Kepler-391 and Its Two Neptune-Like Planets

Kepler-391 is a K2 III-IV star located 2840 light-years away. It is 4.37 billion years old, with a mass of 1.27 solar masses and a radius of 2.88 solar radii, and its temperature is 5040 K. Its reported mass and radius estimates are larger than the Sun's, while its reported temperature is cooler. Two planets are known to orbit this host: Kepler-391 b and Kepler-391 c. Both are Neptune-like worlds discovered by transit in 2014, the method in which a planet passes in front of its star from our viewpoint and causes a small dip in observed brightness.

Kepler-391 b has a radius of 2.5 Earth radii, a reported upper limit of 1100 Earth masses, and an orbital period of 7.42 days. Among the listed planets, it has the shortest year and the smallest radius. Kepler-391 c has a radius of 2.88 Earth radii, a reported upper limit of 450 Earth masses, and an orbital period of 20.5 days. Among the listed planets, it has the longest year and the largest radius. Both planets' reported radius estimates are larger than Earth's. As upper limits, these mass values are reported bounds rather than exact values, and as size classifications, Neptune-like describes radius rather than composition.