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So I've been working on a project where I modify the Solar System, and I originally had a small gas planet(like 5-10 earth massses) named Juno orbiting between Jupiter and Saturn. However, as it turns out, the area between Jupiter and Saturn is rather precarious - when I ran the system in a simulation the two gas giants ejected my precious little Juno from the Solar System within 1 million years or so. I don't want all the worldbuilding work I put into Juno to go to waste, however, so I'm thinking that Juno could be a Lagrangian companion to Jupiter, sharing the giant planet's orbit. The idea isn't too ludicrous - a lot of asteroids(the Trojans) do this exact same thing. However, this is not an asteroid - this is a rather massive planet. When I ran it in the simulation it surprisingly seemed to be stable, but I want to ensure that it remains stable over billions of years. So could my gas dwarf share Jupiter's orbit?

Nip Dip
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    IIRC having a heavy body in L4/L5 Sun-Jup results in a stable system if its mass is "a lot less" than Jup's, but how much is a lot was not written where I read that. Maybe a 1/10 of Jup could hold itself in L5 without interfering on Jup too hard, but again, maybe not. – Vesper Oct 17 '23 at 09:22
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    I'm sure we've had questions about this in the past, but my searching isn't turning up much. Aside from that: good work on simulating your setup... that puts you head and shoulders most other people building fictional planetary systems. Set your sim going at a decently high timestep, go out for the day, and if your world is still there when you get home that's probably good enough for everyone. No-one will bother to prove otherwise, even if they could. – Starfish Prime Oct 17 '23 at 11:12
  • Does this answer your question? Can multiple similar planets share the same orbit without interfering with each other too much? – JBH Oct 17 '23 at 16:27
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    Does this answer your question? How Many Planets Can Safely Share A Single Orbit? – sphennings Oct 17 '23 at 19:13
  • @spohennings Perhaps the answers there are not final. See https://planetplanet.net/2017/05/03/the-ultimate-engineered-solar-system/ – M. A. Golding Oct 17 '23 at 20:27
  • JBH it is possible that the answers to that question are not the last word on the subject. See https://planetplanet.net/2017/05/03/the-ultimate-engineered-solar-system/ – M. A. Golding Oct 17 '23 at 20:28
  • Even in our reality, why should it be problematic for another planet to balance itself 180º against Jupiter in the same orbit? Why could several bodies not share the same orbit, if their masses were suitable?

    Since this is about your own built world, what percentage of readers might notice if you went as far as you could with reality, and fudged the rest?

     – Robbie Goodwin Oct 17 '23 at 21:58
  • By the current definition of a planet, no. Any body less massive than Jupiter but in Jupiter’s orbit wouldn’t have cleared its orbit and so couldn’t be a planet. – Mike Scott Oct 17 '23 at 18:16
  • That is a rather silly thing to say since the question wasn't asking about definitions but orbital stability. According to the IA.U. definition Jupiter itself would not be a planet in that situation. – M. A. Golding Oct 17 '23 at 19:06
  • @M.A.Golding Jupiter would still be a planet, just like Neptune is despite Pluto. A smaller body in an orbital resonance with a planet doesn’t mean that it hasn’t cleared its orbit. – Mike Scott Oct 17 '23 at 19:29
  • This reads more like a comment than a full answer. – sphennings Oct 18 '23 at 02:58
  • @M.A.Golding On the contrary, this is the only correct or relevant answer. If I ask if there’s a cow in a field, telling me that there’s a sheep is uninformative. If I ask if a planet can be somewhere, telling me that some other kind of body can be there is equally uninformative. If I didn’t really mean a planet when I asked about a planet, that’s on me. – Mike Scott Oct 18 '23 at 06:44
  • @MikeScott The fact that the question is using the colloquial definition of planet rather than the official IAU definition as intended for use in professional publications doesn't mean that they're using the word incorrectly. Colloquial definitions are perfectly valid in informal discussions, and literally no one here was or would be confused about the intended meaning of the question. – Idran Oct 18 '23 at 13:35

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The solar system itself is NOT stable over billion of years, as any good N body system does.

And the hypothesis of Theia tells you that a planet in a Lagrangian point is easily perturbed out of it with dramatic consequences.

L.Dutch
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Nip Dip says they ran a simulation of the set up and it seemed to be stable - for the time period covered.

But they ask about stability over a period of billions of years, indicating their simulation didn't cover that long a period.

Astrophysicist Sean Raymond does similar simulations of solar system evolution as part of his job. And he says:

I am available to help out with ideas for books, movies, games or any other projects set on other planets. My specialties are:

World building: creating scientifically accurate settings for storytelling

Understanding what it would feel like to live on other (custom-built) worlds. How would the stars move? What would the weather be like?

Testing the scientific validity of already-created settings.

https://planetplanet.net/about/

And you should consider:

As a rule of thumb, the system is likely to be long-lived if m1 > 100m2 > 10,000m3 (in which m1, m2, and m3 are the masses of the star, planet, and trojan).

https://en.wikipedia.org/wiki/Trojan_(celestial_body)#Stability

So according to that rule of thumb, a Jupiter trojan would have long time stable orbit if the trojan had a mass less than 0.0001 that of Jupiter or less than 0.03178 that of Earth.

I note that some of the imaginary planetary systems Raymond has designed on his blog seem to feature trojans which break that rule of thumb. Raymond has also done simulations with planets in stable rings or arcs. If anyone knows of exceptions to that rule of thumb and is able to calculate them Raymond would seem to be a good choice to start asking.

And if it is impossible for Juno to have a long term enough stable orbit as a Jupiter Trojan, Raymond be able to find a more stable place in the solar system for you to put Juno.

You should also consider the scale of science fiction hardness.

https://tvtropes.org/pmwiki/pmwiki.php/SlidingScale/MohsScaleOfScienceFictionHardness

If you are content to have a low score in that scale, like star Wars, for example, you can put your fictional planet Juno wherever you want without worrying about how plausible the orbit is.

M. A. Golding
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  • It looks like m3 has to be only 1% the mass of m2, not 0.01%, so a trojan of 3 earth masses could work. Maybe I could tweak Juno to be a super-Earth? – Nip Dip Oct 18 '23 at 02:57
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stable over billions of years

Over a handful billion years you will have planetary migration. According to some current models, gas and ice giants will move a lot. There is even an hypothesis suggesting that Jupiter and Saturn moved inward, then outward since they formed.

While L4 and L5 Lagrange points are considered stable, such migration and the the interactions with a protoplanetary disc make your arrangement very improbable.

I think a more plausible way to have such an arrangement is if your planet is a recent capture, having come from another system.

The Square-Cube Law
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