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Assume there was a super advanced alien race that for some reason, wanted to add a new planet, of lets say 1 Earth mass, into the Solar System. Where in the inner Solar System (The part of the solar system inside Jupiter's orbit, so asteroid belt is included) could we put this planet, so that it would have a stable orbit for billions of years?

Note: I would preferably not want have this planet orbit closer to the sun than Mercury, and would like for it to affect the inner planets as little as possible, although I care more about the first condition rather than the second.

And also, the planet needn't be habitable

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    $\begingroup$ Non rhetorical question: what happens if its on Earth's exact orbit but 180 degrees out of phase? I'm guessing something nasty involving Venus or Earth itself due to a perturbation? $\endgroup$
    – user86462
    Aug 26, 2022 at 3:17
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    $\begingroup$ @SeanOConnor You're talking about the counter-Earth theory but such a planet wouldn't stay behind the sun for long. $\endgroup$
    – JBH
    Aug 26, 2022 at 3:48
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    $\begingroup$ @JBH Wow, I had no idea it was such a popular idea. There is nothing new under the sun, or even on the other side of it! $\endgroup$
    – user86462
    Aug 26, 2022 at 4:31
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    $\begingroup$ There are many solar system simulators, some are listed in our list of worldbuilding resources. Have you tried using them? $\endgroup$
    – JBH
    Aug 26, 2022 at 4:31
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    $\begingroup$ There's an online tool / game where you can experiment with various planetary configurations: stefanom.org/spc but finding appropriate orbits is quite tricky. $\endgroup$ Aug 26, 2022 at 19:50

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There is a book about the possibility of human habitable exo planets around other stars. Habitable Planets for Man, Stephen H. Dole, 1964.

https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf

On pages 46 to 52 Dole writes about the spacing of the planets in the Solar System as it was understood in the early 1960s. Dole estimates that the forbidden zones around the planets take up ab about 50 percent of the volume of the Solar System, leaving about another 50 percent which might possibly be occupied by other planets under some conditions.

According to Dole's calculations, the more massive the planet, the larger its forbidden zones, and the farther the planet is from the Sun, the larger its forbidden zone will be. And the more eccentric the planet's orbit is, the larger its forbidden zone will be. The mass of the star is also a factor.

Table 8 on page 50 gives the inner and outer edges of the forbidden zones of the planets. As nearly as I can read the tiny numbers in the table, the gaps between the forbidden zones are wider than the forbidden zones for the inner planets.

Figure 18 on page 51 indicates that there is room for a roughly Earth mass planet and its forbidden zone between the orbits of Mercury and Venus, between the orbits of Venus and Earth, between Earth and Mars, and beyond Mars. It might be possible to put two Earth mass planets and their forbidden zones between Mars and Jupiter.

None of the forbidden zones overlap, except for those of Neptune and Pluto, and the orbit of Pluto is tilted considerably compared to that of Neptune.

I note that it would be unlikely for major planets to form with orbits where their forbidden zones overlap, because those overlapping forbidden zones would interfere with he formation of the planets out of smaller objects.

But if aliens build a new planet in our solar system by using spaceships to move smaller objects into place and assemble a planet out of them, or if the super advanced aliens open a portal to move an already formed planet from a distant star system to an orbit among the already formed planets in our solar system having partially overlapping forbidden zones might not matter much.

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  • $\begingroup$ Putting planets in the Mars-Jupiter gap wouldn't be very healthy for the planets or even for Earth. $\endgroup$ Aug 27, 2022 at 3:40
  • $\begingroup$ It's worth saying that our understanding of the solar system has changed a lot since the 1960s. It looks like Dole's forbidden zones are multiples of each planet's hill radius - which is a decent approximation - but it doesn't take into account the meandering of the planets over billion-year timescales. When you account for this, the equivalent of the forbidden zones become much larger, even overlapping for Mercury and Venus. See for example Figure 5 here: (scholarpedia.org/article/Stability_of_the_solar_system) and the associated paper. $\endgroup$ Aug 31, 2022 at 1:15
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Make Mars a double planet. Or Venus, or of you do not care about Earth much, replace the Moon with the planet. The previous stability should remain more or less the same, apart from some disturbances that might or might not manifest in the long term (as long as the resonances remain stable, you do not have much to worry about).

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If you have the resources to create or bring a new planet - generally, wherever you feel like. You will be probably able to maintain the orbital stability as well.

On the other hand, if you are limited in long-term expenses, the orbital stability becomes an issue. And there, the answer becomes "nowhere in the habitable zone".

Mercury, Venus, Earth and Mars are in a complex orbital relationship, maintained by constant exchange of momentum between the planets. There are hints that the solar cycle is related to the tides the first 3 planets exert at the Sun as well.

How can one save the plot, then?

  • Replace Venus / terraform Venus. It is Earth-mass and with some added water and without the greenhouse effect can be pretty much habitable. And only slightly hotter than the Earth (if at all).

  • Make Mars double. Mars is not much different than the Moon in both mass and composition. Put an Earth-sized planet there and allow Mars to orbit around. A heavier body in the place of Mars will likely have only minor effect on the Earth's orbit. The two Mars moons can be put in an orbital resonance with Mars (3-moon world?), mined into oblivion or stored in the asteroid belt until better ideas arise.

  • A combination thereof (replace Venus, store Venus near Mars where it can cool off for a while).

Beyond Mars it's rather cold, but if you don't mind that, the asteroid belt itself is a place. Not a place to have a planet on its own, but pretty much a place where a planet would be stable. Vacuum it first or prepare for a serious meteorite shower later. You can use the collected material for a little fancy moon as well.

Beyond the asteroid belt it is really, really cold, in particular too cold for almost any type of atmosphere or hydrosphere. If you can bear the heating bill, neither Jupiter, nor Saturn, nor the ice giants beyond, will mind an Earth mass satellite. You will have to rearrange the moon zoo there first in order to avoid crashes.

Don't put anything heavy below Venus orbit even if you don't mind the heat. We like our calendar and seasons pretty much. Thank you in advance.

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The planet would be able to be the most stable in a orbit between earth and Mars because that the biggest gap between notable celestial bodies of the inner solar system. So gravitation pull of any bodies would be most minimal hence being able to keep a steady orbit as long as the sun lives.

Edit:I apologize by I messed up(dyslexic). It rather crowded between mars and the belt you best bet would be between earth and mars.

Another solution would to have an orbit perpendicular of all the other orbit since most bodies in this system tend to stay on one plane.

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    $\begingroup$ There is a reason if between the asteroid belt and Mars there are no bodies of significant size. How can you assume that a planet would be there unperturbed? $\endgroup$
    – L.Dutch
    Aug 26, 2022 at 1:38
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    $\begingroup$ @JBH it's because the gravitational perturbation of the closest planet keep that area free. Which is exactly what the OP is asking to avoid $\endgroup$
    – L.Dutch
    Aug 26, 2022 at 2:47
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    $\begingroup$ sorry, you got it all wrong. Gravity doesn't give little tugs whenever it feels like, gravity acts always. If you were correct, there would be other bodies of significant size between the planets. $\endgroup$
    – L.Dutch
    Aug 26, 2022 at 3:54
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    $\begingroup$ @L.Dutch Doesn't the objection of "if it's unoccupied, it's for a reason" apply to everywhere unoccupied, though? So should we not look for the best of a bad bunch of options? $\endgroup$
    – user86462
    Aug 26, 2022 at 8:01
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    $\begingroup$ @KaffeeByte You can't have a planet in the asteroid belt because it's difficult to form one - in situ - there. Those orbits are indeed stable for billions of years, and if you're okay with occasional impacts from asteroids - that would be a good place for an extra planet! $\endgroup$ Aug 31, 2022 at 1:12
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This is a cool question. The short answer is: probably not.

The long answer requires us to consider a few things:

1. Are the current planets stable?

To discuss this we first need to understand something about dynamical chaos. Let's imagine a situation where you're trying to predict whether a skilled soccer player will make a goal. Furthermore, let's say you're a particular enterprising soccer fan and consequently you know the direction they will kick and the power of kick with some accuracy. In addition, you measured the goal beforehand and know what its boundaries are. If you know all these initial conditions precisely you could simply run a simulation to predict the position of the ball over time and determine - for certain - whether the player makes the goal or not.

Now let's complicate the situation: instead of knowing the initial conditions perfectly, say there is some error. If you conduct your simulation in this case you'll find there are some cases where the ball ends up in the goal, and some cases where it does not. Even if we know exactly how to do the physics, Even if our simulation is perfect, the position of the ball at any given time in the future will be uncertain simply because the initial conditions are.

The solar system as a whole exhibits something called chaos, which, at the risk of oversimplifying, is fundamentally just a version of this same effect we found with the soccer player. The difference is that in the case of chaotic systems the error in a subsequent state grows quickly (typically exponentially) in time. Hence, dynamical studies of the solar system typically involve comparing large sets of simulations statistically - while we can't be confident in any individual simulation - the overall ensemble is interpretable!

enter image description here

Below is a plot from a paper by Laskar, J. & Gastineau, M. unfortunately behind paywall. They integrated identical copies of the solar system for 5 billion years with the only difference being that they shifted initial position of the planet Mercury by just a few meters between simulations. The plots show the resulting maximum eccentricity of the Mercury over time (first panel is using just Newtonian mechanics, and the second panel is using General Relativity). In either case, there are many cases where the eccentricity of mercury can grow extremely large, and most of these cases result in either ejection from the solar system or collision with another planet.

This implies that the current planets - just the 4 we have - aren't super stable as is. This isn't to say that the solar system will definitely shake itself apart - but it does show that the unstable trajectories are possible given what we know about the current state of the solar system.

2. Alright, but what about adding another one?

If we wish to consider the addition of another planet, what's critical is its mass. If the new body has a small mass, it won't really destabilize the orbits of the more massive bodies, and it has a better chance of surviving. There are regions like this in the solar system - for example the "Earth-Mars Belt" (you can read the paper here: since it's public), a region between ~1.09 and ~1.17 AU where small bodies might be able to survive for billions of years.

Finally, we get to your case - what if the added body is the size of the earth? In this case, the body not only needs to survive, but it also has to do so in a system that is being destabilized by its own (earth sized!) gravitational field. Given that the solar system without such a body is barely stable, the addition of such a large body would almost destabilize the entire inner solar system - probably impacting another planet or being ejected. While the only way to find out for sure would be to run a ton of very time-consuming simulations, I'd be very skeptical that such a situation is possible.

3. What about resonances?

There are some situations where planets can be found in what are called "Mean-Motion Resonances" with each other, and this tends to permit stability even in very closely packed systems. The poster child for this type of behavior is the Trappist-1 system, which exhibits several small planets all in resonance with each other. While such a configuration cannot be realized by simply adding a planet to our solar system - it is conceivable that there could exist systems in general with planets more tightly packed than what we have.

**This answer is reproduced from a similar question on the Astronomy StackExchange

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