Pluto’s ongoing identity crisis stirs planet definition debate

Science isn’t usually subject to a popular vote, but Pluto’s status in the Solar System has already gone through two of them. The first vote resulted in astronomers stripping Pluto of its planetary status in 2006. Last September, however, a popular vote of laypeople — the attendees of a public lecture, to be specific — concluded that decision was a mistake. While Pluto’s identity crisis is the common thread that ties these votes together, a much bigger issue at the heart of the matter is: What, precisely, does it mean to be a planet?

That question was the subject of debate at the Harvard-Smithsonian Center for Astrophysics on Sept. 18th, 2014, when three astronomers publicly pondered what the definition of the term planet should be. Two of them, Owen Gingerich and Dimitar Sasselov, provided definitions that would restore Pluto’s planethood; only Gareth Williams argued for keeping the current official definition that excludes it. Ultimately Sasselov’s definition, which allows just about any spherical thing orbiting a star to be dubbed a planet, triumphed in a landslide popular vote of the debate’s audience. While the point was to discuss what constitutes a planet, the spree of social media posts and Internet articles that popped up in the debate’s wake emphasized a different result entirely: According to the debate, Pluto was a planet again!

The debate was the brainchild of David Aguilar, director of public affairs at the Harvard-Smithsonian Center, and he expected those hyped-up headlines. The International Astronomical Union (IAU) vote that removed Pluto from the planetary roster never sat well with him. It took place in Prague at the 2006 annual meeting of the IAU and although there were more than 2,500 astronomers in attendance, the final vote was only 206 to 199 — about one sixth of those in attendance. It established a formal definition of the term planet: Planets must be spherical, orbit the sun and have cleared their neighborhoods, which means they can’t exist in a belt like the asteroid belt or the expanse of small, icy objects that make up the Kuiper belt — which happens to be Pluto’s neck of the woods.

Two parts of this definition particularly bother Aguilar and astronomers like Sasselov who study planets outside our solar system, called exoplanets. First, if an object must orbit the sun it means that the current definition of planet can’t extend beyond our solar system, which causes some confusion in light of the wealth of exoplanet research being carried out. By definition, objects around a foreign star can’t be planets — but the term exoplanet remains in use all the same. Second, Aguilar finds the phrase “cleared its neighborhood,” to be so vague that it’s essentially meaningless. What constitutes a planet’s neighborhood can change depending on how a researcher chooses to define it because it has no set definition of its own.

It seems absurd to Aguilar that the term planet should be so meaningless. “We know what a star is, we know what a moon is, we know what a galaxy is — but we don’t know what a planet is, and we live on one.”

Sasselov proposed the much simpler, debate-winning definition for a planet: The “smallest spherical lump of matter that formed around stars or stellar remnants.” Under this definition, all it takes to be classified as a planet is a round shape and an orbit around something that is or once was a star. By being non-specific to the sun it clears up the issue regarding exoplanets, and its laxer requirements let objects like Pluto and Ceres, the largest object in the asteroid belt, back into the line-up.

For Williams, Aguilar and Sasselov’s definition is too broad and too inclusive. If you allow a “piddly little thing” like Pluto to be a planet, Williams says, then you must admit all other objects that are similar in size to the planetary club. The new members would include at least four more objects, which currently keep Pluto company in the dwarf planet category. Ceres, which had its own 76-year spell as a planet from 1801 to 1877, is the most notable. The Dawn spacecraft will arrive there in March. Haumea and Makemake, two large objects located in the Kuiper belt beyond Pluto, and Eris, located in the far-off scattered disc region of the Solar System, would also receive planetary status under Sasselov’s definition.

Williams and the astronomers who support the IAU definition feared that the solar system’s roster would keep increasing as more and more objects larger than Pluto were discovered. When the vote took place, there was a genuine belief that 50 to 100 new planet candidates within the solar system would be found within the next few years. Rather than have an out-of-control list of planets for school kids to memorize, they thought it would be more palatable to students and teachers to create a stricter definition for the term planet that would cap the solar system at eight planets. “Would you want to be a school kid trying to remember the names of 100 planets?” Williams asked. “I wouldn’t want to remember 20.”

There’s also the fact that Pluto was the planet that never should have been. According to Williams, Pluto was initially dubbed a planet because astronomers at the time were expecting to find a ninth planet to explain some minor deviations in Neptune’s orbit — deviations later proven to be the result of inaccurate equipment. When Clyde Tombaugh, an American astronomer, discovered Pluto in 1930, he declared it the expected ninth planet. Even then, some astronomers wondered whether it truly deserved to be called a planet, but letting Pluto be the expected planet was less fuss than figuring out what it really was, at least until more Pluto-like objects were found, such as the aptly-named Eris, which was named for the Greek goddess of discord.

Defining planet so that the solar system has fewer planets for students to memorize seems ludicrous to Aguilar. “We didn’t stop at 30 presidents or 20 states,” he said, citing other examples of things school kids are often made to commit to memory in class, so why should the solar system be limited to just eight planets in the name of making it easier on students?

It’s also possible, Aguilar suspects, that there’s a political angle to the redefinition. Pluto was the only planet discovered by an American; the inner planets and Jupiter were known since ancient times (they could easily be seen with the naked eye), while Europeans discovered both Uranus and Neptune in the 18th and 19th Centuries, respectively.

Williams agrees that there’s an international element to this, but not in the same way as Aguilar. “It’s very telling that most of the ‘Pluto-is-a-Planet’ advocates come from within the U.S.,” he said. According to him, the only people still raising a fuss over Pluto are school kids and teachers in America. The rest of the world has moved on and accepted the IAU’s definition of a planet while the United States has clung to the hunk of space rock.

It does seem as though the United States is the only nation still up in arms about Pluto, but that doesn’t mean the international astronomical community doesn’t want a better, more comprehensible definition of the term planet. Junichi Watanabe, a Japanese astronomer, was part of the 2006 IAU committees that attempted to define what a planet is. Although he believes the definition he helped reach was sufficient at the time, “the situation is changing,” Watanabe said. “Many Pluto-like objects have been found, so that now it may be good to reconsider the definition of objects in the solar system.”

“What we call it doesn’t change the fact that it’s a very interesting object,” Williams said. No matter what the definition of planet is, Pluto will still be worth studying, either as an abnormal planet or as the trendsetter for its own class of objects, or simply as a notable chunk of rock in orbit around the sun.