Planetars are used in astronomy to represent one of two concepts:

  • Planetars are brown dwarfs which formed through accretion or core collapse within a circumstellar disc, forming like a planet does.
  • Planetars are cold masses smaller than brown dwarfs which do not orbit a star, but are free-floating in space (otherwise known as an interstellar planet).

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.

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

The free-floating planet, known as a planetar, is 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 thus be called a planet. As it exists alone, it is like a star, so it is called a planet-star, or planetar.

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

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