I know there are questions similar to this, but I'm not very scientific minded, and wanted to check with you guys how this would work exactly. I don't care what else I need to add into the equation to make it work. (like more planets, size or distance changes, etc...) As long as it can, plausibly, work.

Large sun as the main mass of gravity/pull (please excuse me if I get terms mixed up), a couple of uninhabitable orbits/planets in close orbit.

Then the first large habitable planet in my crappy drawing in its own orbit - a bit larger than earth so the gravity of that planet would be similar to ours. For that matter - how large could I make that planet and have it still relatively similar to earth?

Then a second orbit with three smaller habitable planets in rotation (again, only slightly smaller than earth, so the gravity would be similar to ours) though I don't mind if I have to make one of them bigger, as long as its still habitable, if not, I'd need to add in an additional uninhabitable planet.

This would mean that the large planet and the three smaller planets would have to each line up at one point. Like the first answer in this post. (though I don't understand half of Michael's answer, hence why I'm asking again, and with more specifics to my problems) My planets would be far enough away from one another though that I wouldn't have to worry about gravity pull or collisions.

Ideally, I would want all four planets to share the similar species. I know that the lone planet, being in a different orbit, would function quite differently, and create different species - but would there be a chance of anything human like with human intelligence?

Similarly with the planets sharing an orbit, would they have a better chance of developing/evolving the same, since they are in the same conditions?

I suppose something to ask on that front, would it work if a large early-stage planet (obviously no life form or anything on it yet) was pulled to the sun, and then somehow broke into three smaller rocks which then created the three planets (or the four, if that could work, the forth being pushed into that first orbit, away from the others?) At least then each world would have been started with the same materials, and then maybe the evolution would be similar - I mean, humans evolved with minor differences on opposite sides of the world? so it could work?

Solar Eclipses are actually a bit important in my story, how would they be affected by this situation? Especially for the smaller tri-planets? EDIT.

What I mean by this is, the tri-planets would still view an eclipse the same as we do, yes? So once or twice a year? But the lone planet might view them more times a year, as three planets are on the same orbit one orbit away from it?

And then beyond from that second main orbit would be more uninhabitable plants in more orbits, etc..

NOT TO SCALE. I don't care how far away the orbits have to be, as long as the planets are habitable. enter image description here

I'm sorry if I'm confusing guys, thanks in advance for your answers.


5 Answers 5


I think you would be best having multiple planets that are gravitationally locked with one another as in a planet/moon system. A prime example would be Pluto and its moons (largest Charon). Charon is so large when compared to Pluto that Pluto doesn't completely command it gravitationally...Charon actually drags Pluto around as well. The result is that the two (as well as the other moons) all revolve around a void space between them whose location within the system is based upon the relative masses of the bodies. The spot that is the center of the system would be closest to Pluto since it is the largest of the system and commands the greatest amount of gravitational pull. The net effect is that the Plutonian system orbits the sun and Pluto seems to wobble (as it is orbiting the Plutonian System's center of gravity). Eclipses of the sun and each other would be common as the bodies would be very close to one another. The effect of seeing "moons" that were of this type would make them appear in the sky several times the size of the host star. Nights would be twilit often times due to reflected light of the other planets. One thing to consider would be how close would be considered too close...if they were too close together, their gravity might cause collisions or they might tear each other apart (that might reduce the size in the sky and amount of reflected sunlight). Lots to research and consider before attempting...tides would also be wild...

As to the prescience of life, if they were in the habitable zone and all had the requisite elements/conditions, it would be more alarming if only one had life while the others didn't. There is a theory that basic life may have "seeded" Earth millions of years ago from elsewhere (Mars?!?) and that it was ejected into space through a collision which later fell onto Earth within space rocks. Some of the theory holds water when you add I. That meteorites on earth have been found to have Martian origin. So ultimately, microscopic life that develops one one would easily traverse and seed the others in a system. Keep in mind that such an event would be highly devastating to the planet of origin and life on it (the impact would have to be great enough to send rocks into space and would likely be considered an extinction event). Life on the planets would evolve separately (with the potential for some microscopic aliens from time to time) and would result in different species on each planet. Also keep in mind that cosmic events would likely affect each planet unless one or two were shielded by the others.

  • $\begingroup$ This was me before I made my account...wish I could claim it... $\endgroup$
    – user21896
    Commented Jun 20, 2016 at 23:16
  • $\begingroup$ Thankyou for your answer. I found it the most helpful. Have been researching more into these concepts. Thankyou XD $\endgroup$
    – Winchester
    Commented Jun 21, 2016 at 2:20

Gravity has very strict rules and as others have said, your ideas are impossible.

It is possible, though it's rare, to have 2 planets sharing an orbit. See here, but never 3. Earth and the mars sized Theia are thought to have shared an orbit for a time. But co-orbitals work best if they're not pulled about by neighboring planets so you'd want those to planets to have lots of space on both sides. Your larger planet on the inside of those two and not far away. That's also wouldn't work.

The co-orbitals has to be at specific orbital distance too, which means, no eclipses and just a dot in the sky. Not very interesting. A permanent morning star for one, a permanent evening star for the other.

There's no way to do what you propose with 4 planets.

The most impressive planet viewed from another planet is Earth, viewed from Venus, though Venus has clouds so it's a terrible planet for star gazing, but if you could see stars from Venus, Earth would look quite impressive.

If you want an interesting visual with 2 planets, where they pass relatively close and perhaps have the occasional eclipse, try something like this. Two planets can share similar orbits and swing past each other every few years. That's perhaps not stable on the scale of hundreds of millions of years either, but it's almost arguable.

Two planets orbiting around each other is possible too, like the Earth-Moon system. That's rare that two planets would do that, but theoretically possible. Both planets would have large tides though and you could have fairly regular eclipses.

The problem with 3 or more is, you can't have 3 planet sized objects in relatively close orbits. There's too much instability with three. Two kind of works though, but the 3rd would need some sizable distance.

@JDługosz you mention L3 and L4, the only stable Lagrange points are L4 and L5. You can have a 2 planet system 60 degrees apart from each other, the planet that's ahead is in L4 to the planet that's behind and the planet that's behind is L5 to the planet that's ahead. This is stable provided there are no other near-by planets and provided that the Sun they both orbit is measurably larger than the 2 planets.

If you add a 3rd planet and all 3 are equal sized, then the first and last planet are in L4 and L5 to the center planet, but they are 120 degrees apart from each other and that's the problem, if they're both equal size to the center planet, they would attract each other at 120 degrees and the saddle points would both become unstable. You can only have objects in both L4 and L5 if they are both comparatively small, or you can have 2 comparatively large objects, but not a 3rd. (I looked, I couldn't find a reference to back this up, and I can't do the math, but I know it to be true and it's logically consistant. The Trojan saddle is quite narrow and the two fairly large objects at 120 degrees from each other in the same orbit would gravitationally attract each other.


  • $\begingroup$ Thankyou for taking the time to comment. You've certainly helped me out a lot. I love the idea of two planets orbiting around one another, so I think that is something I will be using in my story. $\endgroup$
    – Winchester
    Commented Jun 21, 2016 at 2:19
  • $\begingroup$ Very nice writeup. But can you expand (or link) why you can't have 3 similar-sized bodies 60 degrees apart? That is, the middle one has companions at L3 and L4. $\endgroup$
    – JDługosz
    Commented Jun 22, 2016 at 4:04
  • $\begingroup$ @JDługosz answer given above, not room here. $\endgroup$
    – userLTK
    Commented Jun 22, 2016 at 7:17
  • $\begingroup$ It's best to improve the post with details and clarifications, not with comments, anyway, so that's normal. Hmm, but the post is not edited. So what did you mean by "above"? $\endgroup$
    – JDługosz
    Commented Jun 22, 2016 at 7:24
  • $\begingroup$ With your example about two moons with similar orbits (Saturn's moons), that's known as a horseshoe orbit. I tried asking a question specifically about such a scenario, but so far the answers suggest that it doesn't remain stable when scaled up to a star and habitable planets. $\endgroup$
    – Palarran
    Commented Dec 22, 2017 at 18:52

This setup cannot work

Lots of stuff, but here is where it falls apart. You say:

"all four planets to share the same [species]"


"Solar Eclipses are actually a bit important in my story".

If the planets are that similar in size... and so close they can eclipse one another, then the orbits cannot be stable. This system will fly apart, even if you "imported" planets.

In order to make this work you need magic to hold this together, or have it be an artificial, simulated system.

  • $\begingroup$ So not even similar? They don't have to be exactly the same species. So like, a dog and a wolf a similar to each other, but different, and yet still recognizable as a dog species. The eclipses could come from other plants that are further out/in in the solar systems. They don't have to be from these two orbits in particular, just need to be seen from them. Each planets will have their own moon/s. $\endgroup$
    – Winchester
    Commented Jun 19, 2016 at 12:39
  • $\begingroup$ @Winchester I am sorry but it just cannot work. Space and planetary systems are surprisingly empty.... as illustrated here: distancetomars.com This precludes eclipses because the planets are much too small at that distance. And for planets to independently evolve - let's call them canine-like creatures they will need to be fairly similar in size, solar energy input, atmosphere and.... and this is the big kicker: have the exact same randomness in the evolution process. This last thing will not happen. $\endgroup$
    – MichaelK
    Commented Jun 19, 2016 at 12:54
  • $\begingroup$ @Winchester Christopher Hitchens brings this last fact up very well in his book "God Is Not Great", where is he quoting Stephen J Gould. strangewondrous.net/browse/author/g/gould+stephen+jay ...where they both conclude that the entire family tree of vertebrates (of which humans, dogs and mani others are part) all depend on one thing: the survival and thriving of an early prototype called Pikaia Gracilens. If any one little event prevents Pikaia from thriving, every vertebrate in the history of the planet is wiped out. That is how volatile the evolution process is. $\endgroup$
    – MichaelK
    Commented Jun 19, 2016 at 12:59
  • $\begingroup$ Don't apologize XD you're helping me. So if my tri-planets were the same size of earth, with a same sized moon as earths moon, all the same, apart from there being three of them in the same orbit, how does that prevent an eclipse? Wouldn't it still work the same as the ones we actually get? $\endgroup$
    – Winchester
    Commented Jun 19, 2016 at 13:11
  • $\begingroup$ @Winchester It's rather the big inner planet - that close - that messes things up. Either you cannot get the same kind of environment on all four planets... or you cannot get them that close, because the inner planet will throw the other ones out of whack. $\endgroup$
    – MichaelK
    Commented Jun 19, 2016 at 13:12

The problem with the setup as described is the planets will gravitationally interact and kick each other out of their current orbits. You can see this in our own solar system, many planets or moons in the Gas Giant systems orbit in resonant periods with each other, since this provides stability.

When planets are in non resonant orbits, the best way to think of what happens is to think of a man pushing his child on the swing. If he pushes the swing correctly, the child will rise higher and higher, while if not, the swing could come to a stop. Planets "passing" each other in non resonant orbits will interact gravitationally and deliver much the same effect as the man pushing the swing.

Needless to say, being flung out into interstellar space or dropped into a close orbit around the sun will wreak havoc with the planetary biosphere and make much of the rest of your story moot as well.....

Without commenting on the plausibility of all the planets on the habitable zone sharing the same sort of biosphere (maybe they were all settled and terraformed by colonists from one single planet), the best way to get the effect I think you are looking for is to have a much larger star as the central sun, and have the planets in 4 separate orbits with the proper resonant periods so they do not fling each other out of orbit. This will take some calculating power (starting with the size and luminosity of the sun, which defines the habitable zone), and the planets will necessarily have long "years" so eclipses or transits (given the distances, a transit of the sun is far more likely than an eclipse) will be decades or centuries apart, depending on the planet.


Your setup is fine. 2 planets can share an orbit. The most stable setup is for them to be 60 degrees apart (technical term: in each other L4/L5 points). And there is plenty of space for multiple orbits within the habitable zone.

I've written a series of blog posts about how to design a planetary system. The main page is here (with links): https://planetplanet.net/the-ultimate-solar-system/

You will be specifically interested in these two posts: 1. Orbits (with 1 planet per orbit) https://planetplanet.net/2014/05/21/building-the-ultimate-solar-system-part-3-choosing-the-planets-orbits/ 2. Moons and co-orbital planets: https://planetplanet.net/2014/05/22/building-the-ultimate-solar-system-part-4-two-ninja-moves-moons-and-co-orbital-planets/

Hope this helps.

  • $\begingroup$ I agree that two planets could have stable orbits within each other's L4/L5, but is there a mechanism that would allow those planets to form? Gladman, 1993 determined a minimum orbital separation for accretion into a planet. I believe the paper is still valid, it is referenced in newer exoplanet books (here and here). I don't think two planets can form close enough to end up in each others L4/L5. $\endgroup$
    – kingledion
    Commented Dec 22, 2017 at 15:27
  • $\begingroup$ Yes, co-orbital planets are a common outcome of planet formation. If they grow relatively large early on while there is still a dense gas disk, then co-orbital configuration happens in a large fraction (>10%) of systems. I wrote a story about this stuff here: nautil.us/blog/the-mystery-of-the-missing-planets $\endgroup$ Commented Dec 22, 2017 at 16:13
  • $\begingroup$ The harvard abstract links in the post are all dead. $\endgroup$
    – kingledion
    Commented Dec 22, 2017 at 16:40
  • $\begingroup$ @kingledion You're talking about the ADS links? I just tried and they work fine. $\endgroup$ Commented Dec 22, 2017 at 20:19
  • $\begingroup$ Investigating further, it looks like the abstracts might be password protected, I get a page that says " ADS Message: No valid abstract selected for retrieval or not yet indexed in ADS " $\endgroup$
    – kingledion
    Commented Dec 22, 2017 at 20:27

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