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Extraterrestrial life, or alien life (colloquially, 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 humans.
- Overview
- The search for extraterrestrial life
Astrobiology, a term coined for the study of all life anywhere in the 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 has been adduced. However, the design of astrobiological experiments forces critical examination of the generality of assumptions derived from Earth life.
There is an entire spectrum of possibilities for life on another planet. A planet may be lifeless and lack any vestiges of organic matter or fossils. Alternatively, it may be lifeless but contain organic matter or fossils. There may be life having simple or quite complex biochemistry, 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, examination of Earth for life with the full armoury of contemporary scientific instrumentation and knowledge would be 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 and 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 and about 1.7–2 parts per million (106) of methane, but in 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 could have been produced nonbiologically early in Earth’s history, and slow 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. Some methanogenic bacteria live in wetlands (hence the term marsh gas for methane), and others live in the intestinal tracts of cows and other ruminants. Similarly, the large amount of free oxygen gas might be considered a sign of life. The possibility that the photodissociation of water and the subsequent escape to space of hydrogen are the source of oxygen would need to be excluded. Also, spectroscopic detection of such relatively complex reduced organic molecules as terpenes (hydrocarbons given off by plants and found over forests) could be used as a test for life.
By contrast, photographic observations of the daytime Earth from Mars would not necessarily detect life. Even at a resolution of 100 metres (330 feet)—that is, an ability to discriminate fine detail at high contrast only if its components are more than 100 metres apart—cities, canals, bridges, the Great Wall of China (long erroneously believed to be visible from the Moon), highways, and other large-scale 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 fields, highways, and airports. 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 colour of plants.
Scanning of the electromagnetic spectrum offers another technique for detecting life. Domestic television transmissions, the high-frequency end of the AM broadcast band, and radar defense networks make up some of the enormous amount of 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.
Astrobiology, a term coined for the study of all life anywhere in the 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 has been adduced. However, the design of astrobiological experiments forces critical examination of the generality of assumptions derived from Earth life.
There is an entire spectrum of possibilities for life on another planet. A planet may be lifeless and lack any vestiges of organic matter or fossils. Alternatively, it may be lifeless but contain organic matter or fossils. There may be life having simple or quite complex biochemistry, 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, examination of Earth for life with the full armoury of contemporary scientific instrumentation and knowledge would be 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 and 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 and about 1.7–2 parts per million (106) of methane, but in 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 could have been produced nonbiologically early in Earth’s history, and slow 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. Some methanogenic bacteria live in wetlands (hence the term marsh gas for methane), and others live in the intestinal tracts of cows and other ruminants. Similarly, the large amount of free oxygen gas might be considered a sign of life. The possibility that the photodissociation of water and the subsequent escape to space of hydrogen are the source of oxygen would need to be excluded. Also, spectroscopic detection of such relatively complex reduced organic molecules as terpenes (hydrocarbons given off by plants and found over forests) could be used as a test for life.
By contrast, photographic observations of the daytime Earth from Mars would not necessarily detect life. Even at a resolution of 100 metres (330 feet)—that is, an ability to discriminate fine detail at high contrast only if its components are more than 100 metres apart—cities, canals, bridges, the Great Wall of China (long erroneously believed to be visible from the Moon), highways, and other large-scale 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 fields, highways, and airports. 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 colour of plants.
Scanning of the electromagnetic spectrum offers another technique for detecting life. Domestic television transmissions, the high-frequency end of the AM broadcast band, and radar defense networks make up some of the enormous amount of 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.
Jun 2, 2020 · Despite extensive research in this field, at least within the West and the Christian world, the ‘history of extraterrestrial life debate’ has remained less well known. This post discusses some of the major events that have shaped the ‘plurality of the worlds debate’ throughout the centuries.
May 12, 2020 · In this chapter we provide an overview of the extraterrestrial life debate since 1900, drawing largely on the major histories of the subject during this period, The Biological Universe, Life on Other Worlds, and The Living Universe, as well as other published works....
- Steven J. Dick
- 2020
Aug 3, 2021 · And for as long as humans have had words, we have been sharing stories about the presumed builders and occupiers of those vaulted heavens: the gods, spirits, angels, and demons who were, in a sense, the first extraterrestrials.
Sep 5, 2024 · A scientific search for intelligent extraterrestrial life that could communicate beyond its own celestial home was first called for in 1959 by Italian physicist Giuseppe Cocconi and American physicist Philip Morrison.
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The existence of extraterrestrial life is a scientific idea that has been debated for centuries. Initially, the question was purely speculative; in modern times a limited amount of scientific evidence provides some answers.
