Kepler-304: A Cool, Older Star With Four Small Worlds

Kepler-304 is a star located 1420 light-years away, and four planets have been identified in orbit around it. The star itself is estimated to be 2.09 billion years old, with a mass of 0.8 solar masses and a radius of 0.73 solar radii. Its reported temperature is 4820 K, making it cooler than the Sun, and its mass and radius estimates are each smaller than the Sun's.

The planets around Kepler-304 were all found via 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-304 b, c, and d were discovered in 2014, while Kepler-304 e was discovered in 2016.

Kepler-304 b is classified as Neptune-like, with an orbital period of 3.3 days and a radius of 2.86 Earth radii. Kepler-304 c is a Super-Earth with an orbital period of 5.32 days and a radius of 2.17 Earth radii. Kepler-304 d is Neptune-like, orbiting every 9.65 days with a radius of 2.75 Earth radii. Kepler-304 e is Earth-like, with an orbital period of 1.5 days and a radius of 1.2 Earth radii. All four planets have reported radius estimates larger than Earth's.

Among the listed planets, Kepler-304 e has both the shortest year, at 1.5 days, and the smallest radius, at 1.2 Earth radii. Kepler-304 b has the largest radius, at 2.86 Earth radii, and the second-shortest year, at 3.3 days. Kepler-304 c has the second-smallest radius, at 2.17 Earth radii, and the second-longest year, at 5.32 days. Kepler-304 d has the longest year, at 9.65 days, and the second-largest radius, at 2.75 Earth radii.