Planetoides

Planetoide (plural planetoides) is a historical and informal term in astronomy used to denote small planetary bodies that orbit the Sun and are not comets or natural satellites. The word emerged in the 19th century as the first objects between Mars and Jupiter (such as Ceres, Pallas, Juno, and Vesta) were discovered and initially regarded as planets; later discoveries broadened the concept to include many smaller bodies. In contemporary usage, the term is largely obsolete in favor of “minor planet,” a category that covers asteroids, near-Earth objects, and trans-Neptunian objects, as well as dwarf planets when considered part of the minor-planet population.

Planetoides are diverse in size and composition. They can be rocky, metallic, or icy, and their shapes range from irregular to roughly spheroidal. Their orbits around the Sun vary from close, near-Earth trajectories to distant trans-Neptunian paths. Most reside in the main asteroid belt between Mars and Jupiter, while others are classified as near-Earth objects or trans-Neptunian objects. Some planetoides, such as Ceres, Pluto, Haumea, and Eris, are also described as dwarf planets under the current IAU definition.

Observation and study of planetoides provide essential clues about the formation and evolution of the Solar System. In modern terminology, they are cataloged as minor planets, assigned numerical designations and, when appropriate, names following IAU conventions. The term planetoide remains mainly of historical use and appears in some languages as a less formal synonym for minor planets.