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Could two planets in the same solar system sustain life? I have one world that is about the size of our own planet. The other planet is farther away from the first planet think of it like Mars but it is much larger. I want to have life on both planets for a future fight between them over the solar system.
Im not worried about how these life forms look or how they will eventually space travel. I can handle that part of the equation.
Is this theoretically possible?
Mars had liquid water on its surface in the past. It evidently still has intermittent liquid water flows. The biggest thing keeping Mars from being life-supporting (at least in the way we understand carbon-water based life) is its lack of atmosphere.
Supposing Mars was the size of earth, with active tectonic plates and a magnetic field. It would then be able to support an atmosphere for the billions of years needed for life to develop, and then it seem that it is likely that life would develop.
If, in addition to making Mars bigger, we removed Venus, moving both Earth (slightly) closer and Mars much closer to the sun, then development of life might proceed on both planets, one hotter than earth, one colder.
However, even if you consider that both planets develop land-based animal life at the same time, it is highly unlikely that one intelligent species would meet another. It took some 300 million+ years on vertebrate life to develop into intelligent civilization on earth. If development of intelligence on the two planets were off by just 30,000 years (.01 %), then the inhabitants of one would find stone-age inhabitants on the other. Thus it is very unlikely that both planets will get intelligent life at the same time.
Turn the question around: To what extent is it possible to terraform multiple planets in a solar system? If you can come up with plausible scenarios for that, you may be able to also have situations that arose that way naturally.
Unfortunately I have no experience in terraforming. You may want to consult someone experienced.
Consider however:
As @kingledion comments, Mars had running water.
Venus has a greenhouse effect run wild. Had Venus been in Mars orbit it might be quite habitable. This seems reasonable. Mars is a bit of an oddball, being smaller than the earth. As you get further out, the gradient of the gravitational field decreases, so a protoplanet should be able to capture from a larger slice of their lane in the ecliptic. Mind you, the planet boffins have found jupiter sized worlds in Mercury sized orbits, so my mental model of planet formation is clearly lacking. Being fallible sucks.
Similarly suppose a planet at Venus's distance early on had a companion planet or large close moon that broke up into a fairly massive set of rings. Would this cast enough shade to reduce the temperatures to a reasonable level.
Another way to get more habital planets is to stack the orbits closer together. This may not work. See the wikipedia article on Titus-Bode's law. Too close together and their orbits become unstable. Play with it a planetary similator maybe?
How about double planets? They would be tidally locked. See Roche's limit for how close. If you had a companion for earth, the limit for small co-orbiters is about 40,000 km. Larger ones I think will increase this. But suppose you have an earth twin 80,000 km (50K miles) away. From the surface of either planet the other one is 1/3 the distance of the moon, and 4 times the diameter, giving an object that subtends 6 degrees of sky, has 36 times the area, and since the earth is a lot brighter than the charcoal grey of the lunar surface, several hundred times the brightness at night.
Suppose you have a planet that is marginally too close to the sun. The equatorial regions are lifeless deserts. You have the possibility of two entirely separate genesises (genesi?) with different chemistry.
It would be very unlikely, but given the incredible size of the universe, I very much expect that there are a few such places, or at the very least have been or will be at some point in the future.
There are no physical reasons why the presence of intelligent life on a planet would make the evolution of life on nearby planets impossible.
But given the apparent scarcity of planets that have any life and the tiny amount of species on Earth that developed human intelligence (1), I'd say it's really, really unlikely to happen in any given star system. But one or two such systems in the galaxy? Why not?
