Also found in: Dictionary, Thesaurus.
(also Jovian planet), any one of the planets Jupiter, Saturn, Uranus, and Neptune. The major planets are located beyond the asteroid belt. Compared to the terrestrial planets (inner planets), they have large diameters and masses, lower mean densities, thick atmospheres, short periods of rotation, and a large number of satellites. All these characteristics decrease in moving outward from Jupiter to Neptune. They are absent in the most distant planet, Pluto, and hence Pluto is not considered to be a major planet.