An interstellar planet is a hypothetical type of rogue planet that has been ejected from its solar system by a proto-gas giant to become an outcast, drifting in interstellar space. Possibly it formed on its own through gas cloud collapse like a star.
In 1998, David J. Stevenson authored a paper entitled \"Possibility of Life Sustaining Planets in Interstellar Space.\" In this paper, Stevenson theorizes that wandering planets which drift in the vast expanses of cold interstellar space could possibly sustain a thick atmosphere which would not freeze out due to radiative heat loss. The mechanism he proposes which preserves atmosphere formation in these bodies is due to the pressure-induced far infrared radiation opacity of a thick hydrogen-containing atmosphere.
It is thought that during planetary system formation, several small protoplanetary bodies may be ejected from the forming system. With the reduced ultraviolet light associated with its increasing distance from the parent star, the planet's predominantly hydrogen and helium containing atmosphere would be easily confined even by an Earth-sized body's gravity.
It is calculated that for an Earth-sized planet at a kilobar hydrogen atmospheric pressures in which a convective gas adiabat has formed, geothermal energy from residual core radioisotope decay will be sufficient to heat the surface to temperatures above the melting point of water. Thus, it is proposed that interstellar planetary bodies with extensive liquid water oceans may exist. It is further suggested that the bodies are likely to remain geologically active for long periods of time, providing a geodynamo-created protective magnetosphere and possible sea floor volcanism which could provide an energy source for life. The author admits these bodies will be difficult to detect due to the intrinsically weak thermal microwave radiation emissions emanating from the lower reaches of the atmosphere.
Recently, it has been discovered that some interstellar planets have debris discs. If these are considered as stars (brown sub-dwarfs) then the debris would coalesce into planets. If these are considered planets, then the debris will coalesce as moons.
Planetars are used in astronomy to represent one of two concepts:
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.
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
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.
Sub-brown dwarfs or brown sub-dwarfs (also, less commonly, grey dwarf or gray dwarf) are cold masses smaller than the low-mass cut-off for brown dwarfs. These generally are referred to as planets.
However, a sub-brown dwarf is formed in the manner of stars, through the collapse of a gas cloud, and not through accretion or core collapse from a circumstellar disc. The distinction between a sub-brown dwarf and a planet 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.
An alternate definition involves the same mass range (less than a brown dwarf, but in the planetary range), but is free of gravitational attachment with any star. These are generally referred to as free-floating planets.