NASA made a big announcement this week about new exoplanets found orbiting the dwarf star TRAPPIST-1 some 39 light years away. I’ve discussed this particular dwarf star system before (1), as it was already known to have three terrestrial planets in attendance orbiting very close to this cool, fairly dim star (2,3). The dwarf star is approximately one tenth the size of the Sun, and it’s mass places it on the border between a brown dwarf and a red dwarf star. Unusually for a star this small, TRAPPIST-1 has a high metallicity, which actually exceeds that of the Sun (4).
Now, an international team of astronomers, using the Belgian TRAPPIST telescope in Chile and the Spitzer infra-red space telescope, have released details about a further four terrestrial planets in this mini-star system, three of which (e, f and g) are located within it’s habitable zone, where temperatures favour the presence of liquid water (5):
“Researchers led by Michaël Gillon, of the University of Liège in Belgium, have been studying the infrared light emitted by this miniature star and have detected drops in luminosity characteristic of transits, i.e. the passage of astronomical bodies moving across its face. As early as 2015, the first three planets (dubbed b, c and d) had been identified. Tracking the system using TRAPPIST and the space telescope Spitzer, the team was then able to identify four others planets (e, f, g and h) in 2016. Based on the frequency of these transits and the degree of reduction in luminosity of the star, they have demonstrated that these seven planets are all comparable in size to Earth (to within 15%), and orbit very close to their star.” (6)