“Worlds” is a better designation than “earths” in this kind of contemplation since conditions on moons or dwarf planets could prove “just right.” Energy can be supplied by sources other than suns; by geothermal, by gravitation, by chemosynthesis, by radioactivity, etc. Greenhouse gases can warm up worlds distant from their suns. “Dark sides” or “perpetual dawn” regions of hot worlds can be quite stable. Subterranean environments can provide stable conditions where surface conditions are unsuitable. Thick atmospheres can behave like oceans.
If the adaptability and tenacity of life on earth is in any way indicative, worlds which fluctuate between habitable and inhabitable could support earth-like life the likes of which are capable of migration or stints of suspended animation. Worlds on which conditions change gradually (e.g. heat up or cool down) could also allow earth-like life to adapt accordingly. Once upon a time our Earth would have appeared uninhabitable by our present standards of habitability. Radiation resistances, pH resistances, pressure resistances etc. are achieved in earth-life in high degrees.
And we have yet to entertained the anthropic principle. If the life that evolved on earth was that life which was suited to earth-conditions why could not that life that evolves on other worlds have evolved to suit its conditions, conditions we consider “too hot” or “too cold”?
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