Kepler-138c

Kepler-138c is classified as a super-Earth with a radius roughly 1.5 times that of Earth. It is somewhat larger than its neighboring planet Kepler-138b, which is close to the size of Mars.

Kepler-138c orbits an M-dwarf (red dwarf) star, which is cooler and significantly smaller than the Sun. Because M-dwarfs emit less energy, their habitable zones sit much closer to the star, but planets in close orbits around red dwarfs are often exposed to intense stellar flares that can strip away atmospheres.

Some studies of Kepler-138c's measured mass and radius suggest it could have a substantial volatile content—potentially including a significant fraction of water by mass. However, the exact composition remains debated, and it could alternatively be a rocky planet with a thick atmospheric envelope.

The mass was estimated using transit timing variations (TTVs)—small, periodic shifts in exactly when the planet transits its star, caused by the gravitational tug of neighboring planets. In the compact Kepler-138 system, the mutual gravitational perturbations between planets allowed astronomers to infer their masses without traditional radial velocity measurements.