Planetar (astronomy)

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For other uses, see Planetar (disambiguation).

The term planetar is a portmanteau of the words, "planet"+"star". Exactly what a planetar is has not been fully defined.

In astronomy, the term planetar has been used to denote two different kinds of celestial objects:

  • Brown dwarfs — objects with a size larger than planets but smaller than stars, having formed by processes that typically yield planets; and
  • Sub-brown dwarfs — objects smaller than brown dwarfs that do not orbit a star.

Both definitions have been proposed, but neither has been widely adopted by astronomical and planetary-science communities.

Brown dwarf planetars[edit]

Unlike brown dwarf stars, which are formed from the collapse of a gas cloud, planetars are planet-like objects that are formed in the manner of planets, through accretion or core collapse from a circumstellar disc. Both brown dwarfs and planetars are planet-like objects above a certain size. Astronomers are divided into two camps over whether to classify these planet-like objects separately depending on their formation process. Such a planet might also be referred to as a hypergiant planet.[citation needed]

Red dwarf planetars[edit]

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[edit]

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 planetar. In 2003, the IAU Extrasolar Planet Working Group recommended that these objects be called sub-brown dwarfs.

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

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