TRAPPIST-1 h is the outermost and smallest of the seven planets around TRAPPIST-1, a small, cool red dwarf about 40 light-years away. It is about three-quarters of Earth's width and about a third of its mass. Its orbit, every 18.77 days, was the last piece of the system's puzzle.
When a team led by Michaël Gillon announced seven planets in 2017, TRAPPIST-1 h's orbital period had not yet been pinned down. Later that year a team led by Rodrigo Luger used new observations from NASA's K2 mission to measure it at 18.77 days. That matched a prediction based on the other planets' orbits. It showed that TRAPPIST-1 h completes a chain in which every neighbouring trio of planets is linked by a three-body resonance, a repeating pattern in their orbits. The same data showed the star rotating every 3.3 days and producing flares consistent with a low-activity, middle-aged red dwarf.
TRAPPIST-1 h receives only about a seventh of the light Earth gets from the Sun. Luger's team estimated its equilibrium temperature, the temperature set by starlight alone, at about 173 kelvin, far below freezing. In 2021 a team led by Eric Agol used years of transit timing to measure all seven masses, putting TRAPPIST-1 h at about 0.33 Earth masses, and found the planets' densities consistent with rocky worlds. Whether it has any atmosphere is unknown.