Habitable Planet Kepler 186F Could Sustain Life, New Earth Discovered,

Kepler-186f is an exoplanet orbiting the red dwarf Kepler-186, about 500 light-years from the Earth. It is the first planet with a radius similar to Earth’s to be discovered in the habitable zone of another star. NASA’s Kepler spacecraft detected it using the transit method, along with four additional planets orbiting much closer to the star (all modestly larger than Earth). Analysis of three years of data was required to find its signal. The results were presented initially at a conference on 19 March 2014 and some details were reported in the media at the time. The full public announcement was on 17 April 2014, followed by publication in Science.

Kepler-186f orbits a star with about 4% of the Sun’s luminosity with an orbital period of 129.9 days and an orbital radius of about 0.36 or 0.40 times that of Earth’s (compared to 0.39 AU 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 0.22 to 0.40 AU). Kepler-186f receives about 32%, placing it within the conservative zone but near the outer edge, similar to the position of Mars in our Solar System. The stellar flux received by Kepler-186f is similar to that of Gliese 581d.

The only physical property directly derivable from the observations (besides the orbital period) is the ratio of the radius of the planet to that of the central star, which follows from the amount of occultation of stellar light during a transit. This ratio was measured to be 0.021. This yields a planetary radius of 1.11±0.14 times that of Earth, taking into account uncertainty in the star’s diameter and the degree of occultation. Thus, 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)

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Its mass can only be estimated by combining the radius with a density estimate derived from an assumed planetary composition; it could be a rocky terrestrial planet or a lower density ocean planet with a thick atmosphere. However, 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 that would 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 Earth’s (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.

The star hosts four other planets discovered so far, though Kepler-186 b, c, d, and e (in order of increasing orbital radius) are too close to the star, and so too hot, to have liquid water. The four innermost planets are probably tidally locked but Kepler-186f is further out, where the star’s tidal effects are much weaker, so there may not have been enough time for its spin to slow down that much. Because of the very slow evolution of red dwarfs, the age of the Kepler-186 system is poorly constrained, although it is likely to be greater than a few billion years. There is a roughly 50-50 chance it is tidally locked. 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 

As part of the 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. To be detectable, however, such transmissions, if radiated isotropically, would need to be at least 10 times as strong as those from Arecibo Observatory. Given the interstellar distance of 492 light-years, any such signal detectable from Earth in the present would have been emitted prior to 1522.

At nearly 500 light years distant, Kepler-186f is too remote 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 shows 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 upcoming James Webb Space Telescope and future large ground-based telescopes to analyze atmospheres, determine masses and infer compositions

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