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.

Theoretical ideas on the atmospheres of interstellar exoplanets

In 1998, David J. Stevenson authored a paper entitled "Possibility of Life Sustaining Planets in Interstellar Space" [1] (http://www.gps.caltech.edu/faculty/stevenson/pdfs/interstellar_planets.pdf). 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.

Reference

• Stevenson, D. "Life-Sustaining Planets in Interstellar Space?" Nature 400, 32, 1999.
• Article by Stevenson simillar to the Nature article but containing more information, titled: "Possibility of Life Sustaining Planets in Interstellar Space" (http://www.gps.caltech.edu/faculty/stevenson/pdfs/interstellar_planets.pdf)