/ Rogue planet, 400C - how can this be

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pebbles - on 14 Nov 2012
apparently astronomers have "spotted a "rogue planet" - wandering the cosmos without a star to orbit - 100 light-years away."

the report says "The team believe it has a temperature of about 400C and a mass between four and seven times that of Jupiter "
how come its so warm if it hasnt got a star? its pretty big, but the surface temperature of jupiter is still -145 degrees C. So do they mean core temperature or something? Or is it just a typo or the reporter misunderstanding? Any of the astronomer types on here know?

http://www.bbc.co.uk/news/science-environment-20309762
pebbles - on 14 Nov 2012
In reply to pebbles: just checked the actual report and the temperature isnt a typo. so is that mass enough for a planet to heat itself? and if so, then does that mean that you could potentially have some of these things wandering around potentially able to support life without needing a star of their own at all?
richard_hopkins - on 14 Nov 2012
In reply to pebbles:
Here are some of my thoughts
1) Gravitational heating, caused by increased pressure as the planet contracts from when it WAS orbiting a star. This effect significantly warms jupiter. If it were larger then the gravitational compression would be enough to cause it to undergo nuclear fusion in the core and turn into a star.
http://en.wikipedia.org/wiki/Kelvin%E2%80%93Helmholtz_mechanism

2) Radioactive decay, this would be a fairly small effect.

3) Tidal heating caused by an as yet unseen body orbiting it.

jonathan shepherd - on 14 Nov 2012
In reply to richard_hopkins: Here are some of my thoughts.
1. Its a death star, time to leave the planet.
Hugh J - on 14 Nov 2012
In reply to pebbles: It's Nibiru and we've only got 37 days left!
Camm on 14 Nov 2012
In reply to pebbles:
Hats off to the person who got the themometer on there!
pebbles - on 14 Nov 2012
In reply to richard_hopkins: ta! interesting link
Coel Hellier - on 14 Nov 2012
In reply to pebbles:

It's a young planet. That means it is still coalescing out of the stuff it was made out of, and that means it is contracting. As it contracts it releases gravitational potential energy, and that energy gets converted to heat. Indeed, this is how they managed to estimate the age of the planet as 50-120 million years old. This process would be over after a couple of hundred million years old. Jupiter is much cooler because it is much older (4.5 billion years).
pebbles - on 14 Nov 2012
In reply to Coel Hellier: aha! thanks coel. I was genuinely puzzled by how it could be so hot.
no_more_scotch_eggs - on 14 Nov 2012
In reply to Coel Hellier:

what's it doing out there then...? taken a bit of a shunt in an evolving solar system?

and is that quite big for a planet? i thought jupiter was pretty big already. how big can a planet get before it becomes a brown dwarf star...?

cheers
gregor

ps where you been, thought you'd have some comment to make on the eclipse thread....?

;-)
Coel Hellier - on 14 Nov 2012
In reply to no_more_scotch_eggs:

> what's it doing out there then...? taken a bit of a shunt in an evolving solar system?

Most likely yes, some multi-body near collision with something heavier that sling-shotted it out of its solar system. (Or, much less likely, it formed in situ in a region without enough stuff to form anything heavier.)

> and is that quite big for a planet? i thought jupiter was pretty big already. how big can a planet
> get before it becomes a brown dwarf star...?

They're claiming that one as 4 to 7 Jupiter masses, which is fairly common; planet masses peter out at about 20 Jupiter masses. Brown dwarfs are then heavier than about 30 Jupiter masses. You need about 75 M_Jup to burn hydrogen and so be a star. (You can burn lithium at > 13 M_Jup, but nowadays that isn't considered sufficient qualification to be a brown dwarf.)
no_more_scotch_eggs - on 14 Nov 2012
In reply to Coel Hellier:

is it easier to fuse lithium than hydrogen? i thought that it became progressively harder to generate fusion with higher atomic masses...?

cheers

gregor
Coel Hellier - on 15 Nov 2012
In reply to no_more_scotch_eggs:

Sorry, I meant deuterium, not lithium. Deuterium is easier to burn than hydrogen (no involvement of weak-force needed).

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