Eight major or classical planets are currently recognized by the International Astronomical Union (IAU), the body that gives official names to objects in the solar system. The planets are commonly divided into two groups: the inner planets (Mercury, Venus, Earth, and Mars) and the outer planets & (Jupiter, Saturn, Uranus, and Neptune). The inner planets are small and are composed primarily of rock and iron. The outer planets are much larger and consist mainly of hydrogen, helium, and ice. Pluto, historically counted as the ninth planet, does not belong to either group, and was reclassified as a dwarf planet by the IAU in 2006.
- major [ˈmeidʒə] adj. 主要的
- classical [ˈklæsikəl] adj. 传统的，古典的，经典的
- recognized [ˈrekəɡnaizd] adj. 公认的， 经过验证的
- solar system [ˈsəulə - ˈsistəm] n.太阳系
- divide [diˈvaid] v. 划分
- Mercury [ˈmə:kjuri] n. 水星
- Venus [ˈvi:nəs] n.金星
- Earth [ə:θ] n. 地球
- Mars [mɑ:z] n.火星
- Jupiter [ˈdʒu:pitə] n.木星
- Saturn [ˈsætən] n.土星
- Uranus [ˈju:ərənəs] n.天王星
- Neptune [ˈneptju:n] n.海王星
- compose [kəmˈpəuz] v. 组成(be composed of：由……组成)
- primarily [ˈpraimərili] adv.主要地
- consist of [kənˈsist - əv] v. 由……组成
- mainly [ˈmeinli] adv. 主要地
- hydrogen [ˈhaidrədʒən] n.氢
- helium [ˈhi:ljəm] n.氦
- Pluto [ˈplu:təu] n.冥王星
- dwarf planet [dwɔ:f - ˈplænit] n. 矮星
