K2-18 c
The parameters of this planet are disputed or ambiguous in the scientific literature.
K2-18 c is a super-Earth orbiting K2-18 in the constellation Leo. It lies about 124 light-years from Earth and was discovered in 2017 using the radial velocity method.
Is K2-18 c in the Habitable Zone?
K2-18 c orbits inside the inner edge of the habitable zone of K2-18. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of K2-18: 0.120–0.313 AU (conservative: 0.152–0.297 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 ›
Temperature on K2-18 c
The equilibrium temperature of K2-18 c is about 363 K (90 °C) — hotter than anywhere on Earth. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C.
Orbit and Year Length
A year on K2-18 c — one full orbit around K2-18 — lasts 8.96 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.060 AU — closer to its star than Mercury is to the Sun. Its orbit is highly elliptical (eccentricity 0.47), swinging dramatically closer to and farther from its star.
How Was K2-18 c Discovered?
K2-18 c was discovered in 2017 using the radial velocity method, with observations from La Silla Observatory.
The radial velocity method measures the subtle wobble of a star caused by the gravitational pull of an orbiting planet, visible as periodic shifts in the star's light spectrum. The size of the wobble reveals the planet's minimum mass.
How Far Away Is K2-18 c?
K2-18 c is 124.0 light-years (38.0 parsecs) from Earth. Light arriving here tonight left the planet around the year 1902. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 2,182,400 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. K2-18 c scores 0.36, ranking #668 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: K2-18
K2-18
- Surface temperature
- 3,457 K (Sun: 5,772 K)
- Mass
- 0.36 M☉
- Radius
- 0.41 R☉
The K2-18 Planetary System
K2-18 c is one of 2 known planets in the K2-18 system. Its siblings:
- K2-18 b (Super Earth)
K2-18 c — Complete Data
| Mass (best estimate) | 7.51 Earth masses |
|---|---|
| Orbital period | 8.96 days |
| Orbital distance | 0.060 AU |
| Eccentricity | 0.470 |
| Equilibrium temperature | 363 K (90 °C) |
| Earth Similarity Index | 0.36 |
| Distance from Earth | 124.0 light-years (38.0 parsecs) |
| Constellation | Leo |
| Discovery method | Radial Velocity |
| Discovery facility | La Silla Observatory |
| Discovery year | 2017 |
Data: NASA Exoplanet Archive, last updated 2017-12-19. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-18 c
Is K2-18 c habitable?
No — K2-18 c orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is K2-18 c?
K2-18 c is about 124 light-years from Earth in the constellation Leo. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 2,182,400 years to get there.
How long is a year on K2-18 c?
One orbit around K2-18 takes 9.0 Earth days — short enough that 41 of its years would fit into one Earth year.
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