Johannes Kepler: Only One Sun

PART I: When Giordano Bruno promoted the idea of a Plurality of Worlds, science was clearly against it, as Johannes Kepler took pains to point out.

LEFT: “Portrait of Johannes Kepler,” 1620. RIGHT: Diagram of the universe of stars from Johannes Kepler’s ‘Epitome of Copernican Astronomy,’ showing a small sun (the dot at the center) surrounded by large stars.
LEFT: “Portrait of Johannes Kepler,” 1620. RIGHT: Diagram of the universe of stars from Johannes Kepler’s ‘Epitome of Copernican Astronomy,’ showing a small sun (the dot at the center) surrounded by large stars. (photo: Public Domain)

UFOs, ETs and the Strange History of Other Earths


Blockbuster movie franchises, media coverage of UFOs and Navy pilots, and claims about what the discovery of “extraterrestrials” would mean for humankind’s place in the universe all hinge on the idea that our universe abounds in other worlds like our Earth. This prominent idea is commonly associated with scientific progress, dating back to Copernicus in the 16th century. But as this four-part series shows, science in fact has never supported this idea of a “Plurality of Worlds.”

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The story of UFOs abducted more than its fair share of headline space in 2021. Here at the Vatican’s astronomical observatory, we had people from serious news organizations virtually banging on our door, wanting some “Vatican scientist” to say something newsworthy about Little Green Men from outer space, and what world-changing consequences might result were certain blips on Navy aircraft videos shown to be in fact vehicles from another planet.

Scientifically speaking, however, there has never been much to say for the idea of UFOs — going back centuries.


Johannes Kepler and Life on Jupiter

Johannes Kepler could have explained the scientific problems with the UFO story. Kepler, the brilliant astronomer of the early 17th century who first worked out the true mathematical nature of how planets orbit, would not have found problems in the idea of extraterrestrial UFOs itself. He was a big fan of, for example, the idea of intelligent life on Jupiter. All those moons circling Jupiter, he said, surely served the Jovians well, illuminating the skies of that stupendous world.

But no one today associates UFOs with Jupiter. Our studies of Jupiter have revealed that it lacks even a surface to stand on, let alone inhabitants standing there, gazing at its moons. It is a ball of turbulent gas. Its winds far surpass Earth’s fiercest hurricanes. Beneath the colorful Jovian clouds that we see with our telescopes, the density of that gas just increases with depth. Were you somehow to be teleported right now to any spot on (or in) Jupiter, you would die — promptly.

That is true for pretty much any place in the solar system, outside of Earth. When we think today of “extraterrestrials” and UFOs, we do not think of them coming from one of the planets that orbit the sun. Invading Jovians, or better, Martians, are a late-19th- through mid-20th-century idea. The UFOs in H. G. Wells’s 1898 book War of the Worlds came from Mars; likewise, those in Orson Welles’s 1938 radio version of War of the Worlds, and in the 1953 movie version. In Steven Spielberg’s 2005 War of the Worlds movie, however, Mars was not part of the story. Today we imagine our UFOs coming from other suns — from the stars.


One Sun

Kepler would have had none of that. He had no tolerance for the idea of the stars being other suns with planets orbiting them. He believed that there was only one sun in the universe: ours. Actually, he did not believe that; he was scientifically certain of it. The simplest observations, measurements and calculations proved it. Just look at the sky, he said:

If [the stars] are suns having the same nature as our sun, why do not these suns collectively outdistance our sun in brilliance? Why do they all together transmit so dim a light…? When sunlight bursts into a sealed room through a hole made with a tiny pinpoint, it outshines the fixed stars at once. The difference is practically infinite.

In other words, stars look nothing like the sun. Kepler took into account the weak light output of the stars, their apparent sizes in the sky, and their vast distances from Earth as required by Nicolas Copernicus’s sun-centered model of the universe that Kepler strongly supported. Using basic geometry, he showed that stars were huge, and stars were dim. They were nothing like the sun. In 1604 Kepler published a calculation showing that Sirius — the Dog Star, the most prominent star visible in the night sky — had to be larger than the orbit of Saturn. Every last star we can see, even those barely visible to the eye, had to be larger in size than the Earth’s orbit.

In terms of size, the sun was nothing compared to the smallest visible star. But size did not matter, as Kepler saw it; energy and life mattered. In terms of light output, he said, the tiny sun surpasses all the giant stars — combined — by a practically infinite amount. Earth is tinier still, he said, and yet behold …

… our little ball, the little cottage of us all, which we call the Earth: the womb of the growing, herself informed by a certain internal faculty. The architect of marvelous works, she kindles daily so many little living things from herself — plants, fishes, insects — as she may easily scorn the rest of the bulk in view of this her nobility.

And even tinier than Earth are …

… these fine bits of dust, which are called human beings; to whom the Creator has granted such, that in a certain way they may beget themselves, clothe themselves, arm themselves, teach themselves infinite arts, and daily work toward the better; in whom is the image of God; who are, in a certain way, lords of the whole bulk.
And who among us would choose a body the breadth of the universe in exchange for no soul? Let us learn, therefore, what well pleases the Creator, who is the author of both coarse bulks and minute perfections. For he glories not in bulk, but ennobles those that he willed to be small.


Kepler vs. Bruno

Kepler promoted his “single sun” universe in the process of explicitly criticizing the ideas of Giordano Bruno. Bruno had urged that the stars were other suns, which would be orbited by other earths, which would be inhabited. Today, we envision UFOs coming from those sorts of places: other earths around other suns. We envision a universe like Bruno’s, not Kepler’s. 

Bruno had been burned at the stake in 1600. He was burned in some part, perhaps, for his insistent advocacy of countless other suns and earths and how they glorified God, and so forth. He was burned in large part, certainly, for his insistent advocacy of many other ideas that his contemporaries found deeply offensive. Kepler, who had his own ideas about the universe and God, wanted to make it clear that Bruno’s ideas about the universe had no basis in science. Every astronomer who could see the sky, make basic measurements, and do some simple math could reproduce Kepler’s results and see that Bruno was wrong, and that there was just one sun.

But if science showed that there was only one sun, how did Bruno’s idea that there were other suns, and other earths, ever catch on?

Further Reading

Graney, C. M. (2019). The Starry Universe of Johannes Kepler. Journal for the History of Astronomy, 50(2), 155-173.