Is Pluto a Planet?

Why can't astronomers decide on whether or not Pluto is a planet? Ask Science explains the controversy about our faraway neighbor.

Sabrina Stierwalt, PhD
5-minute read
Episode #324

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.

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.

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.

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.

What Is a Dwarf Planet?

That all changed in 2006, when the International Astronomical Union, an organization of professional astronomers and the group that gets to name planetary bodies, voted to reclassify Pluto instead as a dwarf planet, suggesting we had been getting it wrong all this time.

Nothing about Pluto or our knowledge of Pluto changed, but the definition of what makes a planet was rewritten. The vote decided that objects in our solar system would be classified into three categories: the planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune), dwarf planets, and “small solar system bodies.” In order to be classified as a planet, the organization decided, an object had to:

  1. orbit the Sun (so not a satellite like the Moon).
  2. be round or mostly round (so not a potato-shaped asteroid, for example).
  3. clear the neighborhood around its orbit.

Dwarf planets satisfy the first two categories but not the third and small solar system bodies only meet the first criterion.

Pluto both orbits the Sun and is round but it has decidedly not cleared the other so-called small solar system bodies out of its path as Pluto and Charon reside in the Kuiper Belt. This donut shaped region beyond the orbit of Neptune is full of icy bodies that we call Kuiper Belt Objects or KBOs.  Thus, after more than 75 years of calling Pluto a planet, the icy orb was officially demoted to the status of dwarf planet.

So posters in classrooms and museums everywhere had to be remade and lectures updated, but finally, it was settled that there are only eight planets in our solar system...Or are there?


Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Sabrina Stierwalt, PhD

Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.