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  2. 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.

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    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.

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    Learn about the scientific questions and challenges of finding life beyond Earth. Explore the universal criteria for life, the possible forms and sources of energy, and the current efforts and discoveries in the search for extraterrestrial intelligence.

  3. Sep 10, 2021 · NASA has not found any evidence of aliens, but is exploring various environments in the solar system and beyond for signs of life. Learn more about the NASA Astrobiology Program and its mission to answer the profound question: Is there life beyond Earth?

    • Is E.T. phoning us from Proxima Centauri? The answer to weird signals happening in the universe is never aliens, until maybe it is. Earlier this month, researchers announced that they had captured a very mysterious beam of energy in the radio part of the electromagnetic spectrum at 980 megahertz, coming from the closest star to our own.
    • Alien bacteria might live in the clouds of Venus. Astrobiologists were a-twitter with anticipation and skepticism in September when news broke of potential evidence of life in the upper clouds of Venus.
    • 'Oumuamua could still be an alien artifact. Two years ago, scientists spotted a cigar-shaped object hurtling through the solar system. Dubbed 'Oumuamua, the entity is considered by most to be an interstellar comet flung out from around another star.
    • Navy declassifies UFO videos but don't believe the hype. A fair number of Earthlings don't care what ambiguous evidence scientists come up with to show that aliens are out there.
  4. Oct 25, 2023 · In the context of life, “extraterrestrial” would mean an organism originating and capable of surviving outside of Earth. “It’s a broad term that brackets a whole range of methods for looking for life in different locations and different environments,” says Chris Impey, a university distinguished astronomy professor at the University of Arizona.

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  5. Scientists are becoming increasingly optimistic that extraterrestrial life is out there. Here, an astrobiologist explains why.

  6. Extraterrestrial life probably exists. How do we search for aliens? MAGAZINE. Life probably exists beyond Earth. So how do we find it? With next-generation telescopes, tiny space probes, and...

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