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I have returned to this forum from Astronomy with this question because it immediately got down-voted. Maybe these hypothetical questions are more appropriate here? I am happy to take advice from Worldbuilding forum veterans.

I am in the process of building a hypothetical solar system very similar to ours with only a few changes. I would like to test the viability of my modifications mathematically. Procedural answers are most welcome. As I have been advised in the comments by Angry Muppet and JBH, I am adding additional information for clarity below:

My objective for moving planets is two-fold: (1) that I can create calendars and astrology specific to my "Earth," and (2) that Venus and Mars will orbit on the furthest edges of the habitable zone and therefore be able to have liquid water in some quantity somewhere on their surfaces.

My concern is that by moving my hypothetical Venus, Earth, and Mars so close together they will affect each other so significantly during conjunctions that my solar system build is no longer viable, i.e., the hypothetical planetary orbits will not be stable over billions of years as ours have been.

As advised by L.Dutch, I have edited the title question to specify the hypothetical semi-major axes.

As advised by planetmaker from Astronomy, I am adding this additional information: planetmaker asked "How big a disturbance (in planetary orbit) is too big?" and "What time scales are we talking about?" An orbital disturbance is too big if the hypothetical Venus, Earth, and Mars cannot plausibly have retained their proposed semi-major axes long enough for habitable worlds to evolve. "Habitable" should be taken to mean a world with a liquid water ocean, temperate climate, and breathable air in at least some regions, which indicates the evolution of complex, oxygen-producing lifeforms. (I am aware that Mars' size is considered responsible in part for the loss of its oceans. I may increase the size of my Martian planet if necessary, but right now it is not relevant.) This leads me to suggest that a timescale of 4.5 billion years should be considered as baseline.

Thank you for your help!

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    $\begingroup$ Could you make it clearer what the starting conditions are (what's happened in the last million years or so), then tell us what your objective is - "significantly affect each other's orbits" needs expanding on to tell us exactly what you're after. $\endgroup$ Sep 3, 2022 at 2:32
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    $\begingroup$ Excellent, if you add those details to the body of the question (comments are by their nature ephemeral) then it sounds like we could be in business for attracting answers. Might I suggest also adding the tag science-based for good measure to ping additional talent to help. You can click on the edit button, if you're not familiar. $\endgroup$ Sep 3, 2022 at 2:56
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    $\begingroup$ Please edit your question to include any responses and clarifications you're tempted to give in comments. Never trust that people will read the comments to better understand your question. $\endgroup$
    – JBH
    Sep 3, 2022 at 3:14
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    $\begingroup$ "significantly affect each other's orbits" is a rather vague and unspecific term. They already do affect each other orbits, and since gravity has infinite range they will always do for any distance. $\endgroup$
    – L.Dutch
    Sep 3, 2022 at 3:45
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    $\begingroup$ FYI it's stable at 10 million years. It's running a little slow because it's competing with other simulations I'm running. I'm guessing it will have reached 100 million by tomorrow. I'll let you know. $\endgroup$ Sep 21, 2022 at 19:42

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Final answer: the system remains stable.

Let me explain exactly what I did. I ran an N-body simulation including the following planets:

  • Venus at 0.85 AU (other orbital parameters unchanged)
  • Earth at 1.1 AU (same)
  • Mars at 1.4 AU (same)
  • Jupiter and Saturn on their current orbits

I didn't include Mercury (which would shorten the timestep needed for the simulation) or the ice giants (their influence should be minimal).

I ran the simulation for 80 million years with a 10-day timestep. (I know I said 100 million years but I needed the machine for a separate simulation, and in general systems that go unstable do so relatively quickly).

The planets' orbits remained nice and stable for the full duration of the simulation. Their orbital eccentricities and inclinations stayed in roughly the same range as they do on their current orbits (Earth's eccentricity goes between about zero and 0.05, same for Venus, a little higher for Mars -- see blog post here).

I fully expect the planets' orbits to remain stable indefinitely, so it's plausible to have a story as you describe. FYI the inner edge of the habitable zone is at about 0.95ish AU (not 0.85), at least for an atmosphere like Earth's. I suppose that if you're using Venus as an analog for a tidally-locked planet, then the inner edge is a bit closer, so maybe 0.85 is OK.

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    $\begingroup$ Sean Raymond, thank you so much for performing this simulation and helping me to text the viability of this hypothetical solar system! As per your final comments about Venus, yes, both my "Venus" and "Mars" are designed to have SOME habitable zones, but they are not as preferrable for human habitable as Earth. For instance, my Venus IS tidally locked, and humans live mainly around a band between the dark and light side, where liquid water can exist. Again, THANK YOU FOR YOUR HELP! $\endgroup$
    – JM Yaden
    Sep 22, 2022 at 17:43
  • $\begingroup$ I would also like to thank James K, planetmaker, L.Dutch, Angry Muppet, and JBH for their input on this question. I am most grateful for your help! $\endgroup$
    – JM Yaden
    Sep 22, 2022 at 17:47

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