Kepler-391 c
Kepler-391 c 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 c?
Kepler-391 c has a radius of 2.88 times that of Earth. Its mass is 450 times that of Earth, giving it a density of 400.00 g/cm³ — comparable to rocky planets like Earth (5.51 g/cm³).
Is Kepler-391 c in the Habitable Zone?
Kepler-391 c 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 c
The equilibrium temperature of Kepler-391 c is about 662 K (389 °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 179 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on Kepler-391 c — one full orbit around Kepler-391 — lasts 20.5 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.164 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.27).
How Was Kepler-391 c Discovered?
Kepler-391 c 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 c?
Kepler-391 c 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 c scores 0.20, ranking #3,903 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 c is one of 2 known planets in the Kepler-391 system. Its siblings:
- Kepler-391 b (Super Earth)
Kepler-391 c — Complete Data
| Radius | 2.880 Earth radii (0.257 Jupiter radii) |
|---|---|
| Mass | 450.00 Earth masses (1.416 Jupiter masses) |
| Density | 400.00 g/cm³ (Earth: 5.51) |
| Orbital period | 20.50 days |
| Orbital distance | 0.164 AU |
| Eccentricity | 0.270 |
| Equilibrium temperature | 662 K (389 °C) |
| Stellar irradiation | 179.00× 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 c
Is Kepler-391 c habitable?
No — Kepler-391 c orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is Kepler-391 c?
Kepler-391 c 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 c compared to Earth?
Kepler-391 c has 2.88 times the radius of Earth and about 450 times its mass.
How long is a year on Kepler-391 c?
One orbit around Kepler-391 takes 20.5 Earth days — short enough that 18 of its years would fit into one Earth year.
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