Kepler-186f

Kepler-186f (also known by its Kepler object of interest designation KOI-571.05) is a candidate Earth-sized exoplanet orbiting within the habitable zone of the red dwarf star Kepler-186, the outermost of five planets discovered around the star by NASA's Kepler space telescope. It is located about 580 light-years (180 parsecs) from Earth in the constellation of Cygnus. Kepler-186f orbits its star at a distance of about 0.43 AU (64,000,000 km; 40,000,000 mi) from its host star with an orbital period of roughly 130 days, and a radius around 1.17 times that of Earth. As one of the more promising candidates for habitability, it was the first planet with a radius similar to Earth's to be discovered in the habitable zone of another star. However, key components still need to be found to determine its habitability for life, including an atmosphere, and its composition and if liquid water can exist on its surface.

Discovery and follow-up studies Analysis of three years of data was required to find its signal. NASA's Kepler space telescope detected it using the transit method (in which the dimming effect that a planet causes as it crosses in front of its star is measured), along with four additional planets orbiting much closer to the star (all modestly larger than Earth). The results were presented initially at a conference on 19 March 2014 and some details were reported in the media at the time. The planet was announced on 17 April 2014, simultaneously with publication of a scientific paper in Science. Some follow-up studies have indicated that Kepler-186f (like Kepler-452b) may still fall below the statistical threshold for confirmation, and so should still be considered a planet candidate. The false positive probability was estimated to be 4% by a 2019 study and 20% by a 2025 study.

Physical characteristics

Mass, radius and temperature The only physical property directly derivable from the observations (besides the orbit) is the size of the planet relative to the central star, which follows from the amount of occultation of stellar light during a transit. This ratio was measured to be 0.021, giving a planetary radius of 1.17 ± 0.08 times that of Earth. The planet is about 11% larger in radius than Earth (between 4.5% smaller and 26.5% larger), giving a volume about 1.37 times that of Earth (between 0.87 and 2.03 times as large). A very wide range of possible masses can be calculated by combining the radius with densities derived from the possible types of matter from which planets can be made. For example, it could be a rocky terrestrial planet or a lower density ocean planet with a thick atmosphere. A massive hydrogen/helium (H/He) atmosphere is thought to be unlikely in a planet with a radius below 1.5 R🜨. Planets with a radius of more than 1.5 times that of Earth tend to accumulate the thick atmospheres which make them less likely to be habitable. Red dwarfs emit a much stronger extreme ultraviolet (XUV) flux when young than later in life. The planet's primordial atmosphere would have been subjected to elevated photoevaporation during that period, which would probably have largely removed any H/He-rich envelope through hydrodynamic mass loss. Mass estimates range from 0.32 M🜨 for a pure water/ice composition to 3.77 M🜨 if made up entirely of iron (both implausible extremes). For a body with radius 1.11 R🜨, a composition similar to that of Earth (i.e., 1/3 iron, 2/3 silicate rock) yields a mass of 1.44 M🜨, taking into account the higher density due to the higher average pressure compared to Earth. That would make the force of gravity on the surface 17% higher than on Earth. The estimated equilibrium temperature for Kepler-186f, which is the surface temperature without an atmosphere, is said to be around 188 K (−85 °C; −121 °F), somewhat colder than the equilibrium temperature of Mars.

Host star

The planet orbits Kepler-186, an M-type red dwarf star which has a total of five known planets. The star has a mass of 0.54 M☉ and a radius of 0.52 R☉. It has a temperature of 3755 K and is about 4 billion years old, about 600 million years younger than the Sun, which is 4.6 billion years old and has a temperature of 5,778 K (5,505 °C; 9,941 °F). The star's apparent magnitude, or how bright it appears from Earth's perspective, is 14.62. This is too dim to be seen with the naked eye, which can only see objects with a magnitude up to at least 6.5–7 or lower.

Orbit Kepler-186f orbits its star with about 5% of the Sun's luminosity with an orbital period of 129.9 days and an orbital radius of about 0.40 times that of Earth's (compared to 0.39 AU (58 million km; 36 million mi) for Mercury). The habitable zone for this system is estimated conservatively to extend over distances receiving from 88% to 25% of Earth's illumination (from 0.23 to 0.46 AU (34 to 69 million km; 21 to 43 million mi)). Kepler-186f receives about 32%, placing it within the conservative zone but near the outer edge, similar to the position of Mars in the Solar System.

Habitability

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