Planetar (astronomy)

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".Fact|date=February 2007

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.Fact|date=February 2007

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.

ee also

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

References

* arXiv: [http://arxiv.org/abs/astro-ph/0105154%20v2%2014%20May%202001 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: 25/08/2006)
* Royal Astronomical Society: [http://www.ras.org.uk/index.php?option=com_content&task=view&id=405&Itemid=2 FREE-FLOATING PLANETS CONFIRMED] Thursday, 29 March 2001 (accessed: 25/08/2006)
* [http://search.nature.com/search/?sp-q=Lonely+planets+float+free&sp-x-9=cat&sp-s=date&sp-q-9=NEWS&submit=go&sp-a=sp1001702d&sp-sfvl-field=subject%7Cujournal&sp-t=results&sp-x-1=ujournal&sp-p-1=phrase&sp-p=all [e-mail]] (subscription required): [http://www.nature.com/news/2001/010404/full/010404-9.html Lonely planets float free] "Tom Clarke" 04 Apr 2001 (accessed: 25/08/2006)

External links

* [http://www.space.com/scienceastronomy/060605_planemos.html Strange New Worlds Could Make Miniature Solar Systems] Robert Roy Britt (SPACE.com) 05 June 2006 11:35 am ET
* [http://www.dtm.ciw.edu/boss/definition.html Working Group on Extrasolar Planets - Defintion of a "Planet"] POSITION STATEMENT ON THE DEFINITION OF A "PLANET" (IAU) 2003