Although most of the planet candidates cataloged by NASA’s Kepler space telescope have diameters in the range of 2 to 4 times that of the Earth, little Kepler-138 b is only about the size and mass of Mars.
Kepler-138 b is one of three planets orbiting the red dwarf star Kepler-138, which is located 200 light-years from Earth in the constellation Lyra. The gravity of Kepler-138’s three planets pulls them into slightly different positions than they would occupy otherwise. This enables scientists to calculate the mass of the planets.
The innermost planet is Kepler-138 b, which has a density about the same as the rocky planet Mars. The middle planet, Kepler-138 c, has a density about the same as Earth, and the outermost planet (Kepler-138 d) is less than half as dense, indicating a large proportion of light materials such as water and hydrogen.