TRAPPIST-1 is a cool M-dwarf (about 0.09 solar masses, 2,566 K, 0.12 solar radii). TRAPPIST-1 hosts seven Earth-sized worlds in tight, resonant orbits. At least three may lie in the habitable zone, making this cool red dwarf one of the most scrutinised systems in the search for temperate rocky planets. TRAPPIST-1 b is a 1.4-Earth-mass planet, with an orbital period of 1.5 days, at 0.012 AU from its star, discovered in 2016 via transit. Mutual gravitational tugs between the planets can slowly reshape their orbits, preserving clues to the system's assembly.