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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
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.
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.
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 planetar. In 2003, the IAU Extrasolar Planet Working Group recommended that these objects be called sub-brown dwarfs.
- arXiv: Infrared Spectroscopy of Substellar Objects in Orion P. W. Lucas, P. F. Roche, France Allard, Peter H. Hauschildt Mon, 14 May 2001 09:08:51 GMT (accessed: 25 August 2006)
- Royal Astronomical Society: FREE-FLOATING PLANETS CONFIRMED Thursday, 29 March 2001 (accessed: 25 August 2006)
- news@Nature.com (subscription required): Lonely planets float free Tom Clarke 04 Apr 2001 (accessed: 25 August 2006)
- Strange New Worlds Could Make Miniature Solar Systems Robert Roy Britt (SPACE.com) 5 June 2006 11:35 am ET
- Working Group on Extrasolar Planets - Definition of a "Planet" POSITION STATEMENT ON THE DEFINITION OF A "PLANET" (IAU) 2003