最近，科学家在主小行星带中也发现了一些彗星——这些主带彗星可能是类地内行星上水的关键来源。
Recently, scientist have also discovered comets in the main asteroid belt -these main-belt comets might be a key source of water for the inner terrestrial planets.
水星在地内行星中是最小的行星（直径4 880公里），同时也是靠太阳最近的行星（5800万公里）。
The planet Mercury is the smallest of the inner planets (4, 880 km/3, 032 mi in diameter), and the closest to the Sun (58 million km/36 million mi - or 3.2 light minutes).
同样，由于它们的相对位置，四颗类木行星被称为外行星，而类地行星被称为内行星。
Also, because of their relative locations, the four Jovian planets are known as the outer planets, while the terrestrial planets are known as the inner planets.
大约7000个已知的近地天体在内行星之间疯狂运行，其轨道路径收到太阳重力和热量的影响。
About 7,000 known NEOs loop wildly among the inner planets, following paths that shift in response to gravity and the Sun's heat.
大约7000个已知的近地天体在内行星之间疯狂运行，其轨道路径收到太阳重力和热量的影响。
About 7,000 known NEOs loop wildly among the inner planets, following paths that shift in response to gravity and the Sun's heat.
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