Exoplanets · Space Codex
Kepler-186f
Found by Kepler in 2014, this world is only 17% wider than Earth and sits at the cool outer edge of its red dwarf's habitable zone.
First Earth-size planet found in a habitable zone
Artist's concept. No telescope has seen Kepler-186f as more than a point of light, and most exoplanets not even that. What we know comes from how a planet dims or tugs its star. Any picture of its surface, colour or sky, anywhere, is an illustration, not an observation.
At a glance
Key points
- Only 17% wider than Earth
- At the outer edge of the habitable zone
- Too faint for atmosphere studies
How it was found
Kepler stared at one patch of sky in Cygnus and Lyra for four years, logging the brightness of each star every half hour. Kepler-186, a dim red dwarf about half the Sun's width, showed four small planets on orbits of 4 to 22 days. A fifth, much rarer dip every 130 days became Kepler-186f, announced in Science in April 2014 as the first Earth-sized planet found in a habitable zone.
Cold but not hopeless
With about a third of Earth's sunlight, it would need a thick greenhouse atmosphere to keep any water liquid. Early illustrations painted it Earth-like; that was imagination, not data.
A planet on probation
Kepler-186f was never weighed; it was validated statistically, by showing that a planet was far likelier than any false alarm. A 2019 re-examination of Kepler's final catalogue found that its signal no longer reaches the 99% confidence used for that test, once Kepler's own instrumental false alarms are counted. The Exoplanet Archive now marks it controversial. That does not show the planet is false, only that Kepler's data alone cannot settle it.
Habitability is speculation
Even if it is real, nothing is known about its surface. A width of 1.17 Earths suggests rock but does not prove it, and with no mass there is no density. Whether it spins freely or keeps one face to its star is debated. Any talk of oceans or life is a hypothesis, not a finding.
What we still don't know
Its mass, composition and whether it has an atmosphere. The star is too faint for JWST to study the planet's air.
Missions
- 2009–2018
Kepler / K2
NASACompletedTransit survey that showed small planets are common.
Related entries
- Exoplanets: the censusPlanets around other stars
- Kepler-452bAn 'Earth cousin' that may not be real
Sources
- Quintana et al. 2014, Science 344:277 (arXiv) (opens in a new tab)
- NASA Exoplanet Archive: Kepler-186 overview (opens in a new tab)
- Burke et al. 2019, AJ (arXiv) (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.