For some of us, the shock has still never quite settled in. That distant ball of ice Pluto is still tightly woven in our minds as a true planet and even has its very own place within our planetary moniker: "My Very Excellent Mother Just Sent Us Nine Pizzas". But back in 2006, the International Astronomical Union (IAU) stripped Pluto of its planetary status declaring the relatively small compact of ice much closer to a satellite in its new second-tier title as "dwarf planet".
Just like that, in the outcome of a decision that seemingly appeared out of the blue, Pluto was no longer on our shortlist for space exploration and it was out of the planetary club. But some still did not accept the decision as final: if Pluto could be taken off of the list, then substantial evidence to support its case could certainly put it back on. And the fight towards reinstatement began. Even Harvard University's Smithsonian Center for Astrophysics made a point for Pluto's planetary status saying in a press release "a dwarf fruit tree is still a small fruit tree, and a dwarf hamster is still a small hamster."
So to get the conversation started, the Harvard-Smithsonian Center recently held a debate, and the audience overwhelmingly agreed: "Pluto IS a planet".
Discussing the three factors that the IAU demand's for a planet to be admitted to the list, the debate primarily focused on the topic for why Pluto was originally removed from the list in that it was unable to prove that it was the gravitationally dominant body in its area by creating its own polarizing force.
Pluto is small, with a radius of 750 miles dwarfed considerably by that of Earth which is greater than five times that. And with a circumference of only about 4,500 miles, the small sphere of ice is even smaller than our moon. And while we know that our moon has some gravitational force, that causes our shifts in the tides, for Pluto's parts of space that is far from being the most powerful gravitational body on the block.
The debate was argued by three familiar faces: Owen Gingerich (who chaired the IAU Planet Definition Committee) defined Pluto from a historical viewpoint, Gareth Williams (Associate Director of the Minor Planet Center) argued Pluto was not a planet, and Dimitar Sasselov (Director of the Harvard Origins of Life Initiative) countered the arguments by supporting Pluto's planetary status from the exoplanetary scientist's perspective.
Firstly Sasselov tackled the underlying criteria for a planet to be on the list pointing out that the requirement that it "orbit around the sun" and not just any star completely excluded all exoplanets existing just beyond our solar system. And once the "sun-centric" viewpoint was called into question, he instead offered a new definition for Pluto and all planets: "A planet is the smallest spherical lump of matter that formed around stars or stellar remnants".
And with that, the public opinion shifted. The debate was over, and Sasselov and Pluto had won. Though while the outcome may not have changed the binding word of the IAU just yet, the victory in changing the public opinion is the first of many wins in getting Pluto its planetary status back.
