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?
Answer
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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