In its efforts to comprehend our universe, science must often resort to categorizing and naming things which were once nameless and unclassified. Once Pluto had been discovered in February of 1930, it took until May to decide upon its name. This frozen orb had circled the sun undetected for billions of years without any sort of name or designation. Even for a few months after it was found, it managed to continue on that orbit without a name. And now, suddenly it was Pluto.
Is this because the actual identity of this object was in fact Pluto all along? Had astronomers discovered an essence of Pluto-ness to the thing that made it necessary to call it Pluto? Or did they just pull that name out of their proverbial hat? The true story is that the name was suggested by an eleven year old school girl named Venetia Burney. Clyde Tombaugh - the discoverer - and his colleagues at Lowell Observatory liked her idea, and voted to keep it. In other words, the name was simply made up.
Things that have names are easier to point to and talk about. That's why scientists find themselves continuously making up names for things. In some fields of study, it has been traditional to apply Latin and Greek labels. They sound important, but they are just labels. They are not the thing itself.
So Pluto had a name now, but what the hell
was it? This question cannot be answered without resorting to some sort of classification scheme. If something fits into some sort of pigeon-hole, if we have a file folder to drop it into, then we can have a general sense of what sort of thing it is. At first, Pluto was considered to be a planet. For decades, children were taught in grade school that the solar system has nine planets. In 2006, astronomers voted to make an adjustment to the classification scheme. They redefined the word "planet." The new definition changed Pluto's designation from planet to dwarf planet. And now children are taught that we have eight planets. Yet, obviously the number of major celestial entities out in our solar system has not changed. Pluto is still the same size as it was before its demotion. It's just that some astronomers re-arranged their file cabinet, and the concept of Pluto (not Pluto itself!) wound up in a different folder.
Many people felt betrayed, as though they had been lied to in science class all those years. Even some astronomers continue to dissent against the International Astronomical Union's 2006 definition of planet, and re-categorization of Pluto. Because of his high profile as a popularizer of science, astrophysicist Neil deGrasse Tyson felt that he was taking more than his share of the heat for the controversial Pluto downgrade. So in 2009 he published an informative and amusing book on the subject, which I can highly recommend, called
The Pluto Files.
Tyson tells the story behind the IAU decisions which took such a toll on Pluto's status. But knowing the background is small comfort for those who cherish the traditional nine planet system that they had to memorize in primary school. And perhaps there is no harm in allowing a few die-hards to run around calling Pluto a planet. After all, controversy is the best publicity. Any excuse to discuss science, astronomy, and Pluto in particular is a good thing. However, for curious persons who don't mind tossing tradition aside when updated information comes along, the details about Pluto's changed designation provide some interesting insights about the solar system as a whole.
Starting in 1992, several new planet-like objects were found beyond Pluto's orbit. They were designated as Trans-Neptunian Objects, or TNOs. They all inhabit a region of scattered rocks and ice and dust known as the Kuiper Belt. One of these objects, Eris, discovered in 2005, is actually more maassive than Pluto. This is what initially caused scientists to question the appropriateness of the traditional nine planet scheme. Meanwhile, new theories were brewing to explain the origin of TNOs within the context of the whole solar system's history. Pluto's highly eccentric orbit and proximity to the Kuiper Belt made it evident that Pluto had much more in common with a TNO than it did with any of the inner eight planets. This suggested a common ancestry for Pluto and the TNOs. For many scientists, this seemed like a good opportunity to make some revisions to the nomenclature, to better reflect the presumed origins and evolution of the various entities which compose the solar system.
At a meeting in Prague, Czech Republic, on August 24th, 2006, the IAU passed a resolution to approve an official definition of the term "planet," to be applied only to objects in our solar system. (Other stars were known to possess planets as well. But so little was known about these systems that it was deemed best not to extend the new definition to fit such speculative situations.) Here is the definition that the IAU came up with:
To be considered a planet of the solar system, an object must meet all of these three criteria:
1. It must be in orbit around the Sun.
2. It must be massive enough to be rounded by its own gravity (technically this is called hydrostatic equilibrium).
3. It must have cleared the neighborhood around its orbit.
Because it resides within the dusty Kuiper Belt, Pluto does not meet the third requirement. At this same meeting, the term "dwarf planet" was invoked to characterize Pluto and the other largest Trans-Neptunian Objects within the Kuiper Belt. The scientific urge to sort phenomena into categories was now satisfied on two levels - Pluto's physical traits, and its likely historical origin, were both filed away tidily in the new pigeonhole. And millions of science textbooks were suddenly out of date.
Then, in July of 2015, a spacecraft called New Horizons whipped past Pluto and its five moons, and sent home data and pictures of the dwarf planet. A valentine-shaped region of ice seemed to be sending a love note to us here on Earth. And for the general public the distant entity began to take on more of a personality. Demands to reinstate Pluto's planethood began to resonate in social media comment sections. But scientists have been seeing confirmation that Pluto does indeed belong with the population of Kuiper TNOs. For example, its composition closely resembles that of comets which originate from the Kuiper Belt. And the way the solar wind erodes its surface and blasts the resulting particles into space is also quite comet-like. It's not about how much personality Pluto has. It's about a specific set of criteria which classify it in accordance with the origin and evolution of everything else in its region. So the official word from astronomers is that Pluto is still not a planet.
Personally, I have come to terms with this. My childhood bedroom featured a solar system poster on the wall, with a vague and greenish Pluto taking its proper place as a full-blown planet after Neptune. And I still have my second grade diagram of the nine planets that I drew. The nostalgia for the old-school outlook is still strong within me. But it just makes more sense to lump Pluto in with its similar TNO companions, which are becoming more numerous and well-studied over time.
I just have two objections, concerning the third requirement - sweeping the neighborhood of debris. Firstly, the size of this "neighborhood" is not quantified. Just exactly how far does an orbital neighborhood extend? Do smaller planets have to clear a path as wide as the more massive planets do? And secondly, just how thorough does this cleansing have to be? Watch the night sky for an hour or so, and you will witness several pieces of space rock streaking across the sky as meteors. These are objects that the planet Earth had not cleared out of its path until just that moment. Obviously there are many more where they came from. And Jupiter has two entire populations of asteroids, one preceding it, and one following it, called the Trojans, which will never be cleared out because they are in orbital resonance with Jupiter. So does that mean Jupiter and Earth are not planets? For all we currently know, Pluto may have done a better job of clearing clutter from its orbit than Jupiter or Earth. It's not like we've been able to survey the entirety of Pluto's orbital neighborhood for tiny bits of rock. So as it stands, if the IAU does not bother to clarify these two points any time soon, I think there is still some wiggle room for people to call Pluto a planet if they want to.
