Hey, remember Pluto? Of course you do! When most of us were brought up in school we learned that there were nine known planets in the Solar System, and Pluto was the one that hung out on the outskirts. Then, in 2006, everything changed and astronomers from around the world declared the Pluto didn’t meet the criteria for being called a planet.
The issue at hand was Pluto’s mass, which just wasn’t high enough to give it what astronomers claimed was necessary for all true planets: a clean orbital path around its host star. Pluto had everything except for this “clear neighborhood” requirement, since debris from the nearby Kuiper belt spilled over into Pluto’s own orbit and the much larger Neptune occasionally tugged on Pluto.
Now, over a decade after that messy list of criteria was cemented, planetary scientist Philip Metzger of the University of Central Florida in Orlando says astronomers should seriously rethink their decision to snub Pluto.
“The IAU definition would say that the fundamental object of planetary science, the planet, is supposed to be a defined on the basis of a concept that nobody uses in their research,” Metzger said in a statement. “And it would leave out the second-most complex, interesting planet in our solar system.”
Metzger’s argument isn’t that Pluto meets the stated requirements for being considered a planet — everyone agrees that Pluto doesn’t fit the description set forth by the International Astronomical Union — but rather that the list of criteria is just plain broken.
“We now have a list of well over 100 recent examples of planetary scientists using the word planet in a way that violates the IAU definition, but they are doing it because it’s functionally useful. It’s a sloppy definition,” he says.
His stance is that the one sticking point for those who wanted to strip Pluto of its planetary status — the “clear” orbit requirement — isn’t useful in determining status at all. Instead, Metzger says, the real defining feature of a planet should be whether or not it is massive enough, and creates enough gravitational force, that it becomes spherical.
“It turns out this is an important milestone in the evolution of a planetary body, because apparently when it happens, it initiates active geology in the body,” he explains.
Whether or not the IAU will take this new argument into account is anyone’s guess. Pluto is presently classified as a “dwarf planet,” but Metzger’s reasoning regarding the seemingly arbitrary definition of a planet seems pretty solid. Perhaps Pluto will once again become the ninth planet in the Solar System.
