In the 19th century, astronomers believed that solar systems formed in nebular clouds throughout the universe. But 20th century scientists rejected that idea, arguing that our solar system was an aberration. This mistake derailed the search for exoplanets, and extraterrestrials, for decades.
Scientists of the early 20th century argued that tidal forces had caused the sun to spit out the planets when a rogue star passed too close. It was a kind of drive-by shooting theory of planetary formation known as the "Planetesimal Hypothesis."
The idea held sway as the dominant theory for planetary formation for nearly forty years—during which time it had a profound impact on how we viewed geology, climate, evolution and the possible existence of life on other planets. One geologist even proclaimed it to be "the greatest contribution to theoretical science since the Darwinian renaissance in biology."
In retrospect, that level of enthusiasm is a bit embarrassing, since the Planetesimal Hypothesis was eventually proven to be mind-bogglingly wrong. But, at the time, scientists embraced the idea, since it offered a practical alternative to the existing theory, the "Nebular Hypothesis," which was slowly collapsing beneath the weight of new discoveries.
Spinning a Theory
The French mathematician Pierre-Simon Laplace unveiled the Nebular Hypothesis in 1796, when he published his Exposition du Systéme du Monde (The System of the World).
Brieﬂy, Laplace's theory amounts to this: originally all matter in the solar system was contained in one slowly rotating, huge spherical nebula, roughly 10,000 times the diameter of the sun. With the passage of time, gravity caused the cloud to contract and rotate faster. Gradually, the rotation flattened the cloud so that it took the shape of a spinning disk.
Laplace claimed that, eventually, gravitational attraction was unable to supply the necessary centripetal force to hold everything together. As the spinning disk continued to shrink, outer rings of gas were left behind. Each of these rings coalesced into planets, while the sun would be the remnant of the spinning nebula.
It was an elegant theory that explained, among other things, why the sun and planets all rotated in the same direction. As Laplace himself described it, "this gorgeous planetary scheme, like the blossom, had a bud…within which rested the necessities of its present glorious unfolding!"
But, by the late 19th century, astronomers and physicists discovered anomalies that made it harder and harder to accept Laplace's model. For starters, if the views of Laplace were true, then moons should revolve in the same direction as the rotation of their planets. But astronomers had found that one moon of Saturn and two moons of Jupiter revolved in the opposite direction of their planets. And, while the sun contains 99.9% of the mass of the solar system, the planets carry more than 99% of the system's angular momentum. If the Nebular Hypothesis were correct, then either the sun should be rotating more rapidly, or the planets should be revolving around it more slowly.
A Rocky Start
It was a scientist named Thomas Chrowder Chamberlin who offered an alternative to Laplace's model when, in 1905, he formally presented the Planetesimal Hypothesis.
Chamberlin wasn't an astronomer, physicist or mathematician, but a geologist who had been appointed to the U.S. Geological Survey as the director of its newly created Pleistocene Division, which studied the physical history of the Earth.
Chamberlin was the first geologist to demonstrate that there had been multiple Pleistocene glaciations in North America. But, he was perplexed by his findings, since they appeared to conflict with the Nebular Hypothesis of planetary formation.
Laplace had envisioned a molten, superheated Earth slowly cooling over millions of years. Scientists believe that, as the Earth cooled, a thick crust formed on its surface. Geological features, such as mountains, were thought to have formed as that cooling crust contracted. Moreover, scientists believed that, as the molten Earth continued to cool, the surface of the planet and the climate would become colder and colder, until our world became a frozen wasteland.
But that theory contradicted with Chamberlin's findings. The cycles of glacial formation, expansion and retraction suggested to him that the Earth was not steadily cooling down. (Otherwise, the glaciers would have continued to expand.) In his view, the Earth's climate remained more or less constant. The frigid eras in Earth's past were aberrations, caused by some sort of temporary fluctuations.
Chamberlin was among the first scientists of his era who recognized that the level of carbon dioxide in the atmosphere played a key role in regulating the climate. He theorized that geological changes—such as the formation of mountains—created erosion and weathering that periodically exposed minerals that had been buried beneath the surface. The carbon dioxide in the atmosphere chemically interacted with these minerals, he believed, reducing the overall levels of CO2, creating temporary ice ages.
And this was another point that, in Chamberlin's view, didn't fit in with the Nebular Theory. Recent research had indicated that "thermal contractions" as the planet gradually cooled down could not account for the magnitude of geological changes, such as the formation of mountain ranges. Therefore, some other mechanism must be responsible for physically deforming the Earth's surface.
So, Chamberlin "reverse engineered" the Earth. He needed a new theory for planetary formation that could explain why the Earth's climate remained constant, while also allowing for dynamic geological changes. He began to believe that, instead of a molten beginning, the Earth had been assembled from cold rocky formations in space.
And that's how Chamberlin arrived at the Planetesimal Hypothesis, with the assistance of a young astronomer named Forest Ray Moulton. The two scientists claimed that the building blocks for our solar system had been pulled out of our sun by a larger, traveling star that happened to pass through our celestial neighborhood. This larger star created a tidal force that stretched the sun in two directions—toward and away from the star—causing it to eject material in the form of two, rotating spiral arms, pulled sideways by the passing star. As this material cooled, some of it formed solid bodies of varying sizes that eventually coalesced into the planets, including our Earth. Over time, the Earth continued to grow in size, as meteorites regularly bombarded the planet. This bombardment decreased in intensity, as the finite supply of rocky matter in the solar system grew smaller.
A New Theory of Everything
As the new big thing in science, the Planetesimal Hypothesis had something for everyone.
Now, the popular view in geology was that the Earth had a solid interior, with radioactive elements creating pockets of magma that were responsible for volcanic eruptions. As a 1927 edition of The Science Newsletter reported:
Geologists now tell us that instead of living on a thin crust of rock and earth, floating like slag on a sea of seething lava and molten rock, as we were taught a generation ago, we are living on the outer weathered crust of a globe as rigid as steel and heavier than any of the common rocks…..It is now thought that the great bulk of the interior of the earth, all of it to within 1,800 miles of the surface, is a mass of heavy metal, principally if not wholly iron, or iron and nickel. So that Mother Earth has a heart of iron, heavy but strong.
This new theory about the Earth's structure also appeared to explain the mechanism that caused geological deformations, such as mountains. Chamberlin suggested that some of the planetesimals in the early solar system were denser than others—and, as a result, segments of the Earth's surface had different properties. Continents were composed of lighter materials than ocean basins, which is why they were higher. Gravitational forces were said to be constantly pulling the surface downward. The oceanic basins sank the most—which exerted horizontal pressure on the continents, causing their borders to wrinkle into mountain ranges.
Evolutionary biologists likewise endorsed the Planetesimal Hypothesis. Laplace's theory had envisioned a molten, superheated Earth slowly cooling over millions of years. During that period, life couldn't survive. But, according to Chamberlin's theory, the Earth was formed from already-solid rock. Biologists believed that this extended the timeline allowing for life to evolve.
Moreover, if the Earth had been smaller millions of years ago, gravity would have been weaker. Biologists believed that's why prehistoric life (such as the dinosaurs) were so much bigger than life on Earth today. Lower gravity created a niche for evolution to spawn gigantic creatures. And, weaker gravity, they believed, also enabled the emergence of flying species. (As one biologist wrote, "The evolution of ﬂight under existing conditions seems inconceivable because a creature would require a very highly developed mechanism before the air could appreciably help it to overcome gravity.")
Arguably, the Planetestimal Hypothesis' most profound impact was on how we viewed the universe. The Nebular Hypothesis had suggested that planets were the byproduct of stellar formation, which meant it was likely that all the stars had planets, possibly inhabited by extraterrestrial life. But Chamberlin's theory stipulated that the planets in our solar system were the result of a freak accident. As such, the scientific consensus—which just a few decades earlier had contemplated a "plurality of worlds" in the universe—now favored a paucity of worlds. The perceived chances for life beyond the Earth plummeted.
Over time, piece by theoretical piece, the Planetesimal Hypothesis began to fall apart. The astrophysicist Henry Norris Russell calculated that any planets produced by a stellar encounter would orbit the sun far closer than in our actual solar system. But the fatal blow came in 1939, when astrophysicist Lyman Spitzer proved that any material torn from the hot interior of the sun—no longer held together by strong gravitational forces—would rapidly dissipate into space before it could condense into planetesimals.
Before too long, everything old was new again. Geologists turned their attention back to early 20th century theories of continental drift, which grew into the science of plate tectonics. Laplace was partially vindicated, as variations on the Nebular Hypothesis became the basis for the current theories of solar system formation.
And the universe, once again, was teeming with the possibility of extraterrestrial life.