Sorry, Pluto’s still not a planet

Pluto still isn't a planet—unless Earth's moon is, too

On the 20th anniversary of Pluto's infamous demotion, Mike "Pluto Killer" Brown explains why any reversal of that decision would be driven more by nostalgia than science

Twenty years ago Pluto lost its planethood, and I'm partially to blame.

"Blame" is a strange word to use for what seems like a matter of mere semantics; after all, Pluto exists as an intriguing world worthy of study and exploration, regardless of labels . But after two decades in the rearview, Pluto's demotion still stirs anger and controversy, and many people regard its change in status as a very bad thing.

To everyone upset about this: I'm truly sorry, but Pluto had it coming.

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To understand why, do one thing: gaze up at the moon during a long summer twilight, note the dark patterns of ancient volcanic flows mottling its surface, and ask yourself a simple, sincere question: Is the moon a planet? I think that you, like me, will agree that the obvious answer is no. And that means that you, like me, think that Pluto isn't a planet either.

You might object that your opinion of the moon has no effect on your feelings for Pluto, but equating these two objects is unavoidable for anyone seeking to manufacture a philosophically consistent reason for Pluto's planethood. The main argument, these days, for why Pluto's status should be restored boils down to an astrophysical version of what's best known as "the duck test": if something looks, walks and quacks like a duck, it's probably a duck. In Pluto's case, the idea is that "geophysically" it's a planet, which simply means it looks like a planet (it's round!) and acts like a planet (it probably once had active volcanoes, albeit of water!). Earth's moon—which also has the advantage of being 50 percent larger than Pluto and of having volcanoes that once spewed actual lava instead of mere water—fully deserves planethood by these very same metrics.

Yet, according to the current formal definition from the International Astronomical Union, which was so controversially set on August 24, 2006 , Pluto's "problem" is its failure to "clear the neighborhood around its orbit" rather than anything intrinsic to the world itself. This phrase is a complicated way of simply acknowledging the fact that Pluto sits within the Kuiper Belt , a vast swarm of bodies that go around the sun in similar orbits beyond Neptune. Astronomers call this way of looking at a population a "dynamical" classification. What matters is not just the body itself but also its relationship to the solar system around it. Simply remove the dynamical classification part of the definition, critics say, and Pluto reverts to its former lofty status!

And they're entirely correct. The trouble is that Pluto would not be alone. The reason the moon doesn't qualify as a planet is dynamics—where and how the body orbits. When you acknowledge that you don't think the moon is a planet, you are acknowledging that dynamics actually matter, too. And it's not just Earth's moon that would suddenly join the elite "planetary" club. There are many round, massive, geologically interesting objects in our solar system that, viewed in isolation, look and act somewhat like planets. By one count, if we removed the dynamical part of the classification, we would make planets out of one asteroid, four moons of Jupiter, seven of Saturn, five of Uranus, one of Neptune, and then maybe 10 (or maybe 200!) objects in the Kuiper Belt, where Pluto also lies.

I arguably have more to gain by this looser, more flexible accounting than anyone: I discovered seven of those most distant bodies, so I would instantly become the most prolific planet discoverer in solar system history. This could be nice, I guess, but instead I'd feel somewhat embarrassed. Every time I looked up and saw the moon in the sky, I would be reminded that my newfound historical stature would require me to accepti something that seemed obviously wrong: if I had discovered multiple planets, then our moon definitely would be a planet—which it is not. Surely you'd agree.

I can already anticipate the vociferous boos from Pluto's proplanetary contingent. But what is the true nature of our disagreement? Why would any scientist—or anyone, really—support a definition of the word "planet" that goes against 500 years of common usage and that seems obviously wrong to most people?

Yes, it could be a strongly held belief that the geophysical definition of what makes a planet is of great importance. But, if true, these strongly held beliefs are suspiciously recently formed. No hint of the importance of this definition was even mentioned prior to the discussions that led to Pluto's demotion, though the evidence of the geophysical nature of the moon has been evident to our eyes for centuries. Whether discussing the past of NASA's moonwalking Apollo astronauts or the future of the crewed landings planned for the agency's Artemis program , no one talks about these missions as voyages to another planet.

Similarly, we have sent spacecraft to visit every single one of the moons of the other planets that are on the upgrade list; never, in the previous decades of those visits, did anyone suggest that we were instead visiting planets. We found active volcanoes on Jupiter's moon Io , vast lakes of liquid methane and ethane on Saturn's moon Titan , geysers and a nitrogen atmosphere on Neptune's moon Triton . But as fascinating as each of these bodies was found to be, no one felt that they suddenly deserved planethood.

The real reason for the attempt to promote all of these new bodies, it seems evident, is nostalgia—not just for Pluto but also for a time when our conceptions of the solar system and the universe at lar...