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.”

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