Brown Dwarf Planets

Brown dwarfs are failed stars-- bodies not quite massive enough to sustain nuclear reactions-- but they still might have planets orbiting them.  Researchers are proposing to use NASA's Spitzer Space Telescope to find such worlds, especially Mars-sized ones.

Mars-sized objects seem to be important.  In current planetary formation theory, Mars-sized bodies banging around the early Solar System formed the larger planets, so some researchers think finding such things orbiting brown dwarfs could be an indicator of planets still to be found.

Since brown dwarfs do give off heat, it's also possible that a planet in an extremely tight orbit could support native life.  Alternatively, such a world could also be a good outpost for a star-hopping civilization.