Kepler-51 b
Kepler-51 b is a cold gas giant orbiting Kepler-51 in the constellation Cygnus. It lies about 2,557 light-years from Earth and was discovered in 2012 using the transit method.
How Big Is Kepler-51 b?
Kepler-51 b has a radius of 7.10 times that of Earth, or 0.63 times the radius of Jupiter. Its mass is 2.1 times that of Earth, giving it a density of 0.03 g/cm³ — closer to gas planets like Jupiter (1.33 g/cm³).
Is Kepler-51 b in the Habitable Zone?
Kepler-51 b orbits inside the inner edge of the habitable zone of Kepler-51. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of Kepler-51: 0.634–1.497 AU (conservative: 0.802–1.419 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 Kepler-51 b
The equilibrium temperature of Kepler-51 b is about 543 K (270 °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 Kepler-51 b — one full orbit around Kepler-51 — lasts 45.2 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.251 AU — closer to its star than Mercury is to the Sun. Its orbit is nearly circular (eccentricity 0.040).
How Was Kepler-51 b Discovered?
Kepler-51 b was discovered in 2012 using the transit method, with observations from Kepler.
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 Kepler-51 b?
Kepler-51 b is 2,556.5 light-years (783.8 parsecs) from Earth. Light arriving here tonight left the planet about 2,557 years ago. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 44,994,400 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. Kepler-51 b scores 0.20, ranking #3,854 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: Kepler-51
Kepler-51
- Surface temperature
- 5,674 K (Sun: 5,772 K)
- Mass
- 0.96 M☉
- Radius
- 0.87 R☉
- Age
- 0.7 billion years (Sun: 4.6)
The Kepler-51 Planetary System
Kepler-51 b is one of 4 known planets in the Kepler-51 system. Its siblings:
- Kepler-51 c (Cold Gas Giant)
- Kepler-51 d (Cold Gas Giant)
- Kepler-51 e (Super Earth)
Kepler-51 b — Complete Data
| Radius | 7.100 Earth radii (0.633 Jupiter radii) |
|---|---|
| Mass | 2.10 Earth masses (0.007 Jupiter masses) |
| Density | 0.03 g/cm³ (Earth: 5.51) |
| Orbital period | 45.15 days |
| Orbital distance | 0.251 AU |
| Eccentricity | 0.040 |
| Equilibrium temperature | 543 K (270 °C) |
| Earth Similarity Index | 0.20 |
| Distance from Earth | 2,556.5 light-years (783.8 parsecs) |
| Constellation | Cygnus |
| Discovery method | Transit |
| Discovery facility | Kepler |
| Discovery year | 2012 |
Data: NASA Exoplanet Archive, last updated 2014-05-14. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About Kepler-51 b
Is Kepler-51 b habitable?
No — Kepler-51 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is Kepler-51 b?
Kepler-51 b is about 2,557 light-years from Earth in the constellation Cygnus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 44,994,400 years to get there.
How big is Kepler-51 b compared to Earth?
Kepler-51 b has 7.10 times the radius of Earth and about 2.1 times its mass.
How long is a year on Kepler-51 b?
One orbit around Kepler-51 takes 45.2 Earth days.
Explore Kepler-51 b in the app
Browse, filter and compare 6,000+ exoplanets on iPhone, iPad and Apple Watch — with habitable-zone views, widgets and offline data.
