After a vote by the International Astronomical Union (IAU) in August 2006, Pluto was relegated from major planet to dwarf planet status. Our solar system dropped from nine planets to eight. At the time, many scientists were unhappy with the decision.
Two decades on, the debate is, if anything, even more polarised. For some, Pluto’s downgraded status is the source of their affection for the ex-planet. For others, the IAU’s decision, and the way it was taken, is central to the debate.
The event that sparked the IAU’s 2006 vote was the discovery of a distant celestial body that would come to be called Eris. Eris was out in the Kuiper Belt, beyond Neptune – where Pluto also resides. It was roughly the same size as Pluto (less volume, but more mass).
If Pluto was a planet, the argument went, then so was Eris. And if Eris wasn’t a planet, how could Pluto continue to be one? Eris and Pluto were by no means alone in the Kuiper Belt.
Other smaller bodies were discovered around the same time, including Haumea, Makemake and Sedna. There were likely many more similar objects still to be found. How would schoolchildren manage a planetary roster that might number in the hundreds?
In search of a solution, the IAU came up with a three-point definition of a planet: it must orbit the Sun; it must have sufficient mass to pull itself into a round shape; and it must have “cleared its neighbourhood” – meaning it must gravitationally dominate the area in which it orbits. Nothing beyond Neptune that we knew about qualified; Pluto was out. It was the last point in particular that upset a lot of Pluto advocates, because a body’s ability to clear its neighbourhood depends a lot on the neighbourhood.
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