Gas Giants: Two for the price of one and we don't know it?

[Michael Kilderry]

Although double gas giants may be unlikely, nobody ever expected hot jupiter planets like 51 Pegasi b either, so here's the point my post:

Could extrasolar planets that have been discovered actually be two orbiting tightly around a barycenter between them? Some are quite massive, but in fact these could actually be two worlds, each about half of the mass predicted of the one. Future, more precise doppler shift research in the future could find these out. These would make for interesting giant tide-affected worlds.

What's your opinion?

Michael Kilderry

[Evil Dr Ganymede]

well, there's a double brown dwarf system around epsilon indi:
http://www.solstation.com/stars/eps-indi.htm

They orbit with a separation of 2.65 AU which is pretty darn close on a stellar scale but a respectable distance on a planetary scale. Tidal effects probably aren't that noticeable there if they're that far apart. But that opens the door to the possibility that you can get jovian-type worlds orbiting eachother, but I'd guess it'd be more likely for isolated objects like brown dwarfs rather than jovians within a planetary system.

[Michael Kilderry]

Interesting,

I can think of a scenario that could give us two jovains that orbit eachother.

Say we had the same scenario that formed the Earth and Moon, but out further from the sun, where bigger planets are likely to form. The quciker mass accretion out here could cause this to happen earlier, and still give the planet and moon more time to grow, and eventually give us two gas giants.

Michael Kilderry

[Spaceman Spiff]

Hmm, yes, there might be no reason that two forming gas giants collide and form a gigantic Earth-Moon style 'double planet', if the gas giants were mostly formed, and they are not too close to the parent star.

I say "not too close" because going back to the original post, mentioning 51 Peg, I would expect two gas giants being, say, 300,000km apart would be prone to such a great difference in gravitational pull from 51 Peg just a few million km away - that is, 'tidal disruption' across the pair of gas giants as they orbit their common barycentre. It's probably unstable compared to the lifetime of the star.

Double gas giants at the distance of Jupiter would be very stable though. If Jupiter could hold into Io through to Callisto OK, then it can hold on to a Gas Giant too...

Finding the limit of stability between 5 A.U., and 0.05 A.U. is something many detailed and complicated calculations would be needed for.

By the way, there's one of those 'did you know...?' facts about the Earth and Moon. The gravitational pull of the Sun on the Moon is twice as strong as the gravitational pull of the Earth on the Moon. In that sense, the Moon really orbits the Sun, and the Moon and Earth travel together in a common orbit. I think chris made a post about replotting the Moon's orbit about the Sun instead of the Earth, and was surprised to see the Moon never moves backwards in it's true orbit around the Sun. This is indeed the case.

I think Solar tidal disruption from the Sun was once an explanation for why Venus and Mercury don't have moons. However, I think the Moon could orbit Venus quite stably just like the Earth. The Solar tide increases by only about 2.7 and Venus has 89% the mass of Earth, making the Solar force 6 times greater than Venus. For Mercury, the tide would increase much more. With perihelion at 0.29 A.U., and Mercury weighing 5.5% of Earth, the competing force grows to 750 times...

So, when you feed that back into the moonless Mercury and Venus puzzle, it's not so clear cut. Mars' moons are probably captured asteriods. More likely terrestrial planets simply don't normally form moons.

Maybe double gas giants near to their parent star could do orbits like an Earth-Moon with each other. I wouldn't expect much room for stable moons though...

Well, that's my opinion.

Spiff.

[Matt McIrvin]

This actually brings up one of my hobbyhorses, which is that when thinking about extrasolar planetary systems we really ought to be extrapolating not from our own solar system, but from what we know about extrasolar multiple star systems. We know that, at least for the brighter stars, complex bound multiple star systems are more the rule than the exception in our galaxy, and many of them are absolutely nutty by Sol-system standards: sextuple stars with close spectroscopic binaries in them, and such. It's not much of a stretch to imagine replacing some of those stars with Jupiters.

So in 20/20 hindsight, all those hot Jupiters should not have been such a surprise. And I'd expect gas giants and brown dwarfs to appear in all manner of strange configurations.

[eburacum45]

This is a model I have just made of Anders Sandberg's twin gas giants Bill and Bull from the OA universe;

I had to use 3ds eventually, as I was having difficulty getting the two planets to orbit each other with their poles pointed together.

The original image made by Anders in POV-Ray is here;
http://www.orionsarm.com/worlds/Bill_and_Bull.html