A rogue planet — also known as an interstellar planet, nomad planet, free-floating planet or orphan planet — is a planetary-mass object which has either been ejected from its system or was never gravitationally bound to any star, brown dwarf or other such object, and that therefore orbits the galaxy directly. Astronomers agree that either way, the definition of planet should depend on its current observable state and not its origin.
Larger planetary-mass objects which were not ejected, but have always been free-floating, are thought to have formed in a similar way to stars, and the IAU has proposed that those objects be called sub-brown dwarfs (an example of this is Cha 110913-773444, which may be an ejected rogue planet or may have formed on its own and be a sub-brown dwarf). The closest rogue planet to Earth yet discovered, CFBDSIR 2149-0403, is around 100 light years away.
When a planetary-sized object passes in front of a background star, its gravitational field causes a momentary increase in the visible brightness of the background star. This is known as microlensing. Astrophysicist Takahiro Sumi of Osaka University in Japan and colleagues, who form the Microlensing Observations in Astrophysics (MOA) and the Optical Gravitational Lensing Experiment (OGLE) collaborations, carried out a study of microlensing which they published in 2011. They observed 50 million stars in our galaxy using the 1.8 meter MOA-II telescope at New Zealand's Mount John Observatory and the 1.3 meter University of Warsaw telescope at Chile's Las Campanas Observatory. They found 474 incidents of microlensing, ten of which were brief enough to be planets of around Jupiter's size with no associated star in the immediate vicinity. The researchers estimated from their observations that there are nearly two free-floaters for every star in our galaxy. Other estimations suggest a much larger number, up to 100,000 times more free-floating planets than stars in our Milky Way.
Retention of heat in interstellar space 
In 1998, David J. Stevenson theorized that some planet-sized objects drift in the vast expanses of cold interstellar space and could possibly sustain a thick atmosphere which would not freeze out. He proposes that atmospheres are preserved by 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 that would normally strip the lighter components from an atmosphere, due to 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 object 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 these planets are likely to remain geologically active for long periods, providing a geodynamo-created protective magnetosphere and possible sea floor volcanism which could provide an energy source for life. The author admits these bodies would be difficult to detect due to the intrinsically weak thermal microwave radiation emissions emanating from the lower reaches of the atmosphere, although later research suggests that reflected solar radiation and far-IR thermal emissions may be detectable if one were to pass within 1000 AU of Earth.
A study of simulated planet ejection scenarios has suggested that around five percent of Earth-sized planets with Moon-sized natural satellites would retain their satellites after ejection. A large satellite would be a source of significant geological tidal heating.
Proplyds of planetars 
Recently, it has been discovered that some extrasolar planets such as the planemo 2M1207b, orbiting the brown dwarf 2M1207, have debris discs. If some large interstellar objects are considered stars (sub-brown dwarfs), then the debris could coalesce into planets, meaning the disks are proplyds. If these are considered planets, then the debris would coalesce as satellites. The term planetar exists for those accretion masses that seem to fall between stars and planets.
Known or possible rogue planets 
There is no current way of telling whether these are planets that have been ejected from orbiting a star or were originally formed on their own as sub-brown dwarfs.
In popular culture 
||This article may contain an excessive amount of intricate detail that may only interest a specific audience. (April 2013)|
In the novel When Worlds Collide (serialised 1932) by Edwin Balmer and Phillip Wylie, Earth is first devastated, and then destroyed, by "Bronson Alpha", a gas-giant-sized rogue planet, orbited by "Bronson Beta", an Earth-sized satellite. In the film version (1951) of the novel, Bronson Alpha was reimagined as a dwarf star and renamed "Bellus", while Bronson Beta was designated "Zyra."
In the novel Wolfbane by Frederik Pohl and C. M. Kornbluth (originally serialised in Galaxy in 1957), a rogue planet, populated by strange machines known as Pyramids, steals the Earth from the Solar System, taking it off into interstellar space.
In Fritz Leiber's novel The Wanderer, Earth encounters two ambulatory rogue planets. In the novel The Witches of Karres (1966) by James H. Schmitz, expanded from a 1949 novelette, the rogue planet Karres can be moved through space by means of psychic powers.
The first known use of "rogue planet" as term for such detached worlds occurred in Poul Anderson's Polesotechnic League novel Satan's World (1969). In the British science-fiction television series Space: 1999, the pilot episode (1975) has a rogue planet, Meta, coming near Earth. The rogue planet of Worlorn is the scene of action in George R. R. Martin's novel Dying of the Light.
In the 1980 animated series Thundarr the Barbarian a "runaway planet" passes between the Earth and Moon in the year 1994. The Moon is cracked in half and Earth's civilization is destroyed. The series takes place two thousand years later, when the post-apocalyptic Earth is inhabited by mutants and wizards.
In the first series of Transformers comics published by Marvel Comics, the planet of Cybertron is a rogue planet that was dislodged from its original orbit in the Alpha Centauri system by weapons of mass destruction. In the Red Dwarf books, the Earth becomes a rogue planet when it is torn from its orbit by exploding sewage.
The homeworld of the Founders in Star Trek: Deep Space Nine is a rogue planet in a nebula; it has climatic conditions capable of supporting humanoid life. From the 2nd season of Mainframe Entertainment's War Planets cartoon onward, the titular planets were forced to become rogue planets in order to escape being consumed by the Beast Planet, which they achieved with colossal "World Engine" propulsion systems created by a lost civilization.
The planet Zonama Sekot in the Star Wars fictional universe was first introduced in the novel Rogue Planet and later expanded on in the New Jedi Order series. In the Star Trek: Enterprise episode "Rogue Planet", Enterprise happens upon a rogue planet with an Earth-like atmosphere.
In the online game Cartoon Network Universe: FusionFall, Earth is under attack by Fuse, the ruler of a possibly Saturn-sized rogue planet named Planet Fusion. The video game Metroid Prime 2: Echoes (2004) takes place on a rogue planet named Aether.
See also 
- Orphan Planets: It's a Hard Knock Life, Space.com, 24 Feb 2005, retrieved 5 Feb 2009.
- Free-Floating Planets – British Team Restakes Dubious Claim, Space.com, 18 Apr 2001, retrieved 5 Feb 2009.
- Orphan 'planet' findings challenged by new model, NASA Astrobiology, 18 Apr 2001, retrieved 5 Feb 2009.
- Working Group on Extrasolar Planets – Definition of a "Planet" POSITION STATEMENT ON THE DEFINITION OF A "PLANET" (IAU)
- Rogue planet find makes astronomers ponder theory
- Astronomers spy a planet untethered to any star; there may be many more, Washington Post, 19 Nov 2012. Retrieved 20 Nov 2012.
- Homeless' Planets May Be Common in Our Galaxy by Jon Cartwright, Science Now ,18 May 2011, Accessed 20 may 2011
- Planets that have no stars: New class of planets discovered, Physorg.com, May 18, 2011. Accessed May 2011.
- [T. Sumi, et al. (2011). "Unbound or Distant Planetary Mass Population Detected by Gravitational Microlensing". arXiv:1105.3544v1 [astro-ph.EP].
- "Researchers say galaxy may swarm with 'nomad planets'". Stanford University. Retrieved 29 February 2012.
- Stevenson, David J.; Stevens, CF (1999). "Life-sustaining planets in interstellar space?". Nature 400 (6739): 32. Bibcode:1999Natur.400...32S. doi:10.1038/21811. PMID 10403246.
- Lissauer, J.J. (1987). "Timescales for Planetary Accretion and the Structure of the Protoplanetary disk". Icarus 69 (2): 249–265. Bibcode:1987Icar...69..249L. doi:10.1016/0019-1035(87)90104-7.
- Dorian S. Abbot; Eric R. Switzer (2 Jun 2011). "The Steppenwolf: A proposal for a habitable planet in interstellar space". arXiv:1102.1108.
- Debes, John H.; Steinn Sigurðsson (20 October 2007). "The Survival Rate of Ejected Terrestrial Planets with Moons". The Astrophysical Journal Letters 668 (2): L167–L170. arXiv:0709.0945. Bibcode:2007ApJ...668L.167D. doi:10.1086/523103.
- When Worlds Collide (2012). IMDb
- Stevenson, D. (1999). "Life-sustaining planets in interstellar space?". Nature 400 (6739): 32. Bibcode:1999Natur.400...32S. doi:10.1038/21811. PMID 10403246.
- Article by Stevenson similar to the Nature article but containing more information, titled: "Possibility of Life Sustaining Planets in Interstellar Space"
- 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
- The IAU draft definition of "planet" and "plutons" press release (International Astronomical Union) 2006
- Text of A Pail of Air and original radio broadcast