- any of the four planets closest to the sun: Mercury, Venus, Earth, or Mars.
- any of the planets Mercury, Venus, earth, and Mars, whose orbits lie inside the asteroid belt
- Any of the four planets Mercury, Venus, Earth, and Mars, whose orbits lie nearest the Sun. The inner planets are of similar size and have high densities compared to the larger gas giants among the outer planets. They are composed mostly of rock and metal and are relatively slow to rotate, with solid surfaces, no rings, and few moons. Also called terrestrial planet Compare outer planet. See also inferior planet.