[ plan-it ]
See synonyms for planet on
  1. Astronomy.

    • Also called major planet. any of the eight large heavenly bodies revolving about the sun and shining by reflected light: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, or Neptune, in the order of their proximity to the sun. Until 2006, Pluto was classified as a planet ninth in order from the sun; it has been reclassified as a dwarf planet.

    • a similar body revolving about a star other than the sun.

    • (formerly) a celestial body moving in the sky, as distinguished from a fixed star, applied also to the sun and moon.

  2. Astrology. the sun, moon, Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, or Pluto: considered sources of energy or consciousness in the interpretation of horoscopes.

Origin of planet

1250–1300; Middle English planete (<Old French planète) <Late Latin planēta, planētēs (found only in plural planētae) <Greek (astéres) planḗtai literally, wandering (stars) Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2023

How to use planet in a sentence

  • All the eight planets added together only make one-seven-hundredth part of his weight.

    God and my Neighbour | Robert Blatchford
  • This immense and magnificent globe diffuses heat and light to all the other planets.

    God and my Neighbour | Robert Blatchford
  • The celebrated Kepler discovered his canon for the periodical motion of the planets.

  • But this term would have no mnemonic significance to one who knows the word Mars as meaning only one of the planets.

    Assimilative Memory | Marcus Dwight Larrowe (AKA Prof. A. Loisette)
  • In the nine differently colored circular tracks, rolled little globes representing the planets.

    Fee of the Frontier | Horace Brown Fyfe

British Dictionary definitions for planet


/ (ˈplænɪt) /

  1. Also called: major planet any of the eight celestial bodies, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, that revolve around the sun in elliptical orbits and are illuminated by light from the sun

  2. Also called: extrasolar planet any other celestial body revolving around a star, illuminated by light from that star

  1. astrology any of the planets of the solar system, excluding the earth but including the sun and moon, each thought to rule one or sometimes two signs of the zodiac: See also house (def. 9)

Origin of planet

C12: via Old French from Late Latin planēta, from Greek planētēs wanderer, from planaein to wander

Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Scientific definitions for planet


[ plănĭt ]

  1. In the traditional model of solar systems, a celestial body larger than an asteroid or comet, illuminated by light from a star, such as the Sun, around which it revolves.

  2. A celestial body that orbits the Sun, has sufficient mass to assume nearly a round shape, clears out dust and debris from the neighborhood around its orbit, and is not a satellite of another planet. The eight planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was considered to be a planet until its reclassification in 2006 as a dwarf planet. A planetlike body with more than about ten times the mass of Jupiter would be considered a brown dwarf rather than a planet. See also extrasolar planet inner planet outer planet.

Other words from planet

  • planetary adjective

The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

Cultural definitions for planet


An object in orbit around a star. A planet does not give off its own light; rather, it shines by reflecting sunlight. Planets close to the sun are rocky. Those farther out consist mostly of gases and liquids.

Notes for planet

There are nine major planets, including the Earth, in orbit around our sun, along with many asteroids. (See solar system.)

Notes for planet

Scientists have discovered evidence for the existence of many planets that circle other stars.

The New Dictionary of Cultural Literacy, Third Edition Copyright © 2005 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.