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"Planet" Is What You Say It Is
Simply Use Pluto as a Minimum
Wednesday, June 12, 2019 12:07 pm
Regarding the demotion of Pluto as a planet, an astronomer once tweeted:
So, hey, Pluto is still not a planet. Actually, never was. We just misunderstood it for 50 years. Now, we know better. Nostalgia for Pluto is not a very good planet argument, but that's basically all there is. Now, let's get on with reality.
I'm sure this guy thought he pwned the folks defying Pluto's demotion. There's a nastiness in his tweet, isn't there? Not surprising, considering his handle is @plutokiller.

He's wrong, in any event.

"Planet" is not a designation like "baryon" or "lepton." There is nothing in the design of the universe that relies on some profound distinction between, say, a "planet" and a "dwarf planet." Sorry, but Pluto was a planet for 50 years. We said it was. We did not learn anything new about Pluto that would make it not a planet.

What we did learn is that there's a lot of Pluto-like things out there, such as Eris and Sedna. So is Eris a planet? Is Sedna? If they are, we might end up with hundreds of "planets." Where would it end? Heck, would Ceres get promoted? How about Vesta?

"Planet" would become a useless designation.

So step back for a moment. What has been the historical definition of a planet? "Planet" initially encompassed the objects known as Mercury, Venus, Mars, Jupiter, and Saturn. This, then, is our foundation. What is common about these objects? Think a while and the definition arises:

A planet is not the Sun nor a star. That is, it does not engage in the fusion of elements, nor does it constitute the late-stage remnants of a fusing body.

A planet orbits a star. More precisely, its primary orbit has a star as a focus. Even more precisely: Once one has identified a solar system — a collection of objects gravitationally bound, as a unit, to one or more stars — a planet is an object that orbits not a fellow object but one of the stars.

A planet has sufficient gravity to maintain itself as a sphere. Its mass is essentially in a spherical equilibrium, if you will.

And finally, to keep our definition from including every beachball in a system: A planet is no smaller than Pluto.

Ta da.

You don't have to bother with any nonsense about an object clearing debris from its orbit. You don't have to worry about an absurd proliferation of "planets." This definition satisfies that sense we all have of "I know a planet when I see one."

Yes, Pluto is an edge case. Yes, it seems like it's just an upstart Kuiper-Belt object. But come on — Pluto has a substantial moon, its orbit periodically brings it closer to the Sun than Neptune, and it stood out enough among the objects of our system to be noticed without fancy space-borne telescopes.

Besides, there is a lot to be said for nostalgia. Nostalgia is also respect for continuity. Science doesn't have to be constant — and snarky — upheaval.

P.S. "Dwarf planet" is a stupid term. So is it a planet or not? Is a "giant planet" not a planet? For planet-like objects smaller than Pluto, there should be a distinctive noun, akin to "planetoid."
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