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There are six basic parameters that determine whether an environment is suitable for life as we know it: temperature, pressure, salinity, acidity, water availability, and oxygen content. Advanced life is restricted to a narrow range of these parameters, but primitive microorganisms exist over a much wider range. Data already collected by space probes essentially rule out advanced life on other planets of our solar system; however, given the potential number of planetary systems in the galaxy, there may be as many as 50,000 planets that have earthlike conditions, a fraction of which could have cultures as technologically advanced as our own. Three decades of passive listening with radio telescopes have produced no signals comparable to those radiating from earth, but we do not know if another civilization would produce electromagnetic radiation in detectable amounts.
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