# Modelling a multi-star system with numerous planets and moons

Would it be in ANY way possible to have 3 or more stars within a few light-hours (up to several light-days between the most distant stars) of each other, and for the resulting orbital mash-up to allow for numerous planets to have stable orbits with moons?
These do not necessarily have to be on a single plane, and possibly work with several super-giants providing gravitational wrangling.

It would be nice if the planets could be atmospherically stable/habitable, either by incredible magnetic fields providing significant radiation shielding, or just by the stars being weaker and the planetary atmosphere being high in greenhouse gasses in order to hold onto the heat, or something else.

• This is pretty much a no I think, but largely because it's SO complex. Have you heard of the three body problem? en.wikipedia.org/wiki/Three-body_problem we can't even predict the gravitation effects of 3 objects in orbit most of the time. – sydan Mar 11 '15 at 11:16
• Oh, that I have. Currently, I'm playing around with an idea taken from Mean Field Theory to try to determine a set of 2d planes twisted through 3 dimensions showing the gravitational nullification points, then tweak the position and gravity of the Stars until I get a set of points where two planes collide, and those collisions stay cohesive during stellar orbit (implying gravity would "pool" and allow for rocks to gather). The resultant orbits of the planets wont be circular, perhaps more horseshoe-like, but we'll see – topherg Mar 11 '15 at 12:33
• Are you asking whether a habitable planet can form in a multiple-star system? Or whether a stable orbit can exist between two stars? – JDSweetBeat Mar 11 '15 at 13:10
• For a planet to have a stable orbit between two stars the minimum distance between the stars would be about 1 light year (the beginning of the Orbt cloud. – JDSweetBeat Mar 11 '15 at 13:12
• If ~70 light days is within your acceptable range, you don't have to go far to find it. Alpha Centauri is a ternary system, with Proxima Centauri orbiting the other two stars at 0.2 ligth years. – Diego Sánchez Mar 12 '15 at 17:03

You may be able to temporarily create a system like this, but long term, it would not be stable.

Let's first reduce this system to a binary system to talk about what would happen.

In a binary system, a planet has one of three orbit options:

1. It orbits both stars in a figure-8 pattern, passing through the L1 point.
2. It orbits both stars in a giant elliptical orbit. (P-type orbit, or circumbinary orbit)
3. It orbits one star in a tight elliptical orbit. (S-type orbit)

You can see in the potential diagram above how the figure-8 pattern is stable, but any perturbation would cause it to change into one of the other two orbits.

This problem is exacerbated in a 3/4/more star system. And if you have more than one planet, then you're guaranteed to get the kinds of perturbation that will decay all orbits into S- or P-types.

However, we do know about binary star systems with multiple planets (Kepler-47 is a great example), so this is not an impossible feat. Be warned that more than a couple planets may be impossible, unless they are significantly far from their parent stars.

As far as habitability goes, we honestly don't have a full understanding of the Goldilocks zone around multiple star systems. If the two stars are far enough apart (on the order of light days), then we can roughly approximate an S-type orbit as a single-star orbit, which gives us the Goldilocks zone. If the stars are much closer (on the order of a light hour or less), then it's unlikely that an S-type orbit would be habitable. However, a P-type may be habitable.

A complication here is that we haven't found any non-gas-giants in a P-type orbit. This may simply be a limitation of our current technology. Some astronomers propose that moons of the gas giants could possibly be habitable.

Depending on the assumptions that we're making, there are multiple scenarios where a trinary system could contain habitable planets.

One straightforward scenario would be a single massive star orbited by multiple dwarf stars. The difference in mass between a super-massive star and a dwarf star would (could) be on a par with the difference in mass between the Sun and Jupiter. Habitable planets could potentially exist in an orbit around the super-massive star, or in orbit around either (any?) of the dwarf stars.

In fact, you could have a super-massive star orbited by a dwarf star that was orbited by a habitable planet that was orbited by a potentially habitable moon.

The other possibility is looking at the definitions of 'stable' or 'habitable'. An unstable system could still remain habitable for thousands or millions of years - even if the stability is decaying quickly (on an astronomical scale), civilizations could rise and fall before the changes were noticeable. While this may or may not provide enough time for intelligent life to naturally evolve, even a highly unstable system could potentially remain stable enough to colonize and remain habitable for far longer than human history.

• +1, a very well thought out answer. I was thinking something similar to that (well, habitable moons around larger planets), but having a super-star could provide a suitable vector for a gate-based FTL system with people getting there in a realistic timeframe (in order to seed a gate). But I suppose that an environment could be temporarily terraformed in order to temporarily provision a rich environment for a civilization, even if temporary means the next 10/100/1000 years – topherg Nov 4 '15 at 21:34

I doubt it, for the simple reason that "light hours" is the distance at which some of the planets in our own solar system are currently orbiting.

I can't imagine having another star at the distance of Pluto would do any good to a solar system. Let alone multiple. And they would all somehow have to orbit each other, without stealing each other's planets.

• I am fully anticipating planetary theft (and I have reported it to the police), and have been thinking about creating a three-body braid orbit-like arrangement, but that requires incredibly specific masses, and from what I have seen, is very easy to unbalance causing the system to destabilize – topherg Mar 11 '15 at 12:37

Yes, most certainly that's possible.

Binary and triplet star systems have been observed, and some of them, such as Gliese 667, are known to have planets similar to Earth (Gliese 667 Cc which has Earth similarity index of 85%)

According to this lecture, the average distance between neighbouring stars in the Galactic Center is estimated to be about 1000 AU, or 1 light week, so even without the mashup you can expect to have neighbouring stars a few light days away.

I don't think the close proximity of start will affect anything in terms of habitability. Pluto, which is only 39AU away, get's very little sunlight (you can see "noon on Pluto" here), at few hundred AU, the effect is negligible.