Extraterrestrial life

from Wikipedia, the free encyclopedia

Extraterrestrial life is a term for life forms that are neither native nor created on Earth . The term covers all possibly existing types and manifestations of life of non-terrestrial origin, from the simplest biological systems (e.g. microspheres , prions , viruses and prokaryotes ) through plant and animal life to life forms whose complexity corresponds to that of humans it exceeds. An extraterrestrial being is also called extraterrestrial for short or after the English name alien (German foreigner ). The adjective extraterrestrial is synonymous with the foreign word extraterrestrial .

So far it is not known whether life exists outside the terrestrial biosphere .

Historical considerations

Natural philosophical thoughts on the existence of extraterrestrial life can be traced back to antiquity. For example, in Plutarch's work Das Mondgesicht or Lukian from Samosata's writing Ikaromenipp or the cloud journey, thoughts about living beings beyond the earth can be found. Such texts, however, relate essentially to mythical motifs and do not claim to develop theories about extraterrestrial life with the help of rational argumentation.

Giordano Bruno in the 16th century thought that the universe was infinite and that there were also an infinite number of living beings on other planets in the universe. In the late seventeenth century the astronomer Christiaan Huygens published his work Weltbeschauer, or Reasonable Conjectures, that the planets are no less adorned and inhabited than our earth. Huygens, at the same time one of the founders of the probability theory , realized that he could not come to any reliable knowledge about extraterrestrial life. Nevertheless, some assumptions are more likely than others, so one can at least come to "reasonable guesses". The idea of ​​“reasonable guesswork” strongly influenced the natural philosophy of the 18th century. With the help of analogy arguments and “reasonable assumptions”, Christian Wolff even calculated the size of the inhabitants of Jupiter to be 138191440 of a Parisian foot , i.e. about four meters. Even Immanuel Kant dealt in 1755 in his work by the residents of the stars with the question of whether there was life on other planets.

Speculations about extraterrestrial life increased, especially in the second half of the 19th century, when the theory of evolution became more widespread, which states that life on earth grew larger and larger over periods of billions of years through natural mutation and selection processes Diversity, greater complexity and ultimately intelligence. This idea made it seem possible that life has developed in a comparable way on other planets - especially after the traditional biblical-Christian view of the world lost more and more importance as a result of the Enlightenment and astronomy had shown that our sun was a star below Billions of like stars.

Initially, speculations about extraterrestrial life focused on the closest celestial bodies: the moon and the planets of our own solar system , in particular the two neighboring planets of the earth, Mars and Venus . In addition, it has long been speculated whether our solar system with its planets is a special case in the universe or whether there are large numbers of planets in the universe.

In 1854, William Whewell theorized that Mars had oceans, land, and possibly life forms. After telescope observations of the Martian channels , which later turned out to be an optical illusion, speculations about life on Mars exploded at the end of the 19th century. The American astronomer Percival Lowell published his book Mars in 1895 , followed by Mars and its Canals (Mars and its Canals) in 1906, in which he suggested that the canals were the work of a civilization long past (" Martians "). In the first scientifically substantiated notions of Venus as a cosmic body, this earth-like planet, due to its greater proximity to the sun, was considered a more life-friendly, young and very warm world of prehistoric times, which is characterized by jungles and deserts under the impenetrable cloud cover. This was then also reflected in the later scientific fantasy of literature and film art, especially in the form of various " Venusians ". With the exploration of the real conditions, especially since the first measurement results of the Mariner 2 probe in 1962, it became clear that Venus is not tropical and friendly to life, but very hot and dry.

Today's view

Existence probability

For the existence of intelligent life outside the earth, the facts are cited in particular that there are between 200 and 400 billion stars in the Milky Way alone and that this in turn is only one of more than 100 billion galaxies in the visible universe . The probability of the existence of such a life has been estimated with the Drake equation since 1961 . However, many of the factors used in the Drake equation are controversial. There are also discussions about the extent to which the theoretical result of the Drake equation has practical relevance or how it is to be interpreted at all.

If one restricts the consideration to " intelligent " life, it must be taken into account that it is unknown whether life in a "typical" biosphere will inevitably develop into intelligent life forms sooner or later through evolution, or whether it will only develop in very rare cases comes to such. Intelligent life forms can also become extinct again, so that their average “time window”, measured against the billions of years of life development on planets, may only be very short.

The first planets around alien stars were discovered in 1992. By March 2017, astronomy was able to detect around 3,600 exoplanets , including not only gas giants , but also rocky planets . Furthermore, indications of liquid water outside the earth (which is commonly regarded as one of the necessary conditions for life) have been found in our own solar system , especially on the icy moons of the outer solar system such as Jupiter's moon Europa , giving rise to new speculations about extraterrestrial life existed in our own solar system.

According to an adapted form of the Drake equation, which includes the knowledge about exoplanets known as of 2016, the following was estimated: If the probability that a planet in a habitable zone will produce a “technological species” is greater than about 10 −24 , then humanity is probably not the only case of a technological species in the observable universe.

Hypothesis of interstellar colonizations

Another consideration concerns the possible interstellar spread of life in the Milky Way. If technologically advanced forms of life were capable of interstellar colonization and could also sustain their civilization for millions of years, the entire galaxy could be completely colonized within a few million years. The fact that to this day there are no signs of this is also known as the Fermi paradox .

Some of the reasons are the limited habitable zone in the vicinity of a sun and, according to Włodzisław Duch, the very limited number of chemical elements that can be used for complex systems such as known life forms, which should also invalidate the argument of carbon chauvinism. The Rare Earth hypothesis According to the Fermi paradox is not a paradox. The emergence and the proven continuous development of complex multicellular living beings on earth for billions of years is only due to a comparatively unlikely constellation, primarily astrophysical and geological conditions. Further reasons or hypotheses would be the impossibility of interstellar colonization, distribution patterns, natural self-extinction of intelligent living beings, humanity as one of the first intelligent living beings in the universe, astrophysical explanation, mathematical a priori argument, the great filter, berserk, deadly probes and Dark Forest Theory.


The American astronomer Seth Shostak estimates that there could be 10,000 extraterrestrial civilizations in the Milky Way. Tom Westby and Professor Christopher Conselice of Nottingham University estimate that there could be 36 extraterrestrial civilizations in the Milky Way. With 10,000 civilizations, the average distance between them would be around 1,000 to 2,000 light years. In 36 civilizations, the average distance would be 17,000 ly.


It is obvious that extraterrestrial life forms that developed independently of life on earth could differ more or less clearly from the life forms known to us on earth ( microorganisms , plants , animals ). But it is also conceivable according to the panspermia hypothesis that earthly life did not arise on earth, but was brought to earth by asteroids. Especially simple extraterrestrial life forms could thus be similar to the earthly.

The speculations about the nature of extraterrestrial life forms can be roughly divided into three groups:

  • those who resemble life on earth, especially humanoid life forms, on the principle of convergent evolution ,
  • Life forms that are completely different from earthly life,
  • lower life forms (microorganisms).

Alien life could even be based on completely different chemical elements. The assumption that extraterrestrial life can only be imagined on the basis of carbon is polemically referred to as carbon chauvinism . A study by NASA at the end of 2010 provided evidence that life can also be based on other elements , according to which the bacterium GFAJ-1 incorporates the semi-metal arsenic into its genome; Critics of this study criticize, among other things, contaminated samples and the instability of an arsenic-based genome. In June 2012 it became known that GFAJ-1 - contrary to previous assumptions - only contains free arsenate, but not biochemically integrated arsenic. Instead, the structure of its nucleic acids resembles that of known bacteria.

Some astrobiologists are of the opinion that viruses should also be included in living beings - or at least in a preliminary stage of life (virus-first hypothesis). For this reason, it has already been suggested to look for viruses on celestial bodies such as Mars, instead of focusing exclusively on cells such as bacteria.

Life forms outside the earth

If earth-like life should exist on other objects in the solar system, it would have to be clarified whether this life spread from earth, came to earth from space (panspermy theory), or developed independently of one another in different places. In the space is the Planetary Protection of importance.

Philosophical science criticism and problems

One problem with astrobiology is that there is no generally accepted definition of life. Indeed, while there have been innumerable attempts to define life, none have been found to be complete or even satisfactory. One possible conclusion is that there is no firm dividing line between “animate” and “inanimate”. As a working definition, in large parts of exobiology, especially when it comes to the direct search within the solar system, the term “life in the form we know” is used.

Proponents of exobiology use precisely this argument to turn against the rare earth hypothesis, because it only searches for precisely those circumstances that have led to our form of life on earth. However, all circumstances must be taken into account that can potentially lead to life. In particular, the application of the anthropic principle to arrive at statements about the frequency of intelligent life in the universe therefore appears to them to be inappropriate.

There are also thoughts on very exotic life forms that are not based on carbon (carbon chauvinism), take on planetary dimensions (a biosphere as "a" living being) or even live in interplanetary and interstellar space. These considerations are mostly assigned to the field of science fiction .

Life in our solar system

In theory, life could exist on other planets of the solar system outside of Earth . In the astrobiological department of NASA , for example, it is assumed that  life can or could exist on the planets Venus and Mars as well as on some larger moons such as those of Jupiter - especially Europe , but also Ganymede and Callisto . The Saturn moon Titan occupies a special position , on which conditions similar to those of the primordial earth could prevail under a dense atmosphere of nitrogen and methane. According to the current state of knowledge, the most life-friendly conditions in the solar system outside of the earth seem to be the only 500 km large Saturn moon Enceladus , an ice moon .

Both with the innermost planet Mercury and with the outer worlds of ice from Uranus onwards , the possibility of life is in fact excluded. On Mercury the day and night temperatures (and thus also the fluctuations) are too extreme (−180 ° C to 460 ° C), on the outer planets the temperature is permanently too low (below −190 ° C) for life to arise to let.

Meteorites and comets

During investigations on meteorites , for example ALH 84001 , traces were found that could be fossilizations of extraterrestrial microorganisms. This is controversial because the traces found cannot be explained biologically. Since the emergence of astrobiology, no find has been made that clearly shows traces of extraterrestrial life forms. Amino acids - important building blocks of living beings on earth - have already been detected outside of the solar system and also on meteorites (e.g. the Murchison meteorite ) and the comet Wild 2 .

In the meantime, it has been experimentally proven that meteorites like the Murchison meteorite have catalytic abilities: Their material can cause amino acids and precursors of sugar molecules to be formed from simple molecules like formamide .

After the NASA Johnson Space Center found traces of biomorphic (biomorphic: "something that resembles a biological form or shape") traces in the meteorite Nakhla , a fragment of the meteorite was broken up for further investigation in 2006 to identify possible contamination with terrestrial organisms To be able to rule out investigations. Various complex carbonaceous materials were found in it, which contained dendrite-like pores and channels in the rock, similar to the effects of bacteria in stones that are known from the earth. According to the majority of scientists, the similarity of the shapes with those of living organisms is not enough to prove that bacteria once lived on Mars.

At the beginning of March 2011, NASA astrobiologist Richard Hoover published research results, according to which fossil remains of extraterrestrial organisms had been found in the meteorites Alais, Ivuna and Orgueil, three carbonaceous chondrites . The find is the subject of controversial discussions. On March 7, 2011, NASA distanced itself from the publication of Hoovers in the Journal of Cosmology. Other astrobiologists assume terrestrial contamination and question Richard Hoover's results.

Life in other planetary systems

Probably the most favorable conditions for life are offered by planets and large moons, especially terrestrial ones, in the habitable zone of the respective star with liquid water on their surface. According to the current state of knowledge, only main sequence stars of the spectral classes F – M can be considered for a sufficiently stable habitable zone with only minor changes over several billion years . Smaller stars have a smaller size of the habitable zone, but they have a significantly longer lifespan ( hydrogen burning ) up to a few trillion years. In the case of red dwarfs , however, there is the problem of flares and the bound rotation in the habitable zone, which is why the life-friendliness of these stars is fundamentally doubted by some scientists.

There could also be life beyond the circumstellar habitable zone. An example are massive super-earths further away from their star, which have a significant amount of hydrogen in their atmospheres, which is a very potent greenhouse gas and causes a warmer climate. Another example are ice moons with liquid oceans deep beneath a thick ice crust.

Some scientists believe that the earth is not ideal for life. They assume that there are super-habitable planets that have even better living conditions.


In 2010, the Cranfield Astrobiological Stratospheric Sampling Experiment (CASS-E) was started, which uses a balloon probe to collect samples in the stratosphere , which are then examined for any extraterrestrial microorganisms that may exist . Since the bio-barriers had opened, no samples could be collected.

As part of the Search for Extraterrestrial Genomes Project (SETG), MIT and NASA are developing a device that can process very different samples and detect nucleic acids in them. After field tests in the Atacama Desert and Antarctica , the detector is planned to be used on Mars in 2018 .

Theoretical considerations

Cohen and Stewart use the terms universal ('universal') and parochial ('limited') to categorize characteristics that are very likely or less likely (but possible) to occur in life forms on other, but in the broadest sense Earth-like planets.

  • Universal is the term used to describe principles and physical and chemical functions that have developed independently of one another several times during evolution on Earth, thereby indicating that they represent an evolutionary advantage at different times and in different places.
  • Those characteristics that have so far only occurred once on earth are considered to be limited; in relation to extraterrestrial life, they are considered unlikely, but they are possible.
    • Ingestion and breathing through the same orifice
    • Five fingers
    • self-reflective awareness
    • etc.

Intelligent life forms

There is no one-size-fits-all definition of intelligence . Even transferring the concept of intelligence to the known nonhuman animals is difficult. Nevertheless, attempts are being made to apply this term, vaguely in the sense of human-like or higher cognitive or mental performance, to possible extraterrestrial life.

Even if it is more likely than unlikely that there is extraterrestrial intelligent life, it is believed that it is relatively (to extremely) rare in the universe.

Search & contact

Most people today assume that there are enormous distances between us and extraterrestrial civilizations. In view of this, the following approaches to search and possible contact seem to be the most promising:

  1. communication via radio waves , which in principle can take place over the greatest distances (but at the maximum at the speed of light ),
  2. the space with manned spaceships or unmanned probes or
  3. future technologies that are not yet known to us today.

Science focuses primarily on the search for signs of (primitive) life or its traces on exoplanets , meteorites, our neighboring planets and their moons on the one hand, and on the search for radio signals that could result from intelligent extraterrestrial life in alien solar systems.

In 2009, on the occasion of the International Year of Astronomy, the Pontifical Academy of Sciences dealt with the search for extraterrestrial beings.

Probability of contact with radio waves

Distance factor

If an intelligent life comparable to civilization is not found in the immediate vicinity (0 to 80 light years), communication in the classic sense, via radio waves between humans and extraterrestrials, will probably not take place, since the transit times are too long - at least for our human existence. Only intergenerational communication would be possible.

Factor limitation of the radio and reception range

Through the use of radio waves , TV signals , civil and military radar systems and other sources, our civilization produces an artificial EM signature of the earth (leakage radiation), which extraterrestrial technical civilizations with astronomical research interests within a distance of about 60 to 80 ly can optionally be detected. Estimates (status: 2009) assume around 3000 stars and an unknown number of planetary systems within a distance of 100 ly. Some Seti researchers believe it is possible that military facilities such as B. the long-range phased array radar Don-2N , Cobra Dane , Sea-Based X-Band Radar or HAARP could still be detected at distances of 500 light years and more due to the radiation power used.

SETI therefore tries to search the stars for radio signals up to a range of 500 ly. According to statements (status: 2008) by SETI researcher Seth Shostak , this should be possible by the end of 2025, since by then the computer performance will be strong enough to evaluate all data.

Technological development factor

One hypothesis says that intelligent extraterrestrial civilizations only use radio wave communication for a limited time (around the 1st century) until the advancement of technology leads to other means of communication (possibly that of quantum communication or quantum teleportation or by means of spooky long-range effects ) The probability of detecting a radio signal from an alien civilization is significantly reduced. The exact reason for this is that radio waves only travel at the speed of light and the formation of planets and the evolution of extraterrestrial life and their technology are very unlikely to develop simultaneously and similarly. It would be very unlikely that the "time windows" would match.


With the number of stars in radio range of our EM signature and vice versa (e.g. in the range 100 to 500 ly, 3,000 to 375,000 stars), the number of stars in the Milky Way (e.g. in the range 250 to 400 billion stars ) or number of stars in the habitable zone of the Milky Way , and the estimated probability of existence or the number of intelligent extraterrestrial civilizations in our Milky Way (e.g. in the range 36 to 10,000), the contact probability can be roughly (with e.g. in the range 0.00000027 to 0.015 civilizations, and therefore highly unlikely). In the case that generally extraterrestrial civilization only use radio waves for a limited time due to technological development (e.g. 100 years), the probability of contact is again significantly reduced.

Conversely, the number of extraterrestrial civilizations in the Milky Way can be estimated based on the number of contacts.

Contact attempts

Golden Record Cover with instructions for use

As early as the 19th century, Franz suggested to Paula Gruithuisen to get in contact with the inhabitants of the moon he suspected by planting correspondingly dimensioned swede plantings in the form of the figure of the Pythagorean theorem in the vastness of Siberia .

The search for intelligent extraterrestrial life is known by the abbreviation SETI ( Search for Extraterrestrial Intelligence ). The SETI project is based on the assumption that extraterrestrials casually or deliberately emit electromagnetic signals that could be detected by other intelligent living beings.

In 1919 Guglielmo Marconi made the first attempts to receive radio signals from outside the world, but these could not be confirmed. The SETI has been pursued since 1960, but so far without success. The most spectacular received signal so far is the so-called Wow! Signal , but it is not certain whether it is really of extraterrestrial origin.

When the two interstellar space probes Pioneer 10 and Pioneer 11 were sent out in 1972 , gold plaques, the so-called Pioneer plaques, were attached to the probes in the hope that if the probes were one day found by any intelligent extraterrestrial life forms, they would be found by them humanity would experience. In 1974 a single message was sent from Earth to potential aliens in the form of a radio wave signal, the so-called Arecibo message .

NASA launched the Voyager 1 and Voyager 2 space probes to the outer planets in 1977 . You are now in the border area of ​​the solar system and each carry a golden data disc with image and audio information ( Voyager Golden Record ) about the earth and humanity, which would presumably be readable for extraterrestrial civilizations.

On September 30, 2006, the cultural broadcaster Arte also broadcast CosmicConnexion via a special antenna in the direction of the star Errai . In contrast to previous news, it does not consist of pure information about the earth and humans, but is a more artistic representation of humanity. Arte is also planning its own series, which will also be sent into space by antenna.

For 2015, the European Space Agency was planning a now discontinued space experiment to observe Earth-like exoplanets and look for signs of life on them, the Darwin Telescope named after Charles Darwin . The project Terrestrial Planet Finder , which has been postponed indefinitely, is also uncertain about its implementation .

Potential contact hazards

It is speculated that contact with extraterrestrial life forms could be dangerous, especially if they were superior to humanity. Researchers such as Stephen Hawking and Simon Conway Morris have voiced their concerns about contact with intelligent extraterrestrial beings. For example, Hawking suggested that instead of actively searching for extraterrestrial civilizations, humanity should instead do everything possible to remain undetected. Alien civilizations may only be interested in the earth as a resource source and would want to plunder it. As an example, Hawking cited the discovery of America by Christopher Columbus , which had not turned out positive for the native Americans either. The astronomer Alexander Zaitsev coined the term Darth Vader Scenario for a possible danger scenario, named after a character from the science fiction films Star Wars . There are considerations for planetary defense . The Global Risks Report 2013 of the World Economic Forum indicates a future discovery of extraterrestrial life as a possible X-Factor, the profound effect might have.

See also


  • Aleksandar Janjic: Habitat Universe - Introduction to Exoecology. Springer Nature, Berlin Heidelberg, 2017, ISBN 978-3-662-54786-1 .
  • Aleksandar Janjic: Astrobiology - the search for extraterrestrial life. Springer Nature, Berlin Heidelberg, 2019, ISBN 978-3-662-59492-6 .
  • Karim Akerma: Aliens. Introduction to philosophy. Extraterrestrial thinking from Epicurus to Jonas. Münster 2002, ISBN 3-935363-70-2 .
  • J. Berndorff: Hello? (Cover story) In: PM No. 2/2018, pp. 18–27.
  • Dieter Beste (Ed.): Life in All. Spectrum of Science Dossier 2002, 3, Spektrum-d.-Wiss.-Verlag, Heidelberg 2002, ISBN 3-936278-14-8 , Spektrum.de. ( Memento from October 9, 2014 in the Internet Archive ).
  • Steven J. Dick: Life on Other Worlds. Cambridge University Press, Cambridge UK 1998, ISBN 0-521-62012-0 .
  • Ernst Fasan : Relations with alien intelligences - the scientific basis of metalaw. Berlin Verlag, Berlin 1970.
  • Gerald Feinberg , Robert Shapiro: Life beyond earth - the intelligent earthling's guide to life in the universe. Morrow Quill, New York 1980, ISBN 0-688-08642-X .
  • Linus Hauser: The importance of the question of extraterrestrial life for (Christian) theology. In: T. Myrach, T. Weddigen, J. Wohlwend, SM Zwahlen (eds.): Science and Fiction. Imagination and Reality of Space. Stuttgart / Vienna 2009, 199–218.
  • Jean Heidmann: Bioastronomy - About terrestrial life and extraterrestrial intelligence. Springer-Verlag, Berlin, Heidelberg, New York 1994, ISBN 3-540-57137-X .
  • Michael Michaud: Contact with Alien Civilizations - Our Hopes and Fears about Encountering Extraterrestrials. Springer, Berlin 2006, ISBN 0-387-28598-9 .
  • Heinz H. Peitz among others: The multiplied Christ. Extraterrestrial life and Christian salvation history. Akad. D. Diocese Rottenburg-Stuttgart 2004, ISBN 3-926297-92-1 , PDF online.
  • Michael Schetsche , Martin Engelbrecht (ed.): Of humans and extraterrestrials. Transterrestrial encounters as reflected in cultural studies. Transcript-Verlag, Bielefeld 2008, ISBN 978-3-89942-855-1 .
  • Michael Schetsche, Andreas Anton: In the mirror cabinet. Anthropocentric pitfalls in thinking about communicating with aliens. In: M. Schetsche (Hrsg.): Interspezies-Kommunikation. Requirements and limits. Berlin 2014: Logos-Verlag, ISBN 978-3-8325-3830-9 , pp. 125–150.
  • Jamie Shreeve: Where are the others? (Cover story) In: National Geographic. (German version), March 2019, pp. 42–77.
  • Travis S. Taylor et al .: An Introduction to Planetary Defense - A Study of Modern Warfare Applied to Extra-Terrestrial Invasion. BrownWalker Press, Boca Raton 2006, ISBN 1-58112-447-3 .
  • Karl H. Türk: extraterrestrial intelligence, reality or illusion. QNST-Verlag 1993, ISBN 3-928641-06-9 .
  • Diana G. Tumminia: Alien Worlds - Social and Religious Dimensions of Extraterrestrial Contact. Syracuse Univ. Press, Syracuse 2007, ISBN 978-0-8156-0858-5 .
  • Peter D. Ward : Life as we do not know it - the NASA search for (and synthesis of) alien life. Viking, New York 2005, ISBN 0-670-03458-4 .
  • Hubert Untersteiner: Exobiology - Science of Life in Space. Edition nove 2006, ISBN 3-902546-42-5 .
  • Harald Zaun: SETI - The scientific search for extraterrestrial civilizations. Opportunities, prospects, risks. With a foreword by Harald Lesch . Heise-Verlag, Hannover 2010, ISBN 978-3-936931-57-0 .

Web links

Commons : Alien Life Forms  - collection of images, videos and audio files


Individual evidence

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