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
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Is Pluto a Planet?
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:
- orbit the Sun (so not a satellite like the Moon).
- be round or mostly round (so not a potato-shaped asteroid, for example).
- 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?
Revisiting Pluto’s Status
The 2006 decision was hotly debated at the time and continues to be a point of contention among planetary scientists. At the time of the vote, a group of astronomers and historians presented an argument for a possible definition of a planet that still included Pluto, but their side did not prevail. After the decision, some argued that the vote was not representative of the astronomical community because only 424 astronomers who stuck around for the last day of the two-week long meeting (and could afford to attend the meeting in the first place) voted on the issue. That’s less than 5% of the professional astronomers that make up the IAU. Personally, I was not there to vote and can assure you I was not part of Pluto’s demotion.
Twelve years later, a research paper was published in 2018 arguing for the re-promotion of Pluto to planetary status. The motivation is again not based on any change in our knowledge of the astronomical orb itself, but with our confidence in the definition of what we call a planet in the first place. The new argument points out that there is no physical basis for requiring an object be large enough to knock everything out of its way as it orbits the Sun in order to call it a planet. In other words, this path-clearing ability is not used as an indicator of anything important in the formation and evolution of a planet, so why should it be the deciding factor in what makes a planet a planet?
Pluto is not just a cold rock in space, but a very complex place with icy methane-snow capped mountains and frozen lakes of liquid nitrogen.
In fact, the authors of the new study combed through 200 years worth of results and found only one instance—a study published in early 1800s—that even referenced a solar system body clearing its orbit.
Some argue there are better, more physically relevant, reasons to determine what makes a planet. For example, once an object is massive enough that its gravity is strong enough to make it round, that is when active geology, like volcanos, can kick in. The astronomer Gerard Kuiper, namesake of the Kuiper Belt, maintained in the 1950s that the method of formation of a solar system object should determine what we call it.
And so the debate continues. Since astronomers don’t disagree on what we know about Pluto but rather whether or not that knowledge is enough to make Pluto a planet, the argument could go on for a long time. We now know more about the distant, icy Pluto than ever before since the New Horizons spacecraft visited it in 2015. We’ve learned that it’s not just a cold rock in space, but a very complex place with icy methane-snow capped mountains and frozen lakes of liquid nitrogen. I, for one, do not care what we call it as long as we are inspired to continue to explore it.
Want to test your knowledge of Pluto? Take our quiz! And hear about what it takes to get to Pluto from author David Grinspoon who had a behind-the-scenes look a the epic mission that was over nine years in the making.
Until next time, this is Sabrina Stierwalt with Ask Science’s Quick and Dirty Tips for helping you make sense of science. You can become a fan of Ask Science on Facebook or follow me on Twitter, where I’m @QDTeinstein. If you have a question that you’d like to see on a future episode, send me an email at everydayeinstein@quickanddirtytips.comcreate new email.
Image of Pluto courtesy of NASA.gov.
