Kepler-278 and Its Two Neptune-Like Planets

Kepler-278 is a star located 1440 light-years away. Its spectral type, K2 III-IV, places it among the cooler K-type stars. The star is estimated to be 5.76 billion years old, with a mass of 1.23 solar masses and a radius of 2.86 solar radii. Its temperature is 4970 K. Compared with the Sun, the star's reported mass and radius estimates are larger, while its reported temperature is cooler. Two planets are known to orbit this host: Kepler-278 b and Kepler-278 c. Both were discovered in 2014 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.

Both planets are Neptune-like, a size class that reaches up to four times Earth's radius. Kepler-278 b has a radius of 3.96 Earth radii and a mass of 56 Earth masses. Its mean density is 4.99 g/cm³, which is 0.905 times Earth's. It completes an orbit in 30.2 days. Kepler-278 c has a radius of 3.27 Earth radii and a mass of 34.9 Earth masses. Its mean density is 5.51 g/cm³, 1 times Earth's. Its orbital period is 51.1 days.

Among the listed planets, Kepler-278 b has the shortest year and the largest radius, at 3.96 Earth radii, as well as the largest reported mass, at 56 Earth masses. Kepler-278 c has the longest year and the smallest radius, at 3.27 Earth radii, and the smallest reported mass, at 34.9 Earth masses. Each planet's reported radius and mass estimates are larger than Earth's.