Is Pluto a planet? That’s asking the wrong question

Oh damn, This Again?

Last week, NASA Administrator Jared Isaacman appeared before the U.S. Senate Appropriations Committee to answer questions about the space agency. When Sen. Jerry Moran of Kansas asked Isaacman about Pluto, the administrator responded, “I’m very much in the ‘make Pluto a planet again’ camp.” And I would say that we are currently writing papers on, I think, a position that we would like to bring to the scientific community to revisit this discussion and ensure that Clyde Tombaugh gets the credit that he once received and rightly deserves to receive again.

It’s no coincidence that Tombaugh, who discovered Pluto in 1930, was from Kansas, so Isaacman’s response to a senator from that state isn’t exactly unexpected. Additionally, because Pluto was discovered by an American, some national pride is also at stake.

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

Additionally, the IAU rules regarding planetary status are dubious to say the least.

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

(A) is in orbit around the Sun,
(b) has sufficient mass that its self-gravity can overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) cleared the neighborhood around its orbit.

The first part is pretty obvious and clears up any confusion regarding a planet-sized moon orbiting a giant planet (such as Saturn’s Titan and Jupiter’s Ganymede).

The second rule is problematic for me because achieving plumpness depends on body composition. An object made of water ice will deform more easily than an object made of iron, for example, and for the two to be round, their sizes will be very different. But whatever, GOODbecause it’s everyone’s third Really hated.

This last rule almost makes sense; the idea is that a planet gravitationally dominates its volume of space and any smaller object near its orbit will either be swept away or ejected. But this rule is extremely vague. There are many asteroids whose orbits are similar to Earth’s, which means we could say our world hasn’t exactly “cleared its orbit” – and yet we still call Earth a planet. There are physical ways to better define this idea, but the official rule doesn’t do it.

If you still think this all makes sense, I will note that the “planet” Mercury is not in hydrostatic equilibrium. Worse still for poor Mercury, whom I choose only to prove something, in an article published in the April 2026 issue of American Astronomical Society Research Notestwo astronomers discovered another problem with its “orbit clearance” bona fides. They were studying how the sun can affect small pieces of space debris, causing them to first fragment (via the wonderful YORP effect) and then evaporate.

The researchers calculated how long this 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 the form Seven millions of years to eliminate this debris by gravitation. This means the sun is responsible for clearing Mercury’s orbit, rather than the planet (for now) itself. Since the rule explicitly states a planet “has After cleaning up the neighborhood” (emphasis mine), Mercury’s status as a planet could officially be in doubt.

I am not advocating ignominiously throwing Mercury out of the planetary club! Instead, I point out how stupid it is to have a club in the first place. And if you think the analogy is a stretch, know that in a footnote to the rules, the IAU specifically listed the member objects in its planetary parade and does not include Pluto:

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

This violates the basic principle of even trying to find 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 seems to me perhaps that I am defending the other side, namely 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 those calling for Pluto to be reinstated. In fact, in 2017, a group of astronomers proposed a different planetary definition that would be include Pluto – however, their definition would also include over a dozen outer planet moons, which seems a little too welcoming. I suspect that this rule may have been premeditated to include Pluto, which is just as bad as apparently devising a rule to exclude it.

Actually, I don’t agree 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 about what constitutes a “planet” in the first place.

Rules serve as definitions, clear dividing lines that help you sort objects into different taxonomic categories. But in nature, sooner or later, this always flies in the face of reality, because the more closely we look at different things, the less clear-cut these distinctions become. However you define planetary status, it is not difficult to come up with extreme cases that violate its principles and all common sense. There are objects that technically meet the definition of “planet”, even though everyone would agree that they are not planetary at all; there are others that the rules would exclude but which clearly should be a planet.

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

I can think of better uses of the NASA administrator’s time than the machinations of performative nomenclature; The space agency’s budget is once again threatened with sharp cuts, this time by 23 percent in total and 47 percent fatally injured for scientific research, which could lead to the cancellation of more than 50 scientific missions. Worrying about definitions isn’t so much counting boogying angels as watching the band play on the shipwreck deck. Titanic and asking if their instruments are tuned correctly.

We shouldn’t waste time wondering what to call Pluto. We should generously fund scientists to study Pluto, its siblings, and the rest of the universe.