TRAPPIST-1 is a planetary system about 39.5 light-years away in the constellation Aquarius. With a visible magnitude of 18.8, it is about 100’000 times too faint to be visible with the naked eye. It is made of a star and at least seven planets.
The star, discovered in 1999 thanks to 2MASS, is an ultracool dwarf of spectral type M8 with an age between 3 and 8 billion years. Its is only slightly larger than Jupiter but 83 times more massive, putting it close to the limit between stars and brown dwarfs. Its temperature is about 2550 kelvins (2280) and its metallicity (relative content of elements heavier than hydrogen and helium) is similar to that of the Sun.
The seven planets, detected by transits in 2015 and 2016, have radii between 0.7 and 1.2 times that of the Earth, while the masses of planets b to g are between 0.4 and 1.4 that of Earth (the mass of planet h is not known yet). Given that the star is 2000 times less luminous than the Sun, planets e, f and g are inside the habitable zone of the system. Interestingly, nearest-neighbor planets have period ratios about 8/5, 5/3, 3/2, 3/2, 4/3, and 3/2.
* Michaël Gillon et al., “Temperate Earth-sized planets transiting a nearby ultracool dwarf star”, Nature, 533, 221-224 (2016).
* Michaël Gillon et al., “Seven temperate terrestrial planets around the nearby ultracool dwarf TRAPPIST-1”, Nature, 542, 456-460 (2017).
* Rodrigo Luger et al., “A seven-planet resonant chain in TRAPPIST-1”, Nature Astronomy, 1, article n° 0129 (2017).