noun
inner planet
Astronomical term for planets whose orbits lie inside the asteroid belt: Mercury and Venus. Sometimes also includes Earth and Mars in broader usage.
内惑星は地球よりも太陽に近い軌道を回っている。
Inner planets orbit closer to the sun than Earth does.
外惑星 refers to outer planets (Mars, Jupiter, Saturn, Uranus, Neptune), while 内惑星 refers to inner planets (Mercury, Venus, and sometimes Earth and Mars).
Compound of 内 (nai, 'inner') + 惑星 (wakusei, 'planet'). A straightforward modern astronomical term.