Last year I was happy to get a paper published in an astrobiology journal. “The Challenging History of Other Earths” appeared in the International Journal of Astrobiology in December 2023 — CLICK HERE for it. The paper brings history together with the search for life on other planets! This post is a much-shortened (and footnotes-free) version of that paper.
I research the science of those astronomers in history who opposed Copernican ideas. To some, this subject is arcane, but I find it fascinating. After the paper was unofficially published (on-line, in August), I was invited to give a seminar on it for NASA-Goddard because it connected to the search for life on other worlds (click here and look in the column of titles at right for September 27, 2023). I was thrilled to be able to successfully connect my arcane research with something so “cool” as that search, and NASA.
What I argue in “The Challenging History” is that, from the very beginning of “the Copernican Revolution”, science has challenged the idea of earth-like planets being common in the universe. The fact that science posed this challenge was understood by Johannes Kepler, Jacques Cassini, and William Whewell, in the seventeenth, eighteenth, and nineteenth centuries, respectively. If this all sounds familiar, that is because I often use various Sacred Space Astronomy posts as places to first put down in writing ideas that I think might be interesting and worth further exploration. I have touched on these ideas here at Sacred Space. “The Challenging History” also derives from a series of blogs about other earths that I wrote for the National Catholic Register (click here for those).
The idea that there might be other earths arguably begins with Nicolaus Copernicus (1473-1543) and Giordano Bruno (1548-1600). Copernicus envisioned Earth circling the Sun with Jupiter, Mars, etc.; Bruno, a Copernican, envisioned stars being other suns, circled by other inhabited earths. But Kepler (1571-1630) and Cassini (1677-1756) each argued that the stars could not be suns, and they had solid data supporting their arguments.
Kepler said in his 1610 Conversation with Galileo’s Starry Messenger that the simplest observations, measurements, and calculations proved Bruno wrong. “To use Bruno’s terms, the [fixed stars] are suns,” Kepler wrote, “Nevertheless, let him not lead us on to his belief in infinite worlds, as numerous as the fixed stars and all similar to our own.” Kepler said that if you looked at just the thousand brightest visible stars and brought them together in the sky, they would form an object comparable in apparent size to the Sun. Add in the rest of the stars, especially the 10,000 that Galileo reported being telescopically visible, and the stars will occupy well more space in the sky than the Sun, Kepler said.
Kepler was right. He noted that the majority of stars listed in catalogs of the time were shown as having diameters of at least a thirtieth that of the Sun, a size repeatedly measured by astronomers from Ptolemy in the second century to Tycho Brahe in the sixteenth. Area goes as diameter squared; 30 squared is 900. Just 900 stars would equal the Sun. Throw in the others and they exceed the sun.
So, Kepler said,
If this is true, and if they 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 pin point, it outshines the fixed stars at once. The difference is practically infinite.
In other words, stars are too dim to be suns. Their light is infinitely weaker, practically speaking, despite their combined apparent size rivaling the Sun. And since the stars have measurable apparent diameters, their distances cannot explain this weakness. “Will my opponent tell me that the stars are very far away from us?” Kepler wrote, “This does not help his cause at all. For the greater their distance, the more does every single one of them outstrip the sun in diameter.” Stars had to be huge, given their measured apparent sizes and their vast distances from Earth as required by the Copernican system (that Kepler strongly supported). In 1604, Kepler had shown that Sirius surpassed the orbit of Saturn in size. Every single star visible to the eye had to surpass the orbit of Earth — and be far dimmer than the sun. Cassini reached similar conclusions a century later, but using a telescope. (Astronomers who opposed Copernican ideas saw all these giant stars as ridiculous.)
Today we know the sizes that stars appear to have, whether seen through a telescope or with the eye, are illusory; stars look much larger than they are. Their light is spread out over that illusory size, making them look dim. People did not understand this in Kepler’s time. At that time, solid science — reproducible stuff that all skilled observers could go out and test for themselves — showed that stars were absolutely not suns, and that Bruno was flat out wrong.
Well… Science, schmience. Reproducible, shmeproducable. We love the idea of other earths with people on them.
Bernard Le Bovier de Fontenelle (1657–1757; those dates are correct) wrote a book in 1686 called Conversations on the Plurality of Worlds. This book gave a great boost to the idea of a universe of other suns and other earths (with people on them). It featured a frontispiece (below) showing the solar system as but one system among a vast swarm of suns and planets. From the seventeenth century onward, you can find one astronomer after another echoing that frontispiece, saying that the stars are other suns with other earths orbiting them.
In the 1830s (by which time the illusory nature of star sizes had been worked out), one writer, Thomas Dick, even attempted to estimate the population of the solar system, and of the universe with its swarm of systems. He reckoned that our neighboring planets fairly swarmed with intelligent life. He even calculated how many people could be living on Saturn’s ring! He put the population of the solar system in the tens of trillions (not including any solar population, and yes, lots of people assumed the Sun would be populated). He put the population of the universe at trillions of times larger still.
This is the sort of universe that Mark Twain portrayed in his 1907 Captain Stormfield’s Visit to Heaven. Here the Captain comes to find out that the universe is so full of inhabited worlds that no one has even heard of Earth, to say nothing of the America, California, or San Francisco that he so greatly esteems. When he identifies Earth as “the world” he is answered with “the world! H’m! there’s billions of them!”.
This kind of thinking ran into a problem in the form of Whewell (1794-1866). In 1853, he anonymously published Of the Plurality of Worlds: An Essay, attacking scientifically the idea of a universe widely inhabited with intelligent life. He noted the inverse-square law as it relates to gravitational force, light, and heat radiation. (The law that says that if planet B is twice the distance from the Sun as planet A, it will receive one quarter the heat and light from the Sun, all else equal) Were the Earth a little closer to or further from the sun, then the amount of heat and light it would receive would be significantly changed. Whewell was on to the idea of a “habitable zone” around a star (an idea that implies that the rest of the region around a star is not habitable), although he called it a “temperate zone”. Whewell, who had been elected president of England’s Geological Society, also argued that evidence for the age of the Earth showed that throughout most of Earth’s history it had lacked intelligent life. Even a habitable planet need not be inhabited.
Whewell did not rule out the idea of lower life forms within the solar system. For example, he speculated on life existing on Jupiter, writing that Jovian life forms might be “aqueous, gelatinous creatures; too sluggish, almost, to be deemed alive, floating on their ice-cold water, shrouded forever by their humid skies.” He was not so radical as to think that the planets of the solar system would be lifeless. Gosh no.
But of course, they are lifeless, as best we can tell. If we found some microscopic gelatinous creatures in the frigid waters under the ice of Europa or Enceladus, that would be a huge discovery. Other planets are not all other earths. Planets are a diverse lot.
Ditto for stars. In a post a few weeks ago, I discussed Agnes Mary Clerke (click here for that) and how she described the impact that improved star observations during the nineteenth century had on our idea of what stars were. In brief, she noted how, at that century’s beginning, astronomers had assumed that one star was like another, whereas by its end they had to recognize that there was enormous diversity among stars.
In other words, stars are not all suns. To be clear, today we understand that the sun is a star. But as Clerke observed, the range of what counts as a “star” is very wide. Today we know that there actually are stars that are enormous, comparable in size to the sizes Kepler determined (but not at all dim); these are rare. More common are stars comparable to the Sun. But most common of all are those red dwarfs that are much weaker than the Sun.
Something else happened in the nineteenth century besides Whewell and Clerke that is very important to the idea of other earths and life on them. We figured out that life is not a natural result of matter.
The idea of the “spontaneous generation” of life from matter dates back to at least Aristotle. Given spontaneous generation, life should be the default for any world — maybe even the Sun. But even in the seventeenth century, some scientists were arguing against the idea. By the end of the nineteenth century, science had killed off the idea entirely. It did not die easily. As late as the 1880s, the German botanist Carl Nägeli was arguing that simple living creatures were spontaneously generated all the time, and that higher life forms then evolved from those simple life forms.
But today science tells us that, by whatever means life originated on Earth, it is clear that it originated a long time ago, through a process which we cannot reproduce in a lab or find occurring in nature. Science tells us that nothing living on Earth today just spontaneously sprang to life last night; it is all descended from something living here in the past. This is a huge change from thinking that life just naturally springs from matter; it has huge consequences for the idea of life elsewhere.
So, the idea of other inhabited earths arose from Bruno’s ideas about stars being other suns, at a time when life was thought to naturally spring from matter. But science did not support Bruno’s ideas that in essence said the universe really looks a lot like us — full of worlds “as numerous as the fixed stars and all similar to our own”. Still, those ideas became popular anyway (science, schmience).
What science has always told us, even when it was erroneous, is that the universe is not like us. Today we see tremendous diversity in stars, planets, and planetary systems. As we go around looking for other Earths (and do not misread me — I think we should look for them), we need to understand that we are looking for something whose existence science has challenged for much of history.
If you do not have access to the International Journal of Astrobiology and want a copy of “The Challenging History of Other Earths”, contact the Vatican Observatory and request one.
