See also: Planetoid



planet +‎ -oid


planetoid (plural planetoids)

  1. (astronomy) An asteroid of any size
    • 1856, Robert James Mann, A guide to astromomical science, page 231:
      The group of small bodies that circle round the Sun, outside the orbit of Mars, are known under the designation of the planetoids.
    • 1859, J.D. & E.S. Dana, The American journal of science, page 153:
      Numbering of the Planetoids or Asteroidal Planets.—In numbering the planetoids a difficulty has arisen [...]
    • 1979 Tsunami generation by pelagic planetoid impact
    • 1979, H.A. Zook, “On a New Model for the Generation of Chondrites”, in Lunar and Planetary Science, volume xii, page 1242:
      if one can argue for the early existence of a few largely molten planetoids with dimension of kilometers or tens of kilometers, one can quite as easily argue for very large numbers of such planetoids.
  2. (astronomy) An asteroid-like body in an orbit beyond the asteroid belt, such as a centaur or Kuiper belt object
  3. (astronomy) A larger, planetary, body in orbit around the Sun, such as Vesta or (candidate) dwarf planets such Eris or Sedna
    • 1991 Optimal trajectories for an ion driven spacecraft from earth to the planetoid Vesta
    • 2009, Paul A. Czysz, Claudio Bruno, Future Spacecraft Propulsion Systems, page 377–378:
      After discovering "planetoids", bodies orbiting the Sun, even larger than Pluto's moon Charon ... the question of how Sedna and other planetoids came to be.
    • 2009, Patricia Daniels, Robert Burnham, The New Solar System: Ice Worlds, Moons, and Planets Redefined, National Geographic Books, page 173:
      Eris [...] was the upstart planetoid that knocked Pluto off the planetary lists
  4. (astronomy) A dwarf planet
    • 2006, Mike Brown, “Sedna”, in (Please provide the book title or journal name)[1]:
      We prefer the word "planetoid" as a new word to describe round objects orbiting the sun.
    • 2007, Hannu Karttunen et al., editor, Fundamental Astronomy, 5 edition, page 131:
      A dwarf planet or a planetoid is a celestial body that [...]
    • 2010, Mike Brown, How I Killed Pluto and Why It Had It Coming, page 223:
      'Dwarf planet' is a dumb phrase. For years we've called things like Pluto and Xena 'planetoids'—planetlike.

Usage notesEdit

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.


See alsoEdit

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From French planétoïde.


planetoid m (plural planetoizi)

  1. (astronomy) planetoid