Est-ce qu'on croyait aux extra-terrestres il y a 2000 ans ?

Greek philosophers questioned from antiquity whether life existed on other planets, independent of the question of divinity.

• Democritus posited great diversity of worlds with suns, moons, and potentially unknown creatures. • Epicurus explicates the link between atomism and the existence of infinite worlds in his Letter to Herodotus. • Lucretius reports these ideas to Rome in his poem De Rerum Natura.

Lucretius defends the existence of extraterrestrial life, arguing that vital force and the diversity of terrestrial species must exist everywhere.

• Pythagoreans believed the Moon was populated by animals, plants, and more imposing humans. • Plutarch imagined a dialogue where the Moon could host life like terrestrial deserts.

Aristotle posited that the void does not exist and the entire universe is filled with matter, with each element attracted to its own center.

In this system, there could be neither multiple worlds nor multiple centers: Earth alone is at the center and attracts elements, and consequently, life.

This thought born in antiquity imposed itself during the Christian era and persisted throughout the Middle Ages.

Historical irony: it was Aristotle's thought, which excluded other worlds, that created renewed interest in the question during the Middle Ages, examined through Christian thought.

For the Church, Earth and humanity were at the center of divine creation, with no indication that God had reproduced this creation elsewhere.

• Roger Bacon rejected the plurality of worlds by adopting Aristotelian argument. • Thomas Aquinas argued that perfection lies in unity, so a single world proved God's omnipotence. • Bishop of Paris Étienne Tempier condemned in 1277 thesis 34 on the plurality of worlds.

Although a doctrine was established, the theory remained open: 14th-century thinkers could write about extraterrestrial worlds as long as they specified it was a possibility, not reality.

• William of Ockham defended that God could create an infinity of worlds similar to ours. • Nicholas of Cues, cardinal and friend of the pope, defended the existence of inhabitants on other celestial bodies.

In 1417, Lucretius' poem De Rerum Natura was rediscovered, first appreciated for its style and then for its content in the 16th century.

With Copernicus, Earth revolves around the sun and is merely one star among others. Yet it is populated, so other stars could be too.

• Montaigne rediscovered Lucretius and integrated the theory of extraterrestrial worlds into his Essays. • Giordano Bruno affirmed that extraterrestrial existence was rational necessity and would be burned in 1600 for heresy. • Galileo affirmed he did not believe in inhabitants of other planets but acknowledged having no certain answer.

The idea of multiple worlds gradually became dominant: by the time of Louis XVI, Napoleon, and Benjamin Franklin, many scientists believed in fertile and viable extraterrestrial worlds.

In 1686, Bernard le Bovier de Fontenelle wrote Conversations on the Plurality of Worlds, a popular success with 33 editions during the author's lifetime.

Fontenelle argued that on celestial bodies 300 times larger than Earth, there must rationally be at least some plants, then animals and humans.

• By highlighting the possibility of extraterrestrial life, Fontenelle demonstrated the relativity of viewpoints. • Mercury's inhabitants would be hyperactive and impulsive due to the planet's short year. • This diversity celebrates differences but presents them as natural and insurmountable.

• The Encyclopédia addressed the subject with caution. • Christians like Newton and Huygens defended the existence of multiple worlds. • Theologian Richard Bentley believed God designed other planets for the service of their own inhabitants.

In 1840, German astronomer Mädler drew the first map of Mars. In the 1890s, Percival Lowell in Flagstaff, Arizona observed long straight lines on Mars.

Lowell concluded these lines were not natural but canals built by a Martian civilization to irrigate crop zones. He published Mars and Its Canals in 1906 and Mars as an Abode of Life in 1908.

• Lowell was influenced by Italian astronomer Giovanni Schiaparelli who described canali on Mars. • A translation error led Lowell to understand these as artificial waterways rather than simple paths. • Lowell also read French scientist Camille Flammarion who concluded a superior race inhabited Mars.

The scientific community remained divided: no other astronomer observed the famous lines, even Eugène Antoniadi with a more powerful telescope.

Lowell devised increasingly dubious theories: the canals would transport water by floating enormous icebergs from the poles to the center of arid lands.

• Detractors noted that to be visible from Earth, these canals 80 million kilometers away would need to span hundreds of kilometers wide. • They questioned how such rectilinear lines could avoid borders of Martian nations.

For Lowell, Martians were technologically advanced and had reached great moral maturity with world governance abolishing borders and the notion of countries.

• The idea of Martian canals persisted in popular culture for nearly half a century. • Spectroscopy suggested the existence of Martian vegetation with seasonal variations. • Only in 1964 with Mariner 4 probe did defenders of Lowell's theory admit there was not much to see on Mars.

Lowell Observatory continued operating and it was there that Clyde Tombaugh discovered Pluto in 1930. Science advances even from errors.

• The search for exoplanets continues today. • Scientists continue searching for the distant, different, and viable world that ancient Greeks already dreamed of.

This mathematical formula developed in 1961 estimates the number of extraterrestrial civilizations in our galaxy, resuming the millennia-old logic that infinite worlds make life elsewhere probable.

For millennia, without probes or telescopes, people already imagined viable planets and worlds harboring intelligent life, even more intelligent than ourselves.