Orbital period

KEPLER 186F Discovered And Life After Earth - 2016 Documentary - 2 Hours

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• published: 10 Jun 2016
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Kepler-186f is an exoplanet orbiting the red dwarf Kepler-186,[4][5][6] about 490 light-years (151 pc) from the Earth.[1] 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).[5] Analysis of three years of data was required to find its signal.[7] The results were presented initially at a conference on 19 March 2014[8] and some details were reported in the media at the time.[9][10] The public announcement was on 17 April 2014,[2] followed by publication in Science.[1] Orbital parameters relative to habitable zone[edit] Kepler-186f orbits an M-dwarf 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[1] or 0.40[3] 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).[1] 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.[1] Mass, density and composition[edit] 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.[1] This yields a planetary radius of 1.11±0.14 times that of Earth,[2][5] 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). There is a very wide range of possible masses that can be calculated by combining the radius with densities derived from the possible types of matter that planets can be made from; 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 radii of more than 1.5 times that of Earth tend to accumulate the thick atmospheres that would make them less likely to be habitable.[11] 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.[1] 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⊕,[1] taking into account the higher density due to the higher average pressure compared to Earth. Formation, tidal evolution and habitability[edit] 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 are therefore too hot to have liquid water. The four innermost planets are probably tidally locked but Kepler-186f is in a higher orbit, 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.[3] There is an approximately 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 (see Tidal effects on rotation rate, axial tilt and orbit).[12] Artist's concept of a rocky Earth-sized exoplanet in the habitable zone of its host star, possibly compatible with Kepler-186f’s known data (NASA/SETI/JPL) Kepler-186f's axial tilt (obliquity) is likely very small, in which case it wouldn't have tilt-induced seasons as Earth and Mars do. Its orbit is probably close to circular,[12] so it will also lack eccentricity-induced seasonal changes like those of Mars. However, the axial tilt could be larger (about 23 degrees) if another undetected nontransiting planet orbits between it and Kepler-186e; planetary formation simulations have shown that the presence of at least one additional planet in this region is likely. If such a planet exists, it cannot be much more massive than Earth as it would then cause orbital instabilities.[3]