Rogue planets

Astronomers are closing in on those sunless planets that drift through interstellar space.  The ones featured in such SF novels as George R.R. Martin's Dying of the Light...

http://www.nytimes.com/2011/05/19/science/space/19planets.html
http://www.space.com/11699-rogue-alien-planets-milky-common.html

There may be more sunless planets than there are stars!  However, the latest data indicate they are more likely to be gas giants than terrestroid worlds.