||It has been suggested that this article or section be merged with Sub-brown dwarf and Free-floating planet to Free-floating planet. (Discuss) Proposed since November 2011.|
A rogue planet (also known as an interstellar planet, or orphan planet) is a planetary-mass object that has been ejected from its system and is no longer gravitationally bound to any star, brown dwarf or other such object, and that therefore orbits the galaxy directly. Some astronomers have estimated that there may be twice as many Jupiter-sized rogue planets as there are stars.
Isolated 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.
Cha 110913-773444 may be an ejected rogue planet, or it may have formed on its own and be a sub-brown dwarf. Astronomers believe that either way, the definition of planet should depend on current observable state, and not origin.
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 due to radiative heat loss. 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 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 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 will 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 detected 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 moons would retain their moons after ejection. A large moon 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 moons. The term planetar exists for those accretion masses that seem to fall between stars and planets.
Interstellar planets in popular culture
In the novel When Worlds Collide (1933, serialisation began 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. Fortunately, advance warning enables several groups of survivors to escape to Bronson Beta, whose orbit maps onto that of the destroyed Earth, and is torn away from its former primary by the gravitational impact of the Bronson Alpha/Earth collision. A cinematic version of the book was produced in a 1951 film of the same name.
In the 1951 film, When Worlds Collide, a cinematic incarnation of the 1933 book of the same name, Bronson Alpha was reimagined as a dwarf star and renamed “Bellus”, while Bronson Beta was designated “Zyra.” 
The short story A Pail of Air by Fritz Leiber, which first appeared in the December 1951 issue of Galaxy Magazine and aired on the radio drama X Minus One in March 1956, is narrated by a boy living on Earth after it has been torn from the Sun’s gravity and captured by a passing “dark star”. Although Earth now orbits this “dark star” (which might be a black hole or cool brown dwarf), it shares many characteristics with an interstellar planet.
In the 1959 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 The Wanderer (Fritz Leiber novel), Earth encounters two ambulatory rogue planets. One, The Wanderer, is inhabited by nonconformist felinoid aliens, while The Stranger is a ‘police world’ that is pursuing the renegade felinoids. There are gravitational and tidal upheavals and the Moon is destroyed.
In the 1966 Doctor Who story The Tenth Planet, Mondas, home of the Cybermen, is said be Earth’s twin which was knocked out of Solar orbit during prehistoric times, returning in 1986. In Revenge of the Cybermen, the Cybermen waged war on the inhabitants of a rogue planetoid, the remnants of the destroyed planet Voga. The Star Trek: The Original Series episode “The Squire of Gothos” is set on a rogue planet, uninhabitable except for a small patch maintained by a superhuman being.
In the Warhammer 40,000 universe, the Temple of the Culexus Assassins of the Officio Assassinorum is located deep under the surface of a ‘dead’ rogue planet.
In the Red Dwarf books, the Earth becomes a rogue planet when it is torn from its orbit by exploding sewage.
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. The planet, which is in fact a sentient life form itself, is home to life unlike any other in the galaxy, including organic spaceships.
In the Star Trek: Enterprise episode “Rogue Planet“, Enterprise happens upon a rogue planet with an Earth-like atmosphere. The planet was heated by volcanic vents that sent heat into the atmosphere, thus sustaining the ecology of the planet.
In the novel Sunstorm, by Arthur C. Clarke and Stephen Baxter, the eponymous sunstorm is caused by the impact into the Sun of a gas giant diverted from Altair roughly two thousand years before. As it passed through the Solar System, it was visible as the Star of Bethlehem.