noun
inner planet
Astronomical term for planets whose orbits lie inside the asteroid belt: Mercury and Venus. Sometimes Earth and Mars are included in broader definitions.
内遊星には水星と金星が含まれる。
The inner planets include Mercury and Venus.
外遊星 refers to outer planets (those beyond the asteroid belt), while 内遊星 refers to inner planets.
Compound of 内 (inner) + 遊星 (planet). 遊星 is an older term for 'planet', now largely replaced by 惑星 (わくせい) in modern Japanese.