atmosphere. A 2024 study led by Elsa Ducrot, adding another wavelength, found two possible explanations: a bare rock with a freshly resurfaced surface, or a thick carbon dioxide atmosphere with a high haze. In 2025 Webb measured how its brightness changes around its whole orbit. Gillon's team found a hot day side, about 490 kelvin, and no detectable heat on the night side, and concluded that TRAPPIST-1 b is unlikely to have any substantial atmosphere.