TRAPPIST-1, also designated 2MASS J23062928-0502285, is an ultra-cool red dwarf star that is slightly larger, but much more massive, than the planet Jupiter; it is located 39.6 light-years (12.1 pc) from the Sun in the constellation Aquarius. Seven temperate terrestrial planets have been detected orbiting the star, a larger number than detected in any other planetary system. A study released in May 2017 suggests that the stability of the system is not particularly surprising if one considers how the planets migrated to their present orbits through a protoplanetary disk.
A team of Belgian astronomers first discovered three Earth-sized planets orbiting the dwarf star in 2015. A team led by Michaël Gillon [fr] at the University of Liège in Belgium detected the planets using transit photometry with the Transiting Planets and Planetesimals Small Telescope (TRAPPIST) at the La Silla Observatory in Chile and the Observatoire de l Oukaïmeden in Morocco. On 22 February 2017, astronomers announced four additional exoplanets around TRAPPIST-1. This work used the Spitzer Space Telescope and the Very Large Telescope at Paranal, amongst others, and brought the total number of planets to seven, of which three are considered to be within its habitable zone. The others could also be habitable as they may possess liquid water somewhere on their surface. Depending on definition, up to six could be in the optimistic habitable zone (c, d, e, f, g, h), with estimated equilibrium temperatures of 170 to 330 K (−103 to 57 °C; −154 to 134 °F). In November 2018, researchers determined that planet e is the most likely Earth-like ocean world and would be an excellent choice for further study with habitability in mind.