The UniverseFridays

May 8, 2026

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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

Tinged blue by a high-altitude haze, Pluto appears as a receding crescent in this approximately true color composite image captured by NASA’s New Horizons probe on July 14, 2015. Mountains and other topographic features of Pluto’s surface are silhouetted against the haze.

NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

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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:

(a) is in orbit around the Sun,

(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and

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

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