Which system of seven Earth-sized planets is a key exoplanet target for JWST? It is the TRAPPIST-1 system. This compact planetary system lies about 40 light-years away and contains seven roughly Earth-sized, rocky planets orbiting a small ultracool red dwarf star.
The planets are designated TRAPPIST-1b through TRAPPIST-1h. They orbit extremely close to their star, completing their years in roughly 1.5 to 19 days. Their compact arrangement makes repeated transits especially useful: each time a planet passes in front of the star, starlight can filter through any atmosphere it possesses.
JWST studies those tiny changes in infrared light to search for atmospheric molecules and assess whether the planets have retained substantial atmospheres. Several of the outer planets are commonly identified as lying in or near the star’s habitable zone, where temperatures could potentially allow liquid water under suitable atmospheric conditions.
The system was announced as a seven-planet discovery in 2017. TRAPPIST-1 is not one planet but the name of the star and its planetary system, named after the TRAPPIST telescope involved in its discovery.