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I'm aware that 'tatooine' worlds don't make good habits for planets, normally. From what I've read, either a planet has to orbit one star really closely, or orbit both stars from a really long distance away.

I envisioned a kind of double solar system, where there are two stars that are close enough to orbit one another, but far enough away where they can each have planets of their own.

In such a situation, its possible to have habitable worlds orbiting both stars, but they would be conceivably close enough where intelligent life from these worlds could potentially have some awareness of each other, and possible communication, even with our own current level of technology.

Of course, orbital calculations are beyond me, and from my understanding would require a supercomputer to work out. I don't believe I've ever heard of an actual 'double solar-system' like this, which would lead me to believe that such a thing is not possible.

If it matters, I wasn't thinking of either of these stars as having that many planets. The stars are close enough where outer-planets like in our system simply can't exist. And one only has one planet: a gas giant, with multiple habitable moons. The other is more like our solar system (well, inner solar system anyway, there's only 3 or 4 planets).

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  • $\begingroup$ Hello IXBlackWolfXI, and welcome to Worldbuilding! If you have any questions about how our site works, please take our tour and visit the help center to learn more. $\endgroup$
    – Gryphon
    May 10, 2018 at 20:09
  • $\begingroup$ I am somewhat confused: are you talking about a system that has planets that go between two stars or a system where planets obit around two stars? $\endgroup$
    – Pinion Minion
    May 11, 2018 at 2:45
  • $\begingroup$ I'm talking about a binary star system, in which each star has its own set of planets orbiting around it within its habitable zone. I was thinking of something like two small solar system, except the stars are close enough together to be gravitation-ally bound to each other. $\endgroup$
    – user50663
    May 11, 2018 at 3:05
  • $\begingroup$ As a possible example, the stars for Alpha Centauri (A and B orbit each other, and C orbits both at a much greater distance), are all far enough from each other than planets could be within the habitable zone of each star. $\endgroup$
    – Michael Richardson
    May 17, 2019 at 22:27

3 Answers 3

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Yes, and the planets can orbit in the habitable zones.

This page gives a long list of planets in these S-type orbits, and states

For a few systems (2 so far), there are planets orbiting each member of the binary. In this case, the system is divided into two "binaries", one where the first star is the "central star" and the other star is the perturbing companion, and another one where the roles are reversed.

It's important to note that many of the planets in S-type orbits are quite close to the main star they orbit, meaning that the influence from the other star is small, and the orbit is stable over reasonably timescales. This shouldn't preclude a planet from orbiting the other star in the system.

One thing that's worth noting is that binaries where S-type orbits are possible usually have a large separation - that is, the stars are pretty far apart. This makes it possible for the planets to be in a larger range of orbits, meaning they aren't restricted to orbiting close to one star. Therefore, in wide binaries, it's more likely that a planet could orbit in the habitable zone of one of the stars.

Examples I've been able to find (to be updated):

  • WASP-94, a pair of F-type main sequence stars, each with a close hot Jupiter orbiting it.
  • HD 20781/HD 20782, two G-type stars, each with 1-2 planets (one at 1.3 AU orbiting HD 20782, two at <1 AU orbiting HD 20781).
  • Kepler-132, a pair of G-type main sequence stars, although the structure of the system has been disputed.
  • XO-2, two cool K-type stars, with a confirmed hot Jupiter around one star and two possible planets around the other.

I've found these by perusing a list of exoplanets in binary systems and doing some cross-checking. Doing some simulations of the HD 20781/HD 20782 system indicates that it should certainly be habitable. The two stars are separated by ~9000 AU, both are G-type stars like the Sun, and stable orbits exist around 1 AU, in the habitable zones of the star. The same goes for the other systems, which also have extremely large binary separations.

To make a long story short, yes, this will work.

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  • $\begingroup$ Yes, but orbiting that close means that any planets would almost certainly not be habitable. Such planets would probably all be like mercury and maybe venus. I guess my question should've asked if planets in such a system could find a stable orbit in a habitable zone. $\endgroup$
    – user50663
    May 10, 2018 at 20:16
  • $\begingroup$ I noted in my last paragraph that binaries with large separations allow you a lot more freedom, @lXBlackWolfXl. I don't know the orbital parameters of the two systems in question, but binary stars with separations of thousands of AU exist - more than enough space for planets to orbit stably in the habitable zone. $\endgroup$
    – HDE 226868 ♦
    May 10, 2018 at 20:17
  • $\begingroup$ @lXBlackWolfXl I've updated my answer, and I've found a system of Sun-like stars with stable orbits in the habitable zone of each star. $\endgroup$
    – HDE 226868 ♦
    May 10, 2018 at 20:30
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Here is how you do it:

https://www.youtube.com/watch?v=mVh84wBCP_s&t=0s&list=PLduA6tsl3gyi72fEb-_zWk_yh8XkEWfjT&index=5

Edgar has a really great series on how to calculate everything you need to know about solar systems, as well as how plausible different types of systems are:

https://www.youtube.com/watch?v=IE805Vdm0sk&t=34s&list=PLduA6tsl3gyi72fEb-_zWk_yh8XkEWfjT&index=6

Edgar's series is great, and it seems a shame I don't see him cited here more often.

I'd also check out barycenters.

https://en.wikipedia.org/wiki/Barycenter

You might notice that this particular orbit:

https://en.wikipedia.org/wiki/Barycenter#/media/File:Orbit5.gif

is the one most commonly associated with S-type systems, but that doesn't mean that the stars in such systems always have the same mass, as the gif suggests.

Have fun!

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    $\begingroup$ I'd recommend editing this to add in the relevant information; links can, eventually, break, and having the information in the answer saves it for posterity. $\endgroup$
    – HDE 226868 ♦
    May 10, 2018 at 21:49
  • $\begingroup$ @HDE226868 I agree, but it's a weekday. This way the OP has access to the info sooner, and with the answer on my activity page I don't have to worry about it getting buried or forgetting it when I have time to edit this weekend. $\endgroup$
    – KernelOfChaos
    May 10, 2018 at 21:56
  • $\begingroup$ I wouldn't trust that youtube channel. I've watched a video once (about axial tilt and seasons) and some of his theories sound like nonsense (like the polar and tropical switch). Maybe it's fun to watch, but you'd better double check things with a more serious source. $\endgroup$
    – Ghajini
    May 11, 2018 at 12:04
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It seems strange to me that someone would even ask that question. I think I read that as a general rule, if Planet A orbits star B at distance C, another star D in that system will not destabilize the orbit of planet A if star D never gets any nearer to star B than a distance of about 5 times distance C.

The nearest star system to Earth, Alpha Centauri, is a double star. And it has often been calculated that planets orbiting in the habitable ones of either Alpha Centauri A or Alpha Centauri B would have long lasting stable orbits.

Earth orbits the Sun at a distance of 1 astronomical unit, or AU. The equivalent distance around Alpha Centauri A would be a little larger, and that around Alha cCentauri B would be a little smaller. Since the two stars never get any closer to each other than 11 AU, any planets that might be in their habitable zones should have orbits stable for a long time.

And there are many other binary star systems in which calculations indicate that any planets in the habitable zone of one of the stars should have stable obits. Of course there are many binary star systems where the two stars are too close for planets orbiting one of the stars in the habitable zone to have stable orbits.

There are even stars so close together that a planet could orbit in the habitable zone around both of those stars and have a stable orbit.

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  • $\begingroup$ I've read that it would all but impossible for a star to have a stable orbit in the habitable zone in a binary star system. Also, alpha centauri is a three-star system. And its believed there's a planet orbiting in the habitable zone of the third star, proxima centauri b. Though its believed that the planet is tidally locked to its host star, and also would be constantly hit by solar flares that would strip away its atmosphere rather quickly. $\endgroup$
    – user50663
    May 14, 2018 at 5:31

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