Kepler-251: A Distant Star With Four Known Worlds

Kepler-251 is a star located 3020 light-years away, and four planets have been found in orbit around it. The star is estimated to be 3.63 billion years old, with a mass of 0.95 solar masses and a radius of 0.91 solar radii, making it slightly less massive and slightly smaller than the Sun. Its temperature is 5620 K, cooler than the Sun's.

All four planets were discovered 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; repeated dips help establish orbital periods. Each planet completes one orbit, its year, in a different span of days.

Kepler-251 b is classified as Earth-like, with a radius of 1.33 Earth radii, larger than Earth's, and an orbital period of 4.79 days, the shortest among the listed planets. Kepler-251 c, d, and e are all Neptune-like, each with a radius of 2.77 Earth radii, larger than Earth's. Their orbital periods are 16.5 days, 30.1 days, and 99.6 days, respectively. Among the listed planets, Kepler-251 c has the second-shortest year, Kepler-251 d has the second-longest year, and Kepler-251 e has the longest year. Kepler-251 b has the smallest radius among them.