地内行星是指太阳系中轨道位于地球轨道之内的行星。地内行星包括：水星，金星。
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.
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