The first Earth-sized planet found in another star's habitable zone
Kepler-186f
Orbits Kepler-186 · 580 light-years · discovered 2014 (transit)
Kepler-186f orbits a red dwarf about 580 light-years away in the constellation Cygnus. When its discovery was announced in 2014, it became the first planet close to Earth's size confirmed in the habitable zone of another star — a milestone for NASA's Kepler mission.
It is only slightly larger than Earth, but it sits at the cool outer edge of its star's habitable zone, receiving around a third of the energy Earth gets from the Sun. Its mass, atmosphere and surface are all unknown, and it is too distant for current telescopes to study in detail.
Key facts
- Star
- Kepler-186M1V red dwarf
- Distance
- 580 light-years
- Radius
- 1.17 × Earth
- Mass
- Not measured
- Orbital period
- 129.9 days
- Equilibrium temperature
- 177 K (-96 °C)Before any greenhouse warming
- Starlight received
- 0.30× Earth's
- Surface gravity
- ≈ 1.31 gRough estimate (rocky composition assumed)
- Discovered
- 2014Transit
- Star
- Kepler-186 (M1V red dwarf)
- Distance
- About 580 light-years
- Discovered
- 2014, transit (Kepler)
- Radius
- About 1.17 Earth radii
- Mass
- Not measured — estimated at roughly 1.4–1.7 Earth masses if rocky
- Orbital period
- About 130 days
- Starlight received
- Roughly 30% of what Earth gets from the Sun
- Equilibrium temperature
- Roughly 180 K (about −93 °C), a simple estimate that ignores any atmosphere
- Constellation
- Cygnus
Measured values follow the NASA Exoplanet Archive and discovery papers; many carry large uncertainties. The 3D globe and walkable surface on this page are an artist's impression generated from these numbers.
What we know
Kepler-186f was detected from the slight dimming of its star each time it passes in front of it. The transits give its size — about 17% wider than Earth, following revisions to the star's properties — and its 130-day orbit.
Its mass has never been measured: the star is faint and the planet's pull on it is tiny. A planet this size is likely to be rocky, but it could also hold a substantial amount of water or a thicker gas layer.
Its star
Kepler-186 is a red dwarf with roughly half the Sun's mass and size. It hosts at least five planets, all of similar size. The four inner ones orbit within a few weeks of the star and are far too hot for liquid water, while Kepler-186f orbits well beyond them.
Could it be habitable?
Kepler-186f orbits near the outer edge of the habitable zone, where it receives less energy than Mars does from the Sun. To have liquid water on its surface, it would need an atmosphere with a stronger greenhouse effect than Earth's — perhaps one rich in carbon dioxide.
There is no information about whether such an atmosphere exists. Like other planets around red dwarfs, it may also have faced strong stellar activity early in its history. It is fairer to call Kepler-186f a world of Earth-like size in a potentially habitable orbit than an Earth-like planet.
Orbit and climate
Kepler-186f orbits roughly 0.43 astronomical units from its star — a little farther than Mercury is from the Sun. Because the star is cooler and dimmer, that distance makes it relatively cold. Unlike planets on very tight orbits, it may not be fully tidally locked, so it could have a day–night cycle, though its rotation is unknown.
Discovery
A team led by Elisa Quintana of NASA's Ames Research Center announced Kepler-186f in April 2014, in the journal Science. The planet's existence was validated statistically and with follow-up imaging from large ground-based telescopes, including Keck and Gemini, which ruled out most alternative explanations for the signal.
The Kepler mission
NASA's Kepler space telescope monitored around 150,000 stars from 2009 to 2013 for tiny dips in brightness, revealing thousands of planets before the spacecraft was retired in 2018.
Future observations
At around 580 light-years, Kepler-186f is far too distant and faint for JWST to analyse its atmosphere in any meaningful detail. Its main legacy is statistical: it showed that Earth-sized planets can exist in habitable-zone orbits, which shapes the design of future missions to find and study closer examples.
About the 3D impression
Artist's impression — the real surface has never been seen. The globe and the surface you can walk on are generated from what is measured — size, the starlight it receives and the kind of star it orbits — using the same procedural engine as our imaginary worlds. Seas, ice, clouds and any plant-like colours are possibilities, not observations: around a red dwarf the daylight is redder, so the scene uses warmer skies and darker vegetation.
Earth-like exoplanets compared
| Planet | Distance | Radius | Year | Sunlight |
|---|---|---|---|---|
| Proxima Centauri b | 4.2 ly | — | 11.2 days | 0.65× Earth |
| Ross 128 b | 11.0 ly | — | 9.87 days | 1.38× Earth |
| Teegarden's Star b | 12.5 ly | — | 4.91 days | 1.08× Earth |
| Gliese 667 Cc | 23.6 ly | — | 28.1 days | 0.88× Earth |
| TRAPPIST-1d | 40.5 ly | 0.79 R⊕ | 4.05 days | 1.11× Earth |
| TRAPPIST-1e | 40.5 ly | 0.92 R⊕ | 6.10 days | 0.65× Earth |
| TRAPPIST-1f | 40.5 ly | 1.04 R⊕ | 9.21 days | 0.37× Earth |
| TRAPPIST-1g | 40.5 ly | 1.13 R⊕ | 12.4 days | 0.25× Earth |
| LHS 1140 b | 48.9 ly | 1.73 R⊕ | 24.7 days | 0.43× Earth |
| TOI-700 d | 101 ly | 1.07 R⊕ | 37.4 days | 0.87× Earth |
| Kepler-186f | 580 ly | 1.17 R⊕ | 129.9 days | 0.30× Earth |
| Kepler-22b | 635 ly | 2.10 R⊕ | 289.9 days | 1.01× Earth |
| Kepler-62f | 980 ly | 1.41 R⊕ | 267.3 days | 0.41× Earth |
| Kepler-442b | 1,194 ly | 1.34 R⊕ | 112.3 days | 0.70× Earth |
| Kepler-452b | 1,800 ly | 1.63 R⊕ | 384.8 days | 1.10× Earth |
Surprising facts about Kepler-186f
- Kepler-186f gets less energy from its star than Mars receives from the Sun, even though it orbits only slightly farther out than Mercury.
- The SETI Institute listened for radio signals from the Kepler-186 system in 2014 and found nothing.
- Light reaching us from Kepler-186f today left the planet during the late European Middle Ages.
- Its star's light is redder than sunlight, so at noon the planet's sky would be far dimmer to human eyes than a sunny day on Earth.