GJ 251 b
GJ 251 b is a super-Earth orbiting GJ 251 in the constellation Gemini. It lies about 18 light-years from Earth and was discovered in 2020 using the radial velocity method.
Is GJ 251 b in the Habitable Zone?
GJ 251 b orbits inside the inner edge of the habitable zone of GJ 251. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of GJ 251: 0.102–0.267 AU (conservative: 0.129–0.253 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on GJ 251 b
The equilibrium temperature of GJ 251 b is about 336 K (63 °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. It receives 2.37 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on GJ 251 b — one full orbit around GJ 251 — lasts 14.2 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.081 AU — closer to its star than Mercury is to the Sun.
How Was GJ 251 b Discovered?
GJ 251 b was discovered in 2020 using the radial velocity method, with observations from W. M. Keck 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 GJ 251 b?
GJ 251 b is 18.2 light-years (5.6 parsecs) from Earth. Light arriving here tonight left the planet around the year 2008. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 320,320 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. GJ 251 b scores 0.63, ranking #110 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: GJ 251
GJ 251
- Surface temperature
- 3,342 K (Sun: 5,772 K)
- Mass
- 0.35 M☉
- Radius
- 0.36 R☉
- Luminosity
- 0.0155 L☉
- Age
- 6.8 billion years (Sun: 4.6)
The GJ 251 Planetary System
GJ 251 b is one of 2 known planets in the GJ 251 system. Its siblings:
- GJ 251 c (Super Earth)
GJ 251 b — Complete Data
| Mass (best estimate) | 3.85 Earth masses |
|---|---|
| Orbital period | 14.24 days |
| Orbital distance | 0.081 AU |
| Equilibrium temperature | 336 K (63 °C) |
| Stellar irradiation | 2.37× Earth |
| Earth Similarity Index | 0.63 |
| Distance from Earth | 18.2 light-years (5.6 parsecs) |
| Constellation | Gemini |
| Discovery method | Radial Velocity |
| Discovery facility | W. M. Keck Observatory |
| Discovery year | 2020 |
Data: NASA Exoplanet Archive, last updated 2025-10-30. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About GJ 251 b
Is GJ 251 b habitable?
No — GJ 251 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is GJ 251 b?
GJ 251 b is about 18 light-years from Earth in the constellation Gemini. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 320,320 years to get there.
How long is a year on GJ 251 b?
One orbit around GJ 251 takes 14.2 Earth days — short enough that 26 of its years would fit into one Earth year.
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