K2-138 c
K2-138 c is a mini-Neptune orbiting K2-138 in the constellation Aquarius. It lies about 661 light-years from Earth and was discovered in 2017 using the transit method.
How Big Is K2-138 c?
K2-138 c has a radius of 2.30 times that of Earth. Its mass is 6.3 times that of Earth, giving it a density of 2.80 g/cm³ — between that of rocky and gaseous planets.
Is K2-138 c in the Habitable Zone?
K2-138 c orbits inside the inner edge of the habitable zone of K2-138. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of K2-138: 0.570–1.361 AU (conservative: 0.722–1.290 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
See the full interactive habitable-zone view in the Exoplanet Explorer app ›
Orbit and Year Length
A year on K2-138 c — one full orbit around K2-138 — lasts 3.56 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.045 AU — closer to its star than Mercury is to the Sun. Its orbit is nearly circular (eccentricity 0.045).
How Was K2-138 c Discovered?
K2-138 c was discovered in 2017 using the transit method, with observations from K2.
The transit method watches a star for the tiny, regular dip in brightness that occurs when a planet crosses in front of it. The depth and timing of these dips reveal the planet's size and orbital period.
How Far Away Is K2-138 c?
K2-138 c is 660.7 light-years (202.6 parsecs) from Earth. Light arriving here tonight left the planet around the year 1366. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 11,628,320 years to make the journey.
Earth Similarity Index
The Earth Similarity Index (ESI) scores how physically similar a planet is to Earth, from 0 to 1, based on radius, density, escape velocity and surface temperature. K2-138 c scores 0.25, ranking #3,206 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-138
K2-138
- Surface temperature
- 5,356 K (Sun: 5,772 K)
- Mass
- 0.94 M☉
- Radius
- 0.86 R☉
- Age
- 2.8 billion years (Sun: 4.6)
The K2-138 Planetary System
K2-138 c is one of 6 known planets in the K2-138 system. Its siblings:
K2-138 c — Complete Data
| Radius | 2.299 Earth radii (0.205 Jupiter radii) |
|---|---|
| Mass | 6.31 Earth masses (0.020 Jupiter masses) |
| Density | 2.80 g/cm³ (Earth: 5.51) |
| Orbital period | 3.56 days |
| Orbital distance | 0.045 AU |
| Eccentricity | 0.045 |
| Earth Similarity Index | 0.25 |
| Distance from Earth | 660.7 light-years (202.6 parsecs) |
| Constellation | Aquarius |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2017 |
Data: NASA Exoplanet Archive, last updated 2021-03-12. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-138 c
Is K2-138 c habitable?
No — K2-138 c orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is K2-138 c?
K2-138 c is about 661 light-years from Earth in the constellation Aquarius. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 11,628,320 years to get there.
How big is K2-138 c compared to Earth?
K2-138 c has 2.30 times the radius of Earth and about 6.3 times its mass.
How long is a year on K2-138 c?
One orbit around K2-138 takes 3.6 Earth days — short enough that 103 of its years would fit into one Earth year.
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