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Tom Mazanec
2011-Sep-27, 07:05 PM
Two Jupiter size gas giants collide in a solar system of a red dwarf a few parsecs away, say several AU from the star.
Would we be able to see it?

antoniseb
2011-Sep-27, 07:36 PM
It seems like an unlikely scenario, but lets assume that the two gas giants are in dissimilar orbits (that is they collide when one is at periastron and the other is at apastron, and the SMAs had a ratio of about 1.4, and that the red dwarf is 0.2 solar masses.

The relative velocity *before* falling into each other's gravity wells would be about 3 miles a second (I'm waving my hands, I didn't actually calculate it). The infall would add another 50 miles a second (again, I didn't calculate, just a guess based on the speed of the Galileo Atmospheric Probe). Let's call it 8 million cm/second.

Each object would have a mass of 2x1030 grams, and the collision, if head-on (also unlikely) would take about 1000 seconds for the bulk of the collision. ... so the amount of energy converted to heat would be about 1044 ergs peaking for about 103 seconds... this would be more than 10 million times the energy output of the Sun, so if it were all released as visible light (far from the actual case), it would be a star about magnitude -10 for 20 minutes or so.

Undoing my simplifying assumptions, the reality would be much dimmer in the visible, and much longer duration in the infrared, and most of the energy would be trapped in the bodies of the two planets as heat.

R.A.F.
2011-Sep-27, 07:39 PM
I'm not quite sure what it is you are asking...are you assuming that 2 "Jupiter's colliding" would emit more light than they would separately?

Tom Mazanec
2011-Sep-27, 07:49 PM
I'm not quite sure what it is you are asking...are you assuming that 2 "Jupiter's colliding" would emit more light than they would separately?
I'm not quite sure what you're asking...you need two bodies for a collision. As the above posting estimated, the collision would release about 10^44 eargs, which would look like something that would likely be more visible than just one Jovian sitting there.

R.A.F.
2011-Sep-27, 08:16 PM
Would we be able to see it?

I see no reason not to accept Antoniseb's -10 magnitude "estimate" so the answer appears to be no.

Tom Mazanec
2011-Sep-27, 08:21 PM
I see no reason not to accept Antoniseb's -10 magnitude "estimate" so the answer appears to be no.

Umm...if I interpret the post correctly, it would be about as bright as a crescent moon for 15 or 20 minutes. Actually, he points out that it would actually be dimmer for longer, but I would expect the answer to be "yes", especially since by "we" I meant the human race, with our telescopes.

R.A.F.
2011-Sep-27, 08:24 PM
DOH...you are correct...such a rookie mistake...I shouldn't post when hungry. :)

jfribrg
2011-Sep-27, 08:34 PM
When the two planets approached, would the tidal forces cause both to become elongated? Assuming that they are both gas giants, that might complicate things. You might have two tear-drops colliding, but unless they are on a bulls-eye collision course, there could be a substantial percentage of the mass that doesn't immediately collide, which would reduce the maximum brightness of the collision while lengthening the time that the collision is visible.

eburacum45
2011-Sep-29, 08:14 AM
It seems to be the case that planets do occasionally collide, if this article is correct
http://www.sciencedaily.com/releases/2008/09/080923164646.htm

I note that some planets orbit in a retrograde fashion
http://www.newscientist.com/article/dn17603-planet-found-orbiting-its-star-backwards-for-first-time.html
if a system contains both prograde and retrograde gas giants, collisions could be spectacular and highly luminous. Have any such events ever been recorded, I wonder?