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Well, there is a measure of how a planet could be considered like Earth, called Planetary habitability. Based on this measure, what are the prerequisites needed to consider a planet to be a habitable one?

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Habitable by whom? There are conditions that are uninhabitable by humans, however, many "extremophiles" survive perfectly happy.

Although, if you are talking about humans, here is a small list (all the rest are probably more "nice to have" requisites):

  • Approximately 20% oxygen (more or less depending on the pressure)
  • Temperatures that allow for liquid water
  • Adequate access to water (and food)
  • Adequate protection from radiation.
  • Then a whole host of conditions that wouldn't end human life. Such as deadly pathogens on chemicals in the atmosphere.

All this said, since we only have a sample size of one currently for planetary life, we really don't know what is possible, or how to bound the problem. ANYTHING is just speculation drawn from this one sample. That said, we have found life on our own planet where we never suspected it to be. Life has proven to be nearly unstoppable in propagating throughout every niche on this planet. So, the better question I think would be what are the requisites for abiogenesis? I think once life manages to start on any planet, it will adapt to whatever conditions the planet presents (to within a reasonable degree).

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wrt adequate access to food--this won't really happen unless there is life already on that planet which is fit for eating. Since that is a whole other world of speculation, I'd say access to water and chemicals to grow our own food, such as nitrogen, phosphorus, potassium, etc. –  Carson Myers Jun 3 '11 at 7:09
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@Carson Myers that's why I made it a parenthetical. :) –  Larian LeQuella Jun 3 '11 at 14:55
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Similarly you aren't going to get free oxygen unless there is life around. Any planet with a chemistry to allow free oxygen to be present, without some equilibrium disturbing life, is going to be pretty hostile to life! –  Martin Beckett Jun 27 '11 at 14:56
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I think humans are quite adaptable too.. especially when it comes to oxygen content and pressure. Also, I'd add that relatively low density of asteroids/meteors close to the planet's orbit. Wouldn't be very comfortable otherwise. –  Jus12 Aug 30 '11 at 19:15

We have essentially one criteria for looking for planets that contain life: liquid water. This requirement covers planets that:

  • have water
  • have an average temperature which allows that water to be in liquid form

But that's just because the one trait shared by all life on Earth is that they need water to survive. We don't know what other kinds of life are possible, because we haven't found any on other planets.

But consider that all life on Earth requires water because that is pretty much the one condition that has remained constant through Earth's history (since the beginning of life). Conditions which changed caused extinctions and adaptations (such as the oxygen catastrophe) until we get to the present earth with most of its life drinking water and breathing oxygen. So if the earth could go from being oxygen-free to containing an abundance of oxygen which nearly all life requires to survive (a big change), maybe the water could have also been substituted for something else? After all, life didn't come from water, it evolved to use water as its solvent of choice because there was an abundance of it.

So maybe life is possible without water if it grew up in an environment without it. But how did life get started on Earth? Can we look for that on other planets? Actually, nobody knows. There are some processes we think probably contributed to the process (see the Miller-Urey experiment) which involve the presence of a few elements and some weather conditions. But we don't know the full process (we couldn't make abiogenesis happen in a lab, for example) and even if we did, it's unlikely we could look for that on another planet. We can't even really see other planets, aside from indirect data and small points of light. We don't know what conditions are necessary for abiogenesis, or how to look for it -- even if we find a planet which is nearly identical to Earth, there's no guarantee that abiogenesis happened there.

As far as we know, any planet could contain some kind of life. But to maximize our chances, we're only getting excited about earth-like planets since we already know the kinds of life which can exist there.

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I don't know that I'd say that liquid water is a true requirement for the planet itself. It's more an issue that water is heavy, and so expensive to get there, at least at the present time. If other factors were to change (eg, we manage to figure out teleportation, and it's cost effective to use), then pre-existing water may not be a requirement. I'm not even so sure that the average planetary temp needs to have water in liquid form, provided there were enough power to support the necessary climate control to achieve it. –  Joe Jun 10 '11 at 3:22
    
@Joe I'm talking about the criteria for finding planets which already have life. In my answer I talk about why it's not really a requirement, but that's what we look for anyway. –  Carson Myers Jun 10 '11 at 4:47

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