K2-21 is a star with 0.68 times the mass of the Sun, and 0.65 times its radius. It is located 272.81 light years away from the solar system and is estimated to be 1 billion years old, as compared to the Sun which is roughly 4.6 billion years old.
K2-21 is known to have 2 exoplanets in orbit around it.
K2-21 b was discovered by the K2 observatory, in 2015-10, using the transit method. Its semi-major axis is 0.08 astronomical units, as compared to Earth's which is 1 astronomical unit. The mass of K2-21 b is 4.039 times the mass of Earth. The radius of K2-21 b is 1.840 that of Earth. With a mass below 5 Earth masses, a density of 2.7, and a semi-major axis of 0.076 astronomical units, K2-21 b could, potentially, be an ocean world - a planet with no dry land.
K2-21 c was discovered by the K2 observatory, in 2015-10, using the transit method. Its semi-major axis is 0.11 astronomical units, as compared to Earth's which is 1 astronomical unit. The mass of K2-21 c is 6.773 times the mass of Earth. The radius of K2-21 c is 2.490 that of Earth. At more than 10 Earth masses, K2-21 c 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.