K2-138 is a star with 0.92 times the mass of the Sun, and 0.84 times its radius. It is located 660.74 light years away from the solar system.
K2-138 is known to have 6 exoplanets in orbit around it.
K2-138 b was discovered by the K2 observatory, in 2018-02, using the transit method. Its semi-major axis is 0.03 astronomical units, as compared to Earth's which is 1 astronomical unit. The mass of K2-138 b is 2.827 times the mass of Earth. The radius of K2-138 b is 1.490 that of Earth. At 2.827 Earth masses, K2-138 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.
K2-138 c was discovered by the K2 observatory, in 2018-02, 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 K2-138 c is 5.724 times the mass of Earth. The radius of K2-138 c is 2.260 that of Earth. At more than 10 Earth masses, K2-138 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.
K2-138 d was discovered by the K2 observatory, in 2018-02, using the transit method. Its semi-major axis is 0.06 astronomical units, as compared to Earth's which is 1 astronomical unit. The mass of K2-138 d is 6.614 times the mass of Earth. The radius of K2-138 d is 2.460 that of Earth. At more than 10 Earth masses, K2-138 d 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.
K2-138 e was discovered by the K2 observatory, in 2018-02, 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 K2-138 e is 9.858 times the mass of Earth. The radius of K2-138 e is 3.110 that of Earth. At more than 10 Earth masses, K2-138 e 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.
K2-138 f was discovered by the K2 observatory, in 2018-02, 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 K2-138 f is 7.918 times the mass of Earth. The radius of K2-138 f is 2.730 that of Earth. At more than 10 Earth masses, K2-138 f 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.
K2-138 g was discovered by the K2 observatory, in 2021-02, using the transit method. Its semi-major axis is 0.23 astronomical units, as compared to Earth's which is 1 astronomical unit. The mass of K2-138 g is 10.240 times the mass of Earth. The radius of K2-138 g is 3.180 that of Earth. At more than 10 Earth masses, K2-138 g 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.