Kepler-186f

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). 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, 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. 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 and a radius of 0.52 . It has a temperature of 3755 K and is about 4 billion years old, and has a temperature of . 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 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 ==

s Kepler-186f's location within the habitable zone does not necessarily mean it is habitable; this is also dependent on its atmospheric characteristics, which are unknown. However, Kepler-186f is too distant for its atmosphere to be analyzed by the most advanced instruments such as the James Webb Space Telescope. A simple climate model – in which the planet's inventory of volatiles is restricted to nitrogen, carbon dioxide and water, and clouds are not accounted for – suggests that the planet's surface temperature would be above if at least 0.5 to 5 bars of CO2 is present in its atmosphere, for assumed N2 partial pressures ranging from 10 bar to zero, respectively. Since it is closer to its star than Earth is to the Sun, it will probably rotate much more slowly than Earth; its day could be weeks or months long (see Tidal effects on rotation rate, axial tilt and orbit). Kepler-186f's axial tilt (obliquity) is likely very small, in which case it would not have tilt-induced seasons like Earth's. Its orbit is probably close to circular, One review essay in 2015 concluded that Kepler-186f, along with the exoplanets Kepler-442b and Kepler-62f, were likely the best candidates for being potentially habitable planets. In June 2018, studies suggest that Kepler-186f may have seasons and a climate similar to those on Earth. == Follow-up studies ==

Target of SETI investigation As part of the SETI Institute's search for extraterrestrial intelligence, the Allen Telescope Array had listened for radio emissions from the Kepler-186 system for about a month as of 17 April 2014. No signals attributable to extraterrestrial technology were found in that interval; however, to be detectable, such transmissions, if radiated in all directions equally and thus not preferentially towards the Earth, would need to be at least 10 times as strong as those from Arecibo Observatory. The more well known SETI @ Home search does not cover any object in the Kepler field of view. Another follow-up survey using the Green Bank Telescope has not reviewed Kepler 186f. Given the interstellar distance of , the signals would have left the planet many years ago. Future technology and observations At approximately distant, Kepler-186f is too far and its star too faint for current telescopes or the next generation of planned telescopes to determine its mass or whether it has an atmosphere. However, the discovery of Kepler-186f demonstrates conclusively that there are other Earth-sized planets in habitable zones. The Kepler spacecraft focused on a single small region of the sky but next-generation planet-hunting space telescopes, such as TESS and CHEOPS, will examine nearby stars throughout the sky. Nearby stars with planets can then be studied by the James Webb Space Telescope and future large ground-based telescopes to analyze atmospheres, determine masses and infer compositions. Additionally the Square Kilometer Array would significantly improve radio observations over the Arecibo Observatory and Green Bank Telescope. == Previous names ==

As the Kepler telescope observational campaign proceeded, an initially identified system was entered in the Kepler Input Catalog (KIC), and then progressed as a candidate host of planets to a Kepler Object of Interest (KOI). Thus, Kepler-186 started as KIC 8120608 and then was identified as KOI-571. Kepler-186f was mentioned when known as KOI-571-05 or KOI-571.05 or using similar nomenclatures in 2013 in various discussions and publications before its full confirmation. == Comparison ==

The nearest-to-Earth-size planet in a habitable zone previously known was Kepler-62f with 1.4 Earth radii. Kepler-186f orbits an M-dwarf star, while Kepler-62f orbits a K-type star. A study of atmospheric evolution in Earth-size planets in habitable zones of G-Stars (a class containing the Sun, but not Kepler-186) suggested that 0.8–1.15 R🜨 is the size range for planets small enough to lose their initial accreted hydrogen envelope but large enough to retain an outgassed secondary atmosphere such as Earth's. == In popular culture ==

• Along with five other exoplanets, Kepler-186f was included in Civilization: Beyond Earths exoplanet DLC as a playable map. • Dutch rock band named their 2017 album Kepler-186f after this exoplanet. • Kepler-186f is the location of a future earth colony in the short story "Stars" by Drew Hayden Taylor. • In season 2 of the 2020 Animaniacs reboot, Kepler 186f, alongside 51 Pegasi b & the fictional WB-1, is referenced in the song "Yakko's Big Idea" • In season 12 of The Big Bang Theory episode "The Conjugal Configuration", Kepler 186F is shown on a poster in Neil deGrasse Tyson's office. == See also ==