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SCIENTIFIC AMERICAN JANUARY 2007
What Is a Planet?
The controversial new official definition of "planet," which banished Pluto, has its flaws but by and large captures essential scientific principles
By Steven Soter
Most of us grew up with the conventional definition of a planet as a body that orbits a star, shines by reflecting the star's light and is larger than an asteroid. Although the definition may not have been very precise, it clearly categorized the bodies we knew at the time. In the 1990s, however, a remarkable series of discoveries made it untenable. Beyond the orbit of Neptune, astronomers found hundreds of icy worlds, some quite large, occupying a doughnut-shaped region called the Kuiper belt. Around scores of other stars, they found other planets, many of whose orbits look nothing like those in our solar system. They discovered brown dwarfs, which blur the distinction between planet and star. And they found planetlike objects drifting alone in the darkness of interstellar space.
p3 详细讲了新的definition 可以让Pluto和Eric不符合成为planet 然后好像definition有2个条件 有一个写到5000啥啥 (这里认真看 有题 构筑没理清楚)
The discovery in 2005 of Eris (formerly known as 2003 UB313 or Xena), a KBO even larger than Pluto, brought the issue to a head. If Pluto is a planet, then Eris must also be one, together with scores of other large KBOs; conversely, if Pluto is not a planet, neither are the other KBOs. On what objective grounds could astronomers decide?
In short, the difference between planets and nonplanets is quantifiable, both in theory and by observation. All the planets in our solar system have enough mass to have swept up or scattered away most of the original planetesimals from their orbital zones. Today each planet contains at least 5,000 times more mass than all the debris in its vicinity. In contrast, the asteroids, comets and KBOs, including Pluto, live amid swarms of comparable bodies.
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发表于 2019-9-29 12:33:37
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