The universe could be full of failed stars-- or overachieving gas giants, too dim to see until recent developments in infrared telescopic technology. One has already been discovered that has a temperature as low as 86 degrees! It still gives off too much radiation for humans to live on the surface, but there's great deal of excitement. Objects like this, with mass and radiation levels ranging between those of large planets and those of small stars, might be orbited by satellites like Jupiter's moons. If these objects, with or without satellites, are as common as some astronomers suspect, they could be used as refueling stations for interstellar travel.
Article full of further links at Discovery, found via the Slacktiverse
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