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Extraterrestrial life

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For fiction, see Extraterrestrials in fiction.

A 1967 Soviet Union 16 kopeks postage stamp, with a satellite from an imagined extraterrestrial civilization.

For other uses, see Alien (disambiguation).

Extraterrestrial life is defined as life which does not originate from planet Earth. The existence of life outside the planet is theoretical and all assertions of such life remain disputed.

Hypotheses regarding the origin(s) of extraterrestrial life, if it exists, are as follows: one proposes that it may have emerged, independently, from different places in the universe. An alternative hypothesis is panspermia or exogenesis, which holds that life emerges from one location, then spreads between habitable planets. These two hypotheses are not mutually exclusive. The study and theorization of extraterrestrial life is known as astrobiology, exobiology or xenobiology. Speculated forms of extraterrestrial life range from life with the simplicity of bacteria to sapient or sentient beings.

Suggested locations which might have once developed, or presently continue to host life similar to our own, include the planets Venus^[1] and Mars, moons of Jupiter and Saturn (e.g. Europa,^[2] Enceladus and Titan) and Gliese 581 c and d, recently discovered to be near Earth-mass extrasolar planets apparently located in their star's habitable zone, and with the potential to have liquid water.^[3]

To date, no credible evidence of extraterrestrial life has been discovered which has been generally accepted by the mainstream scientific community. All other proposals, including beliefs that some UFOs are of extraterrestrial origin (see extraterrestrial hypothesis) and claims of alien abduction, are considered hypothetical by most scientists. In 2006, New Scientist published a list of ten controversial pieces of evidence that extraterrestrial life exists,^[4] but scientists do not consider them credible since no direct observational evidence has been encountered.

[edit] Possible basis of extraterrestrial life

Several theories have been proposed about the possible basis of alien life from a biochemical, evolutionary or morphological viewpoint.

1. Alien life, such as bacteria, has been theorised by scientists such as Carl Sagan to exist in our solar system and quite possibly throughout the universe. However, no samples have been found by astronauts.

2. As of yet, no intelligent life has been discovered. However, many scientists, such as Carl Sagan, believe that it is nearly impossible that no other intelligent life exists in the universe.

3. UFO sightings are sightings of unidentified flying objects that may or may not be connected with extraterrestrial intelligent life. Many of these sightings can be dismissed as hoaxes or sightings of Earth based aircraft. However some sightings have gone unexplained, having been reported in some cases by police and airline pilots.

[edit] Biochemistry

Main articles: Biochemistry, Hypothetical types of biochemistry, and Water and life

All life on Earth requires carbon, hydrogen, oxygen, nitrogen, sulfur and phosphorus as well as numerous other elements in smaller amounts, notably minerals; it 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 "star dust", 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 and water 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. 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 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 Van der Waals forces 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 lifeforms 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 lifeforms 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.^[5]

When looked at 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.^[6]

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.^[7]

[edit] 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, 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. These 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. Many also conjecture as to some type of egg-laying amongst extraterrestrial creatures, but mammalian mammary glands might be a singular case.

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 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.^[6]

[edit] Beliefs in extraterrestrial life

[edit] Ancient and early modern ideas

See also: Cosmic pluralism and Ancient astronauts

Beliefs in extraterrestrial life may have been present in ancient Babylon, Assyria, Sumer, Egypt, Arabia, China, India, and South America, although in these societies, cosmology was often associated with the supernatural, and the notion of alien life is difficult to distinguish from that of gods, demons, and such. The first important Western thinkers to argue systematically for a universe full of other planets and, therefore, possible extraterrestrial life were the ancient Greek writer Thales and his student Anaximander in the 7th and 6th centuries B.C. The atomists of Greece took up the idea, arguing that an infinite universe ought to have an