Stevenson, of the California Institute for Technology, says that these small planets could be kicked out of their solar system during formation due to the gravitational effects of larger nearby planets. The ejected planets could harbour a hydrogen atmosphere. Such an atmosphere, if it were sufficiently thick, could help retain heat and may even permit the formation of surface liquid water oceans. Since these rogue planets would be quite distant to any stars, any life on them would have to rely on energy sources other than direct sunlight. The complexity and biomass may be low because the energy source will be small, but it is conceivable that these are the most common sites of life in the universe.
Scientists suspect the 'planet' is actually a small star.
Stevenson admits that his proposal is almost impossible to prove.