Life in the Universe

Life on Exoplanets

What is “life as we know it”?

Ironically, though astronomers search for life beyond Earth, biologists here on Earth have still not conclusively determined what constitutes “life as we know it.” Most scientists agree that the basic definition of a living thing is something that begins an active existence (is born), changes and matures over time (grows), replicates itself through a well-ordered process (reproduces), and then ends its existence (dies). On Earth, all things that go through these steps achieve them through the complex interactions of very large molecules such as ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). Some terrestrial things, however, go through all four of those steps, at least by some definitions, but may or may not be alive; certain viruses, for example, defy easy classification. In the cosmic context, the line between living and nonliving things may be even blurrier; stars, after all, are born, grow, reproduce, and die—all after a fashion, at least. So, are stars alive?


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