Kepler-186f

Kepler-186f was discovered using the transit method by the Kepler space telescope, which monitored the brightness of over 150,000 stars in the Cygnus–Lyra region. The planet's periodic dimming of the host star was first detected in data from the mission's first four quarters. The discovery was announced in April 2014, led by Elisa Quintana of the SETI Institute and NASA Ames Research Center.¹ The team employed sophisticated statistical validation, ruling out false positives with high confidence, and the result was published in Science.¹

Kepler-186f is about 10% larger than Earth, with a radius of approximately 1.17 R⊕, making it the smallest exoplanet confirmed to orbit in another star's habitable zone.¹ Its mass is not precisely known but is estimated between 0.32 and 3.77 Earth masses, depending on its composition.² The planet receives about 32% of the stellar flux that Earth receives from the Sun, placing it near the outer edge of the habitable zone.³ It orbits a dim M1-type red dwarf star that is about half the Sun's size and mass, with an orbital period of 129.9 days.² The atmosphere and surface conditions remain unknown, but climate models suggest that if its atmosphere is similar to Earth's, surface temperatures could allow liquid water.

Kepler-186f holds special importance because it lies within the conservative habitable zone—the region where a rocky planet could maintain liquid water on its surface—of a low-mass star. However, the zone for red dwarfs is narrower and closer to the star, and planets there may be tidally locked, with one hemisphere permanently facing the star.⁴ This could influence atmospheric circulation and potential habitability. Studies have examined the planet's orbital stability and climate, suggesting that despite tidal locking, surface temperatures could still be moderate over much of the globe if oceans and atmospheres exist. The discovery of a similarly sized planet in such a zone around a red dwarf inspired confidence that Earth-like planets are common, as red dwarfs are the most numerous stars in the galaxy.³

Beyond its headline discovery, Kepler-186f has several subtle aspects. Its host star, Kepler-186, is actually part of a five-planet system, with Kepler-186b through e orbiting closer than the habitable zone, all likely too hot for life. The planet was identified in data from only the first six quarters of Kepler observations—a testament to the mission's sensitivity. Researchers have also used the habitable zone definition that factors in cloud and CO2 feedback, placing Kepler-186f in a region where an Earth-like atmosphere would keep water frozen unless greenhouse gases were present. There is a chance that the planet could be a “water world” if its radius is dominated by a thick ice layer, altering its classification. Furthermore, because of its host's red dwarf output, the planet experiences no visible “sunset” as we know it; the star would appear as a large, dim orange-red disk in its sky.⁴ Kepler-186f continues to be a primary target for future atmospheric characterization missions like the James Webb Space Telescope, though direct observation remains beyond current capabilities.⁵

Habitable zone

The region around a star where a rocky planet could maintain liquid water on its surface.

Red dwarf

A small, cool, low-mass star of spectral type M.

Transit method

A technique for detecting exoplanets by measuring periodic dips in a star's brightness.

Tidal locking

A gravitational effect causing one side of a planet to always face its host star.