The Kepler-21 Exoplanetary System
Kepler-21 is a star with 1.41 times the mass of the Sun, and 1.9 times its radius. It is located 355.05 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 using the transit method. Its discovery was announced in 2012-02. Its semi-major axis is 0.04 astronomical units, as compared to Earth's which is 1 astronomical unit. It takes 2.79 Earth days to complete an orbit around its host star, Kepler-21.
Its eccentricity, the extent to which the shape of the exoplanet's orbit departs from a perfect circle (Earth's is 0.0167, which is why the shape of Earth's orbit is circular rather than oval in appearance), is 0.02, which means that when Kepler-21 b is at is at its closest point to its host star, 0.04 astronomical units separates the two, and when it is at its farthest point, this number is 0.04. In the case of Earth, these numbers are 0.9832899 and 1.0167103 respectively.
The mass of Kepler-21 b is 5.08 times the mass of Earth. The radius of Kepler-21 b is 1.639 that of Earth. At 5.08 Earth masses, Kepler-21 b is a so called Super Earth. Super Earths could be terrestrial worlds like Earth, but they could also be ocean worlds or terrestrial worlds wrapped in a substantial atmosphere, in which case some refer to them as Mini Neptunes. No Super Earths are known to exist in our solar system, but if it exists, the so-called Planet Nine could very well be a super Earth, as it is hypothesized to have a mass between five and ten Earth masses.