Kepler-21 is a star with 1.41 times the mass of the Sun, and 1.9 times its radius. It is located 353.95 light years away from the solar system and is estimated to be 3.03 billion years old, as compared to the Sun which is roughly 4.6 billion years old.
Kepler-21 is known to have 1 exoplanets in orbit around it.
Kepler-21 b was discovered by the Kepler observatory, in 2012-02, using the transit method. Its semi-major axis is 0.04 astronomical units, as compared to Earth's which is 1 astronomical unit. The mass of Kepler-21 b is 5.082 times the mass of Earth. The radius of Kepler-21 b is 1.639 that of Earth. At more than 10 Earth masses, Kepler-21 b is an ice giant, a planet that is made up mostly of volatiles like water, amonia and methane, and enveloped by a dense hydrogen and helium atmosphere, much like Uranus and Neptune in our solar system.