The beginnings of modern science shaped how philosophers saw alien life – and how we understand it today

By Philip C. Almond, The University of Queensland

Speculation about extraterrestrials is not all that new. There was a vibrant debate in 17th-century Europe about the existence of life on other planets.

This was the consequence of the transition from a Ptolemaic view, in which Earth was at the centre of the universe and everything revolved around it, to a Copernican view in which the Sun was at the centre and our planet, along with all the others, revolved around it.

It followed that if we were now more like other planets and moons close to us that revolved around the Sun, then they were more like Earth. And if other planets were like Earth, then they most likely also had inhabitants.

Robert Burton’s remarks in his The Anatomy of Melancholy (1621) were common:

If the Earth move, it is a Planet, and shines to them in the Moone, and to the other Planitary inhabitants, as the Moone and they doe to us upon the Earth.

Similarly, the Dutch astronomer Christiaan Huygens (1629–95) believed life on other planets was a consequence of the Sun-centred view of Copernicus. But his speculation on such matters proceeded from the doctrine of the “divine plenitude”. This was the belief that, in his all-powerfulness and goodness, having created matter in all parts of the universe, God would not have missed the opportunity to populate the whole universe with living beings.

In his The Celestial Worlds Discover’d (1698), Huygens suggested that, like us, the inhabitants of other planets would have hands, feet and an upward stance. However, in keeping with the greater size of other planets, particularly Jupiter and Saturn, they might be much larger than us. They would enjoy social lives, live in houses, make music, contemplate the works of God, and so on.

Others were much less confident in speculating on the nature of alien lives.

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