Art Courtesy of Annli Zhu.
It is easy to take for granted the vast scope of scientific understanding that humans have acquired over hundreds of thousands of years on Earth. Today, for example, the origin of the solar system is relatively well understood. But not too long ago, we were searching for an answer among a sea of endless theories.
An article from the 1941 edition of the Yale Scientific Magazine (Vol. 15 No. 4), titled “The Origin of the Solar System,” is a time capsule that provides a firsthand account of the scientific community’s understanding of the solar system’s origins at the time. The writer, Lyman Spitzer Jr., was a well-respected Yale faculty member who earned his PhD in physics from Princeton University in 1939. In his piece, Spitzer sought to analyze contemporary understanding of the origin of the solar system and place doubt on the existing theories of the time.
Eighty years later, this article revisits Spitzer’s analysis. By reflecting on evolving scientific understanding, we can help provide motivation for further scientific advancement, as the cosmos still leaves much to be discovered.
“The rise and subsequent decline of the most important theories of the origin of the solar system are an instructive chapter in the history of science and cast light on the problems which an ultimately successful theory must face,” Spitzer wrote. At the time of his article, there were many different hypotheses surrounding the formation of the solar system, yet none could truly be rigorously proven. In fact, Spitzer criticized two of the leading hypotheses for their lack of concrete evidence.
The first theory he analyzed was the nebular hypothesis, which argued that the sun was first created out of a huge, rotating cloud of gas and dust in space, which was then flattened into a disk. This disk would have subsequently condensed to form the sun and various planets seen in the solar system today. He argued that this hypothesis was impossible due to the conservation of angular momentum, which means that the sun would have to rotate much faster than observed to maintain the known laws of the universe.
The second hypothesis Spitzer examined was coined the encounter theory. This idea proposed that the solar system was formed from the collision of two stars. This would give rise to the planar nature of the observed solar system as well as the debris and planets that orbit around the sun. However, Spitzer was not a fan of this hypothesis either. He argued that the debris pulled from the near-collision of two stars would not have been able to condense into planetary objects like the ones observed today in the solar system. Such rapid cooling of the nebular gasses would have manifested into something more akin to an explosion rather than spherical planets, according to his theoretical calculations. “The origin of the solar system may well be a riddle which science will never wholly solve,” he concluded.
Advancements in Scientific Technology
The foundation of scientific study is rooted in empirical evidence and a rigorous commitment to skepticism. Spitzer’s article did not seek to reject ideation, but rather to adhere to the scientific method’s demands that theories be substantiated with empirical data. “There’s nothing wrong with having an idea of how something might work without being able to go out and measure it,” said Charles Bennett, a professor of physics and astronomy at Johns Hopkins University. “In the end, you need to match simulations with observations to know that what you’re doing is right.”
Spitzer’s critiques shed light on the technological limitations of his time, which prevented many scientists from being able to experimentally prove their conclusions. Spitzer acknowledged that the mathematics required to go back two billion years is an “ambitious calculation” that would probably never be attempted.
Today, the scientific landscape has evolved significantly. Technological advancements, particularly in computing technology and simulation power, have revolutionized our ability to create intricate models of the formation of the solar system. Further, the Hubble Space Telescope has even been used to photograph stars surrounded by accretion disks of gas and dust, signaling an early stage of planetary formation. These new findings have proven pivotal: while Spitzer discounted the nebular hypothesis in his article, now we know it was not far off.
The Winning Hypothesis
Since Spitzer’s time, scientists have been able to conclude that the solar system was very likely created through the collapse of a nebular cloud of gas and dust from a nearby supernova. Today, the “solar nebula hypothesis” is widely accepted to be the most accurate account of the solar system’s origins.
Avi Loeb, a renowned theoretical physicist and professor at Harvard, described the formation of the solar system as a series of events caused by a nearby supernova event, which is a massive explosion of a star that releases immense amounts of energy and material into space. The solar system formed about 4.6 billion years ago—just about one-third of the existence of the universe—from a giant cloud of gas and dust. The shockwaves from the supernova then triggered the collapse of this cloud. As it collapsed, gravity pulled the material together at the center, forming the sun, which is mostly composed of helium and hydrogen.
Over time, small particles of leftover mass in the form of dust and gas began to condense in orbit around the early sun. This then created the rocky planets closest to the sun—where the hydrogen would be absorbed more quickly—leaving the gaseous planets like Jupiter and Neptune further away in orbit. “The universe started from a pretty uniform distribution of matter. There were small differences in the density of matter, and they grew over time because of gravity,” Loeb said. This process led to the diverse and fascinating solar system we know today.
One article, written by Michael Perryman of the University of Bristol, explained that this current “solar nebula hypothesis” accounts for early issues with the nebular hypothesis, such as the angular momentum discrepancy that Spitzer highlighted. Somehow the sun maintained most of the mass, but only a tiny percentage of the angular momentum of the solar system. The new theory reconciled this discrepancy by more chronologically describing the process by which the nebula compressed into a disk before beginning the sequential process of planetary formation.
A Glimpse Forward: Finding Life
Although the mystery of the origin of the solar system is somewhat resolved, there are natural questions that these resolutions leave unanswered. One such question is the existence of extraterrestrial life. By studying the formation of our solar system and the emergence of life on Earth, scientists can gain valuable insights into the potential habitability of other planets and celestial bodies. “Looking for life around stars is a very exciting frontier,” Loeb said. “One can search for signatures of microbial life. But one can also search for intelligent life.”
Loeb highlighted the notion that the laws of physics, chemistry, and biology are universal, suggesting that the conditions conducive to life on Earth may very possibly exist elsewhere in the cosmos. This perspective enables scientists to assess the likelihood of life on exoplanets.
Spitzer, eighty years ago, was simply hoping to figure out how the solar system formed. Now that we know the conditions that led to life within our solar system, we can launch a new quest for life beyond our planet. We have the means to explore the broader cosmic environment and likewise form new hypotheses, critiques, technologies, and conclusions in the ongoing pursuit of alternative forms of life.