TRAPPIST-1 e

The star TRAPPIST-1 is a tiny, cool red dwarf in the constellation of Aquarius, a little more than forty light years from the Sun. Though only about a tenth of the Sun's mass, this star plays host to one of the busiest systems of planets known beyond the Solar System. The system has seven known planets, all following very close orbits around the star (so close that the orbits of all seven would fit easily within the orbit of Mercury around the Sun). TRAPPIST-1 e is the fifth of these planets from its red star, orbiting at a distance of 0.03 AU (or about 4.4 million kilometres) in a period of just over six days (so a 'year' on TRAPPIST-1 e is less than a week long).

TRAPPIST-1 e is notable as being remarkably Earth-like in many ways. It is slightly smaller and less massive than Earth (about nine-tenths of Earth's diameter) which strongly implies that it is a rocky planet. Because the TRAPPIST-1 star puts out significantly less energy than the Sun, its habitable zone - the zone in which liquid water can exist - lies much closer to the star, and TRAPPIST-1 e orbits confortably within this zone. The presence of oceans on the planet, then, while not confirmed, is at least plausible.

The surface temperature of the planet is hard to calculate because the details of its atmosphere (if indeed it has an atmosphere) are not known, but it is at least possible that the planet's surface might be temperate and Earth-like. All these factors combine to give TRAPPIST-1 e a very high Earth Similarity Index (ESI) value of 0.95, though the volatile nature of its star, and the proximity of its orbit, mean that it may be less habitable than this figure suggests.