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According to a BBC report

Astronomers have spotted a "rogue planet" - wandering the cosmos without a star to orbit - 100 light-years away.

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The proximity of the new rogue planet has allowed astronomers to guess its age: a comparatively young 50-120 million years old.

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One tricky part is determining if rogue planet candidates are as massive as the "failed stars" known as brown dwarfs, further along in stellar evolution but without enough mass to spark the nuclear fusion that causes starlight.

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The team believe it has a temperature of about 400C and a mass between four and seven times that of Jupiter - well short of the mass limit that would make it a likely brown dwarf.

Is this heat residual heat from the planet's formation 50-120 million years ago? if not, what is the process that sustains this temperature in the absence of a nearby star?

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Yes this heat is the residual heat from its formation ~ 70 Myr ago. It is currently cooling down, but it should take several billion years to reach temperatures as cold as Jupiter. The structure of this object is really close to Jupiter in fact, even if it's more massive it has roughly the same radius and a similar composition. We can't actually call it a "planet" because it doesn't orbit a star, but a preferred word than "sub-brown dwarf" amongst researchers is "planemo", meaning "planetary mass object". In fact, CFBDSIR2149 might have formed as a planet around some star, which was ejected from its system. So let alone the play on words, "planet / planemo / sub-brown dwarf", it really probably is exactly the same thing as massive giant exoplanets. Except it's freely floating in outer space.

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have a look at http://arxiv.org/abs/1210.0305. The object isn't really a planet, it's more like a mini brown dwarf star just too small to produce any fusion. The temperature is from it's formation, though the paper doesn't go into details of the model it uses for the temperature.

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If the discoverers are correct in their assessment that it is part of the AB Doradus cluster (85% probability), then it is a mini brown dwarf. For a good description, have a look at Phil Plait's Bad Astronomy blog

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