The K2-138 system contains 6 exoplanets. It is located 660.74 light years away from the solar system.
| Mass | 0.94 solar masses |
| Radius | 0.86 solar radiae |
| Temperature | 5356.3 kelvin |
| Stellar Metallicity | 0.14 decimal exponent |
| Age | 2.8 billion years |
At 3.100 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.
| Mass | 3.100 Earth masses |
| Density | 4.900 grams per cubic centimeter |
| Radius | 1.510 Earth radiae |
| Semi-major Axis | 0.03385 AU |
| Eccentricity | 0.048 |
| Orbital Period | 2.353 days |
| Discovery Method | Transit |
| Discovery Facility | K2 |
| Discovery Telescope | 0.95 m Kepler Telescope |
| Discovery Instrument | Kepler CCD Array |
| Discovery Date | 2018-02 |
| Reference | Christiansen et al. 2018 |
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.
| Mass | 6.310 Earth masses |
| Density | 2.800 grams per cubic centimeter |
| Radius | 2.299 Earth radiae |
| Semi-major Axis | 0.04461 AU |
| Eccentricity | 0.045 |
| Orbital Period | 3.560 days |
| Discovery Method | Transit |
| Discovery Facility | K2 |
| Discovery Telescope | 0.95 m Kepler Telescope |
| Discovery Instrument | Kepler CCD Array |
| Discovery Date | 2018-02 |
| Reference | Christiansen et al. 2018 |
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.
| Mass | 7.920 Earth masses |
| Density | 3.100 grams per cubic centimeter |
| Radius | 2.390 Earth radiae |
| Semi-major Axis | 0.05893 AU |
| Eccentricity | 0.043 |
| Orbital Period | 5.405 days |
| Discovery Method | Transit |
| Discovery Facility | K2 |
| Discovery Telescope | 0.95 m Kepler Telescope |
| Discovery Instrument | Kepler CCD Array |
| Discovery Date | 2018-02 |
| Reference | Christiansen et al. 2018 |
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.
| Mass | 12.970 Earth masses |
| Density | 1.800 grams per cubic centimeter |
| Radius | 3.390 Earth radiae |
| Semi-major Axis | 0.0782 AU |
| Eccentricity | 0.077 |
| Orbital Period | 8.261 days |
| Discovery Method | Transit |
| Discovery Facility | K2 |
| Discovery Telescope | 0.95 m Kepler Telescope |
| Discovery Instrument | Kepler CCD Array |
| Discovery Date | 2018-02 |
| Reference | Christiansen et al. 2018 |
With a mass below 5 Earth masses, a density of 0.4, and a semi-major axis of 0.10447 astronomical units, K2-138 f could, potentially, be an ocean world - a planet with no dry land.
| Mass | 1.630 Earth masses |
| Density | 0.400 grams per cubic centimeter |
| Radius | 2.904 Earth radiae |
| Semi-major Axis | 0.10447 AU |
| Eccentricity | 0.062 |
| Orbital Period | 12.758 days |
| Discovery Method | Transit |
| Discovery Facility | K2 |
| Discovery Telescope | 0.95 m Kepler Telescope |
| Discovery Instrument | Kepler CCD Array |
| Discovery Date | 2018-02 |
| Reference | Christiansen et al. 2018 |
With a mass below 5 Earth masses, a density of 0.9, and a semi-major axis of 0.23109 astronomical units, K2-138 g could, potentially, be an ocean world - a planet with no dry land.
| Mass | 4.320 Earth masses |
| Density | 0.900 grams per cubic centimeter |
| Radius | 3.013 Earth radiae |
| Semi-major Axis | 0.23109 AU |
| Eccentricity | 0.059 |
| Orbital Period | 41.968 days |
| Discovery Method | Transit |
| Discovery Facility | K2 |
| Discovery Telescope | 0.95 m Kepler Telescope |
| Discovery Instrument | Kepler CCD Array |
| Discovery Date | 2021-05 |
| Reference | Hardegree-Ullman et al. 2021 |