With just 8% the mass of the Sun, TRAPPIST-1 is very small in stellar terms, only marginally bigger than the planet Jupiter — and though nearby in the constellation Aquarius, it is invisible visually with anything less than powerful telescopes. Astronomers expected that such dwarf stars might host many Earth-sized planets in tight orbits, making them promising targets in the hunt for extraterrestrial life. TRAPPIST-1 is the first such system to be discovered.

Co-author Amaury Triaud, of the University of Cambridge's Institute of Astronomy, explains: "Stars like TRAPPIST-1 belong to the most common type of stars that exist within our Galaxy. The planets that we found are likely representative of the most common sort of planets in the Universe. And that the planets are so similar to Earth bodes well for the search for life elsewhere. Planets orbiting ultra-cool dwarfs, like TRAPPIST-1, likely represent the largest habitable real estate in the Milky Way!"

The planetary orbits are not much larger than that of Jupiter's Galilean moon system, and much smaller than the orbit of Mercury in the Solar System. However, TRAPPIST-1's small size and low temperature means that the energy input to its planets is similar to that received by the inner planets in our Solar System; TRAPPIST-1c, d and f receive similar energy inputs to Venus, Earth and Mars, respectively.

All seven planets discovered in the system could potentially have liquid water on their surfaces, though their orbital distances make some of them more likely candidates than others. Climate models suggest the innermost planets, TRAPPIST-1b, c and d, are probably too hot to support liquid water, except maybe on a small fraction of their surfaces. The orbital distance of the system's outermost planet, TRAPPIST-1h, is unconfirmed, though it is likely to be too distant and cold to harbour liquid water — assuming no alternative heating processes are occurring. TRAPPIST-1e, f, and g, however, are of more interest to planet-hunting astronomers, as they orbit in the star's habitable zone and could host oceans of surface water.

These new discoveries make the TRAPPIST-1 system an even more important target in the search for extra-terrestrial life. Team member Didier Queloz, from the University of Cambridge's Cavendish Laboratory, is excited about the future possibilities: "Thanks to future facilities like ESO's Extremely Large Telescope, or NASA/ESA's soon-to-be-launched James Webb Space telescope, we will be able to measure the structure of the planets' atmospheres, as well as their chemical composition. We are about to start the remote exploration of terrestrial climates beyond our Solar system."