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Kepler-11f
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Kepler-11f is an exoplanet (extrasolar planet) discovered in the orbit of the sun-like star Kepler-11 by NASA's Kepler spacecraft, which searches for planets that transit (cross in front of) their host stars. Kepler-11f is the fifth planet from its star, orbiting one quarter of the distance (.25 AU) of the Earth from the Sun every 47 days. It is the furthest of the first five planets in the system. Kepler-11f is the least massive of Kepler-11's six planets, at nearly twice the mass of Earth; it is about 2.6 times the radius of Earth. Along with planets ''d'' and ''e'' and unlike the two inner planets in the system, Kepler-11f has a density lower than that of water and comparable to that of Saturn. This suggests that Kepler-11f has a significant hydrogen–helium atmosphere.〔 The Kepler-11 planets constitute the first system discovered with more than three transiting planets.〔 Kepler-11f was announced to the public on February 2, 2011 after follow-up investigations at several observatories.〔 Analysis of the planets and study results were published the next day in the journal ''Nature''.〔
==Name and discovery==
Kepler-11, known as KOI-157 when it was first flagged for a transit event,〔 is the planet's host star, and it is included in the planet's name to denote that. Because Kepler-11f was discovered with five other planets, the planets of Kepler-11 were sorted by distance from the host star; thus, since Kepler-11f is the fifth planet from its star, it was given the letter "f." The name "Kepler" is derived from the Kepler satellite, a NASA Earth-trailing spacecraft that constantly observes a small patch of sky between the constellations Cygnus and Lyra for stars that are transited by, in particular, terrestrial planets. As these planets cross in front of their host stars with respect to Earth, a small and periodic dip in the star's brightness occurs; this dip is noted by the spacecraft and tagged for future study. Scientists then analyze the transit event more carefully to verify if the planet actually exists and to gather information on the planet's orbit and composition (if possible).〔
Follow-up observations were conducted at observatories at the W.M. Keck Observatory's Keck 1 telescope in Hawaii; the Shane and Hale telescopes in California; the Harlan J. Smith and Hobby-Eberly telescopes in Texas; telescopes at the WIYN (including MMT) and Whipple observatories in Arizona; and the Nordic Optical Telescope in the Canary Islands. The Spitzer Space Telescope was also used. According to NASA, Kepler-11's system is the most compact and the flattest system yet discovered, surpassing even the Solar System.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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