Assume there are three earth-like worlds in our solar system. These three worlds would also have a single natural satellite roughly at the same mass-distance ratio of Earth to the Moon (about 1.2 percent of each of their masses and about 384,400 km distant give or take).

I'd put the innermost of the three is in the current orbit of Earth and the outermost is where Mars is. Where could the third planet and moon exist between the two that they'd be stable over significant time (billions of years)?

I am assuming all three would be habitable.

Can we fit those other two earth-like planets in our solar system? How?

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    $\begingroup$ This might be more world building than astronomy. That said, your 3 planets might be too close together. Venus, on average, .722 AU, Earth 1.0, Mars 1.52. Earth is 38% farther from the sun than Venus. In your Scenario, Earth distance, middle planet, Mars distance, you have a 23% distance increase between planets. That might be too close for long term stability. I'm not sure where to find the numbers on a safe distance though. Also, 3 similar sized planets each with a similar moon at similar distance - some pretty big statistical improbability there. $\endgroup$
    – userLTK
    Mar 21, 2017 at 3:36
  • $\begingroup$ This definitely belongs on the world building stack exchange. However, how do you expect another Earth-sized planet to share Earth's orbit? That isn't stable at all. $\endgroup$
    – zephyr
    Mar 21, 2017 at 13:01
  • $\begingroup$ @zephyr you must believe in Planet X. :-) $\endgroup$ Mar 21, 2017 at 13:25
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    $\begingroup$ Venus is habitable - look up "floating cities" for some actual scientific models that could work. $\endgroup$ Mar 21, 2017 at 13:26
  • $\begingroup$ Note to the worldbuilders: since this is a migrated question, some issues may arise (it was written with astronomy in mind). I tried to edit it to better fit this site. $\endgroup$
    – Mindwin
    Mar 21, 2017 at 15:10

3 Answers 3


Some speculation that the Earth doesn't have Venus's atmosphere due to the presence of the moon. So given a moon to Venus may be sufficient to thin out the atmosphere over sufficient time. Put Ceres in a 90 minute orbit might do the trick and spin Venus up at the same time. (Wait. Roche limit. Hmm. Another ringed planet. Would a band of rings have a net cooling effect?)

A more massive Mars could hold at atmosphere for more time. A molten core may provide a magnetic field to keep the solar wind from splitting water.

Change the orbital distances with some caution, and a lot of simulation time. Orbital dynamics is tricky. Resonances where the period of one is a small integer ratio with the period of another are particularly tricky.

If you accept a solar system formed with massive terraforming by a previous civilization, or working on a very long time scale, bombarding Mars with enough comets would establish at least a transient (few tens of millions of years) atmosphere. Use something like Ceres, and try for a gentle collision and you would get a mass boost, plus a molten core again. Drag one of the ice moons in and siphon off enough water to fill martian seas.

Venus could be uninhabitable if you can get rid of most of the atmosphere (matter transport to mars...) and paint most of it titanium dioxide white. You want a material that is black in the infrared, and white in the visible to minimize solar absorption, and maximize radiation. Even with this I think you would have a wide equatorial band that was too hot. While your at it hit with enough rocks to give it a decent spin.

Remember also that you have 6 other potential spots in the trojan positions 60 degrees ahead and behind these 3 planets. Could be good parking places for ice moons to thaw, or to have industrial worlds. The energy to move from one trojan point to the other in the same orbit is a matter of how long you are willing to wait.

There are other Lagrange points: * L3 "Counter earth" on the opposite side of the sun: * L1 "Sub solar" where it between earth and sun, with the excess gravitational attraction of the sun balanced by the gravitation of the earth. * L2 "Supra solar" where it is just beyond the earth along the earth sun line.

None of these are long term stable. The counter earth I think is close to neutral stability. L2 and L1 are unstable, and perturbations grow rapidly. If you have the resources to move planets around, however station keeping should not be a big issue.

If you need more real estate, play with orbital dynamics and sharp angles to the ecliptic. These have less coupling, and so you should be able to orbit a planet at right angles to Earth's orbit between Earth and Mars. Note that the energy to reach these orbits is horrendous.

Another solution that is elegant, but difficult is a klemperer rosette. N bodies in an ellipse co-rotating about their mutual center of gravity. This has been used in two stories that I know of: The Fleet of Worlds of the Puppeteers of Niven's known space universe, and in Arthur C. Clarke's novel either Against the Fall of Night or The City and the Stars, where reference is made "It is lovely to watch the coloured shadows on the planets of eternal light."

AFter this, it's time to look at building ringworlds.

  • $\begingroup$ Lagrange points are a very good note - you could easily have two planets on complete opposite sides of the star, at the same distance. There's probably no way that could form naturally, but assuming the system is manufactured by a super-advanced alien civ, that's definitely possible, and stable. $\endgroup$
    – SpaceMouse
    Mar 22, 2017 at 13:19

The Habitable Zone for our star actually reaches from about Venus to about Mars: https://en.wikipedia.org/wiki/Circumstellar_habitable_zone so if you wanted to, you could stretch the distance out.

That being said, your question was basically "could there be another earth-size planet in-between where Mars and Earth is, effectively", which the answer is likely "No" - when the planets were forming, they each were clearing a path for themselves out of the stellar nebula. An earth-like object would of course, be gravitationally dominant out to a certain distance, since it "scoops up" all the material in that range of orbit from the sun. During the merger phase, two objects of similar size would eventually mess up each other's orbits enough that only one would become dominant (or they'd crash into each other, like what happened with the earth and a mars-sized object). See https://en.wikipedia.org/wiki/Nebular_hypothesis#Formation_of_planets for a general explanation of how planets form around stars.

So short answer, you'd likely have to have the three planets at the locations Venus - Earth - Mars are now, rather than putting another planet in-between Earth and Mars; it simply wouldn't survive the planet formation stage.


You could do it this way:

  • Change the radius of the Earth to 9000 km, and drop its density to 3300 kg/m$^3$ (same as the moon). This will double its mass and provide .85g of surface gravity

  • Increase the radius of the moon to 2400 km (same as Ganymede) and increase its density to 6300 kg/m$^3$ (same as Earth). This will increase its mass by a factor of 5 and increase its surface gravity to 0.43g. Adjust the Earth-Moon distance for stability.

  • Increase Mars's density to that of Mercury (5400 kg/m$^3$). This increases its mass by about 30% and surface gravity of 0.52g

  • Increase the strength of the sun by 20%.

Now you have three 'planets', with roughly similar surface gravity, all within the habitable zone of a star. Alternately, you can use Venus as the third planet and make the sun weaker, if you want two colder planets and one hotter.

This is a configuration that would be stable for a long time. Now, I don't know if the planets could naturally evolve this way, but that's another question.

  • $\begingroup$ One problem with all this is the question if the planets could sustain surface water for long periods of time. The reason Mars, for example, doesn't have surface water despite being in the Habitable Zone now is because it ran out of fuel in its core, and then the CO2 cycle stopped, and it lost its ability to sustain an atmosphere. If you mess around with planetary density, the question becomes how much fuel the core would have, and then if the CO2 cycle would last, powering a planet that size. So although these numbers work, gravity-wise, they might not sustain surface water long enough. $\endgroup$
    – SpaceMouse
    Mar 21, 2017 at 16:52
  • $\begingroup$ @SpaceMouse It takes a long time to lose an atmosphere. I think that if life developed, it would maintain an atmosphere around the planet. Microbes can do all sorts of awesome things, like liberate gasses from the soil. There are plenty of oxygen and carbonate rich rocks on Mars, plenty to sustain an atmosphere if microbes keep putting them into the atmosphere. $\endgroup$
    – kingledion
    Mar 21, 2017 at 16:57
  • $\begingroup$ Unfortunately it's a little more complicated than that. You need tectonic activity to have large scale redepositing of carbon in the atmosphere. The question about how and why Mars' carbon cycle stopped has been addressed before: ncbi.nlm.nih.gov/pmc/articles/PMC1664679 Microbes or not, without tectonic activity, the planet becomes dead. $\endgroup$
    – SpaceMouse
    Mar 21, 2017 at 17:01
  • $\begingroup$ I forgot to mention, without a strong, active core, you don't have a strong magnetosphere, and then there's literally no way surface life can survive - the solar wind would blow the atmosphere away, and solar radiation would sterilize the surface. Mars has a magnetosphere still, but it's very weak, and is a pale echo of what an active-core Mars magnetosphere would look like. Microbes or not, once the solar wind has blown most of your gas-phase oxygen and hydrogen away, your planet is pretty doomed, life-wise. $\endgroup$
    – SpaceMouse
    Mar 21, 2017 at 17:24
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    $\begingroup$ If Mars had a moon, there's a chance, maybe a pretty good chance it would have a magnetic field. A magnetic field and greater gravity should retain most of the atmosphere, so I think that much isn't an issue. It may be the Earth's slowing down by the tug of the moon that generates the spin variation that generates Earth's magnetic field, not the solid/liquid core. Or it could be some of both. The details are unknown, but I don't think it's a big problem to say all 3 planets have gravity and magnetic fields to retain their atmospheres. $\endgroup$
    – userLTK
    Mar 21, 2017 at 18:38

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