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.
A continuing effort is being made to detect primitive life within our solar system. According to the Russian biologist A. I. Oparin, life can appear as the result of progressive development of organic matter from nonorganic. The principal constituents of organic matter—hydrogen, carbon, nitrogen, and oxygen—are among the most abundant atomic elements in the universe. Oparin assumed that on earth these elements combined to form simple hydrocarbons and that the hydrocarbons combined to form the precursors of life, such as amino and nucleic acids. Once these precursor molecules existed in the earth's primitive seas, they spontaneously interacted to form increasingly complex structures, until self-replicating molecules like deoxyribonucleic acid (DNA) were created, leading the way to protein synthesis.
American chemists Stanley L. Miller and Harold C. Urey provided experimental support for Oparin's theory, by discovering that when a mixture of methane, ammonia, water, and hydrogen is exposed to an electric discharge, amino acids are formed. The composition of this gas mixture is similar to the atmosphere of Jupiter. The same result has been obtained by exposing the gas mixture to ultraviolet radiation, which exists in outer space. Further support for Oparin's theory came with the discovery of organic molecules like ammonia and formaldehyde in the interstellar medium. A labeled-release experiment contained on the Viking landers which analyzed the surface of Mars in 1976 detected what could be organic activity.
Evidence of organic material and, possibly, fossils of microscopic bacteria have been found in certain carbonaceous chondrite meteorites, most notably the Orgeuil meteorite, which fell in France in 1864, the Murchison meteorite, which fell in Australia in 1969, and the Allan Hills martian meteorite, found in Antarctica in 1984, but most scientists believe either that such organic traces result from terrestrial contamination or that the data have been misinterpreted. Some scientists believe that the moon, Mars, and Venus have already been contaminated by microorganisms carried on space probes. Conversely, fears that returning Apollo astronauts could introduce destructive alien organisms into earth's biosphere led NASA to quarantine them for as long as two weeks, and lunar rock samples were kept carefully isolated.
See D. Goldsmith, The Hunt for Life on Mars (1997); P. Day, ed., The Search for Extraterrestrial Life: Essays on Science and Technology (1998); S. J. Dick, Life on Other Worlds: The 20th-Century Extraterrestrial Life Debate (1998).