Kepler-398: A Cool, Old Star With Three Small Worlds

Kepler-398 is a star located 578 light-years away, and three planets orbit it. The star is 2.34 billion years old, with a mass of 0.72 solar masses and a radius of 0.67 solar radii. Its temperature is 4560 K, which is cooler than the Sun, and both its mass and radius estimates are smaller than the Sun's.

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-398 b and Kepler-398 c were discovered in 2014, while Kepler-398 d was discovered in 2016.

Kepler-398 b is a Sub-Earth with a radius of 0.93 Earth radii and an orbital period of 4.08 days, the shortest year among the listed planets. Kepler-398 d is also a Sub-Earth, with a radius of 0.88 Earth radii and an orbital period of 6.83 days. Among the listed planets, Kepler-398 d has the smallest radius and the second-shortest year, while Kepler-398 b has the second-smallest radius.

Kepler-398 c is classified as Earth-like, meaning its radius stays within 1.5 times Earth's radius. It has a radius of 1.01 Earth radii and an orbital period of 11.4 days, the longest year among the listed planets. Kepler-398 c also has the largest radius among the listed planets.

The host star's mass is 0.72 solar masses, its radius is 0.67 solar radii, and its temperature is 4560 K.