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search for extraterrestrial lifelife,
although there is no universal agreement as to a definition of life, its biological manifestations are generally considered to be organization, metabolism, growth, irritability, adaptation, and reproduction.
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 within the solar system and throughout the universe. Philosophical speculation that there might be other worlds similar to ours dates back to the ancient Chinese and Greeks. However, the achievements of space exploration and molecular biology have turned speculation into experimentation.

Parameters of a Suitable Environment for Life

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.

The Search for Primitive Life

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 other scientists have argued either that such organic traces result from terrestrial contamination or that the data have been misinterpreted. However, an analysis of the Murchison meteorite that extracted material from its core found evidence of 70 amino acids and many other organic compounds. 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).


(eks-oh-bÿ-ol -ŏ-jee) The theories concerning ‘living’ systems that may exist on other planets or their satellites in the Solar System or on planetary systems of other stars, the methods of detecting them, and the study of their origin, development, distribution, etc. The probability of this extraterrestrial life occurring at some time on some planetary system of some star depends on the fraction of the possible 1020 stars in the Universe – 1011 in our Galaxy – that have planetary systems and on the fraction of these planetary systems that could support, on one or more members, these ‘living’ organisms.

The degree to which exobiology may differ from terrestrial biology is a matter of dispute. There are no universal laws of biology as there are assumed to be for physics; organisms could evolve on planets or satellites moderately different from Earth, in an unrecognizable form. There is no absolute definition of life but it is believed that any living material is built up from the common elements carbon, hydrogen, oxygen, and nitrogen and that the presence of water is probably necessary for sustaining life. In addition the environment should have an atmosphere to act as a shield from high-energy cosmic and stellar radiation, and to aid respiration where necessary, and the temperature should be reasonably uniform.

The course and rate of evolution of living organisms probably depends on the type of star in the planetary system, and its stability, as well as the planetary environment. The conditions may not be appropriate for the origin and evolution of living organisms in this present era. If they can form, however, many scientists believe that these organisms will inevitably become more developed. The number of life forms that might at this moment be ‘intelligent’ in a human sense and also able to communicate this intelligence is very uncertain: one group of scientists has suggested that at present there could be as many as one million advanced civilizations in our Galaxy; other estimates have been very much lower. Communications, however, are severely limited by distance: signals from a star 100 light-years (30.7 parsecs) away take 100 years to reach Earth.

There is to date no direct evidence for extraterrestrial life. Searches for microorganisms in the soils of the Moon and Mars have been unsuccessful. Searches for extraterrestrial intelligence (SETI) have also been made and are once more in progress.



an experimental scientific discipline concerned with the search for and investigation of extraterrestrial life forms. Exobiology is primarily interested in the determination of the limits of the study of the mechanisms of survival of terrestrial organisms under extreme environmental conditions, the elucidation of pathways of abiogenic synthesis of the most important bio-organic compounds and the stages of prebiological evolution, and the establishment of criteria for the existence and development of automated methods of discovering life on other planets by means of automated biological laboratories.

It has been established that many terrestrial microorganisms or their spores can retain their viability at low temperatures (below –70°C), in a vacuum (as low as 10–7 to 10–10 mm Hg), and at low air humidities (1–2 percent). The abiogenic synthesis of amino acids, lipids, nucleotides, sugars, and other biologically important substances has been effected by the action of ultraviolet rays on simple compounds, such as water, ammonia, carbon monoxide, and methane. This has made it possible to advance the concept in exobiology of the possibility of life on other planets, first and foremost on Mars, which is constructed on a carbon-organic aqueous base. In the search for extraterrestrial life, exobiology uses various methods and devices to identify complex organic compounds, including gas chromatography, mass spec-trometry, and optical instruments that record the absorption spectra and luminescence of substances from extraterrestrial soils. Functional methods are used to determine active metabolism by recording parameters of growth, reproduction, and gas exchange of microorganisms during incubation of soil specimens on combined nutrient media of complex composition. Only the combined use of various methods according to a single program in the automated biological laboratory can eventually bring success.

In 1976 the space probe Viking (USA), with an automated laboratory on board, successfully landed on Mars. However, the first exobiological experiment on Mars provided no evidence of life on the planet. Further experiments may be carried out both by means of automated biological laboratories and by the direct study of soil samples brought from Mars to the earth. Similar investigations carried out on lunar soil specimens indicate the absence of life on the moon in the past and present.


Calvin, M. Khimicheskaia evoliutsiia: Molekuliarnaia evoliutsiia, vedushchaia k vozniknoveniiu zhivykh sistem na Zemle i na drugikh planetakh. Moscow, 1971. (Translated from English.)
Osnovy kosmicheskoi biologii i meditsiny, vol. 1, part 3. Moscow, 1975.



The search for and study of extraterrestrial life.