TRAPPIST-1 c

The second of seven rocky worlds that follow close orbits around the faint red dwarf TRAPPIST-1 in Aquarius. All seven of these planets follow tight orbits that would fit easily within that of Mercury around the Sun, but the star TRAPPIST-1 emits only a feeble amount of energy in comparison with the Sun, and so several of these planets nonetheless fall within its habitable zone.

TRAPPIST-1 c is the second-nearest planet to the star (only TRAPPIST-1 b follows a closer orbit) at a distance of just 2.3 million kilometres or 0.02 AU (that is, less than 2% of the distance from the Sun to Earth). It is comparable in size and mass to Earth, although slightly larger, and is variously categorised as an Earth-mass planet or a super Earth. It completes each orbit of the star TRAPPIST-1 in a period of two days and ten hours.

Despite its proximity to its star, the dim red light of TRAPPIST-1 is so faint that TRAPPIST-1 c receives a similar amount of energy to Venus in the Solar System, and like Venus it orbits on the inner edge of the habitable zone. For this reason, it was initially hypothesised that the planet might also be similar to Venus in other ways, perhaps being swathed in thick atmosphere holding the heat received from the star. Further analysis has shown that this is not the case, and if TRAPPIST-1 c has an atmosphere at all, it is lacking in the greenhouse gases needed to trap significant quantities of heat.

The planet's orbit is so close to its star that it is likely to be tidally locked, permanently showing only one face to TRAPPIST-1 so that, viewed from the surface of the planet, its star would remain fixed in the sky. The planet's close orbit also implies that it will be subject to tidal forces from the star's gravity, meaning that it is likely to be geologically active.