What Can We Predict about Alien Life on Earth-Like Exoplanets?

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At first blush, this seems like an unknowable matter. We’ve been able to glean a surprising amount of information about an ever swelling number of exoplanets based on spectrometry, speed of orbit, distance from parent star and so on. However, our only direct experience is with Earth, a sample size of one. Are we at the limits of what can be determined about Earth-like exoplanets…