Is Pluto a Planet?

Why can’t astronomers decide on whether or not Pluto is a planet? Everyday Einstein explains the controversy about our faraway neighbor

By Everyday Einstein Sabrina Stierwalt

NASA, Johns Hopkins University Applied Physics Laboratory and Southwest Research Institute

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Named after the Roman King of the underworld, Pluto has served as a surprising source of conflict in the astronomical community and for lovers of space everywhere. Growing up, I was taught in school that our Sun hosted nine planets. And as a professional astronomer, I went on to teach this nine-planet solar system model in elementary and middle schools around the globe.

But in a contentious decision in 2006, Pluto was officially stripped of its planetary status leaving our solar system with only eight planets and making me a liar to school children. Now that debate is being reignited with some astronomers calling for the reinstatement of Pluto as a planet, arguing that the icy world should never have been demoted in the first place.

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So what is the deal? Is Pluto a planet? Can we rest assured that the Pluto we grew up with is the real Pluto?

Discovery of Pluto

In his studies of the gas giants Uranus and Neptune, Percival Lowell suggested there must be an as-of-yet undiscovered ninth planet to explain the wobbles their orbits. Lowell never found the mystery planet despite extensive searches, but astronomer Clyde Tombaugh did finally detect Pluto using the Lowell Observatory, so-named in honor of Lowell’s contributions, in 1930. The mystery of the wobbles was not entirely solved, however, since Pluto still didn’t appear large enough to cause them, until Pluto’s companion Charon—which is about half the size of Pluto in diameter—was discovered almost 50 years later.

Pluto was publicly announced on Lowell’s birthday as the planet he had searched for and children across the U.S. learned the acronym “My Very Excellent Mother Just Served Us Nine Pizzas” to remember the order of the nine planets in our solar system.

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