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exobiology or astrobiology, search for extraterrestrial life 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).

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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



the scientific discipline devoted to the study of life in the universe in all its forms. Astrobiology is based on scientific achievement in the areas of astronomy, biology, and biochemistry. In solving certain problems, astrobiology is intimately connected with space biology and space medicine, disciplines which arose along with man’s active penetration of cosmic space.

An important problem in astrobiology is the study of the circumstances of the origin and development of life on earth as a cosmic body under primordial terrestrial conditions in the presence of a basic atmosphere. In such an atmosphere fairly complicated organic compounds, which might have served as the basis for the development of life, could have formed in the presence of external irradiation or electrical discharges. Gradually this life could have formed an extensive biosphere, whose cosmic role has been indicated by V. I. Vernadskii. As a result of the photosynthesis carried on by plants, the earth’s atmosphere gradually became oxidized; thus, the presence of oxygen in the composition of the atmosphere of any planet is a sufficient (though not an obligatory) indication of the presence of life on that planet.

Scientific information about the physical nature of the various planets obtained up to the middle of the 20th century testifies to the fact that life is not possible on every body in the solar system. In particular, it has been established that life is practically impossible on the moon, on Mercury, and on Venus. On Mars, in spite of its extremely rarefied atmosphere (absolute pressure of the order of 1 kilonewton/m2, that is, 10 millibars), the negligible quantity of water vapor in the absence of liquid water, and the low temperature (-55° C on the average), certain terrestrial forms of life could still exist, as has been demonstrated in laboratory experiments. However, a definitive solution of the problem of the existence of life on other planets can be reached only by the direct inspection of these planets by the appropriate space vehicles. The flights of the American Apollo spaceships confirmed the conclusions of astrobiology about the absence of life on the moon. Advances in astronautics permit one to hope that the solution of this problem with regard to the other bodies in the solar system is also near. Projects for experiments devised to discover life on other bodies with the help of automatic apparatus are based on the assumption that life on these planets has the same hydrocarbon basis as on earth. Life organized on another basis (ammonia, silicon) is not very likely. The major argument in favor of the universality of life on a hydrocarbon basis is that, as is shown by the detailed study of the primordial meteoritic matter (carbonaceous chondrites), the formation of extremely complex hydrocarbon compounds (anthracene, phenanthrene, and even the basic elements of deoxyribonucleic acid—the purine bases adenine and guanine) could have taken place in the preplanetary stage in the primordial gas-dust nebula; later this organic material became part of the planets as they formed, and under favorable conditions it determined the development of life on these planets.

A special problem of astrobiology concerns the search for life outside of the solar system. A significant number of stars in our galaxy could have planets with the appropriate circular orbits, sufficient masses, and constant irradiation suitable for the existence of life and even of civilizations. The number of such civilizations with a higher level than that on earth is estimated on the basis of various (sometimes considerably arbitrary) assumptions to be from about 1,000 to hundreds of millions. However, even in the latter case, only one such star out of many hundreds would be located at a distance of the order of 10 or even 100 parsecs (1 parsec = 30.86 x 1012 km) from the earth. Therefore, it is completely unrealistic, for the present, to think of sending any kind of spaceship into galactic space to establish direct contact with extraterrestrial civilizations. The establishment of contact with other civilizations with the aid of radio signals is more realistic. Such attempts to establish contact with possible civilizations near the stars τ Ceti and ∊ Eridani (distance of 3.9 and 3.5 parsecs), which could be presumed to have planetary systems, were begun by F. Drake (USA) in 1960 at the Green Bank Radio Observatory; positive results have not been obtained. In order to establish such contact, it is necessary to choose correctly the direction of signal emission, the length of the radio waves, the content of the broadcast, and the code. These problems are being studied in many scientific institutions in the USSR, the USA, and other countries.


Liubarskii, K. A. Ocherki po astrobiologii. Moscow, 1962.
Shklovskii, I. S. Vselenndia, zhizn’, razum, 2nd ed. Moscow, 1965.
Mezhzvezdnaia sviaz’. [A collection of articles, edited by G. W. Cameron.] Moscow, 1965. (Translated from English.)
Firsoff, V. A. Zhizn’vne Zemli. Moscow, 1966. (Translated from English.)
Ursul, A. D. Osvoenie kosmosa. Moscow, 1967.
Vnezemnye tsivilizatsii. Edited by S. A. Kaplan. Moscow, 1969.


The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


An approach to the scientific study of the living universe which seeks to understand the origin and evolution of life on earth, to determine if life exists elsewhere in the universe, and to predict the future of life on earth and in the rest of the universe.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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