This is the supposition: Venus is too hot, Mars is too cold. If they switched places then Solar energy would change to make both more Earth-like. We might as well have had three instead of just one "Earth" in the solar system if the roulette of early planet formation had played out just a little bit differently.

That's of course too simple. But wouldn't it help a lot if it were the case from the beginning that Venus had had Mars' mass, and Mars had had Venus' mass? Or do other factors dominate mass and distance from the Sun?

Would Venus still have had a thick, but not too thick, atmosphere if it were at 1.5 AU, because it has mass enough to keep one, and wouldn't that have kept it warmer than Mars is today? Including flowing water on its surface under its atmospheric pressure?

Would a Mars at 0.7 AU have been warmer and maybe have had a passing atmosphere created from melting volatiles during a longer era in its history?

  • $\begingroup$ You'll probably be better off by placing an "umbrella" in the Lagrange point between Venus and Sun. If it's big enough, the CO2 may even freeze completely - after a while. $\endgroup$ Jul 13, 2015 at 19:49
  • $\begingroup$ If Venus could be given a moon similar to ours, given a greater tilt to its axis and moved somewhere into the goldilocks zone, and given a significant amount of time, wouldn't that be enough? With a moon, Venus's core could begin to generate a magnetosphere $\endgroup$
    – Jamie
    Feb 8, 2017 at 4:18
  • $\begingroup$ One ingredient question to be asked and answered before judging possible replies to this question would be: would the perturbations in the inner solar system still leave the system as near to stability as it is now, if the masses of Venus and Mars were interchanged? That stability question could be answered (by anyone with the time to do it) by using the software resources linked here [space.stackexchange.com/questions/23408/… and swapping two of the gravitational mass constants provided. $\endgroup$
    – terry-s
    Nov 11, 2022 at 19:20

4 Answers 4


This is a very interesting question. Of course, as you noted, you have simplified things quite a bit; there are other factors besides temperature that affect habitability.

Regarding Venus, you probably know that Venus is extremely hot at its surface not just because it is closer to the Sun, but because it has a thick CO2 atmosphere and is warmed by the greenhouse effect. There are, in fact, two things about Venus that would prevent it from being habitable wherever you put it. One is the lack of a magnetosphere, which is necessary to prevent ionizing radiation (particularly from the solar wind) from reaching the planet's surface. The lack of a magnetosphere appears to be due to the lack of a geodynamo on Venus, which has to do with the structure of its core. Second, Venus appears to lack tectonic plates, which you may know are responsible for earthquakes here on Earth. Interestingly, tectonic plates play a major role in controlling the buildup of CO2 in the atmosphere (see here for details). Lacking tectonic plates, Venus is doomed to have a large CO2 atmosphere wherever you put it, which would not make it a nice place to live.

Mars, on the other hand, is a very different matter. It has both a magnetosphere (albeit it is very weak) and it likely has tectonic plates (although last I heard it is thought to only have two). One of the reasons that NASA has sent so many probes to Mars is that it was thought to be habitable at one point. It is thought that Mars's growth was stunted because of gravitation effects from Jupiter and Saturn. So, in another universe, Mars could have ended up very much like another Earth.

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    $\begingroup$ Just wondering about something, you mention that no matter where Venus is it will be 'doomed to have a large CO2 atmosphere'... Now my teeny brain wants to say, "That's okay because plants 'breathe' CO2 and emit Oxygen in return." So if indeed the planet were overwhelmed with CO2, wouldn't/couldn't that be acceptable for at least providing an environment for plants to thrive... and they could change the atmosphere for animals by proxy? $\endgroup$
    – MegaMark
    Oct 12, 2015 at 14:48
  • $\begingroup$ For a sizable fraction of Earth's history, it had a CO2-dominated atmosphere as well -- until microbes got good enough at photosynthesis. Plate tectonics (which neither Mars nor Venus have, and haven't had for a long time) is necessary to sequester the carbon that the microbes bound into minerals into the Earth mantle. $\endgroup$ Nov 10, 2022 at 20:41
  • $\begingroup$ Magnetic field: This is more an issue with the mantle than with the core. To drive a magnetic field, you need convection in the core, for which you need a way to create a temperature gradient, for which you need the mantle to transport heat away from the core. On Earth, this works because the mantle is itself convecting (plate tectonics being the surface expression of it). But both Mars and Venus have stagnant mantles that cannot transport enough heat to produce core convection, and so there is no magnetic field. $\endgroup$ Nov 10, 2022 at 20:43
  • $\begingroup$ Venus's massive atmosphere would greatly reduce the need for a magnetic field. $\endgroup$ Jun 17, 2023 at 17:31

If, Venus had started out at Mars' position, it is possible it would've ended up evolving into an ice giant. At the greater distance of Mars the Sun's solar wind is weaker and would've stripped off much less of Venus' atmosphere, even in comparison to the amount Earth lost, as Earth is considerably closer to the Sun than Mars. So, Venus would perhaps have been left with a thick helium envelope and become a mini-Neptune/super-Earth rather than a second Earth. And also, if Mars had evolved at the distance of Venus, closer in, it would've been stripped of any appreciable atmosphere and possibly would've evolved into another Mercury, a grey and airless world.

  • $\begingroup$ Doesn't the effect of solar wind on the atmosphere go the other way too? CO2 is partly frozen on Mars. Would a substantial part of Venus' atmosphere freeze to ice if it was as far out as Mars? Would ice on Mars melt to form atmosphere if it was where Venus is? Is/was there really any substantial amounts of helium available within the asteroid belt? $\endgroup$
    – LocalFluff
    Jul 14, 2015 at 1:34

Simply "swapping" position would not in and of itself be cause for a habitable planet. The key is "changing" position. If Venus was to undergo orbital migration it could conceivably move into Earths current position and cool down, continue to develop a more stable atmosphere, and produce oceans. Then further orbital migration could cause the atmosphere to collapse and marine succession. The question then is would it become like Ceres and Jupiter. Simulating a vector reversal, where Mars undergoes orbital migration and moves into the position currently taken by Earth, would warming it up cause it to further develop an atmosphere, and a marine environment. Then, when its vector moves it to the spot currently taken by Venus would marine succession and atmospheric reduction appear as Venus is to day. From physical appearance alone I would say there is a natural progression where the planets evolve as the orbital migration causes their vector to move away from the Sun.


If Mars was placed near the Sun and Venus relocated to Mars' current orbit, neither one would be habitable. To make both Mars and Venus habitable(in Theory), a large lunar moon is required because it would cause a tidal crust movement that would help to ignite the planet's magnetic field. For Venus, Mercury would become its new moon as it could not only ignite a strong magnetic field, but it could correct the planet's rotation by making it rotate in a normal 24hr Earth rotation, and allow it CO2 atmosphere to cool down as it will be able to recycle it into the ground. As for Mars, Jupiter's moon Io would be ideal as it can increase the planet's surface pressure and magnetic field. Both Venus and Mars are capable of terraforming by themselves. All that's needed is a large lunar moon like Io and Mercury respectively.

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    $\begingroup$ How would a magnetic field help habitability? AFAIK magnetic fields hardly interact with biology at all. And atmospheres are obviously independent of magnetic fields, since Titan and Venus both have thicker atmospheres than Earth without any magnetic fields, and Ganymede and Mercury both lack atmosphere in spite of having magnetic fields. Plate tektonics does seem to have other important effects though, like recycling carbon dioxide, the gas which gives birth to all life. I don't know if the Moon helps that happen. Do you have some source? $\endgroup$
    – LocalFluff
    Jun 13, 2017 at 11:55
  • $\begingroup$ @LocalFluff There are articles that say the Moon may play a key role in Earth's magnetic field. Earth's rotation and Coriolis effect might be a key factor to, and it might have more to do with the specific temperature and liquid becoming solid and releasing heat. But there's no consensus on the cause. sciencedaily.com/releases/2016/04/160401075118.htm That said, I think the this answer, even with this moon article, still needs more evidence. Venus has virtually no water for example. You need water to sequester CO2. Venus has no plate tectonics. $\endgroup$
    – userLTK
    Jun 13, 2017 at 15:20

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