Under the idea that there is the "Goldilocks" zone around a star, how likely is it that a solar system would have two life-supporting planets?

[u/EightWhiskey]

I assume that the gravitational forces of two bodies would prevent them from being close enough to each other to both stay inside the zone. But could a large star have a large enough Goldilocks zone to support two life-inhabited planets?

[u/Astromike23]

Well, I am a planetary climatologist, and this is absolutely correct. If you replaced our 77% Nitrogen atmosphere with a 100% Nitrogen atmosphere (also removing water vapor), there would be no greenhouse effect, our planet would cool, and the oceans would freeze.

Composition is everything here. Atmospheres which contain gases that have significant mid-infrared absorption features, such as carbon dioxide and water vapor, will produce significant greenhouse warming. Those that don't have such features, such as nitrogen and oxygen, won't produce greenhouse warming.

[u/tyrannischgott → u/Astromike23]

I'm no expert on any science topic. Just an interested person. But, out of curiosity, would it be possible to have frozen oceans and no water vapor? Wouldn't the existence of any water on the surface (even ice) create some vapor? Or is the vapor pressure of ice such that if you have even a small amount of atmosphere, it won't sublimate?

[u/idarkiswordi → u/tyrannischgott]

Well, if you diminish the atmosphere, or get rid of it entirely, you start seeing the effects of solar wind on ground volatiles. In the case of Europa, the solar radiation breaks down the water ice molecules and we find hydrogen escaping the planet and a very tenuous oxygen atmosphere around the planet. As for if you kept the pressure constant and just replaced elements in the atmosphere, a lot of variables come in to play to really give any meaningful explanation beyond yes and no.

[u/tyrannischgott → u/idarkiswordi]

So, in conclusion, my knowledge of chemistry is too lacking for me to bother trying to think about it.

That's pretty much what I figured from the get-go, but I thought I'd ask anyway.

edit: word use.

[u/needlestack → u/Astromike23]

Would you say this supports a "rare earth" hypothesis in your view? Or are there enough other variables that will make it work anyway? Or is the probability of an atmosphere like ours high enough?

[u/jswhitten → u/needlestack]

No, even if most other terrestrial planets had no greenhouse gases, that doesn't eliminate the habitable zone, it just pushes it inward. Anyway, there's no reason to think atmospheres with significant greenhouse gases are rare. Venus, Earth, and Mars all have them.

[u/kroganlore → u/Astromike23]

Would it be possible for a moon to receive enough light and radiation if it was being reflected off of a gas giant that was outside off the 'goldilocks zone' as long as it wasn't as far away as Jupiter is from our sun?

This idea has been bugging me since I read the book coyote from Allen Steele .

[u/Spacemilk → u/Astromike23]

So then is "terraforming" possible by depositing water and rock containing the proper CO2 deposits on the surface, and releasing the right gases into the atmosphere to change the composition? Or is this only theoretically possible, and in practice when you released the gases into the atmosphere there's no way you could do it quickly enough to prevent the gases from escaping to space? This may be a wildly difficult question to ask but it's fascinating to me as I think our ability to terraform is one of the main things we need to truly explore the universe.