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Planetar (astronomy)

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Planetar is a term used in astronomy that refers to one of two things:

  • Brown dwarfs - objects intermediate in size between planets and stars.
  • Interstellar planets - planetars that are cold masses smaller than brown dwarfs and do not orbit a star, but are free-floating in space.

Both definitions have been proposed, but neither has achieved wide usage in the astronomical and planetary science communities.

Brown dwarf planetars

Planetars are planet-like objects that are more massive than the low-mass cut-off for brown dwarfs. These generally are referred to as brown dwarfs. However, a planetar is formed in the manner of planets, through accretion or core collapse from a circumstellar disc, and not through the collapse of a gas cloud. The distinction between a planetar and a brown dwarf is unclear, astronomers are divided into two camps as whether to consider the formation process of a planet as part of its division in classification. Such a planet might also be referred to as a hypergiant planet.[citation needed]

Red dwarf planetars

Hypothetically an ultra-giant planet may result from planetary formation large enough to become a red dwarf. Perhaps even larger stars may form from discs of gas of Population III protostars.[citation needed]

Unbound planet planetars

Interstellar planetary mass objects, also known as planetars, are called such, because a portion of the astronomy community defines a planet as something that must orbit a star. Any planetary-mass object which does not orbit a star, cannot according to that rule be called a planet. As it exists alone like a star, it is called a planet-star, or shorter planetar. In 2003, the IAU Extrasolar Planet Working Group recommended that these objects be called sub-brown dwarfs.

Some of these planemo harbour debris discs akin to proplyds. The planemo 2M1207b has been discovered to harbour a disc.

See also

References

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