Kepler-138: A Small Red Dwarf and Its Four Known Worlds

Kepler-138 is an M1 V star, a small, cool red dwarf, located 218 light-years away. Its reported mass is 0.56 solar masses and its radius is 0.53 solar radii, both smaller than the Sun's corresponding values. The star's reported temperature is 3810 K, cooler than the Sun's, and its age is 4.68 billion years. Four planets have been reported orbiting this host.

Kepler-138 b is a Sub-Earth, meaning its radius is smaller than Earth's. Its reported radius is 0.64 Earth radii and its mass is 0.07 Earth masses, both smaller than Earth's. Its mean density is 1.47 g/cm³, 0.267 times Earth's. It completes an orbit every 10.3 days.

Kepler-138 c and Kepler-138 d are both Super-Earths, with reported radii of 1.51 Earth radii each, larger than Earth's. Their reported masses, 2.3 Earth masses and 2.1 Earth masses respectively, are also larger than Earth's. Kepler-138 c's mean density is 3.68 g/cm³, 0.668 times Earth's, while Kepler-138 d's is 3.36 g/cm³, 0.61 times Earth's. Their orbital periods are 13.8 days and 23.1 days.

Kepler-138 e has a reported mass of 0.43 Earth masses, smaller than Earth's, and an orbital period of 38.2 days.

All four planets were found by transit, except Kepler-138 e, which was detected through transit timing variations. Among the listed planets, Kepler-138 b has the shortest year by reported estimates at 10.3 days, while Kepler-138 e has the longest at 38.2 days. Among the listed planets, Kepler-138 b has the smallest reported mass at 0.07 Earth masses, and Kepler-138 c has the largest at 2.3 Earth masses.