PLANETOID

Etymology

Noun

planetoid (plural planetoids)

(astronomy) An asteroid of any size

(astronomy) An asteroid-like body in an orbit beyond the asteroid belt, such as a centaur or Kuiper belt object

(astronomy) A larger, planetary, body in orbit around the Sun, such as Vesta or (candidate) dwarf planets such Eris or Sedna

(astronomy) A dwarf planet

Usage notes

The term "planetoid" has never been precisely defined. At first, it was a synonym for asteroid; whereas "asteroid" referred to the star-like image seen through a telescope, "planetoid" referred to its planet-like orbit. Though it approached the popularity of "asteroid" ca. 1915, this usage was never dominant, and largely ceased by ca. 1980. Even before then the etymology of the term was reanalyzed as meaning planet-like in form, and started being used for larger asteroids such as Vesta which had planet-like geologies (that is, were planetary bodies). There was an increase in such usage after 2000 with the discovery of planetary bodies in the Kuiper belt and beyond, which many felt were not appropriately called "asteroids" and concomitant with doubts as to the appropriate definition of "planet". Sedna, for example, was called a "planetoid" in its discovery announcement.