Kepler-18 and Its Three Worlds

Kepler-18 is a star located 1,410 light-years away. It is 10 billion years old, has 0.972 solar masses, and a radius of 1.11 solar radii, larger than the Sun's. Its temperature is 5,350 K, cooler than the Sun's. Three planets orbit this host.

All three planets were discovered by transit in 2011, the method in which a planet crosses in front of its star and causes a small dip in brightness. Kepler-18 b is a Super-Earth with a mass of 6.9 Earth masses and a radius of 2 Earth radii, both larger than Earth's. Its mean density is 4.75 g/cm³, 0.863 times Earth's. It completes an orbit in 3.5 days.

Kepler-18 c and Kepler-18 d are both Jupiter-like planets. Kepler-18 c has a mass of 17.3 Earth masses and a radius of 5.49 Earth radii, each larger than Earth's. Its mean density is 0.576 g/cm³, 0.105 times Earth's. Its orbital period is 7.64 days. Kepler-18 d has a mass of 16.4 Earth masses and a radius of 6.98 Earth radii, both larger than Earth's. Its mean density is 0.266 g/cm³, 0.0482 times Earth's. Its orbital period is 14.9 days.

Among the listed planets, Kepler-18 b has the shortest year and the smallest radius and reported mass. Kepler-18 c has the second-shortest year and the second-smallest radius, while its reported mass is the largest. Kepler-18 d has the longest year, the largest radius, and the second-smallest reported mass.