Kepler-391 b
Kepler-391 b is a super-Earth orbiting the K2 III-IV star Kepler-391 in the constellation Cygnus. It lies about 2,835 light-years from Earth and was discovered in 2014 using the transit method.
How Big Is Kepler-391 b?
Kepler-391 b has a radius of 2.50 times that of Earth. Its mass is 1,100 times that of Earth, giving it a density of 380.00 g/cm³ — comparable to rocky planets like Earth (5.51 g/cm³).
Is Kepler-391 b in the Habitable Zone?
Kepler-391 b orbits inside the inner edge of the habitable zone of Kepler-391. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of Kepler-391: 1.677–4.048 AU (conservative: 2.124–3.838 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on Kepler-391 b
The equilibrium temperature of Kepler-391 b is about 930 K (657 °C) — hot enough to melt many metals. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C. It receives 690 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on Kepler-391 b — one full orbit around Kepler-391 — lasts 7.42 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.083 AU — closer to its star than Mercury is to the Sun. Its orbit is highly elliptical (eccentricity 0.46), swinging dramatically closer to and farther from its star.
How Was Kepler-391 b Discovered?
Kepler-391 b was discovered in 2014 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-391 b?
Kepler-391 b is 2,835.1 light-years (869.2 parsecs) from Earth. Light arriving here tonight left the planet about 2,835 years ago. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 49,897,760 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-391 b scores 0.20, ranking #3,845 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: Kepler-391
Kepler-391
- Spectral type
- K2 III-IV
- Surface temperature
- 5,038 K (Sun: 5,772 K)
- Mass
- 1.27 M☉
- Radius
- 2.88 R☉
- Luminosity
- 4.6000 L☉
- Age
- 4.4 billion years (Sun: 4.6)
The Kepler-391 Planetary System
Kepler-391 b is one of 2 known planets in the Kepler-391 system. Its siblings:
- Kepler-391 c (Super Earth)
Kepler-391 b — Complete Data
| Radius | 2.500 Earth radii (0.223 Jupiter radii) |
|---|---|
| Mass | 1,100.00 Earth masses (3.461 Jupiter masses) |
| Density | 380.00 g/cm³ (Earth: 5.51) |
| Orbital period | 7.42 days |
| Orbital distance | 0.083 AU |
| Eccentricity | 0.460 |
| Equilibrium temperature | 930 K (657 °C) |
| Stellar irradiation | 690.00× Earth |
| Earth Similarity Index | 0.20 |
| Distance from Earth | 2,835.1 light-years (869.2 parsecs) |
| Constellation | Cygnus |
| Discovery method | Transit |
| Discovery facility | Kepler |
| Discovery year | 2014 |
Data: NASA Exoplanet Archive, last updated 2025-09-17. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About Kepler-391 b
Is Kepler-391 b habitable?
No — Kepler-391 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is Kepler-391 b?
Kepler-391 b is about 2,835 light-years from Earth in the constellation Cygnus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 49,897,760 years to get there.
How big is Kepler-391 b compared to Earth?
Kepler-391 b has 2.50 times the radius of Earth and about 1,100 times its mass.
How long is a year on Kepler-391 b?
One orbit around Kepler-391 takes 7.4 Earth days — short enough that 49 of its years would fit into one Earth year.
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