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# 9.6 Safronov and Planet Formation

Bold scientific theories can have obscure origins. In 1969, a little-known Soviet scientist wrote a small paperback book summarizing several decades of work by Victor Safronov and his colleagues. Safronov proposed a theory of how the Solar System formed. He described how asteroid-like bodies orbiting the early Sun would have collided with each other and joined together, or accreted, into planets. What seemed like an obscure book was destined to play an important role in our understanding of the origin of the Solar System. The book's rise to prominence illustrates both the international character of science and how science builds broad theories by combining the research of many individuals.

In the decades before Safronov's book was published, scientific study of the origin of planets hadn't yet begun in earnest. Few scientists worked on this problem, and without the aid of space exploration they didn't have the wealth of data we now have about the Solar System. In America, a few scientists were making telescopic observations of comets and asteroids, and a few others were studying the chemistry of meteorites to get clues about how the planets formed. During this time, Russian scientists were almost completely isolated from scientists in the West because of the Cold War. They approached the problem differently, from a mostly theoretical perspective. The mathematician Safronov was the most important member of this Russian group. He worked with the idea that the planets must have formed in a disk-shaped system of dust grains, ice grains, and gas particles, all orbiting in the same direction around the early Sun.

Safronov's breakthrough came when he calculated the effect on such a system when the particles collide with each other. He knew that the particles would follow elliptical orbits, and they would occasionally collide due to overlapping orbits and turbulence in the disk. Using equations he developed to represent such reasoning, he calculated the speeds at which neighboring particles would collide with each other. Particles traveling at fast speeds relative to each other would just disrupt each other. But particles with slower relative speeds would stick together, like snowflake clusters in a snowstorm, resulting in larger particles. As they grew bigger, the particles' own gravity would make them stick together. Larger objects would attract more mass, and the biggest planetesimals would just keep getting bigger, until they had gathered everything within their gravitational reach. In a few million years, the disk of dust and ice particles would eventually aggregate into a few large, planetary sized bodies in a process called accretion. Safronov realized that the nature of the collisions also explains the planets' generally similar rotation rates and low axial tilts.

If not for the Cold War, Safronov's results would have been published in international journals, and they would have had an immediate effect on scientists in the West. However, because of Cold War competition, Soviets did not publish much in the West. In 1969, Safronov gathered his many papers and summarized his theories in a book, which he wrote in Russian. Finally, in 1972, a circuitous translation program produced the small, gray paperback edition of his life’s work in English. At last, Safronov's science had overcome sociopolitical hurdles and appeared on the world stage.

Other scientists were slow to accept the implications of Safronov's work. In 1975, the suggestion that the Moon was formed by a catastrophic impact with the early Earth was roundly criticized. To Western researchers, giant impacts seemed like wild speculation, dreamed up solely to explain the Moon. Finally, in 1984, after Safronov’s ideas had been read and accepted, researchers agreed that impacts played a role in shaping our Moon and other features of the solar system. American and European researchers have extended Safronov's ideas into complex computer models, which show how systems of particles orbiting the early Sun could have aggregated into a handful of planets. Eventually, Safronov traveled to the United States and other countries, and received scientific prizes for his work.

Japanese scientists came up with another theory of Solar System formation in the 1970s, called the Kyoto model. This theory stressed the importance of gas (as opposed to dust and other small solid particles) in the early Solar System. In this theory, the gas would cause drag effects that would slow down the dust particles. This also helped to account for the formation of the large gaseous planets, which the Safronov model has difficulty explaining. Because there was not much interaction between Japanese and Western scientists, the Kyoto theory was also largely ignored by the western world until more recently.

These stories show how international politics can create barriers to the spread of scientific ideas. Yet, in the end, the pen (and now the keyboard) is once again mightier than the sword. The Cold War did not prevent Safronov’s theoretical work from being accepted. The barriers between Eastern and Western scientists are dissolving. The real key to acceptance has always been the foundation of science: direct observational evidence. In this case, evidence came from studies of meteorites, asteroids, lunar samples, computer modeling, and lab experiments.