Scientists Renew Debate over Pluto's Planet Status

Mar 1, 2017

A decade ago Flagstaff suffered a blow when Pluto was “demoted” to a dwarf planet. It was discovered at Lowell Observatory in Flagstaff in 1930 and is a source of pride for the city. But astronomers voted on the definition of a planet and Pluto didn’t make the cut. Now, planetary scientists say that definition is both overly complicated and incomplete.  They’ve suggested a different one. Melissa Sevigny from the Arizona Science Desk spoke with two Lowell scientists about the definition, Will Grundy and Gerard van Belle.   

Lowell Observatory
Credit Molly Baker


Gerard, you were actually present in 2006 when the International Astronomical Union famously demoted Pluto and changed the definition of a planet. Tell me about that.

Gerard van Belle: That was a very interesting meeting. We first showed up and they were circulating a newspaper—there’s actually an internal newspaper, it’s a big enough meeting—talking about the proposed definition at the time …. By the end of the meeting, a little footnote had been attached to it, which was a secondary part of the definition, which is in colloquial terms: big enough to a bully. When the planet is moving around its orbit, does it push everything else out of the way, does it “clear its zone”? And that is what was adopted and that’s what “demoted” Pluto.

Can I ask, how did you cast your vote?

Gerard van Belle: I voted against this definition because it’s just pretty much silly. If you take Earth and move it to the Kuiper Belt, it ceases to be a planet because it won’t have time to clear its orbit.

Will Grundy, you’re part of a group that has proposed a new definition for planets.

Will Grundy: Well, it’s not actually a new definition, it’s really more of a documentation of the usage that planetary scientists have been, by and large, using all along. And that’s just simply an object that’s large enough to be round, but not large enough to be a star…. So all of the giant planets have satellites big enough to be round, and then there’s quite a few bodies in the Kuiper Belt large enough to be spherical, there’s maybe a couple in the asteroid belt, and of course our own Moon. In total we count up a little over 100 objects that would probably qualify, and there’s probably more to be discovered.      

OK, so there’s no such thing as too many.

Gerard van Belle: No, more the merrier. … And we don’t get worked up about students have to encounter more than 100 elements on the periodic chart or 50 states in the United States. It’s just how the definition fits and they cope perfectly well with these things.

Will Grundy: If you can find me a six-year old that complains there’s too many dinosaurs…..

Gerard van Belle: My kids have practically memorized 715 Pokémon, so they can do these things, you know!   

So this definition is simple enough for an elementary school student to understand. Why does that matter?

Will Grundy: Little kids have an innocence and an open mindedness that makes them a really useful test for a concept. If you can’t explain a concept to a kid, you probably don’t understand it well enough yourself.

Gerard van Belle: This is one of those things that makes it into a fairly simple thing to describe, and it makes sense to a four year old, and if the definition doesn’t make sense to somebody at that age than maybe it’s a little too complicated.

Why do you think this debate matters so much?

Gerard van Belle: Let me put a fine point on it: if the “demotion” of Pluto had happened 10 years earlier, prior to the Congressional vote on whether or not to fund the mission New Horizons to go there, that vote might have gone a very different way, if the members of Congress had not been thinking of sending something to a planet. So there are very real consequences about these sorts of things.

The new definition will be presented at the Lunar and Planetary Science conference this month.