ℹ️ Skipped - page is already crawled
| Filter | Status | Condition | Details |
|---|---|---|---|
| HTTP status | PASS | download_http_code = 200 | HTTP 200 |
| Age cutoff | PASS | download_stamp > now() - 6 MONTH | 0.3 months ago |
| History drop | PASS | isNull(history_drop_reason) | No drop reason |
| Spam/ban | PASS | fh_dont_index != 1 AND ml_spam_score = 0 | ml_spam_score=0 |
| Canonical | PASS | meta_canonical IS NULL OR = '' OR = src_unparsed | Not set |
| Property | Value |
|---|---|
| URL | https://www.britannica.com/science/extraterrestrial-life |
| Last Crawled | 2026-04-13 22:09:50 (7 days ago) |
| First Indexed | 2016-07-19 21:55:16 (9 years ago) |
| HTTP Status Code | 200 |
| Meta Title | Extraterrestrial life | Definition, SETI, & Facts | Britannica |
| Meta Description | Extraterrestrial life refers to life that may exist or may have existed in the universe outside of Earth. The search for extraterrestrial life encompasses many fundamental scientific questions, including ‘What are the basic requirements for life?’ and ‘Are there other planets like Earth?’ |
| Meta Canonical | null |
| Boilerpipe Text | 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.)
Universal criteria
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
.
Photosynthesis
Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.
Thermodynamically,
photosynthesis
based on stellar radiation may be the optimal source of energy for extraterrestrial life. Photosynthetic organisms and the radiation they receive are not in
thermodynamic equilibrium
. On Earth, for example, a green
plant
may have a temperature of about 300 K (23 °C, or 73 °F); the
Sun
’s temperature is about 6,000 K. (K = kelvin. On the Kelvin temperature scale, in which 0 K [−273 °C, or −460 °F] is
absolute zero
, 273 K [0 °C, or 32 °F] is the freezing point of water, and 373 K [100 °C, or 212 °F] is the boiling point of water at one atmosphere pressure.) Photosynthetic processes are possible because energy is transported from a hotter object (the Sun) to a cooler object (Earth). Were the source of radiation at the same or at a colder temperature than the photosynthesizer, no photosynthetic activity would be possible. For this reason, the idea that a subterranean green plant will photosynthesize by use of thermal
infrared radiation
emitted by its surroundings is
untenable
. Equally unfeasible is the idea that a cold
star
, with a surface temperature similar to that of Earth, could sustain photosynthetic organisms.
Trusted knowledge for those who want to know more.
SUBSCRIBE
One can use these conditions to establish the limits for the chemical requirements of life. When atoms chemically combine, the energy necessary to separate them is called the
bond energy, and the measure of this energy determines how tightly the two atoms are bound to each other. Bond energies generally vary from about 10 electron volts (eV) to about 0.03 eV.
Covalent bonds
, where
electrons
are shared between atoms, tend to be more energetic than hydrogen bonds, where a hydrogen atom is shared between atoms, and hydrogen bonds in turn are more energetic than
van der Waals forces
, which arise from the attraction of the electrons of one atom for the nucleus of another. Atoms, free or bound, move with an average
kinetic energy
corresponding to about 0.02 eV. The higher the temperature, the more atoms move with energy sufficient to break a given bond spontaneously.
Specific atoms have circumscribed functions in modern
biology
, but, aside from structure and the need for the liquid interaction medium, they may not be fundamental. The energy-rich phosphate bonds in
adenosine triphosphate
(ATP), about as energetic as the hydrogen bonds, are in fact of relatively low energy. Cells store large numbers of these bonds to drive a molecular
degradation
or synthesis. One expects the energy currency on high-temperature worlds to be much more energetic per bond and on low-temperature worlds to be much less energetic per bond.
In
The Fitness of the Environment
(1913), American biochemist
Lawrence Joseph Henderson
first stressed the advantages of carbon and water for life in terms of comparative chemistry. Henderson was struck by the fact that the very atoms needed are exactly those that are around. It remains a remarkable fact that the atoms most useful for life have very high cosmic abundances.
AI-generated answers
from Britannica articles. AI makes mistakes, so verify using Britannica articles. |
| Markdown | [](https://www.britannica.com/)
[](https://www.britannica.com/)
[SUBSCRIBE](https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=global-nav&utm_campaign=blue-evergreen)
[SUBSCRIBE](https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=global-nav-mobile&utm_campaign=blue-evergreen)
Login
https://premium.britannica.com/premium-membership/?utm\_source=premium\&utm\_medium=nav-login-box\&utm\_campaign=evergreen
[SUBSCRIBE](https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=hamburger-menu&utm_campaign=blue)
[Ask the Chatbot](https://www.britannica.com/chatbot)
[Games & Quizzes](https://www.britannica.com/quiz/browse) [History & Society](https://www.britannica.com/History-Society) [Science & Tech](https://www.britannica.com/Science-Tech) [Biographies](https://www.britannica.com/Biographies) [Animals & Nature](https://www.britannica.com/Animals-Nature) [Geography & Travel](https://www.britannica.com/Geography-Travel) [Arts & Culture](https://www.britannica.com/Arts-Culture) [ProCon](https://www.britannica.com/procon) [Money](https://www.britannica.com/money) [Videos](https://www.britannica.com/videos)
[extraterrestrial life](https://www.britannica.com/science/extraterrestrial-life)
- [Introduction](https://www.britannica.com/science/extraterrestrial-life)
- [Universal criteria](https://www.britannica.com/science/extraterrestrial-life#ref279235)
- [The search for extraterrestrial life](https://www.britannica.com/science/extraterrestrial-life/The-search-for-extraterrestrial-life)
- [Life in the solar system](https://www.britannica.com/science/extraterrestrial-life/Life-in-the-solar-system)
- [The Moon and Mercury](https://www.britannica.com/science/extraterrestrial-life/Life-in-the-solar-system#ref279238)
- [Martian “vegetation” and “canals”](https://www.britannica.com/science/extraterrestrial-life/Life-in-the-solar-system#ref279239)
- [Venus](https://www.britannica.com/science/extraterrestrial-life/Venus)
- [Jovian planets](https://www.britannica.com/science/extraterrestrial-life/Venus#ref279241)
- [Europa, other Jovian moons, comets, and asteroids](https://www.britannica.com/science/extraterrestrial-life/Venus#ref279242)
- [Life beyond the solar system](https://www.britannica.com/science/extraterrestrial-life/Life-beyond-the-solar-system)
- [The Drake equation and extrasolar life](https://www.britannica.com/science/extraterrestrial-life/Life-beyond-the-solar-system#ref279244)
- [Searching for technical civilizations](https://www.britannica.com/science/extraterrestrial-life/Searching-for-technical-civilizations)
[References & Edit History](https://www.britannica.com/science/extraterrestrial-life/additional-info) [Related Topics](https://www.britannica.com/facts/extraterrestrial-life)
[Images & Videos](https://www.britannica.com/science/extraterrestrial-life/images-videos)
[](https://cdn.britannica.com/60/239460-050-21C85CE4/Newly-discovered-earth-sized-planet-TOI-700-e-orbits-within-the-habitable-zone-of-its-star.jpg) [](https://cdn.britannica.com/22/94122-050-93F6BAE0/trees-plants-carbon-dioxide-sunlight-sugars-water.jpg) [](https://cdn.britannica.com/66/73766-050-C3198E26/neutrons-map-data-Mars-surface-spacecraft-levels-2001.jpg) [](https://cdn.britannica.com/69/118369-050-EB6A7AA3/Earth-satellites-composite-images-US-Air-Force.jpg) [](https://cdn.britannica.com/61/96161-050-4563DDA3/Shortwave-microwave-telephone-telecommunication-antennas-types-buildings.jpg) [](https://cdn.britannica.com/12/96612-004-20AD7080/Image-surface-probe-Titan-Huygens-High-Resolution.jpg) [](https://cdn.britannica.com/29/23329-050-E4D2ABF7/Earth-impact-basins-side-Moon-way-spacecraft-December-7-1992.jpg) [](https://cdn.britannica.com/16/75516-050-C40070F2/Photo-mosaic-Mercury-Mariner-10-spacecraft-1974.jpg) [](https://cdn.britannica.com/24/73524-050-3C2F3432/result-seasons-Mars-planet-inclination-plane-cap.jpg) [](https://cdn.britannica.com/68/118368-050-47C28A3F/Eugene-Antoniadi-Mars-map-observations-Hubble-Space-1894.jpg)
Ask Anything
Quick Summary
[Science](https://www.britannica.com/browse/Science) [Astronomy](https://www.britannica.com/browse/Astronomy)
CITE
Share
Feedback
External Websites
[](https://cdn.britannica.com/60/239460-050-21C85CE4/Newly-discovered-earth-sized-planet-TOI-700-e-orbits-within-the-habitable-zone-of-its-star.jpg)
[Planet TOI 700e](https://cdn.britannica.com/60/239460-050-21C85CE4/Newly-discovered-earth-sized-planet-TOI-700-e-orbits-within-the-habitable-zone-of-its-star.jpg) An illustration of the Earth-size planet TOI 700e orbiting within the habitable zone of its star. The habitable zone is the area around a star where liquid water, and thus possibly life, can survive on a planet's surface. Its Earth-size sibling, TOI 700d, can be seen in the distance.
(more)
# extraterrestrial life
Homework Help
Written by
[Carl Sagan Astronomer and science writer Carl Sagan (1934-96) was a highly popular and influential figure in the United States and controversial in scientific, political, and religious circles for his views on extraterrestrial...](https://www.britannica.com/contributor/Carl-Sagan/2564)
Carl Sagan[All](https://www.britannica.com/science/extraterrestrial-life/additional-info#contributors)
Fact-checked by
[Britannica Editors Encyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree....](https://www.britannica.com/editor/The-Editors-of-Encyclopaedia-Britannica/4419)
Britannica Editors
Last updated
Mar. 10, 2026
•[History](https://www.britannica.com/science/extraterrestrial-life/additional-info#history)
 Britannica AI
Ask Anything
Quick Summary
Table of Contents
Table of Contents
Quick Summary
Ask Anything
**extraterrestrial life**, [life](https://www.britannica.com/science/life) that may exist or may have existed in the [universe](https://www.britannica.com/science/universe) outside of [Earth](https://www.britannica.com/place/Earth). The search for extraterrestrial life [encompasses](https://www.merriam-webster.com/dictionary/encompasses) many fundamental scientific questions. What are the basic requirements for life? Could life have arisen elsewhere in the [solar system](https://www.britannica.com/science/solar-system)? Are there other [planets](https://www.britannica.com/science/planet) like Earth? How likely is the [evolution](https://www.britannica.com/science/evolution-scientific-theory) of intelligent life?
*[(Read Britannica’s biography of Carl Sagan, co-author of this entry.)](https://www.britannica.com/biography/Carl-Sagan)*
## Universal criteria
No one knows which aspects of living systems are necessary, in the sense that living systems everywhere must have them, and which are [contingent](https://www.merriam-webster.com/dictionary/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](https://www.britannica.com/science/carbon-chemical-element), [hydrogen](https://www.britannica.com/science/hydrogen), [nitrogen](https://www.britannica.com/science/nitrogen), and other elements, uses [water](https://www.britannica.com/science/water) as its interaction medium. [Phosphorus](https://www.britannica.com/science/phosphorus-chemical-element), as [phosphate](https://www.britannica.com/science/phosphate) bound to an organic residue, is required for [energy](https://www.britannica.com/science/energy) storage and transport; [sulfur](https://www.britannica.com/science/sulfur) is involved in the three-dimensional configuration of [protein](https://www.britannica.com/science/protein) molecules; and other elements are present in smaller concentrations. Must these particular [atoms](https://www.britannica.com/science/atom) 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](https://www.merriam-webster.com/dictionary/criteria) can be used. The major atoms should tend to have a high cosmic abundance. Structural [molecules](https://www.britannica.com/science/molecule) of organisms at the temperature of the [planet](https://www.britannica.com/science/planet) in question should not be so extremely stable that [chemical reactions](https://www.britannica.com/science/chemical-reaction) 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](https://www.britannica.com/science/solid-state-of-matter) are inappropriate because of their inertness. The medium, most likely a [liquid](https://www.britannica.com/science/liquid-state-of-matter) but possibly a very dense [gas](https://www.britannica.com/science/gas-state-of-matter), must be stable in a number of respects. It should have a large [temperature](https://www.britannica.com/science/temperature) range (for a liquid, the temperature difference between [freezing point](https://www.britannica.com/science/freezing-point) and [boiling point](https://www.britannica.com/science/boiling-point) should be large). The liquid should be difficult to [vaporize](https://www.britannica.com/science/vaporization) and to freeze; in general, it should be difficult to change its temperature. The interaction medium needs to be an excellent [solvent](https://www.britannica.com/science/solvent-chemistry). 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](https://www.britannica.com/science/ultraviolet-radiation) 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](https://www.britannica.com/science/radiation) might permit useful chemical reactions to occur). Finally, it is [imperative](https://www.merriam-webster.com/dictionary/imperative) that conditions allow the existence of [autotrophy](https://www.britannica.com/science/autotroph) (the ability of an organism to synthesize at least some of its own nutrients) or other means of net production of necessary [compounds](https://www.merriam-webster.com/dictionary/compounds).
[](https://cdn.britannica.com/22/94122-050-93F6BAE0/trees-plants-carbon-dioxide-sunlight-sugars-water.jpg)
[Photosynthesis](https://cdn.britannica.com/22/94122-050-93F6BAE0/trees-plants-carbon-dioxide-sunlight-sugars-water.jpg)Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.
(more)
Thermodynamically, [photosynthesis](https://www.britannica.com/science/photosynthesis) based on stellar radiation may be the optimal source of energy for extraterrestrial life. Photosynthetic organisms and the radiation they receive are not in [thermodynamic equilibrium](https://www.britannica.com/science/thermodynamic-equilibrium). On Earth, for example, a green [plant](https://www.britannica.com/plant/plant) may have a temperature of about 300 K (23 °C, or 73 °F); the [Sun](https://www.britannica.com/place/Sun)’s temperature is about 6,000 K. (K = kelvin. On the Kelvin temperature scale, in which 0 K \[−273 °C, or −460 °F\] is [absolute zero](https://www.britannica.com/science/absolute-zero), 273 K \[0 °C, or 32 °F\] is the freezing point of water, and 373 K \[100 °C, or 212 °F\] is the boiling point of water at one atmosphere pressure.) Photosynthetic processes are possible because energy is transported from a hotter object (the Sun) to a cooler object (Earth). Were the source of radiation at the same or at a colder temperature than the photosynthesizer, no photosynthetic activity would be possible. For this reason, the idea that a subterranean green plant will photosynthesize by use of thermal [infrared radiation](https://www.britannica.com/science/infrared-radiation) emitted by its surroundings is [untenable](https://www.merriam-webster.com/dictionary/untenable). Equally unfeasible is the idea that a cold [star](https://www.britannica.com/science/star-astronomy), with a surface temperature similar to that of Earth, could sustain photosynthetic organisms.
Explore Britannica Premium\!
Trusted knowledge for those who want to know more.
[SUBSCRIBE](https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=inline-cta&utm_campaign=shorter-2026)
One can use these conditions to establish the limits for the chemical requirements of life. When atoms chemically combine, the energy necessary to separate them is called the bond energy, and the measure of this energy determines how tightly the two atoms are bound to each other. Bond energies generally vary from about 10 electron volts (eV) to about 0.03 eV. [Covalent bonds](https://www.britannica.com/science/covalent-bond), where [electrons](https://www.britannica.com/science/electron) are shared between atoms, tend to be more energetic than hydrogen bonds, where a hydrogen atom is shared between atoms, and hydrogen bonds in turn are more energetic than [van der Waals forces](https://www.britannica.com/science/van-der-Waals-forces), which arise from the attraction of the electrons of one atom for the nucleus of another. Atoms, free or bound, move with an average [kinetic energy](https://www.britannica.com/science/kinetic-energy) corresponding to about 0.02 eV. The higher the temperature, the more atoms move with energy sufficient to break a given bond spontaneously.
Specific atoms have circumscribed functions in modern [biology](https://www.britannica.com/science/biology), but, aside from structure and the need for the liquid interaction medium, they may not be fundamental. The energy-rich phosphate bonds in [adenosine triphosphate](https://www.britannica.com/science/adenosine-triphosphate) (ATP), about as energetic as the hydrogen bonds, are in fact of relatively low energy. Cells store large numbers of these bonds to drive a molecular [degradation](https://www.merriam-webster.com/dictionary/degradation) or synthesis. One expects the energy currency on high-temperature worlds to be much more energetic per bond and on low-temperature worlds to be much less energetic per bond.
Key People:
[Carl Sagan](https://www.britannica.com/biography/Carl-Sagan)
[Paul Davies](https://www.britannica.com/biography/Paul-Davies)
[Freeman Dyson](https://www.britannica.com/biography/Freeman-Dyson)
[Percival Lowell](https://www.britannica.com/biography/Percival-Lowell)
*(Show more)*
Related Topics:
[extraterrestrial intelligence](https://www.britannica.com/science/extraterrestrial-intelligence)
[The Fermi Paradox: Where Are All the Aliens?](https://www.britannica.com/story/the-fermi-paradox-where-are-all-the-aliens)
[Fermi paradox](https://www.britannica.com/science/Fermi-paradox)
[Kardashev scale](https://www.britannica.com/science/Kardashev-scale)
[alien](https://www.britannica.com/science/alien-extraterrestrial-life)
*(Show more)*
[See all related content](https://www.britannica.com/facts/extraterrestrial-life)
In *[The Fitness of the Environment](https://www.britannica.com/topic/The-Fitness-of-the-Environment)* (1913), American biochemist [Lawrence Joseph Henderson](https://www.britannica.com/biography/Lawrence-Joseph-Henderson) first stressed the advantages of carbon and water for life in terms of comparative chemistry. Henderson was struck by the fact that the very atoms needed are exactly those that are around. It remains a remarkable fact that the atoms most useful for life have very high cosmic abundances.
Britannica AI
*chevron\_right*
Extraterrestrial life
*close*
[AI-generated answers](https://www.britannica.com/about-britannica-ai) from Britannica articles. AI makes mistakes, so verify using Britannica articles.
# The search for extraterrestrial life
[](https://cdn.britannica.com/66/73766-050-C3198E26/neutrons-map-data-Mars-surface-spacecraft-levels-2001.jpg)
[Mars](https://cdn.britannica.com/66/73766-050-C3198E26/neutrons-map-data-Mars-surface-spacecraft-levels-2001.jpg)Global map of Mars in epithermal (intermediate-energy) neutrons created from data collected by the 2001 Mars Odyssey spacecraft. Odyssey mapped the location and concentrations of epithermal neutrons knocked off the Martian surface by incoming cosmic rays. Deep blue areas at the high latitudes mark the lowest levels of neutrons, which scientists have interpreted to indicate the presence of high levels of hydrogen. The hydrogen enrichment, in turn, is suggestive of large reservoirs of water ice below the surface.
(more)
[*Astrobiology*](https://www.britannica.com/science/astrobiology), a term coined for the study of all [life](https://www.britannica.com/science/life) anywhere in the [universe](https://www.britannica.com/science/universe) (including Earth), has replaced *exobiology*, the study of extraterrestrial life exclusively and therefore criticizable as “a science that lacks a subject matter.” Unlike exobiology, astrobiology respects the scientific possibility that life beyond Earth may never be found. Indeed, no evidence for life beyond [Earth](https://www.britannica.com/place/Earth) has been adduced. However, the design of astrobiological experiments forces critical examination of the generality of assumptions [derived](https://www.britannica.com/dictionary/derived) from Earth life.
There is an entire spectrum of possibilities for life on another planet. A [planet](https://www.britannica.com/science/planet) may be lifeless and lack any vestiges of organic matter or [fossils](https://www.britannica.com/science/fossil). Alternatively, it may be lifeless but contain organic matter or fossils. There may be life having simple or quite complex [biochemistry](https://www.britannica.com/science/biochemistry), [physiology](https://www.britannica.com/science/physiology), and behaviour. Even intelligent life with a technical civilization may be found. Confirmation of any of these possibilities would be of great scientific importance.
The search for extraterrestrial life is most clearly grasped by imagining the reverse situation. For example, if humans were on [Mars](https://www.britannica.com/place/Mars-planet), examination of Earth for life with the full armory of contemporary scientific instrumentation and knowledge would be [illuminating](https://www.merriam-webster.com/dictionary/illuminating). Both remote and in situ testing might be attempted. In remote testing, light of any wavelength reflected from or emitted by the target planet can be examined. Remote-sensing methods seek thermodynamic disequilibrium, especially in the fluid phases ([atmosphere](https://www.britannica.com/science/atmosphere) and [hydrosphere](https://www.britannica.com/science/hydrosphere)) of the planet. With in situ studies, samples of a planet must be acquired by instrumentation that lands there and performs experiments.
Chemical, mechanical, or spectral disequilibria may also be sought. Earth’s atmosphere contains large amounts of molecular [oxygen](https://www.britannica.com/science/oxygen) and about 1.7–2 parts per million (106) of [methane](https://www.britannica.com/science/methane), but in [thermodynamic equilibrium](https://www.britannica.com/science/thermodynamic-equilibrium) the abundance of methane should be less than one part in 1035. This huge discrepancy implies that some process continuously and rapidly generates methane on Earth such that methane increases to a very large steady-state abundance before it can be oxidized. Although the methane disequilibrium mechanism need not be biological (e.g., relatively stable aromatic [hydrocarbons](https://www.britannica.com/science/hydrocarbon) could have been produced nonbiologically early in Earth’s history, and slow [degradation](https://www.merriam-webster.com/dictionary/degradation) may then have led to a continuous loss of methane from the planetary subsurface), a biological explanation seems more likely. As seen from Mars, the methane discrepancy could be considered as a preliminary positive test for life on Earth. Indeed, the methane abundance on Earth is due to [bacteria](https://www.britannica.com/science/bacteria). Some methanogenic bacteria live in [wetlands](https://www.britannica.com/science/wetland) (hence the term *marsh gas* for methane), and others live in the intestinal tracts of [cows](https://www.britannica.com/animal/cow) and other [ruminants](https://www.britannica.com/animal/ruminant). Similarly, the large amount of free oxygen gas might be considered a sign of life. The possibility that the photodissociation of [water](https://www.britannica.com/science/water) and the subsequent escape to space of [hydrogen](https://www.britannica.com/science/hydrogen) are the source of oxygen would need to be excluded. Also, spectroscopic detection of such relatively complex reduced organic molecules as [terpenes](https://www.britannica.com/science/terpene) (hydrocarbons given off by plants and found over [forests](https://www.britannica.com/science/forest)) could be used as a test for life.
[](https://cdn.britannica.com/69/118369-050-EB6A7AA3/Earth-satellites-composite-images-US-Air-Force.jpg)
[Earth at night](https://cdn.britannica.com/69/118369-050-EB6A7AA3/Earth-satellites-composite-images-US-Air-Force.jpg)Earth at night as seen in a composite of images taken by satellites of the Defense Meteorological Satellite Program of the U.S. Air Force.
(more)
By contrast, photographic observations of the daytime Earth from Mars would not necessarily detect life. Even at a resolution of 100 meters (330 feet)—that is, an ability to discriminate fine detail at high contrast only if its components are more than 100 meters apart—cities, canals, [bridges](https://www.britannica.com/technology/bridge-engineering), the [Great Wall of China](https://www.britannica.com/topic/Great-Wall-of-China) (long erroneously believed to be visible from the [Moon](https://www.britannica.com/place/Moon)), highways, and other large-scale [accoutrements](https://www.merriam-webster.com/dictionary/accoutrements) of Earth’s technical civilization would be extremely difficult to discern. As resolution progressively improves, it becomes increasingly easy to distinguish the regular geometric patterns of [cultivated](https://www.merriam-webster.com/dictionary/cultivated) fields, highways, and [airports](https://www.britannica.com/technology/airport). However, these are products of recent civilization; thus, only 100,000 years ago no clear sign of life would have been visible with remote-sensing techniques. Today lights of the largest cities are detectable from Mars, as are seasonal changes in the color of plants.
[](https://cdn.britannica.com/61/96161-050-4563DDA3/Shortwave-microwave-telephone-telecommunication-antennas-types-buildings.jpg)
[telecommunication antennas](https://cdn.britannica.com/61/96161-050-4563DDA3/Shortwave-microwave-telephone-telecommunication-antennas-types-buildings.jpg)Short-wave, microwave, cellular telephone, and other types of telecommunication antennas receive and send messages from high ground near Phoenix, Arizona.
(more)
Scanning of the [electromagnetic spectrum](https://www.britannica.com/science/electromagnetic-spectrum) offers another technique for detecting life. Domestic television transmissions, the high-frequency end of the AM broadcast band, and [radar](https://www.britannica.com/technology/radar) defense networks make up some of the enormous amount of [energy](https://www.britannica.com/science/energy) put out by Earth into space at certain radio frequencies. According to an estimate made by the Russian astrophysicist Iosif S. Shklovskii, if this radiation were to be interpreted as ordinary thermal emission, the implied temperature of Earth would be hundreds of millions of degrees. This radio “brightness temperature” of Earth would have steadily increased over the last several decades. The frequency and the time variation of these signals are not purely random noise.
In situ studies by vehicles that enter Earth’s atmosphere and land on the surface could detect life at many places on Earth. However, there are many other places where large organisms are infrequent such that life-detection attempts based solely on television searches for large animals would be inconclusive. Of course, if such an experiment were successful—for example, if the camera recorded a cavorting [dolphin](https://www.britannica.com/animal/dolphin-mammal), a strolling [camel](https://www.britannica.com/animal/camel), or a flying [peacock](https://www.britannica.com/animal/peacock)—it would provide quite convincing evidence of life.
Although the open [ocean](https://www.britannica.com/science/ocean), the [Gobi Desert](https://www.britannica.com/place/Gobi), and [Antarctica](https://www.britannica.com/place/Antarctica) are relatively devoid of large life-forms, they are—like other, less-barren [ecosystems](https://www.britannica.com/science/ecosystem)—replete with microorganisms. A television camera coupled to an optical or [electron microscope](https://www.britannica.com/technology/electron-microscope) might be an optimal life detector. The 17th-century Dutch microscopist [Antonie van Leeuwenhoek](https://www.britannica.com/biography/Antonie-van-Leeuwenhoek) had no difficulty in identifying as alive the little “animalcules” he found in a drop of water, even though nothing similar had been seen before in [human](https://www.britannica.com/topic/human-being) history.
Metabolic and chemical [criteria](https://www.merriam-webster.com/dictionary/criteria) might be used for detecting life with in situ studies. The fixation of gas (such as [carbon dioxide](https://www.britannica.com/science/carbon-dioxide)) when [illuminated](https://www.merriam-webster.com/dictionary/illuminated) might be due to [photosynthesis](https://www.britannica.com/science/photosynthesis) or chemosynthesis. Direct tests of [soil](https://www.britannica.com/science/soil) or [water](https://www.britannica.com/science/water) for [optical activity](https://www.britannica.com/science/optical-activity) might be made. Organic molecules could certainly be sought with [gas chromatography](https://www.britannica.com/science/gas-chromatography), [mass spectrometry](https://www.britannica.com/science/mass-spectrometry), or remote [analytic](https://www.merriam-webster.com/dictionary/analytic) chemistry. The detection of organic matter would then lead to experiments that would determine if it was biological in origin.
In general, many tests for life are intrinsically [ambiguous](https://www.merriam-webster.com/dictionary/ambiguous). There remains the omnipresent problem of contamination. Any [spacecraft](https://www.britannica.com/technology/spacecraft) might carry living organisms from the home planet and report them as detected on the target planet. Great care must be taken to ensure that the spacecraft is rigorously sterilized and travels without life from home.
Even the detection of significant quantities of extraterrestrial organic matter can be misleading. [Carbonaceous chondrite](https://www.britannica.com/science/carbonaceous-chondrite) [meteorites](https://www.britannica.com/science/meteorite) fall on Earth from the [asteroid](https://www.britannica.com/science/asteroid) belt. They contain up to 4 percent organic matter by mass. This matter has been [ascertained](https://www.merriam-webster.com/dictionary/ascertained) to be of nonbiological origin. Microscopic inclusions also have been detected. The most abundant of these inclusions are mineralogical in origin. Highly structured inclusions, such as filaments or microspheres with central dots, are rare and sometimes the result of obvious contamination (one inclusion contained [ragweed](https://www.britannica.com/plant/ragweed) [pollen](https://www.britannica.com/science/pollen)). Claims of the extraction of viable microorganisms from the interiors of carbonaceous chondrites were not supported by subsequent evidence. These meteorites are porous and “breathe” [air](https://www.britannica.com/science/air) in and out during their entry into the atmosphere and during their storage prior to study. Significant opportunities exist for contamination after their arrival on Earth because of the ubiquity of microorganisms. Some bacteria extracted from a [meteorite](https://www.britannica.com/science/meteorite) were facultative aerobes. As no planet in the solar system except Earth harbors significant quantities of oxygen gas, it is unlikely that the electron-transfer multienzyme pathways required for oxygen respiration evolved in the asteroid belt. Nevertheless, the large amounts of organic matter found in carbonaceous chondrites suggest that organic [molecule](https://www.britannica.com/science/molecule) production occurs with great [efficiency](https://www.merriam-webster.com/dictionary/efficiency) in certain extraterrestrial locations. This production may serve as a natural [precursor](https://www.merriam-webster.com/dictionary/precursor) to life elsewhere.
No single unambiguous “life detector” exists. Instruments of great generality that make few ambiguous assumptions about the nature of extraterrestrial life require luck (e.g., an animal or [protist](https://www.britannica.com/science/protist) must walk or swim by during the operating lifetime of the camera) or the solution of difficult instrumental problems (e.g., the acquisition and preparation of samples for remote electron microscopic examination). Highly sensitive instruments, such as [metabolism](https://www.britannica.com/science/metabolism) detectors, are directed at organisms presumably vastly more abundant than animals. These instruments critically depend on assumptions that are basically informed guesses (e.g., that extraterrestrial organisms eat [sugars](https://www.britannica.com/science/sugar-chemical-compound)). Therefore, an [array](https://www.britannica.com/dictionary/array) of both very general and very specific instruments is recommended to establish, or preclude, the existence of extraterrestrial life in the [solar system](https://www.britannica.com/science/solar-system).
Load Next Page
Feedback
Thank you for your feedback
Our editors will review what you’ve submitted and determine whether to revise the article.
*print* Print
Please select which sections you would like to print:
*verified*Cite
While every effort has been made to follow citation style rules, there may be some discrepancies. Please refer to the appropriate style manual or other sources if you have any questions.
Select Citation Style
Sagan, Carl, Margulis, Lynn, Sagan, Dorion. "extraterrestrial life". *Encyclopedia Britannica*, 10 Mar. 2026, https://www.britannica.com/science/extraterrestrial-life. Accessed 13 April 2026.
Copy Citation
Share
Share to social media
[Facebook](https://www.facebook.com/BRITANNICA/) [X](https://x.com/britannica)
URL
<https://www.britannica.com/science/extraterrestrial-life>
External Websites
- [Physics LibreTexts - Life beyond Earth](https://phys.libretexts.org/Courses/HACC_Central_Pennsylvania%27s_Community_College/Astronomy_103%3A_Introduction_to_Planetary_Astronomy/15%3A_Astrobiology_and_the_Search_for_Extraterrestrial_Intelligence/15.03%3A_Life_Beyond_Earth)
- [NASA Technical Reports Server - Extraterrestrial Life in the Universe (PDF)](https://ntrs.nasa.gov/api/citations/19900013148/downloads/19900013148.pdf)
- [Open Library Publishing Platform - Fanshawe College Astronomy - The Search for Extraterrestrial Life](https://ecampusontario.pressbooks.pub/fanshawecastronomy/chapter/16-4-the-search-for-extraterrestrial-life/)
- [The Royal Society Publishing - Philosophical Transactions of the Royal Society A - The detection of extra-terrestrial life and the consequences for science and society (PDF)](https://royalsocietypublishing.org/rsta/article-abstract/369/1936/499/114265/The-detection-of-extra-terrestrial-life-and-the?redirectedFrom=PDF)
- [LiveScience - How Do Scientists Search for Extraterrestrial Life?](https://www.livescience.com/59153-how-to-search-for-extraterrestrial-life.html)
- [Cambridge University Press - Cambridge Core - Evidence and traces of extraterrestrial life](https://www.cambridge.org/core/journals/international-journal-of-astrobiology/article/evidence-and-traces-of-extraterrestrial-life/FE4A464C5B611EE4660FF1F0238B66B4)
- [European Space Agency - Science & Exploration - Extraterrestrial life](https://www.esa.int/Science_Exploration/Human_and_Robotic_Exploration/Exploration/Extraterrestrial_life)
- [Space - SETI & the Search for Extraterrestrial Life](https://www.space.com/33626-search-for-extraterrestrial-intelligence.html)
- [BBC - The truth about life on other planets - and what it means for humans](https://www.bbc.com/news/articles/cp8jwj90ejno)
- [Frontiers - How Will We React to the Discovery of Extraterrestrial Life?](https://www.frontiersin.org/articles/10.3389/fpsyg.2017.02308/full)
- [PNAS - State-of-the-art instruments for detecting extraterrestrial life](https://www.pnas.org/doi/10.1073/pnas.98.3.797)
Britannica Websites
Articles from Britannica Encyclopedias for elementary and high school students.
- [extraterrestrial life - Student Encyclopedia (Ages 11 and up)](https://kids.britannica.com/students/article/extraterrestrial-life/274243) |
| Readable Markdown | **extraterrestrial life**, [life](https://www.britannica.com/science/life) that may exist or may have existed in the [universe](https://www.britannica.com/science/universe) outside of [Earth](https://www.britannica.com/place/Earth). The search for extraterrestrial life [encompasses](https://www.merriam-webster.com/dictionary/encompasses) many fundamental scientific questions. What are the basic requirements for life? Could life have arisen elsewhere in the [solar system](https://www.britannica.com/science/solar-system)? Are there other [planets](https://www.britannica.com/science/planet) like Earth? How likely is the [evolution](https://www.britannica.com/science/evolution-scientific-theory) of intelligent life?
*[(Read Britannica’s biography of Carl Sagan, co-author of this entry.)](https://www.britannica.com/biography/Carl-Sagan)*
## Universal criteria
No one knows which aspects of living systems are necessary, in the sense that living systems everywhere must have them, and which are [contingent](https://www.merriam-webster.com/dictionary/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](https://www.britannica.com/science/carbon-chemical-element), [hydrogen](https://www.britannica.com/science/hydrogen), [nitrogen](https://www.britannica.com/science/nitrogen), and other elements, uses [water](https://www.britannica.com/science/water) as its interaction medium. [Phosphorus](https://www.britannica.com/science/phosphorus-chemical-element), as [phosphate](https://www.britannica.com/science/phosphate) bound to an organic residue, is required for [energy](https://www.britannica.com/science/energy) storage and transport; [sulfur](https://www.britannica.com/science/sulfur) is involved in the three-dimensional configuration of [protein](https://www.britannica.com/science/protein) molecules; and other elements are present in smaller concentrations. Must these particular [atoms](https://www.britannica.com/science/atom) 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](https://www.merriam-webster.com/dictionary/criteria) can be used. The major atoms should tend to have a high cosmic abundance. Structural [molecules](https://www.britannica.com/science/molecule) of organisms at the temperature of the [planet](https://www.britannica.com/science/planet) in question should not be so extremely stable that [chemical reactions](https://www.britannica.com/science/chemical-reaction) 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](https://www.britannica.com/science/solid-state-of-matter) are inappropriate because of their inertness. The medium, most likely a [liquid](https://www.britannica.com/science/liquid-state-of-matter) but possibly a very dense [gas](https://www.britannica.com/science/gas-state-of-matter), must be stable in a number of respects. It should have a large [temperature](https://www.britannica.com/science/temperature) range (for a liquid, the temperature difference between [freezing point](https://www.britannica.com/science/freezing-point) and [boiling point](https://www.britannica.com/science/boiling-point) should be large). The liquid should be difficult to [vaporize](https://www.britannica.com/science/vaporization) and to freeze; in general, it should be difficult to change its temperature. The interaction medium needs to be an excellent [solvent](https://www.britannica.com/science/solvent-chemistry). 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](https://www.britannica.com/science/ultraviolet-radiation) 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](https://www.britannica.com/science/radiation) might permit useful chemical reactions to occur). Finally, it is [imperative](https://www.merriam-webster.com/dictionary/imperative) that conditions allow the existence of [autotrophy](https://www.britannica.com/science/autotroph) (the ability of an organism to synthesize at least some of its own nutrients) or other means of net production of necessary [compounds](https://www.merriam-webster.com/dictionary/compounds).
[Photosynthesis](https://cdn.britannica.com/22/94122-050-93F6BAE0/trees-plants-carbon-dioxide-sunlight-sugars-water.jpg)Diagram of photosynthesis showing how water, light, and carbon dioxide are absorbed by a plant to produce oxygen, sugars, and more carbon dioxide.
Thermodynamically, [photosynthesis](https://www.britannica.com/science/photosynthesis) based on stellar radiation may be the optimal source of energy for extraterrestrial life. Photosynthetic organisms and the radiation they receive are not in [thermodynamic equilibrium](https://www.britannica.com/science/thermodynamic-equilibrium). On Earth, for example, a green [plant](https://www.britannica.com/plant/plant) may have a temperature of about 300 K (23 °C, or 73 °F); the [Sun](https://www.britannica.com/place/Sun)’s temperature is about 6,000 K. (K = kelvin. On the Kelvin temperature scale, in which 0 K \[−273 °C, or −460 °F\] is [absolute zero](https://www.britannica.com/science/absolute-zero), 273 K \[0 °C, or 32 °F\] is the freezing point of water, and 373 K \[100 °C, or 212 °F\] is the boiling point of water at one atmosphere pressure.) Photosynthetic processes are possible because energy is transported from a hotter object (the Sun) to a cooler object (Earth). Were the source of radiation at the same or at a colder temperature than the photosynthesizer, no photosynthetic activity would be possible. For this reason, the idea that a subterranean green plant will photosynthesize by use of thermal [infrared radiation](https://www.britannica.com/science/infrared-radiation) emitted by its surroundings is [untenable](https://www.merriam-webster.com/dictionary/untenable). Equally unfeasible is the idea that a cold [star](https://www.britannica.com/science/star-astronomy), with a surface temperature similar to that of Earth, could sustain photosynthetic organisms.
Trusted knowledge for those who want to know more.
[SUBSCRIBE](https://premium.britannica.com/premium-membership/?utm_source=premium&utm_medium=inline-cta&utm_campaign=shorter-2026)
One can use these conditions to establish the limits for the chemical requirements of life. When atoms chemically combine, the energy necessary to separate them is called the bond energy, and the measure of this energy determines how tightly the two atoms are bound to each other. Bond energies generally vary from about 10 electron volts (eV) to about 0.03 eV. [Covalent bonds](https://www.britannica.com/science/covalent-bond), where [electrons](https://www.britannica.com/science/electron) are shared between atoms, tend to be more energetic than hydrogen bonds, where a hydrogen atom is shared between atoms, and hydrogen bonds in turn are more energetic than [van der Waals forces](https://www.britannica.com/science/van-der-Waals-forces), which arise from the attraction of the electrons of one atom for the nucleus of another. Atoms, free or bound, move with an average [kinetic energy](https://www.britannica.com/science/kinetic-energy) corresponding to about 0.02 eV. The higher the temperature, the more atoms move with energy sufficient to break a given bond spontaneously.
Specific atoms have circumscribed functions in modern [biology](https://www.britannica.com/science/biology), but, aside from structure and the need for the liquid interaction medium, they may not be fundamental. The energy-rich phosphate bonds in [adenosine triphosphate](https://www.britannica.com/science/adenosine-triphosphate) (ATP), about as energetic as the hydrogen bonds, are in fact of relatively low energy. Cells store large numbers of these bonds to drive a molecular [degradation](https://www.merriam-webster.com/dictionary/degradation) or synthesis. One expects the energy currency on high-temperature worlds to be much more energetic per bond and on low-temperature worlds to be much less energetic per bond.
In *[The Fitness of the Environment](https://www.britannica.com/topic/The-Fitness-of-the-Environment)* (1913), American biochemist [Lawrence Joseph Henderson](https://www.britannica.com/biography/Lawrence-Joseph-Henderson) first stressed the advantages of carbon and water for life in terms of comparative chemistry. Henderson was struck by the fact that the very atoms needed are exactly those that are around. It remains a remarkable fact that the atoms most useful for life have very high cosmic abundances.
[AI-generated answers](https://www.britannica.com/about-britannica-ai) from Britannica articles. AI makes mistakes, so verify using Britannica articles. |
| Shard | 62 (laksa) |
| Root Hash | 5455945239613777662 |
| Unparsed URL | com,britannica!www,/science/extraterrestrial-life s443 |