HAT-P-21 is a star with 1.24 times the mass of the Sun, and 1.21 times its radius. It is located 904.36 light years away from the solar system and is estimated to be 10.2 billion years old, as compared to the Sun which is roughly 4.6 billion years old.
HAT-P-21 is known to have 1 exoplanets in orbit around it.
HAT-P-21 b was discovered by the HATNet observatory, in 2011-12, using the transit method. Its semi-major axis is 0.05 astronomical units, as compared to Earth's which is 1 astronomical unit. The mass of HAT-P-21 b is 1548.660 times the mass of Earth. The radius of HAT-P-21 b is 12.442 that of Earth. At more than 50 Earth masses, HAT-P-21 b is a gas giant, a planet whose mass is mostly made up of hydrogen and helium, like Jupiter and Saturn in our solar system.