[trekker7]

Do they believe that only Earth-like planets can support life? Or is it that Earth-like planets are one type of planet that supports life, with many other possibilities? I would think the first assertion to be short-sighted.

[Silentio]

I agree with you. We know that our planet and the qualities that it possesses are appropriate for sustaining life. Who's to say a form of life couldn't arise on a planet incapable of sustaining life as we know it. Would that life, then, view our planet as incapable of sustaining life? If it did bump into us, would an intelligence arising on a planet "incapable" of supporting life even see us as life forms?