K2-18 b
K2-18 b is a super-Earth orbiting K2-18 in the constellation Leo. It lies about 124 light-years from Earth and was discovered in 2015 using the transit method. It orbits within the habitable zone of its star — the region where liquid water could exist on a planet's surface.
How Big Is K2-18 b?
K2-18 b has a radius of 2.37 times that of Earth. Its mass is 8.9 times that of Earth, giving it a density of 4.11 g/cm³ — between that of rocky and gaseous planets.
Is K2-18 b in the Habitable Zone?
K2-18 b orbits within the optimistic habitable zone of K2-18 — the broader region where liquid water might be possible under favorable atmospheric conditions.
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.
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Temperature on K2-18 b
The equilibrium temperature of K2-18 b is about 284 K (11 °C) — in a range broadly comparable to Earth, whose equilibrium temperature is 255 K. 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 b — one full orbit around K2-18 — lasts 32.9 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.143 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.20).
How Was K2-18 b Discovered?
K2-18 b was discovered in 2015 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 K2-18 b?
K2-18 b 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 b scores 0.70, ranking #59 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 b is one of 2 known planets in the K2-18 system. Its siblings:
- K2-18 c (Super Earth)
K2-18 b — Complete Data
| Radius | 2.370 Earth radii (0.211 Jupiter radii) |
|---|---|
| Mass | 8.92 Earth masses (0.028 Jupiter masses) |
| Density | 4.11 g/cm³ (Earth: 5.51) |
| Orbital period | 32.94 days |
| Orbital distance | 0.143 AU |
| Eccentricity | 0.200 |
| Equilibrium temperature | 284 K (11 °C) |
| Earth Similarity Index | 0.70 |
| Distance from Earth | 124.0 light-years (38.0 parsecs) |
| Constellation | Leo |
| Discovery method | Transit |
| Discovery facility | K2 |
| Discovery year | 2015 |
Data: NASA Exoplanet Archive, last updated 2019-03-18. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About K2-18 b
Is K2-18 b habitable?
K2-18 b orbits within the habitable zone of K2-18, the region where liquid water could exist on a planet's surface. Whether it is actually habitable depends on its atmosphere and composition, which remain unknown.
How far away is K2-18 b?
K2-18 b 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 big is K2-18 b compared to Earth?
K2-18 b has 2.37 times the radius of Earth and about 8.9 times its mass.
How long is a year on K2-18 b?
One orbit around K2-18 takes 32.9 Earth days — short enough that 11 of its years would fit into one Earth year.
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