Kepler-81 and Its Three Planets

Kepler-81 is an M-type main-sequence star located 1140 light-years away. Its spectral type, M0 V, places it among the small, cool red dwarfs. The star's reported mass estimate is 0.69 solar masses and its radius estimate is 0.65 solar radii, both smaller than the Sun's. Its reported temperature of 4390 K is cooler than the Sun's, and its age is given as 3.63 billion years. Three planets are known to orbit this star.

All three planets were found by the transit method, in which a planet passes in front of its star from our viewpoint and causes a small dip in observed brightness. Kepler-81 b and Kepler-81 c were both discovered in 2012, while Kepler-81 d was discovered in 2014. Kepler-81 b and Kepler-81 c are classed as Super-Earths, a size class that reaches 2.5 times Earth's radius, while Kepler-81 d is Earth-like, a class that stays within 1.5 times Earth's radius.

Kepler-81 b has an orbital period of 5.96 days and a radius of 2.42 Earth radii. Kepler-81 c orbits in 12 days with a radius of 2.37 Earth radii. Kepler-81 d completes an orbit in 20.8 days and has a radius of 1.21 Earth radii. Among the listed planets, Kepler-81 b has the shortest year and the largest radius by reported estimates. Kepler-81 c has the second-shortest year and the second-smallest radius, while Kepler-81 d has the longest year and the smallest radius. The reported radius estimates of Kepler-81 b and Kepler-81 c are each larger than Earth's.