# Kepler-186f
**Kepler-186f** is an exoplanet about 500 light-years from Earth in the constellation Cygnus, announced on April 17, 2014 as the first Earth-size planet found orbiting within the habitable zone of another star, the range of distances where liquid water could pool on a planet's surface.[1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/) It is the outermost of five roughly Earth-sized planets that transit the M dwarf (red dwarf) star [Kepler-186](https://www.edgepedia.org/kepler-186), and it was detected by NASA's Kepler space telescope through the small dips in starlight caused when a planet crosses its star's disk.[2](https://pubmed.ncbi.nlm.nih.gov/24744370/) • [3](https://www.nature.com/articles/508434e)
| Key fact | Detail |
|---|---|
| Radius | 1.11 ± 0.14 Earth radii (discovery paper); 1.17 Earth radii in the NASA Exoplanet Archive[2](https://pubmed.ncbi.nlm.nih.gov/24744370/) • [4](https://exoplanetarchive.ipac.caltech.edu/overview/Kepler-186%20f) |
| Orbit | 129.9-day period at 0.432 AU from its star; eccentricity 0.04[5](https://science.nasa.gov/exoplanet-catalog/kepler-186-f/) |
| Host star | M dwarf of 0.47 ± 0.05 solar radii, about half the size and mass of the Sun[2](https://pubmed.ncbi.nlm.nih.gov/24744370/) • [1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/) |
| Distance | About 500 light-years (NASA release); 177.594 ± 0.774 parsecs by Gaia parallax[1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/) • [4](https://exoplanetarchive.ipac.caltech.edu/overview/Kepler-186%20f) |
| Energy received | About one-third of what Earth receives from the Sun, placing it near the outer edge of the habitable zone[1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/) |
| Mass | 1.71 Earths (catalog estimate); the mass is not directly measured[5](https://science.nasa.gov/exoplanet-catalog/kepler-186-f/) • [1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/) |
| Discovery | Transit method, Kepler mission; announced 2014 in *Science* 344, 277[2](https://pubmed.ncbi.nlm.nih.gov/24744370/) |
## Discovery
The planet was found by a team led by Elisa Quintana, a research scientist at the [SETI Institute](https://www.edgepedia.org/seti-institute) working at NASA's Ames Research Center, using data from the [Kepler space telescope](https://www.edgepedia.org/kepler-space-telescope), which continuously measured the brightness of more than 150,000 stars.[1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/) • [3](https://www.nature.com/articles/508434e) The discovery paper, published in *Science* on April 18, 2014, reported a planet of 1.11 ± 0.14 Earth radii as the outermost of five planets transiting a star of 0.47 ± 0.05 solar radii.[2](https://pubmed.ncbi.nlm.nih.gov/24744370/)
Before this find, planets known in habitable zones were all at least 40 percent larger than Earth, which made their composition hard to interpret; Kepler-186f, at close to Earth's size, was described by NASA as more reminiscent of Earth.[1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/) Paul Hertz, NASA's Astrophysics Division director, called the discovery a significant step toward finding worlds like our own.[1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/)
## The Kepler-186 system
The system holds five confirmed planets. The four inner companions, Kepler-186b, c, d, and e, orbit every 4, 7, 13, and 22 days respectively, all measuring less than 1.5 times Earth's size and all too hot for life as we know it.[1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/) The Exoplanet Archive lists their periods as 3.89, 7.27, 13.34, and 22.41 days, with radii between 1.07 and 1.4 Earth radii.[4](https://exoplanetarchive.ipac.caltech.edu/overview/Kepler-186%20f)
Kepler-186f itself orbits at 0.432 AU with a 129.9-day period and a catalog eccentricity of 0.04.[5](https://science.nasa.gov/exoplanet-catalog/kepler-186-f/) At high noon on its surface, its star would appear only as bright as our Sun does about an hour before sunset.[1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/)
## Habitability
The intensity and spectrum of the star's radiation place Kepler-186f in the stellar habitable zone, meaning that if the planet has an Earth-like atmosphere and water at its surface, some of that water is likely to be liquid.[6](https://arxiv.org/abs/1404.5667) Being in the habitable zone does not by itself establish habitability; as co-author Thomas Barclay of the Bay Area Environmental Research Institute put it, the surface temperature depends strongly on what kind of atmosphere the planet has, and he described Kepler-186f as an Earth-cousin rather than an Earth-twin.[1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/)
**Composition.** The planet's mass and composition are not directly measured; at roughly 500 light-years it is too distant for the radial-velocity wobble measurements that would give a density.[1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/) • [7](https://time.com/66649/almost-earth-a-newly-discovered-planet-could-be-a-lot-like-ours/) Previous research suggests a planet of this size is likely to be rocky, since planets below about 1.5 Earth radii are more likely rocky than gaseous.[1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/) • [8](https://science.nasa.gov/wp-content/uploads/2023/05/4a.20181010_AstrobiologyStrategyfortheS4LintheUniverse.pdf)
**Climate modeling.** A 1-D climate study by Emeline Bolmont of the University of Bordeaux and colleagues found that the planet's surface temperature could be raised above 273 K with 0.5 to 5 bars of carbon dioxide, depending on how much nitrogen is present, making Kepler-186f a case study of an Earth-sized planet in the cooler regions of a cool star's habitable zone.[9](https://arxiv.org/abs/1404.4368) The same study found that tides have likely driven the four inner planets to near-synchronous rotation with very low obliquities, while Kepler-186f's tidal evolution is slow enough that its current spin state depends on the planet's dissipation rate and the star's age; it could be pseudo-synchronous or still evolving, with obliquity potentially as high as about 85 degrees if the system is young or dissipation is weak.[9](https://arxiv.org/abs/1404.4368)
**Stellar environment.** The host is an M dwarf, a class that makes up 70 percent of the stars in the [Milky Way](https://www.edgepedia.org/milky-way), and Quintana noted that the first signs of other life in the galaxy may well come from planets orbiting such stars.[1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/) M dwarfs can emit high radiation levels that affect planetary atmospheres, a consideration for any habitability assessment of planets around them.[10](https://time.com/5440210/kepler-exoplanets/)
## Validation status
Kepler-186f was confirmed statistically rather than by radial-velocity mass measurement, a standard approach for small, long-period Kepler planets whose signals are beyond Doppler surveys.[11](https://www.nasa.gov/wp-content/uploads/2015/07/ms-r1b.pdf) A 2019 re-analysis by Christopher J. Burke of NASA Ames and colleagues, using the final Data Release 25 pipeline statistics, found that the multi-planet prior enhancement for Kepler-186f is suppressed relative to earlier catalogs and that the planet no longer achieves 99 percent confidence in the planetary hypothesis; its detection parameters fall in a region contaminated by systematic false alarms.[12](https://arxiv.org/abs/1901.00506) The NASA Exoplanet Archive accordingly flags the planet as "Confirmed Controversial" while retaining its confirmed disposition, and notes that independent photometric detection or radial-velocity measurements could eliminate the false-alarm hypothesis.[4](https://exoplanetarchive.ipac.caltech.edu/overview/Kepler-186%20f) • [12](https://arxiv.org/abs/1901.00506)
## Follow-up prospects
Because Kepler-186f orbits a faint star hundreds of parsecs away, atmospheric characterization is difficult. A 2020 study of atmospheric characterization prospects placed Kepler-186f among planets with very low expected transmission-spectroscopy signal relative to brighter targets such as [TRAPPIST-1](https://www.edgepedia.org/trappist-1) b.[13](https://www.aanda.org/articles/aa/pdf/2020/10/aa38616-20.pdf) NASA's Astrobiology Strategy notes that Kepler's habitable-zone planets, including Kepler-186f, are too distant for the follow-up needed to assess their habitability, and that nearer discoveries such as TRAPPIST-1 and [LHS 1140](https://www.edgepedia.org/lhs-1140) provide more practical targets for the [James Webb Space Telescope](https://www.edgepedia.org/james-webb-space-telescope).[8](https://science.nasa.gov/wp-content/uploads/2023/05/4a.20181010_AstrobiologyStrategyfortheS4LintheUniverse.pdf)
## Significance
The discovery confirmed that planets the size of Earth exist in the habitable zones of stars other than the Sun.[1](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/) Later Kepler statistics project that roughly half of Sun-like stars may host Earth- and super-Earth-sized planets in their conservative habitable zones, a rate estimated from the same dataset that produced Kepler-186f.[8](https://science.nasa.gov/wp-content/uploads/2023/05/4a.20181010_AstrobiologyStrategyfortheS4LintheUniverse.pdf)
## References
1. [NASA's Kepler Telescope Discovers First Earth-Size Planet in 'Habitable Zone', NASA, April 17, 2014](https://www.nasa.gov/news-release/nasas-kepler-telescope-discovers-first-earth-size-planet-in-habitable-zone/)
2. [Quintana, E. V. et al. (2014). An Earth-sized planet in the habitable zone of a cool star. Science 344, 277–280.](https://pubmed.ncbi.nlm.nih.gov/24744370/)
3. [Earth twin spotted in habitable zone, Nature news](https://www.nature.com/articles/508434e)
4. [Kepler-186 overview, NASA Exoplanet Archive](https://exoplanetarchive.ipac.caltech.edu/overview/Kepler-186%20f)
5. [Kepler-186 f, NASA Science Exoplanet Catalog](https://science.nasa.gov/exoplanet-catalog/kepler-186-f/)
6. [Quintana, E. V. et al. (2014). An Earth-sized Planet in the Habitable Zone of a Cool Star, arXiv:1404.5667](https://arxiv.org/abs/1404.5667)
7. [Almost Earth: A Newly Discovered Planet Could Be a Lot Like Ours, TIME](https://time.com/66649/almost-earth-a-newly-discovered-planet-could-be-a-lot-like-ours/)
8. [NASA Astrobiology Strategy (2018)](https://science.nasa.gov/wp-content/uploads/2023/05/4a.20181010_AstrobiologyStrategyfortheS4LintheUniverse.pdf)
9. [Bolmont, E. et al. (2014). Formation, tidal evolution and habitability of the Kepler-186 system, arXiv:1404.4368](https://arxiv.org/abs/1404.4368)
10. [The Kepler Space Telescope Found Science and Art in Exoplanets, TIME](https://time.com/5440210/kepler-exoplanets/)
11. [Discovery and validation of Kepler-452b, Astronomical Journal (NASA-hosted PDF)](https://www.nasa.gov/wp-content/uploads/2015/07/ms-r1b.pdf)
12. [Burke, C. J. et al. (2019). Re-Evaluating Small Long-Period Confirmed Planets From Kepler, arXiv:1901.00506](https://arxiv.org/abs/1901.00506)
13. [A&A 642, A49 (2020): atmospheric characterization prospects for small planets](https://www.aanda.org/articles/aa/pdf/2020/10/aa38616-20.pdf)
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*Topic: Encyclopedia › Physical world and mathematics › Astronomy › Cosmology and observation › Observational techniques: astrometry, photometry, spectroscopy*
*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*
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