Kepler-9: A Star and Its Three Planets

Kepler-9 is a star located 2050 light-years away, and three planets have been found orbiting it. The star is 2 billion years old, holds 1.02 solar masses, spans 0.958 solar radii, and has a temperature of 5770 K. Its reported radius estimate is smaller than the Sun's. All three planets were discovered by transit, the method in which a planet passes in front of its star from our viewpoint and causes a small dip in observed brightness.

Kepler-9 b and Kepler-9 c are both Jupiter-like planets, a size class that exceeds 4 times Earth's radius. Kepler-9 b has a mass of 43.4 Earth masses, a radius of 8.29 Earth radii, and an orbital period of 19.2 days; its mean density is 0.42 g/cm³, 0.0762 times Earth's. Kepler-9 c has a mass of 29.9 Earth masses, a radius of 8.08 Earth radii, and an orbital period of 39 days; its mean density is 0.312 g/cm³, 0.0567 times Earth's. Both planets' reported radius and mass estimates are larger than Earth's.

Kepler-9 d is a Super-Earth, a class reaching 2.5 times Earth's radius. It has a radius of 1.64 Earth radii and an orbital period of 1.59 days. Its reported radius estimate is larger than Earth's.

Among the listed planets, Kepler-9 d has the shortest year by reported estimates, at 1.59 days, while Kepler-9 c has the longest, at 39 days; Kepler-9 b has the second-shortest, at 19.2 days. Kepler-9 b also has the largest radius by reported estimates, at 8.29 Earth radii, while Kepler-9 d has the smallest, at 1.64 Earth radii, and Kepler-9 c has the second-smallest, at 8.08 Earth radii.