any of the four planets with an orbit inside the asteroid belt; Mercury, Venus, Earth, or Mars
See terrestrial planet.
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
(plural inner planets)