Astronomers strip Pluto of its status as a planet天文学家取消冥王星行星地位

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With one vote, toys and models of the solar system became instantly obsolete, forcing teachers and publishers to scramble to update textbooks and lessons used in classrooms for decades.

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Discovered in 1930 by the American Clyde Tombaugh, the icy rock of Pluto has traditionally been considered the ninth planet, farthest from the sun in the solar system.

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However, the definition of a planet, approved after a heated debate among 2,500 scientists from the International Astronomical Union (IAU) meeting in Prague, drew a clear distinction between Pluto and the other eight planets.

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The scientists agreed that, to be called a planet, a celestial body must be in orbit around a star while not itself being a star.

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It must be large enough in mass for its own gravity to pull it into a nearly spherical shape and have cleared the neighborhood around its orbit.

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Pluto was disqualified because its oblong orbit overlaps Neptune's. Xena also does not make the grade of being a planet, and will also be known as a dwarf planet.

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The agreed-upon definition -- the first time the IAU has tried to define scientifically what a planet is -- comes in sharp contrast to the draft sent around to delegates at the General Assembly last week.

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The new definition creates a second category called "dwarf planets," as well as a third category for all other objects, except satellites, known as small solar system bodies.

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From now on -- or 'at least for the time being' joked one delegate -- traditional planets will be restricted to eight: Mercury, Venus. Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

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It's an issue mainly for the public, not really for scientists.

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Officials at the American Museum of Natural History in New York had been at the vanguard of the movement to demote Pluto and were feeling vindicated on Thursday.

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请冷玉将此贴移至书院

冷玉沉音

书院帖子可以慢慢浏览，潜心细读

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来自世界各地的天体物理学家在布拉格投票，取消冥王星的地球第九大行星资格，原因是它按照新的行星定义，不具备围绕一颗形体运转的独立的运行轨道（他与海王星的轨道交叉重叠，没有依靠自身的质量形成绕太阳旋转的独立轨道），而且其直径很小，质量不大

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这样，太阳系便只剩下八大行星。课本和科普文章都得重新修订。从小知道太阳系有九大行星的我们也必须改变观念了