Extraterrestrial life (from the Latin words: extra ("beyond", or "not of") and terrestris ("of or belonging to Earth")) is defined as life that does not originate from Earth.
Hypothetical forms of extraterrestrial life range from simple bacteria-like organisms to sapient beings far more advanced than humans.
Hypothetical forms of extraterrestrial life range from simple bacteria-like organisms to sapient beings far more advanced than humans.
It is currently unknown whether any such forms of life exist or ever existed.
The development and testing of theories about extraterrestrial life is known as exobiology or astrobiology; the term astrobiology however also covers the study of life on Earth, viewed in its astronomical context.
Background
Various controversial claims have been made for evidence of extraterrestrial life.
A less direct argument for the existence of extraterrestrial life relies on the vast size of the observable Universe.
According to this argument, supported by scientists such as Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth.
One possibility is that life has emerged independently at many places throughout the Universe.
Another possibility is panspermia or exogenesis, in which life would have spread between habitable planets.
These two hypotheses are not necessarily mutually exclusive.
Suggested locations at which life might have developed, or which might continue to host life today, include the planets Venus and Mars; moons of Jupiter, such as Europa; moons of Saturn, such as Titan and Enceladus; and extrasolar planets, such as Gliese 581 c, g and d, recently discovered to be near Earth mass and apparently located in their star's habitable zone, with the potential to have liquid water.
Beliefs that some unidentified flying objects are of extraterrestrial origin (see Extraterrestrial hypothesis), along with claims of alien abduction,[6] are dismissed by most scientists.
Most UFO sightings are explained either as sightings of Earth-based aircraft or known astronomical objects, or as hoaxes.
Possible basis of extraterrestrial life
Several theories have been proposed about the possible basis of alien life from a biochemical, evolutionary or morphological viewpoint.
Alien life, such as bacteria, has been theorized by scientists such as Carl Sagan to exist in the Solar System and quite possibly throughout the Universe. No samples have been found.
Biochemistry
Biochemistry, Hypothetical types of biochemistry, and Water and life
All life on Earth requires carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur (CHNOPS) as well as numerous other elements in smaller amounts.
Life also requires water as the solvent in which biochemical reactions take place.
Sufficient quantities of carbon and the other major life-forming elements, along with water, may enable the formation of living organisms on other planets with a chemical make-up and average temperature similar to that of Earth.
Because Earth and other planets are made up of "stardust", i.e. relatively abundant chemical elements formed from stars which have ended their lives as supernovae, it is very probable that other planets may have been formed by elements of a similar composition to the Earth's.
The combination of carbon, hydrogen and oxygen in the chemical form of carbohydrates (e.g. sugar) can be a source of chemical energy on which life depends, and can also provide structural elements for life (such as ribose, in the molecules DNA and RNA, and cellulose in plants).
Plants derive energy through the conversion of light energy into chemical energy via photosynthesis.
The development and testing of theories about extraterrestrial life is known as exobiology or astrobiology; the term astrobiology however also covers the study of life on Earth, viewed in its astronomical context.
Background
Various controversial claims have been made for evidence of extraterrestrial life.
A less direct argument for the existence of extraterrestrial life relies on the vast size of the observable Universe.
According to this argument, supported by scientists such as Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth.
One possibility is that life has emerged independently at many places throughout the Universe.
Another possibility is panspermia or exogenesis, in which life would have spread between habitable planets.
These two hypotheses are not necessarily mutually exclusive.
Suggested locations at which life might have developed, or which might continue to host life today, include the planets Venus and Mars; moons of Jupiter, such as Europa; moons of Saturn, such as Titan and Enceladus; and extrasolar planets, such as Gliese 581 c, g and d, recently discovered to be near Earth mass and apparently located in their star's habitable zone, with the potential to have liquid water.
Beliefs that some unidentified flying objects are of extraterrestrial origin (see Extraterrestrial hypothesis), along with claims of alien abduction,[6] are dismissed by most scientists.
Most UFO sightings are explained either as sightings of Earth-based aircraft or known astronomical objects, or as hoaxes.
Possible basis of extraterrestrial life
Several theories have been proposed about the possible basis of alien life from a biochemical, evolutionary or morphological viewpoint.Alien life, such as bacteria, has been theorized by scientists such as Carl Sagan to exist in the Solar System and quite possibly throughout the Universe. No samples have been found.
Biochemistry
Biochemistry, Hypothetical types of biochemistry, and Water and life
All life on Earth requires carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur (CHNOPS) as well as numerous other elements in smaller amounts.
Life also requires water as the solvent in which biochemical reactions take place.
Sufficient quantities of carbon and the other major life-forming elements, along with water, may enable the formation of living organisms on other planets with a chemical make-up and average temperature similar to that of Earth.
Because Earth and other planets are made up of "stardust", i.e. relatively abundant chemical elements formed from stars which have ended their lives as supernovae, it is very probable that other planets may have been formed by elements of a similar composition to the Earth's.
The combination of carbon, hydrogen and oxygen in the chemical form of carbohydrates (e.g. sugar) can be a source of chemical energy on which life depends, and can also provide structural elements for life (such as ribose, in the molecules DNA and RNA, and cellulose in plants).
Plants derive energy through the conversion of light energy into chemical energy via photosynthesis.
Life, as currently recognized, requires carbon in both reduced (methane derivatives) and partially-oxidized (carbon oxides) states.
It also appears to require nitrogen as a reduced ammonia derivative in all proteins, sulfur as a derivative of hydrogen sulfide in some necessary proteins, and phosphorus oxidized to phosphates in genetic material and in energy transfer.
Adequate water as a solvent supplies adequate oxygen as constituents of biochemical substances.
Pure water is useful because it has a neutral pH due to its continued dissociation between hydroxide and hydronium ions.
As a result, it can dissolve both positive metallic ions and negative non-metallic ions with equal ability.
It also appears to require nitrogen as a reduced ammonia derivative in all proteins, sulfur as a derivative of hydrogen sulfide in some necessary proteins, and phosphorus oxidized to phosphates in genetic material and in energy transfer.
Adequate water as a solvent supplies adequate oxygen as constituents of biochemical substances.
Pure water is useful because it has a neutral pH due to its continued dissociation between hydroxide and hydronium ions.
As a result, it can dissolve both positive metallic ions and negative non-metallic ions with equal ability.
Furthermore, the fact that organic molecules can be either hydrophobic (repelled by water) or hydrophilic (soluble in water) creates the ability of organic compounds to orient themselves to form water-enclosing membranes.
The fact that solid water (ice) is less dense than liquid water (within specific temperature ranges) also means that ice floats, thereby preventing Earth's oceans from slowly freezing.
Without this quality, the oceans could have frozen solid during the Snowball Earth episodes.
Additionally, the hydrogen bonds between water molecules give it an ability to store energy with evaporation, which upon condensation is released.
This helps to moderate the climate, cooling the tropics and warming the poles, helping to maintain the thermodynamic stability needed for life.
Carbon is fundamental to terrestrial life for its immense flexibility in creating covalent chemical bonds with a variety of non-metallic elements, principally nitrogen, oxygen and hydrogen.
Carbon dioxide and water together enable the storage of solar energy in sugars, such as glucose.
The oxidation of glucose releases biochemical energy needed to fuel all other biochemical reactions.
The ability to form organic acids (–COOH) and amine bases (–NH2) gives rise to the possibility of neutralization dehydrating reactions to build long polymer peptides and catalytic proteins from monomer amino acids, and with phosphates to build not only DNA (the information-storing molecule of inheritance), but also ATP (the principal energy "currency" of cellular life).
Due to their relative abundance and usefulness in sustaining life, many have hypothesized that life forms elsewhere in the universe would also utilize these basic materials.
However, other elements and solvents could also provide a basis for life.
The fact that solid water (ice) is less dense than liquid water (within specific temperature ranges) also means that ice floats, thereby preventing Earth's oceans from slowly freezing.
Without this quality, the oceans could have frozen solid during the Snowball Earth episodes.
Additionally, the hydrogen bonds between water molecules give it an ability to store energy with evaporation, which upon condensation is released.
This helps to moderate the climate, cooling the tropics and warming the poles, helping to maintain the thermodynamic stability needed for life.
Carbon is fundamental to terrestrial life for its immense flexibility in creating covalent chemical bonds with a variety of non-metallic elements, principally nitrogen, oxygen and hydrogen.
Carbon dioxide and water together enable the storage of solar energy in sugars, such as glucose.
The oxidation of glucose releases biochemical energy needed to fuel all other biochemical reactions.
The ability to form organic acids (–COOH) and amine bases (–NH2) gives rise to the possibility of neutralization dehydrating reactions to build long polymer peptides and catalytic proteins from monomer amino acids, and with phosphates to build not only DNA (the information-storing molecule of inheritance), but also ATP (the principal energy "currency" of cellular life).
Due to their relative abundance and usefulness in sustaining life, many have hypothesized that life forms elsewhere in the universe would also utilize these basic materials.
However, other elements and solvents could also provide a basis for life.
Silicon is most often deemed to be the probable alternative to carbon.
Silicon life forms are proposed to have a crystalline morphology, and are theorized to be able to exist in high temperatures, such as on planets which are very close to their star.
Life forms based in ammonia (rather than water) have also been suggested, though this solution appears less optimal than water.
From a chemical perspective, life is fundamentally a self-replicating reaction, but one which could arise under a great many conditions and with various possible ingredients, though carbon-oxygen within the liquid temperature range of water seems most conducive.
Suggestions have even been made that self-replicating reactions of some sort could occur within the plasma of a star, though it would be highly unconventional.
Several pre-conceived ideas about the characteristics of life outside of Earth have been questioned.
For example, NASA scientists believe that the color of photosynthesizing pigments on extrasolar planets might not be green.
Evolution and morphology
In addition to the biochemical basis of extraterrestrial life, many have also considered evolution and morphology.
Science fiction has often depicted extraterrestrial life with humanoid and/or reptilian forms. Aliens have often been depicted as having light green or grey skin, with a large head, as well as four limbs i.e. fundamentally humanoid.
Other subjects, such as felines and insects, etc., have also occurred in fictional representations of aliens.
A division has been suggested between universal and parochial (narrowly restricted) characteristics. Universals are features which are thought to have evolved independently more than once on Earth (and thus, presumably, are not too difficult to develop) and are so intrinsically useful that species will inevitably tend towards them.
The most fundamental of these is probably bilateral symmetry, but more complex (though still basic) characteristics include flight, sight, photosynthesis and limbs, all of which are thought to have evolved several times here on Earth.
There is a huge variety of eyes, for example, and many of these have radically different working schematics and different visual foci: the visual spectrum, infrared, polarity and echolocation. Parochials, however, are essentially arbitrary evolutionary forms.
These often have little inherent utility (or at least have a function which can be equally served by dissimilar morphology) and probably will not be replicated. Intelligent aliens could communicate through gestures, as deaf humans do, or by sounds created from structures unrelated to breathing, which happens on Earth when, for instance, cicadas vibrate their wings, or crickets rub their legs.
Attempting to define parochial features challenges many taken-for-granted notions about morphological necessity.
Skeletons, which are essential to large terrestrial organisms according to the experts of the field of gravitational biology, are almost assured to be replicated elsewhere in one form or another.
The assumption of radical diversity amongst putative extraterrestrials is by no means settled.
Silicon life forms are proposed to have a crystalline morphology, and are theorized to be able to exist in high temperatures, such as on planets which are very close to their star.
Life forms based in ammonia (rather than water) have also been suggested, though this solution appears less optimal than water.
From a chemical perspective, life is fundamentally a self-replicating reaction, but one which could arise under a great many conditions and with various possible ingredients, though carbon-oxygen within the liquid temperature range of water seems most conducive.
Suggestions have even been made that self-replicating reactions of some sort could occur within the plasma of a star, though it would be highly unconventional.
Several pre-conceived ideas about the characteristics of life outside of Earth have been questioned.
For example, NASA scientists believe that the color of photosynthesizing pigments on extrasolar planets might not be green.
Evolution and morphology
Science fiction has often depicted extraterrestrial life with humanoid and/or reptilian forms. Aliens have often been depicted as having light green or grey skin, with a large head, as well as four limbs i.e. fundamentally humanoid.
Other subjects, such as felines and insects, etc., have also occurred in fictional representations of aliens.
A division has been suggested between universal and parochial (narrowly restricted) characteristics. Universals are features which are thought to have evolved independently more than once on Earth (and thus, presumably, are not too difficult to develop) and are so intrinsically useful that species will inevitably tend towards them.
The most fundamental of these is probably bilateral symmetry, but more complex (though still basic) characteristics include flight, sight, photosynthesis and limbs, all of which are thought to have evolved several times here on Earth.
There is a huge variety of eyes, for example, and many of these have radically different working schematics and different visual foci: the visual spectrum, infrared, polarity and echolocation. Parochials, however, are essentially arbitrary evolutionary forms.
These often have little inherent utility (or at least have a function which can be equally served by dissimilar morphology) and probably will not be replicated. Intelligent aliens could communicate through gestures, as deaf humans do, or by sounds created from structures unrelated to breathing, which happens on Earth when, for instance, cicadas vibrate their wings, or crickets rub their legs.
Attempting to define parochial features challenges many taken-for-granted notions about morphological necessity.
Skeletons, which are essential to large terrestrial organisms according to the experts of the field of gravitational biology, are almost assured to be replicated elsewhere in one form or another.
The assumption of radical diversity amongst putative extraterrestrials is by no means settled.
While many exobiologists do stress that the enormously heterogeneous nature of life on Earth foregrounds an even greater variety in outer space, others point out that convergent evolution may dictate substantial similarities between Earth and extraterrestrial life.
These two schools of thought are called "divergionism" and "convergionism" respectively.
These two schools of thought are called "divergionism" and "convergionism" respectively.
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