Rogue planet: Nobody's Child

A rogue planet (also known as an interstellar planet, free-floating planet or orphan planet) is an object which has equivalent mass to a planet and is not gravitationally bound to any star, and that therefore moves through space as an independent object. Several astronomers claim to have detected such objects (for example, Cha 110913-773444), but those detections remain unconfirmed pending visitation.Some astronomers refer to these objects as "planets", usually because they believe such objects were planets that were ejected from orbit around a star.However, others believe that the definition of 'planet' should depend on current observable state, and not origin. Additionally, these objects may form on their own (sub brown dwarf)through gas cloud collapse similar to star formation; in which case they would never have been planets.

Retention of heat in interstellar space:

In 1998, David J. Stevenson authored a paper entitled "Possibility of Life Sustaining Planets in Interstellar Space."In this paper, Stevenson theorizes that some wandering objects, that Stevenson refers to as "planets", 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 the bodies 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.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 as stars (brown sub-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.