The concept of extraterrestrial is quite old. Long before human civilization had developed a scientifically accurate understanding of the cosmos, people all over the world looked up to the sky and wondered what was out there. Some ancient societies populated this vast and mysterious expanse of gods: entities responsible for creating the sun, moon, and stars. Others considered these same celestial bodies similar to Earth, and therefore inhabited by organisms not very different from us.
In his book aliens, science and technology writer, Wade Roush traces the history of extraterrestrial speculation over nearly two and a half millennia. This story begins with the ancient Greeks and extends to the most recent Mars rover expeditions. Along the way, Roush illustrates how the world we live in shapes how we conceive of the worlds that might exist in space. The history of extraterrestrial speculation is not only the history of science, but also the history of religion and popular culture.
In the past, people were often persecuted for thinking differently. This was true even in ancient Greece, a setting renowned for its pioneering philosophers. When the philosopher Anaxagoras – who sought to provide scientific explanations for seemingly supernatural phenomena like eclipses and rainbows – suggested that “the moon is not a god but a great rock and the sun a burning rock “, he was arrested and sentenced to death. Anaxagoras is said to have met a similar fate to Socrates, but was banished instead of killed thanks to the plea of his friends.
Anaxagoras also considered the possibility that the moon was inhabited, a highly controversial supposition that contradicts the prevailing view of the cosmos as described by Plato and Aristotle. Plato, who separated reality between forms and shadows, refused to recognize the existence of other worlds than ours. Aristotle also rejected the so-called theory of the plurality of worlds because it could not be reconciled with his understanding of gravity, which posited the Earth as the one and only center of the universe.
Our understanding of astronomy does not rest on the shoulders of Plato and Aristotle, but on their often forgotten contemporaries. Anaximander, writes Roush, “was the first to propose that the Earth is a body floating in an infinite vacuum, supported by nothing”. Democritus, assuming that there were an infinite number of atoms, argued that there must also be an infinite number of worlds. “It seems absurd,” one of his pupils reportedly said, “that in a great field only one stem grows and in infinite space only one world exists.”
Belief in the existence of other worlds spread among a number of philosophers, including Epicurus, who once wrote to the historian Herodotus that “there are an unlimited number of cosmoi, and some are like this and others are different”. This belief, although weak, survived in ancient Rome. “Nothing in the universe is single and alone,” wrote the Roman poet Lucretius, “and therefore in other regions there must be other lands inhabited by different tribes of men and races of beasts” .
The scientific revolution
Although Plato and Aristotle lived in a pre-Christian world, their ideas about the universe helped shape the doctrine of the Christian faith. In the Middle Ages, this faith proclaimed that the Earth was created by God as the center of the universe. The story of Jesus Christ, who sacrificed himself to absolve man’s sins, reestablished mankind as the most important of all creations. If other worlds existed, they could not be inhabited. For if they were, that would automatically diminish the significance of the crucifixion.
Church doctrines did not prevent the Polish polymath Nicolaus Copernicus from writing On the revolution of the celestial spheres, but they stopped him from publishing it. The book, which was not published until his death in 1543, mapped an interplanetary system organized not around the Earth but around the sun. This “heliocentric” model explained phenomena that the Aristotelian model could never account for, including retrograde motion. He also confronted Copernicus readers, as Roush puts it, “with the idea that we live on a planet unlike any other.”
The Dominican friar, mathematician and cosmological theorist Giordano Bruno did not wait for his death to share his ideas on the universe. In three dialogues, published between 1584 and 1591, Bruno hypothesized that some distant stars could also be suns, that these suns orbited their own planets and, last but not least, that some of these planets could be inhabited by life. similar to Earth. Although these views may have been annoying to the Church, Bruno’s interest in magic and the occult is likely what led to his arrest in 1592. Refusing to recant, he was burned at the stake after seven years of imprisonment and torture.
The German astronomer Johannes Kepler lived and worked under different circumstances. Kepler was born in Germany after the Protestant Reformation, which meant he could publish his research without fear of persecution. He was particularly influenced by Galileo Galilei’s discovery of Jupiter’s moons, which revolved around the planet in the same way that the Earth revolves around the sun. “Each planet,” Kepler concluded after reading Galileo, “is served by its own satellites. From this reasoning, we infer with the highest degree of probability that Jupiter is inhabited.
A new age of disbelief
Not all participants in the scientific revolution believed in the existence of extraterrestrials. Galileo, a devout Catholic, considered speculation about extraterrestrials “blasphemous”. British polymath William Whewell argued against the pluralism of worlds theory to defend the special bond between God and humanity. Ironically enough, his religiously motivated thesis, set forth in an 1853 book titled Of the plurality of worldsproved scientifically more accurate than the pagan astronomers he criticized.
Roush summed up Whewell’s ingenious but convoluted essay as follows: “If Earth had, in fact, been uninhabited for most of its history, then it would not be surprising if other distant planets were also empty.” Most stars, Whewell adds in his own words, “lie in a nebular region, which can easily be uninhabitable. And where that nebular region, marked by zodiacal light, ends, the world of life begins, c that is, on Earth. He concludes that the absence of life does not make the cosmos less interesting or majestic.
Whewell found an ally in Alfred Russell Wallace, a British naturalist co-credited with formulating the theory of evolution by natural selection alongside Charles Darwin. “Our Earth is almost certainly the only inhabited planet in our solar system,” Wallace wrote in 1903, at a time when the scientific community was actively debating whether Mars harbored intelligent life. Even if life did exist elsewhere in the universe, Wallace was adamant that it could never reach the levels of complexity we find on Earth.
Although Whewell and Wallace were ignored by their contemporaries, their writings ushered in a new era of cosmological pessimism – namely, a continuing period in which extraterrestrials are the stuff of science fiction and every adventure more in outer space (whether by exploration or observation) fails to provide any evidence pointing to the existence of beings that Democritus and Anaximander believed should exist.