The Rare Earth Hypothesis is the unproven supposition that although microscopic, sludge-like organisms might be relatively common in planetary systems, the evolution and long-term survival of larger, more complex, and even intelligent organisms are very rare. The observations on which this hypothesis is based are as follows: (1) Microbial life existed as soon as Earth's environment made it possible, and this nearly invincible form of life flourished over most of Earth history, populating a broad range of hostile terrestrial environments. (2) The existence of larger and more complex life occurred only late in Earth history, it occurred only in restricted environments, and the evolution and survival of this more fragile variant of terrestrial life seem to require a highly fortuitous set of circumstances that could not be expected to exist commonly on other planets. This hypothesis can be tested.
Throughout human history, people have wondered what lies beyond the limits of the known world. This instinctive obsession has driven humans (and perhaps other species) to expand their own territories. This haunting question permeates mythology and religion and has provoked some of the deepest of human thoughts. In earlier times, the phrase beyond the known world may have referred to regions only hundreds to thousands of miles distant. In modern times, these musings extend to actual worlds—to other planets. Over the past century and a half, the great advances in science and in our understanding of nature and physical processes have refined our ability to imagine other worlds realistically and evaluate the possibilities of life beyond Earth. We now actually have the knowledge and technological tools to begin the serious search for alien life, and for the first time in history, we have the capability to test the Rare Earth Hypothesis. The tests that can be done are of two types. One consists of efforts to detect the presence of microbial life in other bodies in the solar system. The discovery of living microorganisms or fossil evidence of microorganisms would support the contention that microbial life originates readily, that it forms frequently, and that we might expect it to occur in numerous bodies that have warm, wet environments somewhere in their interiors. The search for microbial life can be done in the solar system by sending specialized probes to seek life directly with in situ analysis techniques.
The second test of the Rare Earth Hypothesis is the search for evidence of advanced life forms, which might range from simple, multicellular organisms to large animals. We see no evidence of advanced life in the solar system, except on Earth, so the main search for advanced life will focus on planetary systems around nearby stars. These searches will be conducted via large, space-borne telescopes. Both the in situ detection of microbial life and the telescopic detection of advanced life are in the planning stages, and both have high priority with funding agencies in the United States and Europe. This is a very exciting time. It is our first opportunity to actually study the processes that lead to the origin, evolution, and survival of life in the Universe.
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