K2-138: A K-Type Dwarf and Six Transiting Planets

K2-138 is a K1 V star located 661 light-years away. As a main-sequence K-type dwarf, it is somewhat cooler and smaller than the Sun. Its mass is 0.93 solar masses, its radius is 0.86 solar radii, and its temperature is 5380 K. Six planets are known to orbit this star, all discovered by the transit method.

K2-138 b is a Super-Earth with a mass of 3.1 Earth masses, a radius of 1.51 Earth radii, and an orbital period of 2.35 days. K2-138 c is also a Super-Earth, with a mass of 6.31 Earth masses, a radius of 2.3 Earth radii, and a period of 3.56 days. K2-138 d is a Super-Earth with a mass of 7.92 Earth masses, a radius of 2.39 Earth radii, and a period of 5.4 days.

K2-138 e is Neptune-like, with a mass of 13 Earth masses, a radius of 3.39 Earth radii, and a period of 8.26 days. K2-138 f is Neptune-like, with a mass of 1.63 Earth masses, a radius of 2.9 Earth radii, and a period of 12.8 days. K2-138 g is Neptune-like, with a mass of 4.32 Earth masses, a radius of 3.01 Earth radii, and a period of 42 days.

Among the listed planets, K2-138 b has the shortest year, at 2.35 days, while K2-138 g has the longest, at 42 days. K2-138 b also has the smallest radius, at 1.51 Earth radii, and K2-138 e has the largest, at 3.39 Earth radii. In mass, K2-138 f is the smallest at 1.63 Earth masses and K2-138 e the largest at 13 Earth masses. The mean density of K2-138 b is 4.96 g/cm³, 0.9 times Earth's. By contrast, K2-138 f has a mean density of 0.367 g/cm³, 0.0666 times Earth's.