The Annunciation, with Saint Emidius by Carlo Crivelli

Early Ideas About Extraterrestrial Life: What Might Inhabitants from Other Planets Look Like?

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The Christian Church attempted to censor Galileo’s findings in the first decades of the 17th century, but it was a time of expanding knowledge, so it did not take very long for the information to get out. When Galileo’s observation became widely known, people started wondering if these other worlds were like our own. Did life exist on them? Did people live there? Even the Church finally decided that such speculation was not blasphemous. As the truth of the plurality of worlds became accepted, it was assumed that God would never knowingly create a world for no reason.

It was decided that if other worlds in space did exist, their only purpose could be to provide a home to humanlike creatures. As Thomas Burnet asked in his book The Sacred Theory of the Earth (1681):

God himself formed the Earth . . . he formed it to be inhabited. This is true, both of the Earth and of every habitable World whatsoever. For to what purpose is it made habitable, if not to be inhabited? We do not build houses that they should stand empty, but look out for Tenants as fast as we can.

It did not take long before several books were published speculating about what sort of life might exist on the planets. Some authors assumed that any sentient life existing on the other planets would necessarily have to be humanlike. Other authors took a looser definition of what constituted “human”, with the idea that what was most important was the quality and nature of the mind, not the form of the shell that bore it.

Early astronomers started to question whether other worlds would be like our own. Copernicus’ Conversation with God

Early astronomers started to question whether other worlds would be like our own. Copernicus’ Conversation with God ( Public domain )

Journey into Speculation

The great German astronomer Johannes Kepler wrote what might be the first science fiction novel, Somnium, which was published in 1634 (a few years after his death). As a serious scientist, he described the Moon and the sort of creatures that might live on it as accurately as the knowledge of the time allowed. The Moon was an incredibly alien world, he told his readers. Nights were 15 Earth days long “and dreadful with uninterrupted shadow”. The cold at night was more intense than anything experienced on Earth, while the heat of day was terrific. Animals that lived on the Moon adapted to these harsh conditions. Some went into hibernation, while others evolved hard shells and other protection.

As the 17th century progressed, the concept that the planets were not only inhabitable but inhabited was taken for granted. In 1656 the Jesuit priest and writer Athanasius Kircher, sent the hero of The Ecstatic Journey touring the heavens with an angel as his guide. In the course of these journeys through the celestial world, the Moon was actually found to be quite habitable, with a varied terrain that included mountains, oceans, lakes, islands and rivers. In Paradise Lost (1667), John Milton has the angel Raphael and Adam discussing the possibility of life on other worlds, including the Moon. But the angel cautions Adam, saying that it is dangerous to think about such matters since God does not intend human beings to comprehend everything about his creation: “Dream not of other worlds, what creatures there live, in what state, condition or degree.”

The Flammarion engraving, Paris 1888

The Flammarion engraving, Paris 1888 ( Public domain )

A French mathematician named Bernard de Fontenelle was not afraid to dream about such things and wondered what sort of creatures might exist on the planets in his book Entretiens sur la Pluralité des Mondes (Conversations on the Plurality of Worlds ; 1689). In fact, he not only asked the question, but he also attempted to answer it. And he did so in a way that was completely unique. De Fontenelle explained that although the planets were worlds very much like our own, conditions there would probably be vastly different. For example, Mercury would be incredibly hot because it is so close to the Sun. If life existed on the planets it would necessarily have to reflect an adaptation to these very specific conditions.

Conversations on the plurality of worlds, 1715

Conversations on the plurality of worlds, 1715 ( Public domain )

The problem de Fontenelle faced was the simple fact that scientists didn’t know enough. All he had to work with were the approximate sizes of the planets and their approximate distances from the Sun. Other than being able to make rough estimates of the planets’ surface temperatures based on their relative distances from the Sun, he knew nothing at all about the nature of the conditions on them. He had no way of knowing what a planet’s atmosphere might be like – or even for sure if it had one. Still, de Fontenelle didn’t let a little impediment like that inhibit his imagination and he proceeded to describe the creatures that lived on the other planets in great detail. The inhabitants of Mercury, he declared, were exuberant, excitable and quick-tempered. They “resemble the Moors of Granada, a small, black people, burned by the Sun, full of wit and fi re, always in love, writing verse, fond of music, arranging festivals, dances and tournaments every day.”


The writer doesn't know about the book "Cosmotheoros" of Christiaan Huygens (1698).
There is a description and a drawing of a birdlike alien, can't find it.
Live on other planets was impossible according the religious fanatic Newton.

Huygens was the man who was right about the theory of Light, NOT Isaac Newton.
Just because Huygens was Dutch, his legacy is pushed away in history.

Please Check Your Sources !!


I’m aware of Huygen’s Cosmotheoros but I had to draw the line somewhere given the constraints of space—there is only one chapter in the book devoted to this specific subject, so I couldn’t be encyclopedic. I am sympathetic, too, regarding your concerns about Huygens’ work on light. Like Leibniz, who independently developed calculus, his contribution has been overshadowed by Newton. But, again, if I had used Huygens as my example it would have required more explanation than space allowed since most readers would have been unfamiliar with his work on optics. Perhaps future editions of the book will allow me to include “Cosmotheoros,” which I would like to do.

It was not "The Christian Church" that censored Galileo’s findings as you wrote. It was Catholic church. It is an important difference.

Martin Luther was in fact among the first religious leaders to condemn Copernicus and Galileo, well before the Catholic Church did. The latter actually resisted doing so for some time, but ultimately gave in to pressure. I discuss the complex history of all this in my book, “Recentering the Universe.”

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