Steppenwolf Planets

This article http://arxiv.org/PS_cache/arxiv/pdf/1102/1102.1108v1.pdf looks at the possibility of rogue planets in interstellar space harboring life.  The only source of heat would be from the planetary core -- residual heat of formation and radioactive decay.  But given a thick enough insulating layer of ice, that could be enough.

4.

DISCUSSION AND CONCLUSION

A Steppenwolf planet’s lifetime will be limited by the decay of the geothermal heat flux, which is determined by the half-life of its stock of radioisotopes (

40K, 238U,232Th) and by the decay of its heat of formation.

These decay times are

1−5 Gyr, so its lifetime is thus comparable to planets in the traditional habitable zone of main-sequence stars (Kasting et al. 1993).

If a Steppenwolf planet harbors life, it could have originated in a more benign era before ejection from the host star. Alternatively, life could originate after ejection around hydrothermal vents, which are a proposed location for the origin of life on Earth (Baross & Hoffman 1985). If life can originate and survive on a Steppenwolf planet, it must be truly ubiquitous in the universe.

We have shown that a rogue planet drifting through interstellar space could harbor a subglacial liquid ocean despite its low emission temperature, and so might be considered habitable. Such an object could be detected and followed-up using current technology if it passed within

O(1000 AU) of Earth.