Space, Physics, and Math

Pluto’s ongoing identity crisis stirs planet definition debate

If you think this is just about Pluto, you’re thinking too small

February 13, 2015
Money talks, and in this case it’s saying that guests of Lowell Observatory really think Pluto ought to be a planet. [Photo Credit: Flickr user Cheryl Colan]

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.


About the Author

Jennifer Hackett

Jennifer Hackett is a graduate of the College of William and Mary, where she studied physics and history. She’s always been fascinated by science but writing and communication have been her specialty; working as a science journalist enables her to pursue her passion while utilizing her skills. She particularly enjoys writing about the intersection between science and politics, and enjoys any excuse to bring up nuclear topics or the Manhattan Project. When not writing about things like space probes or weird particles, she enjoys sewing, video games, and anything involving atomic culture.



If size is the decisive factor, then, since the centre of gravity between Pluto and Charon is between the two bodies of mass (and I read somewhere that there may be an atmosphere stretching between them) then the space Pluto and its partner inhabit is pretty massive. Jupiter, after all, is mostly atmosphere. If roundness is the criteria then Pluto is a planet. It is not difficult to remember the names of Planets if you also are taught the mythology attached. Teach schoolkids the mythological stories and they will easily remember the names of the planets in their own solar system.

The claim that the US is the only nation that still rejects the IAU definition and that most of the world has accepted the IAU definition and moved on is blatantly false. As an amateur astronomer who runs a blog about Pluto, I regularly hear from people all over the world ranging from professional astronomers to amateur astronomers to educators to members of the general public who continue to reject the IAU definition and consider Pluto–and all dwarf planets–as planets. Many educators around the world teach the controversy rather than promote one definition as gospel truth, and that is the way it should be.

The claim that only Americans still object to the IAU definition is a straw man argument made by supporters of that definition in an attempt to discredit their opponents and make it appear that the IAU definition is universally accepted, which it is not.

Pluto was never improperly designated as a planet to those who adhere to the geophysical planet definition, which is what Sasselov proposed. The geophysical planet definition rejects the notion that an object has to clear its orbit to be considered a planet and that dwarf planets are not planets at all. Ironically, the person who first coined the term “dwarf planet” is none other than Alan Stern, New Horizons Principal Investigator. Within days of the controversial 2006 vote, Stern led a petition drive; his petition rejecting the IAU definition was signed by hundreds of professional astronomers. Somehow, the media ignored this and wrongly treated the IAU definition as gospel truth rather than as one view in an ongoing debate.

According to the geophysical definition, Ceres should never have been demoted either. Nineteenth-century astronomers’ telescopes were not powerful enough to resolve Ceres into a disk, so no one had any idea Ceres is spherical and therefore rounded by its own gravity. No one knew it is different from the hundreds of other objects in the asteroid belt (excluding Vesta and Pallas, which deserve an intermediate category between asteroid and dwarf planet). Today, we know that Ceres is spherical and is therefore a small planet, complete with the complex processes all planets have.

The notion that we cannot have too many planets because kids won’t be able to memorize them has no scientific merit whatsoever. Memorization is not important to learning. We don’t ask kids to memorize the names of all the rivers and mountains on Earth, just to know what makes something a river or a mountain. The only reason for the traditional memorization of planets’ names is because little was known about the planets other than their names until the start of robotic exploration of the solar system. Today, instead of adhering to an archaic teaching method like memorizing a list of names, we should be teaching kids the different subtypes of planets and their defining characteristics.

Pluto should be defined as a planet, and it is under your definition. There is nothing wrong with having multiple definitions of what a planet is, as most words in the Oxford English Dictionary do in fact have multiple defdinitions. My reasons for saying this, however, are more personal than scientific. I knew Clyde well, and I am aware of how unhappy he was towward the end of his life about the idea that his dsignature discovery wpould be taken from him.

Hildy J says:

Ultimately, a planet should have a unambiguous definition. The most obvious is mass. If you look at the mass of objects within the solar system, there is, for whatever reason, a large gap between Mercury (330.2 yg) and Ganymede (148.2 yg). FYI, Pluto is way down at 13.1 yg. Picking a round number between Mercury and Ganymede – 250 yg – and adding the caveat that a planet must orbit a star or former star seems the easiest and most logical solution.

Ron Barr says:

Why does the IAU not simply create classifications of planets? Class A for rocky bodies with atmospheres, Class B for rocky bodies without atmospheres Class C for gas giants, Class D for dwarf planets, etc.

Bion Shelden says:

I am an American educator of 11-year old children, and I have no problem with Pluto being demoted from planetary status. I do, however, think it’s not unreasonable to ask that said demotion occur via a clear and unambiguous definition, which clearly has not been done yet. And it should not be done because Pluto poses an educational problem (Mr. Kornfeld’s point above about memorizing all the rivers is a wonderful argument).

Ultimately, I suspect, we will find ourselves with a continuum upon which to tag our labels. Some bodies may actually fit in multiple catergories. This will no doubt make some of today’s elementary school teachers uncomfortable, but I see this as an opportunity to teach (slightly) higher level thinking, instead of mere memorization. We would do well (again borrowing from Mr. Kornfeld) to “teach the controversy”; nothing could be better to stir up some young minds.

Jamesg says:

Perhaps a planet should be defined by what is inside of it.
Perhaps the fact that a celestial body has enough mass to support a moon should be part of the criteria.

Dileep Sathe says:

As the definition of a planet is being debated, I would like to note that I have questioned the third condition in the definition of a planet, adopted by the IAU in 2006. (Referenc: Dileep V. Sathe / Planetary trouble / IAU C46 / NL66 / p. 19 / 28/03/07)

David Ellis says:

I want to add to the article stating that there is a clear definition of what is and isn’t a star or galaxy. You can very clearly look at a star and say it is a star. If it is not big enough to fuse hydrogen it is not a star. There is an actual difference between stars and brown dwarfs with almost no exceptions. By the IAU’s current definition, a 400 km across body with a clear neighborhood would be a planet, but a theoretical double system with 2 Earth mass planets would not.

The fact is that there is no absolute difference between dwarf planet and normal. So why are we trying to create one? The only reason for having dwarf planets is to not have too many planets. Dwarf planet literally means small planet. What is a small planet? Since Pluto once qualified as a planet I personally think it should be the limit; anything with less gravity than Pluto is a dwarf planet. Any two bodies that orbit a barycenter between the two, but both otherwise qualify as a planet, are still planets, but also binary planets. The Pluto-Charon system would be considered a sub-binary system due to Charon not qualifying as a planet if by itself.

The Pluto gravity limit is obviously an opinion and may be raised or lowered. It should be defined by gravity and not size due to objects like Eris being more “planet-like” than Pluto, but smaller in size. This would bring the rules for being a planet to the following:

1. Not a star.
2. Have a Barycenter within or in-between the object and any orbiting satellites.
3. Have gravity equal to or greater than the planet Pluto.

Dwarf planets would:

1. Fail only rule 3 of the Planetary Definition.
2. Have enough gravity to obtain hydrostatic equilibrium.

If an object fails rule 2 it is a moon, and rule 1 is self explanatory.

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