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?
