Pluto and the folly of planethood

The problem with Pluto isn’t its planetary or nonplanetary status—it’s our insistence on declaring the world must be one or the other

By Phil Plait edited by Lee Billings

Oh geez, this again?

Last week NASA Administrator Jared Isaacman appeared before the U.S. Senate Appropriations Committee to answer questions about the space agency. When Senator Jerry Moran of Kansas asked Isaacman about Pluto, the administrator replied: “I am very much in the camp of ‘make Pluto a planet again.’ And I would say we are doing some papers right now on, I think, a position that we would love to escalate through the scientific community to revisit this discussion and ensure that Clyde Tombaugh gets the credit he received once and rightfully deserves to receive again.”

Not so coincidentally, Tombaugh, who discovered Pluto in 1930, was from Kansas, so Isaacman’s answer to a senator from that state isn’t terribly unexpected. Also, because Pluto was discovered by an American, there is some national pride at play, as well.

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But it’s not up to NASA to classify Pluto as anything. That responsibility lies with the International Astronomical Union (or IAU), which famously demoted Pluto to the status of “dwarf planet” in a vote held in 2006. That event was contentious; of the roughly 9,000 IAU members at the time, only a few hundred were there for the vote, and only a very few of those present were planetary scientists.

Moreover, the IAU’s rules for planetary status are dubious, to say the least.

According to the IAU, a planet is a celestial body that:

The second rule I have issues with because achieving roundness depends on what the body is made of. Something composed of water ice will deform more easily than an object made of iron, for example, and for both to be round, their sizes will be wildly different. But whatever, fine, because it’s the third one everyone really hates.

This final rule almost makes sense; the idea is that a planet dominates its volume of space gravitationally, and any smaller objects in its orbital vicinity will either get swept up or ejected. But this rule is extremely vague. There are lots of asteroids with similar orbits to Earth, meaning we could say our world hasn’t exactly “cleared its orbit”—yet we still call Earth a planet. There are physical ways to better define this idea, but the official rule doesn’t do so.

If you still think this all makes sense, I’ll note that “planet” Mercury is not in hydrostatic equilibrium. Worse yet for poor Mercury, which I’m picking on only to prove a point, in a paper published in the April 2026 issue of Research Notes of the American Astronomical Society, a pair of astronomers found another problem with its “orbit-clearing” bona fides. They were studying how the sun can affect small pieces of space debris, causing them first to fragment (via the wonderfully named YORP effect) and then to evaporate.

The researchers calculated how long such solar destruction of debris in Mercury’s orbit takes and arrived at a timescale of about four million years. Mercury, on the other hand, takes more like seven million years to gravitationally clear away that debris. This means the sun is responsible for clearing Mercury’s orbit, rather than the planet (for now) itself. Given that the rule explicitly states a planet “has cleared the neighbourhood” (emphasis mine), Mercury’s status as a planet could officially be in doubt.

I am not advocating ignominiously tossing Mercury out of the planet club! Instead I’m pointing out how silly it is to have a club in the first place. And if you think that analogy is a stretch, mind you that in a footnote to the rules, the IAU specifically listed the member objects in its planetary parade and didn’t include Pluto:

“The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.”

This violates the basic tenet of even trying to come up with a definition in the first place! Why try to define something when you’re just going to tell everyone what’s on the list by fiat anyway?

It may seem I’m arguing the other side, that we should call Pluto a planet, but be careful! Just because I don’t like the IAU rules doesn’t mean I agree with the side clamoring for Pluto’s reinstatement. In fact, in 2017 a group of astronomers proposed a different planetary definition that would include Pluto—however, their definition would also include more than a dozen moons of the outer planets as well, which seems a bit too welcoming. I suspect this rule may have been premeditated to include Pluto, which is just as bad as seemingly designing a rule to exclude it.

In fact, I disagree with both sides. I don’t think we should make rules that specifically carve out an exception for Pluto—just as we shouldn’t rig the rules to include it. I don’t think we should have rules for what makes a “planet” in the first place.

Rules are for definitions, sharp dividing lines that help you sort objects into different taxonomic bins. But in nature, sooner or later, this always runs afoul of reality because the closer you look at different things, the less sharp such distinctions become. No matter how you define planetary status, it’s not difficult to come up with edge cases that violate its tenets and all common sense. There are objects that technically meet the “planet” definition, even though everyone would agree they’re not planetary at all; there are others that the rules would exclude but that clearly should be a planet.

A “planet” is a concept. It’s like colors or continents—it’s a category with very fuzzy borders, and no matter how fine a razor we use to divide up those categories, the borders will stubbornly remain impossible to define. Nature is very clear that this is how things work: objects exist along a spectrum, and differences are only clear if you look at two spots far enough apart on that range. Pretending otherwise is like arguing over how many angels can dance on the head of a pin.

I can think of better uses for the NASA administrator’s time than performative nomenclature machinations; the space agency’s budget is once again under threat of vicious slashes, this time by 23 percent overall and a mortally wounding 47 percent to science research, which could result in the cancellation of more than 50 science missions. Fretting over definitions is not so much counting boogying angels as it is watching the band playing on the deck of the sinking Titanic and asking if their instruments are properly tuned.

We shouldn’t be wasting time arguing over what to call Pluto. We should be generously funding scientists to study Pluto, its siblings and the rest of the universe.