行星是绕恒星公转、质量小于太阳质量千分之七的近似球形的天体。我们居住的星球是太阳系八大行星中的一员,它们按照与太阳由近及远的顺序排列分别是:水星、金星、地球、火星、木星、土星、天王星、海王星。行星并非太阳系所独有,天文学家还发现了1800多颗太阳系外行星,其中有些系外行星的位置还处在生命宜 The planet is an orbiting sphere that revolves around a star and has a mass less than seven thousandths of the mass of the sun. The planet we live in is one of the eight major planets of the solar system. They are arranged in the order of the sun and the sun: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Planets are not unique to the solar system. Astronomers have also discovered more than 1,800 extrasolar planets, some of which are located outside their own life