Is Pluto technically considered a planet?

By Sam Groves | Editor in Chief


The dwarf planet Pluto and its satellites, photographed by the Hubble Space Telescope in 2006. (NASA)

In 2006, the International Astronomical Union (IAU) voted to downgrade Pluto from a planet to a dwarf planet. Pluto’s planetary status has been the subject of public controversy ever since.

The New Mexico House of Representatives and the Illinois State Senate both passed resolutions taking issue with the IAU vote—the New Mexico resolution even created a “Pluto Planet Day.” And at the end of the year in 2006, the American Dialectical Society declared “plutoed” the word of the year.

(To “pluto,” for the untold millions who’ve forgotten the neologism or never heard it in the first place, is to “demote or devalue someone or something.”)

Recently, this controversy was reignited. In late September, the Harvard-Smithsonian Center for Astrophysics hosted a debate between three astronomers over Pluto’s planetary status and the definition of a planet.

At the end of the debate, the audience participated in an informal poll on what they thought the definition of a planet should be. The loosest definition—the one that would have made Pluto an official planet again—won in a landslide.

News outlets reacted swiftly. “Is demoted planet Pluto making a comeback?” asked an unusually sober CNN headline. “Wait, what? Pluto a planet again?” asked USA Today, with the timid air of someone who might have gotten that question wrong on the test. “The People Have Voted: Pluto is a Planet!” declared TIME Magazine in a cavalier article that puts the cart way before the horse.

No, Pluto is not a planet again—certainly not because about 100 people in a classroom, most of them ordinary citizens, raised their hands to say that it is. But for those to whom that fact is greatly disappointing, let’s remember exactly why Pluto isn’t a planet.

The definition of a planet approved by the IAU has three points. To be a planet, and object must:

  1. Orbit the Sun,
  2. Have sufficient mass for gravity to sculpt it into a sphere, and
  3. Have “cleared its orbital neighborhood” of other objects.

Pluto is disqualified under the third point, because Pluto is in the Kuiper belt, a kind of junkyard of small solid objects that orbit the Sun from 30 to 50 Astronomical Units away (that’s roughly 30 to 50 times the distance from Earth to the Sun). Pluto’s irregular orbit (another reason to doubt its planetary status) takes it exactly within that range: Pluto is 30 AUs from the Sun at its nearest point, and 49 AUs at its farthest.

How densely populated is the Kuiper belt? There are at least 70,000 objects orbiting within it, and astronomers believe there are many more. Of these 70,000 objects, three are classified as dwarf planets.

Add those three dwarf planets to myriad other objects orbiting the Sun of comparable size to Pluto—the largest of which, Eris, is actually bigger than Pluto—and you’ve got a problem. If Pluto qualifies as a planet, there are as many as sixteen other objects orbiting the Sun that would also have to qualify, and possibly dozens more we haven’t yet identified.

“Do we want schoolchildren to have to remember so many?” asked astronomer Gareth Williams, who participated in the debate. “No, we want to keep the numbers low.”

Harvard astronomer Owen Gingerich, another participant, noted that “‘Planet’ is a culturally defined word that has changed its meaning over the ages.” This, he says, is why the IAU’s definition of a planet is invalid.

He’s not wrong: the definition of the word “planet” has never been concrete. But this is exactly the problem that the IAU was trying to rectify in 2006. Our knowledge of the Solar System is increasing at such a rate that a shifting definition of the word “planet” is becoming unsustainable. To achieve a fixed definition, the IAU had to draw the line somewhere, and unfortunately for the former ninth planet and its diehard fans, Pluto just happened to be the most natural place.

Categories: Opinion

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