by Tanja Schumann (TU Berlin), March 2025
Of the about 5800 known exoplanets discovered as of March 2025, about 2000 are part of planetary systems with more than one confirmed planet around the same star. Most of those are two-planet systems (about 500).
However, there is one system with seven known exoplanets: TRAPPIST-1. Its planets, discovered in 2016 and 2017, range between 0.3 and 1.4 times the mass of Earth, which means they are all rocky planets. All of them complete their orbit in less than 20 days and have an orbital radius less than one fifth of that of Mercury, the innermost planet of our Solar System. The structure of this planetary system is therefore more similar to the system of Jupiter’s moons than to our solar system. The planets probably orbit TRAPPIST-1 in a tidally locked rotation.
Artistic representation of the planets of the TRAPPIST-1 system, and in comparison the inner planets of the solar system. The planet sizes are to scale. The extent of the habitable zone is shown in green. Image credit: NASA/JPL-Caltech
But because TRAPPIST-1 is an M-dwarf star, and as such is quite a bit less hot than our Sun, three of its planets lie in the star’s habitable zone, i.e. the range around the star where the existence of liquid water on the surface might be possible.
The planets of the TRAPPIST-1 system are very close to orbital resonances with their respective neighbours, meaning in the time it takes one planet to complete a whole number of orbits, their closest neighbour also completes a whole number of orbits. See an animation of this behaviour here.