Once there were nine planets in our solar system. Then there were eight. And now, if a proposal by a bunch of NASA scientists were to be approved, there would be at least 110 planets around the sun. Try coming up with a mnemonic device for that.

The big name backing the proposal is Alan Stern, former NASA associate administrator and principal investigator on the New Horizons mission to Pluto. Yeah, it's about Pluto again. Eleven years have now passed since the International Astronomical Union declared there to be eight proper planets and "demoted" Pluto to dwarf planet status alongside the big asteroid Ceres and Pluto's neighbors out in the Kuiper Belt. Stern and company still think the decision is "bullshit."

This proposal to the IAU suggests that the term planet should apply basically to anything that's big and round and isn't a star. Or, to put it more technically, they would define a planet like this:

A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters.

You can see where they're coming from here. Pluto angst aside, there are some big problems with the 2006 definition of planethood, such as the requirement that a planet has "cleared the neighborhood" around its orbit. For one thing, no planet in our solar system has accomplished this entirely. For another, clearing the area requires a bigger and bigger planet the further you get from the sun, so, as ScienceAlert points out, even a world the size of Earth wouldn't get to be a planet if it were as far away as Pluto.

Or course, Stern's new definition isn't great, either. By eliminating requirements such as clearing the neighborhood and orbiting the sun, it could mean that the moon, and the big, round moons around the other traditional planets, are now "planets" in their own right.

It's easy to feel weary of all this semantic warfare. As Stern's own New Horizons mission revealed, Pluto is not some cold dead space rock but a fascinating, wonderful place worth studying whether you call it "planet" or not. Same goes for the mystical watery moons of Jupiter and Saturn that would attain planethood under the looser definition. Who cares about classification schemes when there's an ocean on Europa?

Stern's team does, and not just because they're Pluto fans. They open their proposal by writing:

In the mind of the public, the word 'planet' carries a significance lacking in other words used to describe planetary bodies. ... To wit: a common question we receive is, 'Why did you send New Horizons to Pluto if it's not a planet anymore?'

We certainly hope it's not true that people can't care about anything not called "planet." It's a big damn universe out there.

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