About the image. Drawn as a rocky world with seas under a red dwarf's warm light. Only its size and orbit are measured: its mass, atmosphere and surface are unknown, and the seas are imagined.
1.2×as wide as our Sun looks from Earth
26%of the light Earth gets
The sky from Kepler-186f
Kepler-186 would look 1.2× as wide as the Sun does from Earth, and the planet gets 26% of the light Earth gets from the Sun.
Its colour comes from its surface temperature of 3 755 K (3 480 °C); the Sun's is about 5 772 K (5 500 °C).
How fast Kepler-186f spins is unknown, so the length of its day is too.
Apparent size = star radius ÷ orbit; light = size² × (temperature ÷ 5 772 K)⁴. Both from the values on this page. The landscape is imagined.
On 17 April 2014 a team led by Elisa Quintana announced a planet barely larger than Earth orbiting in the habitable zone of another star, the range of distances where liquid water could exist on the surface. It was the first Earth-sized planet ever found there. The previous closest match, Kepler-62f, was 40 percent wider than Earth. It took three years of Kepler data to pick out its small, repeated dip in starlight.
A red dwarf’s outer planet
Kepler-186 is a red dwarf about half the Sun’s mass, with five known planets. Kepler-186f is the outermost. Its year lasts 130 days, and it receives only about a third of the light Earth gets from the Sun, which puts it near the cold outer edge of the habitable zone. Climate models suggest it could stay above freezing if it has an atmosphere with between half a bar and five bars of carbon dioxide. Whether it has any atmosphere at all is unknown, and it is too far away for even the James Webb Space Telescope to tell.
Its mass has never been measured. Depending on what it is made of, it could weigh anywhere from a third of Earth to nearly four times as much.
Four hotter neighbours
The other four planets in the system are all too hot for life as we know it. Kepler-186b, c, d and e circle the star every 4, 7, 13 and 22 days, and each is less than 1.5 times the size of Earth. NASA’s announcement placed the system about 500 light-years away in the constellation Cygnus; the current estimate is about 580. Kepler scientist Thomas Barclay described Kepler-186f as “an Earth-cousin rather than an Earth-twin”.
Seasons and tilt
Nobody knows how far Kepler-186f is tilted or how fast it spins, but simulations suggest that a small tilt could stay steady for a long time. In 2018 Yutong Shan and Gongjie Li modelled how the spin axes of Kepler-186f and Kepler-62f would change over time. For a 24-hour day, tilts below about 40 degrees stayed stable. In their models, slight tilts between the planets’ orbits, of around 3 degrees, could still make the axis swing by up to about 20 degrees, and an undetected companion or a moon could unsettle it further.
Real or not?
Kepler-186f was confirmed statistically, not by measuring the pull it exerts on its star. Later re-analyses of the Kepler data put the chance that the signal is a false alarm at 4 percent in 2019 and 20 percent in 2025, enough for some researchers to treat it as a candidate again. In 2014 the Allen Telescope Array listened to the system for signs of technology and heard nothing.
Its orbit, to scale
Kepler-186f goes around once every 130 days, at 0.432 AU from its star. Dashed circles show Mercury around the Sun, on the same scale.
Kepler-186f: equilibrium temperature. Logarithmic scale from 100 K to 10 000 K. Reference values: NASA, IAU.
How long to get there?
Kepler-186f is about 580 light-years away.
Light299 792 km/s580 years
Breakthrough Starshotplanned light sail, 20% of light speed2 900 years
Parker Solar Probefastest spacecraft so far, 192 km/s905 622 years
Voyager 1leaving the Solar System, 17 km/s10.2 million years
At Voyager 1's speed the trip would take about 127 852 human lifetimes of 80 years. Bars are on a logarithmic scale. Speeds: NASA, Breakthrough Initiatives.
Gallery
A rocky planet with patches of cloud, lit from the left by a large, dim red star, an impression of Kepler-186f. AI-generated painting.A small orange sun reflected in calm dark water, with rocks along the shore, an impression of a shore on Kepler-186f. AI-generated painting.Kepler-186f, 3D artist's impression, rendered from the interactive model.Kepler-186f, 3D artist's impression, from another angle, rendered from the interactive model.Kepler-186f, 3D artist's impression, close-up, rendered from the interactive model.
Timeline
2014Announced on 17 April as the first Earth-sized planet in a habitable zone.
2014The Allen Telescope Array listens for signals and hears none.
2019A re-analysis puts the chance of a false alarm at 4%.
2025Another re-analysis puts it at 20%.
Questions people ask
Could Kepler-186f have life?
Nobody knows. It is Earth-sized and in the habitable zone, but its mass, atmosphere and surface are all unknown.
How big is Kepler-186f?
About 17% wider than Earth. Its mass has never been measured.
Is Kepler-186f tidally locked?
Possibly. It orbits close to a dim star, where tidal locking is common, but its rotation has never been measured.