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In astronomy, a trojan is a minor planet or moon that shares the orbit of a planet or larger moon, wherein the trojan remains in the same, stable position relative to the larger object. In particular, a trojan remains near one of the two trojan points of stability – designated and – which lie approximately 60° ahead of and behind the larger body, respectively. Trojan points are two of five types of Lagrangian points, and a trojan is a type of Lagrangian object. They are one type of co-orbital object. In this arrangement, the massive star and the smaller planet orbit about their common barycenter. A much smaller mass located at one of the Lagrangian points is subject to a combined gravitational force that acts through this barycenter. Hence the object can orbit around the barycenter with the same orbital period as the planet, and the arrangement can remain stable over time. The Jupiter trojans account for most known trojans in the Solar System. They are divided into the Greek camp () in front of and the Trojan camp () trailing behind Jupiter in their orbit. More than 6,000 have been found so far and more than a million Jupiter trojans larger than one kilometer are thought to exist, whereas only a few Mars trojans (7) and Neptune trojans (13) have been found to date. Numerical calculations of the orbital dynamics involved indicate that Saturn and Uranus probably do not have any primordial trojans. The discovery of the first Earth trojan, , was announced by NASA in 2011.〔(【引用サイトリンク】title=NASA's WISE Mission Finds First Trojan Asteroid Sharing Earth's Orbit )〕 Unlike ''trojan minor planets'' that share the orbit with a planet, a trojan moon is a moon orbiting near the trojan point of another, larger moon. All known trojan moons are part of the Saturn system. Telesto and Calypso are trojans of Tethys, and Helene and Polydeuces of Dione. There is also a theoretical concept of a trojan planet, a planet that orbits at the trojan point of another, larger planet. Such a pair of co-orbital exoplanets was already thought to exist in another star system, but this claim was later retracted.〔(【引用サイトリンク】title=Two planets found sharing one orbit )〕 == Trojan minor planets == In 1772, the Italian–French mathematician and astronomer Joseph-Louis Lagrange obtained two constant-pattern solutions (collinear and equilateral) of the general three-body problem.〔(''Essai sur le Problème des Trois Corps'' )〕 In the restricted three-body problem, with one mass negligible (which Lagrange did not consider), the five possible positions of that mass are now termed Lagrangian points. The term "trojan" originally referred to the "trojan asteroids" (Jupiter trojans) that orbit close to the Lagrangian points of Jupiter. These have long been named after characters from the Trojan War of Greek mythology. By convention, the asteroids orbiting near the point of Jupiter are named after the characters from the Greek side of the war, whereas those orbiting near the of Jupiter are from the Trojan side. There are two exceptions, which were named before the convention was put in place, the Greek-themed 617 Patroclus and the Trojan-themed 624 Hektor, which were assigned to the wrong sides. Astronomers estimate that the Jupiter trojans are about as numerous as the asteroids of the asteroid belt. Later on, objects were found orbiting near the Lagrangian points of Neptune, Mars, Earth, Uranus, and Venus. Minor planets at the Lagrangian points of planets other than Jupiter may be called Lagrangian minor planets.〔Whiteley, Robert J.; and Tholen, David J.; "A CCD Search for Lagrangian Asteroids of the Earth–Sun System", ''Icarus'' 136:1, November 1998:154–167〕 * Seven Mars trojans are known: 5261 Eureka, , – the only trojan in the leading cloud at , and ,〔(【引用サイトリンク】title=List Of Martian Trojans )〕〔de la Fuente Marcos, C.; de la Fuente Marcos, R.; ''Three new stable L5 Mars Trojans'', Monthly Notices of the Royal Astronomical Society: Letters, Volume 432, Issue 1, p.31-35 ()〕 as well as , and , which have not yet been accepted by the Minor Planet Center. * Nine Neptune trojans〔(【引用サイトリンク】accessdate=2012-12-18 )〕 are known, but large Neptune trojans are expected to outnumber the large Jupiter trojans by an order of magnitude.〔Chiang, Eugene I. & Lithwick, Yoram; ''Neptune Trojans as a Testbed for Planet Formation'', The Astrophysical Journal, 628, pp. 520–532 (Preprint )〕 * was confirmed to be the first known Earth trojan in 2011. It is located in the Lagrangian point, which lies ahead of Earth. * was identified as the first Uranus trojan in 2013; it is located in the Lagrangian point. * is a temporary Venus trojan, the first to be identified. * Asteroids Ceres and Vesta have temporary trojans.〔(A population of main belt asteroids co-orbiting with Ceres and Vesta )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Trojan (astronomy)」の詳細全文を読む スポンサード リンク
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