Kepler-138, also known as KOI-314, is a red dwarf star about 219 light-years away in the constellation Lyra, which hosts four known planets. Kepler-138 c & d are both about 1.5 times the size of Earth; they were originally estimated to have very different masses and densities, but were found in 2022 to be similarly low-density, volatile-rich "ocean worlds". Kepler-138 b is the second-least massive exoplanet with a measured mass, between that of Mars and Mercury, and also has a low density, though it's certainly rocky given how small it is. Kepler-138 e is almost half the mass of Earth; it was discovered through transit timing variations of the other planets, and is the least massive exoplanet found by this method.
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