It's been nearly two decades since astronomers voted on the definition of a planet and demoted Pluto. Some scientists are still unhappy with the decision, and recently a group in California proposed a new definition.
KNAU’s Melissa Sevigny spoke with Gerard van Belle of Lowell Observatory about how the proposal falls short for Pluto fans but is reigniting the debate.
Let’s start with a little backstory. In 2006 astronomers got together at a meeting and voted on the definition of a planet. And you were there at that meeting, right?
I was there at Prague in 2006… and that was very exciting.
And famously that is the moment when Pluto was demoted from planetary status.
It’s true, yes, we came into the meeting, and there was actually a proposal on the table that would have included Pluto and Ceres and Eris and Makemake and all these other things as planets, and kind of a new rogue definition was introduced at the last minute. That effectively tossed out Pluto and ostracized these other bodies as well.
Since 2006 there have been a lot of, let’s say, complaints in the planetary science community about the accepted, current definition of a planet. Tell me about some of the complaints with that definition.
Well, the complaints start off with it just being completely unnecessary. We don’t have definitions for the word “star” or “galaxy” or this sort of thing. And so why do we need it for planet? We should just leave it to discussion and consensus in the scientific community. There are other reasons why it’s just a really, really stupid definition. For example, it stipulates that the body has to orbit the sun. And in astronomy, we’ve actually discovered more than 5,000 exoplanets around other stars basically in the intervening time…. Having just 8 planets in the universe seems overly protective of this word.
Just recently some scientists at the University of California-Los Angeles have introduced a new definition for discussion. I think the idea is they want to bring it to a vote…. First of all, they’re saying that a planet can orbit a star other than the sun, so this is a good expansion.
Yes.
But also they’re giving it mass limits, so it has to be a certain size, this big, but no bigger.
Yeah, so these mass limits, they come from a good place. But my argument would be: just start at that good place, just say “big enough to be a ball,” and you’re done. Rather than saying, well, if it’s a big enough to be ball, and then 100 times bigger than that because of this clustering analysis that we did—that’s essentially what’s going on here, and it’s not something I could explain to a kindergartner.
And for those of us in Flagstaff, what we care about—Pluto still would not be reinstated.
Pluto would be not be reinstated under this suggested definition from the California group…. The actual answer in the end is that Pluto doesn’t care what we call it. This is only a human construction—the word “planet” or “star” or any these other words—to help us have a mental shorthand to think about what it is we’re looking at. It’s basically because the universe is big and wild and wooly out there, and we need to put things in boxes to help us figure it out, and sometimes those boxes get in the way of actually seeing what’s there.
If this definition came up for a vote tomorrow, would you be yes or no?
That’s a tough one. It’s the classic conundrum of politics, and astro-politics is no different, which is: do you vote to replace something really, really stupid with something less stupid? I would have to think about that.
I won’t put you on the spot and make you give an answer. Gerard van Belle, thank you so much for speaking with me.
Thanks, Melissa, it’s always fun.
