Cosmic pluralism proposed that other worlds may have their own animals and plants. The modern concept of extraterrestrial life is based on assumptions that were not commonplace during the early days of
astronomy. The first explanations for the celestial objects seen in the
night sky were based on mythology. Scholars from
Ancient Greece were the first to consider that the universe is inherently understandable and rejected explanations based on supernatural incomprehensible forces, such as the myth of the Sun being pulled across the sky in the chariot of
Apollo. They had not developed the
scientific method yet and based their ideas on pure thought and speculation, but they developed precursor ideas to it, such as that explanations had to be discarded if they contradict observable facts. The discussions of those Greek scholars established many of the pillars that would eventually lead to the idea of extraterrestrial life, such as Earth being round and not flat. The cosmos was first structured in a
geocentric model that considered that the sun and all other celestial bodies revolve around Earth. However, they did not consider them as worlds. In Greek understanding, the world was composed by both Earth and the celestial objects with noticeable movements.
Anaximander thought that the cosmos was made from
apeiron, a substance that created the world, and that the world would eventually return to the cosmos. Eventually two groups emerged, the
atomists that thought that matter at both Earth and the cosmos was equally made of small atoms of the
classical elements (earth, water, fire and air), and the
Aristotelians who thought that those elements were exclusive of Earth and that the cosmos was made of a fifth one, the
aether. Atomist
Epicurus thought that the processes that created the world, its animals and plants should have created other worlds elsewhere, along with their own animals and plants. Aristotle thought instead that all the earth element naturally fell towards the center of the universe, and that would make it impossible for other planets to exist elsewhere. Under that reasoning, Earth was not only in the center, it was also the only planet in the universe. Cosmic pluralism, the plurality of worlds, or simply pluralism, describes the philosophical belief in numerous "worlds" in addition to Earth, which might harbor extraterrestrial life. The earliest recorded assertion of extraterrestrial human life is found in ancient scriptures of
Jainism. There are multiple "worlds" mentioned in Jain scriptures that support human life. These include, among others,
Bharat Kshetra,
Mahavideh Kshetra,
Airavat Kshetra, and
Hari kshetra. Medieval Muslim writers like
Fakhr al-Din al-Razi and
Muhammad al-Baqir supported cosmic pluralism on the basis of the
Qur'an.
Chaucer's poem
The House of Fame engaged in medieval thought experiments that postulated the plurality of worlds. However, those ideas about other worlds were different from the current knowledge about the structure of the universe, and did not postulate the existence of planetary systems other than the Solar System. When those authors talk about other worlds, they talk about places located at the center of their own systems, and with their own stellar vaults and cosmos surrounding them. The Greek ideas and the disputes between atomists and Aristotelians outlived the fall of the Greek empire. The
Great Library of Alexandria compiled information about it, part of which was translated by Islamic scholars and thus survived the end of the Library. Baghdad combined the knowledge of the Greeks, the Indians, the Chinese and its own scholars, and the knowledge expanded through the
Byzantine Empire. From there it eventually returned to Europe by the time of the
Middle Ages. However, as the Greek atomist doctrine held that the world was created by random movements of atoms, with no need for a
creator deity, it became associated with
atheism, and the dispute intertwined with religious ones. Still, the Church did not react to those topics in a homogeneous way, and there were stricter and more permissive views within the church itself. The first known mention of the term 'panspermia' was in the writings of the 5th-century BC
Greek philosopher
Anaxagoras. He proposed the idea that life exists everywhere.
Early modern period By the time of the
late Middle Ages there were many known inaccuracies in the geocentric model, but it was kept in use because
naked eye observations provided limited data.
Nicolaus Copernicus started the
Copernican Revolution by proposing that the planets revolve around the sun rather than Earth. His proposal had little acceptance at first because, as he kept the assumption that orbits were perfect circles, his model led to as many inaccuracies as the geocentric one.
Tycho Brahe improved the available data with naked-eye observatories, which worked with highly complex
sextants and
quadrants. Tycho could not make sense of his observations, but
Johannes Kepler did: orbits were not perfect circles, but ellipses. This knowledge benefited the Copernican model, which worked now almost perfectly. The invention of the
telescope a short time later, perfected by
Galileo Galilei, clarified the final doubts, and the
paradigm shift was completed. Under this new understanding, the notion of extraterrestrial life became feasible: if Earth is but just a planet orbiting around a star, there may be planets similar to Earth elsewhere. The astronomical study of distant bodies also proved that physical laws are the same elsewhere in the universe as on Earth, with nothing making the planet truly special. The new ideas were met with resistance from the Catholic church. Galileo was
tried for the heliocentric model, which was considered heretical, and forced to recant it. The best-known early-modern proponent of ideas of extraterrestrial life was the Italian philosopher
Giordano Bruno, who argued in the 16th century for an infinite universe in which every star is surrounded by its own
planetary system. Bruno wrote that other worlds "have no less virtue nor a nature different to that of our earth" and, like Earth, "contain animals and inhabitants". Bruno's belief in the plurality of worlds was one of the charges leveled against him by the
Venetian Holy Inquisition, which tried and executed him. The heliocentric model was further strengthened by the postulation of the
theory of gravity by Sir
Isaac Newton. This theory provided the mathematics that explains the motions of all things in the universe, including planetary orbits. By this point, the geocentric model was definitely discarded. By this time, the use of the scientific method had become a standard, and new discoveries were expected to provide evidence and rigorous mathematical explanations. Science also took a deeper interest in the mechanics of natural phenomena, trying to explain not just the way nature works but also the reasons for working that way. There was very little actual discussion about extraterrestrial life before this point, as the Aristotelian ideas remained influential while geocentrism was still accepted. When it was finally proved wrong, it not only meant that Earth was not the center of the universe, but also that the lights seen in the sky were not just lights, but physical objects. The notion that life may exist in them as well soon became an ongoing topic of discussion, although one with no practical ways to investigate. The possibility of extraterrestrials remained a widespread speculation as scientific discovery accelerated.
William Herschel, the discoverer of
Uranus, was one of many 18th–19th-century astronomers who believed that the
Solar System is populated by alien life. Other scholars of the period who championed "cosmic pluralism" included
Immanuel Kant and
Benjamin Franklin. At the height of the
Enlightenment, even the
Sun and Moon were considered candidates for extraterrestrial inhabitants.
19th century Speculation about life on Mars increased in the late 19th century, following telescopic observation of apparent
Martian canals – which soon, however, turned out to be optical illusions. Despite this, in 1895, American astronomer
Percival Lowell published his book
Mars, followed by
Mars and its Canals in 1906, proposing that the canals were the work of a long-gone civilisation.
Spectroscopic analysis of Mars's atmosphere began in earnest in 1894, when U.S. astronomer
William Wallace Campbell showed that neither water nor oxygen was present in the
Martian atmosphere. By 1909 better telescopes and the best perihelic opposition of Mars since 1877 conclusively put an end to the canal hypothesis. As a consequence of the belief in the
spontaneous generation there was little thought about the conditions of each celestial body: it was simply assumed that life would thrive anywhere. This theory was disproved by
Louis Pasteur in the 19th century. Popular belief in thriving alien civilisations elsewhere in the solar system still remained strong until
Mariner 4 and
Mariner 9 provided close images of Mars, which debunked forever the idea of the existence of Martians and decreased the previous expectations of finding alien life in general. The end of the spontaneous generation belief forced investigation into the origin of life. Although
abiogenesis is the more accepted theory, a number of authors reclaimed the term "panspermia" and proposed that life was brought to Earth from elsewhere.
Kelvin (1871),
Hermann von Helmholtz (1879) and, somewhat later, by
Svante Arrhenius (1903). The science fiction genre, although not so named during the time, developed during the late 19th century. The expansion of the genre of
extraterrestrials in fiction influenced the popular perception over the real-life topic, making people eager to jump to conclusions about the discovery of aliens. Science marched at a slower pace, some discoveries fueled expectations and others dashed excessive hopes. For example, with the advent of telescopes, most structures seen on the Moon or Mars were immediately attributed to Selenites or Martians, and later ones (such as more powerful telescopes) revealed that all such discoveries were natural features.
Recent history The search and study of extraterrestrial life became a science of its own,
astrobiology. Also known as
exobiology, this discipline is studied by the
NASA, the
ESA, the
INAF, and others. Astrobiology studies life from Earth as well, but with a cosmic perspective. For example,
abiogenesis is of interest to astrobiology, not because of the origin of life on Earth, but for the chances of a similar process taking place in other celestial bodies. Many aspects of life, from its definition to its chemistry, are analyzed as either likely to be similar in all forms of life across the cosmos or only native to Earth. Astrobiology, however, remains constrained by the current lack of extraterrestrial life-forms to study, as all life on Earth comes from the same ancestor, and it is hard to infer general characteristics from a group with a single example to analyse. The 20th century came with great technological advances, speculations about future
hypothetical technologies, and an increased basic knowledge of science by the general population thanks to
science divulgation through the mass media. The public interest in extraterrestrial life and the lack of discoveries by mainstream science led to the emergence of
pseudosciences that provided affirmative, if questionable, answers to the existence of aliens.
Ufology claims that many
unidentified flying objects (UFOs) would be spaceships from alien species, and
ancient astronauts hypothesis claim that aliens would have visited Earth in antiquity and prehistoric times but people would have failed to understand it by then. Most UFOs or
UFO sightings can be readily explained as sightings of Earth-based aircraft (including
top-secret aircraft), known
astronomical objects or weather phenomenons, or as
hoaxes. Looking beyond the pseudosciences,
Lewis White Beck strove to elevate the level of public discourse on the topic of extraterrestrial life by tracing the evolution of philosophical thought over the centuries from ancient times into the modern era. His review of the contributions made by
Lucretius,
Plutarch,
Aristotle,
Copernicus,
Immanuel Kant,
John Wilkins,
Charles Darwin and
Karl Marx demonstrated that even in modern times, humanity could be profoundly influenced in its search for extraterrestrial life by subtle and comforting archetypal ideas which are largely derived from firmly held religious, philosophical and existential belief systems. On a positive note, however, Beck further argued that even if the search for extraterrestrial life proves to be unsuccessful, the endeavor itself could have beneficial consequences by assisting humanity in its attempt to actualize superior ways of living here on Earth. By the 21st century, it was accepted that multicellular life in the Solar System can only exist on Earth, but the interest in extraterrestrial life increased regardless. This is a result of the advances in several sciences. The knowledge of planetary habitability allows to consider on scientific terms the likelihood of finding life at each specific celestial body, as it is known which features are beneficial and harmful for life. Astronomy and telescopes also improved to the point exoplanets can be confirmed and even studied, increasing the number of search places. Life may still exist elsewhere in the Solar System in unicellular form, but the advances in spacecraft allow to send robots to study samples in situ, with tools of growing complexity and reliability. Although no extraterrestrial life has been found and life may still be just a rarity from Earth, there are scientific reasons to suspect that it can exist elsewhere, and technological advances that may detect it if it does. Many scientists are optimistic about the chances of finding alien life. In the words of SETI's Frank Drake, "All we know for sure is that the sky is not littered with powerful microwave transmitters". Drake noted that it is entirely possible that advanced technology results in communication being carried out in some way other than conventional radio transmission. At the same time, the data returned by space probes, and giant strides in detection methods, have allowed science to begin delineating
habitability criteria on other worlds, and to confirm that at least other planets are plentiful, though aliens remain a question mark. The
Wow! signal, detected in 1977 by a SETI project, remains a subject of speculative debate. On the other hand, other scientists are pessimistic.
Jacques Monod wrote that "Man knows at last that he is alone in the indifferent immensity of the universe, whence which he has emerged by chance". In 2000, geologist and
paleontologist Peter Ward and
astrobiologist Donald Brownlee published a book entitled
Rare Earth: Why Complex Life is Uncommon in the Universe. In it, they discussed the
Rare Earth hypothesis, in which they claim that Earth-like life is rare in the
universe, whereas
microbial life is common. Ward and Brownlee are open to the idea of evolution on other planets that is not based on essential Earth-like characteristics such as DNA and carbon. As for the possible risks, theoretical physicist
Stephen Hawking warned in 2010 that humans should not try to contact alien life forms. He warned that aliens might pillage Earth for resources. "If aliens visit us, the outcome would be much as when
Columbus landed in
America, which didn't turn out well for the
Native Americans", he said.
Jared Diamond had earlier expressed similar concerns. On 20 July 2015, Hawking and Russian billionaire
Yuri Milner, along with the
SETI Institute, announced a well-funded effort, called the
Breakthrough Initiatives, to expand efforts to search for extraterrestrial life. The group contracted the services of the 100-meter
Robert C. Byrd Green Bank Telescope in West Virginia in the United States and the 64-meter
Parkes Telescope in New South Wales, Australia. On 13 February 2015, scientists (including
Geoffrey Marcy,
Seth Shostak,
Frank Drake and
David Brin) at a convention of the
American Association for the Advancement of Science, discussed
Active SETI and whether transmitting a message to possible intelligent extraterrestrials in the
Cosmos was a good idea; one result was a statement, signed by many, that a "worldwide scientific, political and humanitarian discussion must occur before any message is sent". ==Government responses==