Planetar

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In astronomy, a planetar is either of two things:

• A brown dwarf — an object intermediate in size between planets and stars — but having formed similarly to planets.
• A free-floating planet (also called an interstellar planet), including rogue planets and sub-brown dwarfs — 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. The term is a portmanteau: "planet"+"star".

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 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.

See also

Star portal

• Sub-brown dwarf
• Interstellar planet
• Brown dwarf
• Giant planet
• Planemo
• Fusor

References

• 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)

External links

• 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

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