Search results
Extraterrestrial life, alien life, or colloquially simply aliens, is life which does not originate from Earth. No extraterrestrial life has yet been conclusively detected. Such life might range from simple forms such as prokaryotes to intelligent beings, possibly bringing forth civilizations that might be far more advanced than the human species.
- Overview
- Universal criteria
- GeneratedCaptionsTabForHeroSec
extraterrestrial life, life that may exist or may have existed in the universe outside of Earth. The search for extraterrestrial life encompasses many fundamental scientific questions. What are the basic requirements for life? Could life have arisen elsewhere in the solar system? Are there other planets like Earth? How likely is the evolution of intelligent life?
(Read Britannica’s biography of Carl Sagan, co-author of this entry.)
No one knows which aspects of living systems are necessary, in the sense that living systems everywhere must have them, and which are contingent, in the sense that they are the result of evolutionary accidents such that elsewhere a different sequence of events might have led to different properties of life. In this respect the discovery of even a single example of extraterrestrial life, no matter how elementary in form or substance, would represent a fundamental revolution in science. Do a vast array of biological themes and counterpoints exist in the universe, or are there places with living fugues, compared with which Earth’s one tune is a bit thin and reedy? Or is Earth’s the only tune around?
Life on Earth, structurally based on carbon, hydrogen, nitrogen, and other elements, uses water as its interaction medium. Phosphorus, as phosphate bound to an organic residue, is required for energy storage and transport; sulfur is involved in the three-dimensional configuration of protein molecules; and other elements are present in smaller concentrations. Must these particular atoms be the atoms of life everywhere, or might there be a wide range of atomic possibilities in extraterrestrial organisms? What are the general physical constraints on extraterrestrial life?
In approaching these questions, several criteria can be used. The major atoms should tend to have a high cosmic abundance. Structural molecules of organisms at the temperature of the planet in question should not be so extremely stable that chemical reactions are impossible, but neither should they be extremely unstable, or else the organism would fall to pieces. A medium for molecular interaction must be present. Solids are inappropriate because of their inertness. The medium, most likely a liquid but possibly a very dense gas, must be stable in a number of respects. It should have a large temperature range (for a liquid, the temperature difference between freezing point and boiling point should be large). The liquid should be difficult to vaporize and to freeze; in general, it should be difficult to change its temperature. The interaction medium needs to be an excellent solvent. A fluid phase must be present on the planet in question, for material must cycle to the organism as food and away from the organism as waste.
The planet should therefore have an atmosphere and some liquid near the surface, although not necessarily a water ocean. If the intensity of ultraviolet light or charged particles from its sun is intense at the planetary surface, then some area, perhaps below the surface, should be shielded from this radiation (although some forms or intensity of radiation might permit useful chemical reactions to occur). Finally, it is imperative that conditions allow the existence of autotrophy (the ability of an organism to synthesize at least some of its own nutrients) or other means of net production of necessary compounds.
Students save 67%! Learn more about our special academic rate today.
Learn More
Learn about the possible forms and requirements of life beyond Earth, based on scientific criteria and evidence. Explore the history and methods of the search for extraterrestrial intelligence (SETI) and the Fermi paradox.
Sep 29, 2023 · 29 September 2023. By Pallab Ghosh,Science correspondent. ESA. Europa, one of Jupiter's icy moons is the most likely place in our solar system to be home to alien life. Many astronomers are no...
Oct 28, 2023 · Extraterrestrial Life. What's the best evidence we've found for alien life? News. By Patrick Pester. published 28 October 2023. Don't expect an alien invasion anytime soon. An artist's...
- Patrick Pester
Feb 1, 2023 · How can scientists find aliens that don't use DNA or RNA or other nucleic acids? This article explores the challenges and possibilities of looking for life as we don't know it, based on complexity and disequilibrium signals.
Nov 2, 2021 · One scientific discovery about extraterrestrial life may give us a small new insight into the universe, but it may be exaggerated by researchers or misunderstood by the public. The discovery of...
How do scientists look for signs of life beyond Earth? This article explores the latest discoveries, technologies, and challenges of the quest for alien planets and civilizations.
