Sorry, Pluto’s still not a planet | Scientific American
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August 24, 2026<br>6 min read<br>Add Us On GoogleAdd SciAm<br>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
By Mike Brown
Pluto’s thin, hazy atmosphere shows its blue color in this view from NASA’s New Horizons probe, captured as the spacecraft looked back at the world after its close encounter on July 14, 2015.
NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Twenty years ago Pluto lost its planethood, and I’m partially to blame.<br>“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.<br>To everyone upset about this: I’m truly sorry, but Pluto had it coming.<br>On supporting science journalism<br>If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.<br>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.<br>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.<br>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!<br>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.<br>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,...