Planetar (astronomy)

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.^{[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

• 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

v t e Star Evolution Formation Pre-main-sequence star Main sequence Horizontal branch Asymptotic giant branch Dredge-up Instability strip Red clump PG1159 star Mira variable Planetary nebula Protoplanetary nebula Luminous red nova Luminous blue variable Wolf–Rayet star Supernova impostor Supernova Hypernova Hertzsprung–Russell diagram Color–color diagram Protostars Molecular cloud H II region Bok globule Young stellar object Herbig–Haro object Hayashi track Hayashi limit Henyey track Orion T Tauri FU Orionis Herbig Ae/Be Luminosity class Subdwarf Dwarf Blue Red White Black Brown Subgiant Giant Blue Red Bright giant Supergiant Blue Red Yellow Hypergiant Yellow Blue straggler Spectral classification O B A F G K M Be OB Subdwarf B Late-type Peculiar Am Ap/Bp Oscillating Barium Carbon CH Extreme helium Lambda Boötis Lead Mercury-manganese S Shell Technetium Remnants White dwarf Black dwarf Helium planet Neutron star Pulsar Magnetar Stellar black hole Compact star Quark Exotic Stellar core: EF Eridani B Theoretical stars Dark-matter star Frozen star Quasi-star Thorne–Żytkow object Iron star Nucleosynthesis Alpha process Triple-alpha process Proton–proton chain Helium flash CNO cycle Lithium burning Carbon burning Neon burning Oxygen burning Silicon burning S-process R-process Fusor Nova Remnants Structure Core Convection zone Microturbulence Oscillations Radiation zone Atmosphere Photosphere Starspot Chromosphere Corona Stellar wind Bubble Asteroseismology Eddington luminosity Kelvin–Helmholtz mechanism Properties Designation Dynamics Effective temperature Kinematics Magnetic field Magnitude Absolute Mass Metallicity Rotation UBV color Variability Star systems Binary Contact Common envelope Multiple Accretion disc Planetary system Earth's Solar System Earth-centric observation of Pole star Circumpolar star Magnitude Apparent Extinction Photographic Color Radial velocity Proper motion Parallax Photometric-standard star Lists Star names Arabic Chinese Most massive Least massive Largest Brightest Historical Most luminous Nearest Nearest bright Stars with exoplanets Brown dwarfs Collapsars Planetary nebulae Novae Notable supernovae Supernova remnants Supernova candidates Timeline of stellar astronomy Related articles Substellar object Brown dwarf Sub-brown dwarf Planet Star cluster Association Open cluster Globular cluster Super star cluster Galactic year Galaxy Supercluster Helioseismology Guest star Constellation Asterism Gravity Intergalactic star Infrared dark cloud Star portal

v t e Exoplanetology Types Terrestrial Carbon planet Coreless planet Desert planet Ice planet Iron planet Lava planet Ocean planet Sub-Earth Super-Earth Gas giant Carbon giant Chthonian planet Eccentric Jupiter Helium planet Hot Jupiter Hot Neptune Ice giant Mini-Neptune Puffy planet Super-Jupiter Other types Solid giant Brown dwarf Circumbinary planet Double planet Mesoplanet Planetar Protoplanet Pulsar planet Sub-brown dwarf Systems Binary star Exocomet Exomoon Exoplanet Extragalactic planet Interstellar comet Planetary system Rogue planet Trojan planet Retrograde planet Mean-motion resonances Titius–Bode laws Planets orbiting red dwarfs Planets orbiting white dwarfs Planets orbiting brown dwarfs Planets in globular clusters Detection Direct imaging list Microlensing list Pulsar timing list Radial velocity list Transit method list Transit-timing variation Astrometry Polarimetry Habitability Circumstellar habitable zone Earth analog Earth Similarity Index Extraterrestrial liquid water Goldilocks planet Potentially habitable exoplanets Catalogues NASA Exoplanet Archive Exoplanet Data Explorer Extrasolar Planets Encyclopaedia Catalog of Nearby Habitable Systems NASA Star and Exoplanet Database Lists Exoplanetary host stars Exoplanet extremes Exoplanet firsts Kepler Multiplanetary systems Stars with proplyds Terrestrial candidates Unconfirmed exoplanets Other Exoplanet phase curves Exoplanet skies Sudarsky exoplanet classification Discoveries of exoplanets Search projects