# Thread: Planets around multiple stars?

1. ## Planets around multiple stars?

There's a section there about stable orbits in binary systems, with a diagram from a paper by Trilling et al showing how the Spitzer telescope detected disks around multiple stars ( http://www.solstation.com/images/bi2sep.jpg ). I've read the paper, and it says that in most cases you'd have a disk around both stars if they're close together, a disk around only one star if they're widely separated, and no disk around either star if they're in between.

Problem is, that doesn't tally at all with existing orbital dynamics. Theoretically at least, you should be able to have stable orbits around each star in an intermediate system (assuming the stars don't get too close to eachother in their orbits, that is), or each star in a widely separated system. (see e.g. Long-Term Stability of Planets in Binary Systems (Holman, Matthew J.; Wiegert, Paul A.) The Astronomical Journal, Volume 117, Issue 1, pp. 621-628).

But the results from Spitzer seems to be implying that this doesn't happen, and I can't see a reason why you cannot have planets in stable orbits around each star. Right now I'm running a simulation in Gravity Simulator of two stars orbiting eachother with a semimajor axis of 30 AU and a eccentricity of 0.5, with asteroid belts containing 100 objects around each star between 0.5 and 10 AU. Initially a bunch of asteroids get stripped off from each star during closest approach (which I expected), but after about 12 orbits (1200 years or so) they seem to have stabilised, with each star having a smaller remaining bunch of asteroids in the closer and more stable orbits around them beyond the other star's reach. So while that's a pretty short timescale, unless something changes drastically it doesn't look like both stars are in any danger of losing their disks entirely (right now, it looks like the more massive star has 50 objects in orbit around it, and the less massive star has 40. So I guess if you turn that into percentage of material remaining, one is down to 50% and the other is down to 40%, but they don't seem to be losing much more material because they've already lost most the asteroids in the least stable orbits during the earlier perihelia).

So are we missing something here? The Spitzer results seem to be somewhat interpretative - they're not actually detecting the disks directly as far as I can tell, they're just detecting an IR excess that implies there are disks there. So could the excess IR signature be indicative of something else? Or could there be systems with fully-formed planets around them and no detectable IR excess because there's not enough excess dust remaining in the system? (as a secondary point, if we were looking at the Sun from outside the system from say 10 pc away, how much dust would we be able to detect around it? Would any excess IR show up at all?)

My suspicion is that planets are possible around all these orbital configurations, so long as the orbital dynamics allows them to be there - there's no reason that I can see why they shouldn't be able to exist within specific orbital ranges otherwise. If we're seeing excess IR from dust then that's a bonus confirmation (well, at least it shows there's dust and planet-forming material there, implying that there could be planets too), but are we really justified in definitively saying "if there's no excess IR, there's no planets".

Anyone got any thoughts on this?
Last edited by EDG; 2009-Dec-27 at 09:40 PM.

2. ## gas versus discrete bodies

Spitzer is seeing disks around stars when the disks are made largely of gas and very small dust particles. There are a number of physical processes which act on these gas and dust particles which do NOT act on large, discrete, planet-sized bodies. For example, gas pressure, radiation pressure, magnetic field effects on ionized gas, etc.

It is possible to demonstrate one set of stable conditions for large, planet-like bodies in a double-star system, and a different set of stable conditions for gaseous disks in the same systems.

3. Sure, but then what's Spitzer telling us? IIRC they were looking at reasonably young stars (Type A and F) - if they're saying that those stars don't tend to have disks around them early in their histories (when we're looking at them now), then wouldn't that mean they shouldn't have planets that form from them later on?

Or are you saying that while it might be theoretically possible for disks to form around all multiple stars, there's something else that's making them disappear before they have the chance to form planets around those intermediately-separated stars? If so (and I don't know if anyone knows the answer to this) then what could that be?

4. Originally Posted by EDG_
Sure, but then what's Spitzer telling us? IIRC they were looking at reasonably young stars (Type A and F) - if they're saying that those stars don't tend to have disks around them early in their histories (when we're looking at them now), then wouldn't that mean they shouldn't have planets that form from them later on?
Yes.

5. So when you have two widely separated stars (closer to the 500 AU end of the range), why would only one of them retain its disk? It seems to me that this is far enough for both to survive - it's too far for heat or radiation pressure from the one star to have any effect in dispersing the other disk.

Personally I think the sample they're using might be too small for some of the configurations. They only saw three systems in the 3-50 AU range, 12 in the 50-500 AU range, and 7 in the 0-3 AU range. I'm not sure that one can really say anything for sure about the 3-50 AU range given that - maybe they just got unlucky. It's a good start but I'd want to see a much more extensive survey of stars before I would be convinced by their conclusions.

6. Wow 500 AU, that should be easy. But then at 500 AU any planet in the system wouldn't see 2 suns it would only see 1 or even no stars, unless said stars are super giants. The sun is not much more then a star at 30-50 AU forget a planet at something like 400+ AU given the hill sphere of one of the stars.

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Don't Jupiter and Saturn, very lightweights compared to a star, by being able to assemble and maintain a set of satellites and debris, prove that satellites will form and remain in multiple star systems? If they were more massive, their satellite systems would be even more stable.

Don't Jupiter and Saturn, very lightweights compared to a star, by being able to assemble and maintain a set of satellites and debris, prove that satellites will form and remain in multiple star systems? If they were more massive, their satellite systems would be even more stable.
If Jupiter and Saturn were stellar mass instead, then we'd have all sorts of problems with their own orbital stability and that of the other planets. And their satellites probably wouldn't survive the experience. I'd suspect that we'd end up with stellar-Saturn thrown out of the system and stellar-Jupiter in a different orbit around the Sun, and we'd probably lose Mars and possibly Earth as well depending on how crazy things got.

9. Originally Posted by WayneFrancis
Wow 500 AU, that should be easy. But then at 500 AU any planet in the system wouldn't see 2 suns it would only see 1 or even no stars, unless said stars are super giants. The sun is not much more then a star at 30-50 AU forget a planet at something like 400+ AU given the hill sphere of one of the stars.
The Sun at 500 AU is still brighter than the full Moon as seen from Earth, albeit pointlike. Even an itzy 0.01 LSun red dwarf at 500 AU would be well brighter than Venus at its brightest.

10. Originally Posted by EDG_
If Jupiter and Saturn were stellar mass instead, then we'd have all sorts of problems with their own orbital stability and that of the other planets. And their satellites probably wouldn't survive the experience. I'd suspect that we'd end up with stellar-Saturn thrown out of the system and stellar-Jupiter in a different orbit around the Sun, and we'd probably lose Mars and possibly Earth as well depending on how crazy things got.
Whether stable orbits exist or not in a binary system is fairly non-trivial, as the differential equations involved don't lend themselves to the various tests of system stability, such as Routh or Lyapunov. I believe that there are numerous papers posted on the arXiv web site; do a search there and I believe you'll find that there are stable orbits within the CHZ of both Alpha Centauri A & B.

11. Originally Posted by swampyankee
Whether stable orbits exist or not in a binary system is fairly non-trivial, as the differential equations involved don't lend themselves to the various tests of system stability, such as Routh or Lyapunov. I believe that there are numerous papers posted on the arXiv web site; do a search there and I believe you'll find that there are stable orbits within the CHZ of both Alpha Centauri A & B.
Well, we've got somewhat distracted from the point that I was asking about.

I know there are stable orbits in multiple star systems. It's just that Spitzer seem to be telling us that there won't necessarily be anything in those stable orbits, since it depends on the exact orbital configuration of the system.

I can see how it is less likely for there to be planets in the binaries with intermediate separation (3-50 AU - I guess things like extra radiation pressure from the other star on closest approach etc might disperse the disks too quickly, or may thin the disks out too much for planets to form), I'm just having trouble seeing why only one star has a disk and the other doesn't in the much more widely separated ones (50-500 AU).

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Ah, Centaurus A & B fit the close in model?

13. Originally Posted by EDG_
Well, we've got somewhat distracted from the point that I was asking about.

I know there are stable orbits in multiple star systems. It's just that Spitzer seem to be telling us that there won't necessarily be anything in those stable orbits, since it depends on the exact orbital configuration of the system.

I can see how it is less likely for there to be planets in the binaries with intermediate separation (3-50 AU - I guess things like extra radiation pressure from the other star on closest approach etc might disperse the disks too quickly, or may thin the disks out too much for planets to form), I'm just having trouble seeing why only one star has a disk and the other doesn't in the much more widely separated ones (50-500 AU).
Sorry for misunderstanding. I believe that some planets have been detected in multiple systems, so it's evident that there is no intrinsic reason why binaries can't have planets.

14. Originally Posted by swampyankee
Sorry for misunderstanding. I believe that some planets have been detected in multiple systems, so it's evident that there is no intrinsic reason why binaries can't have planets.
Yes, but IIRC those planets are orbiting close binaries - i.e. they're orbiting around both stars. The Spitzer data shows that this is possible.

Alpha Centauri would be "intermediate" separation (3-50 AU) which would mean that neither star should have planets.