Over the past 250 years, the number of "planets" in our solar system has ranged from six to nine — and, briefly, even 11 — depending on what astronomers knew at the time and how they defined a planet. As the United States prepares to celebrate its 250th anniversary, that changing tally offers a unique lens on humanity's evolving understanding of the cosmos since 1776.
Throughout history, astronomers have discovered new worlds, identified entirely new classes of celestial objects and repeatedly revised the very definition of a planet. The result is a surprisingly complicated answer to one of astronomy's most basic questions: How many planets are there in our solar system?
"The change in the number of recognized planets well represents how science is done," Kevin Schindler, historian and public information officer at Lowell Observatory, told Space.com. "Scientists discover something — a planet, dinosaur fossil, or beetle, for example — study it and classify it. With further study, and by s...
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