In article ,
(Steve Pope) writes:
It seems that free-floating planets have been discovered
with magnitudes in the +15 to +20 range.

What reference have you seen? I've seen some candidates but wasn't
aware that any are yet confirmed.

Since these
objects are too small to ever have undergone nuclear
fusion, why are they luminous? Is it just from the
friction of condensing?

That's it, though I'd probably say "release of gravitational
potential energy," which means the same thing but sounds more
scientific. :-) There's probably some contribution from radioactive
decay as well. In our own solar system, Jupiter radiates more energy
than it receives from the Sun. The extra presumably comes from
leftover heat of formation. It just takes a long time to escape.
Similarly, the Earth gets hotter as you dig down; again that's partly
leftover heat of formation and partly from radioactive decay. (Last
I heard, radioactive decay is the main contributor for Earth.)

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Steve Willner Phone 617-495-7123
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