TRAPPIST-1 is an ultra-cool red dwarf star with seven known planets. It lies in the constellation Aquarius approximately 40.66 light-years away from Earth, and it has a surface temperature of about 2,566 K (2,290 °C; 4,160 °F). Its radius is slightly larger than Jupiter's and it has a mass of about 9% of the Sun. It is estimated to be 7.6 billion years old, making it older than the Solar System. The discovery of the star was first published in 2000.
Observations in 2016 from TRAPPIST–South (Transiting Planets and Planetesimals Small Telescope project) at La Silla Observatory in Chile and other telescopes led to the discovery of two terrestrial planets in orbit around TRAPPIST-1. In 2017, further analysis of the original observations identified five more terrestrial planets. The seven planets take between 1.5 and 19 days to orbit the star in circular orbits. They are all likely tidally locked to TRAPPIST-1, and it is believed that each planet is in permanent day on one side and permanent night on the other. Their masses are comparable to that of Earth and they all lie in the same plane; seen from Earth, they pass in front of the star. This placement allowed the planets to be detected: when they pass in front of the star, its apparent magnitude dims.
