Is Pluto a planet? The experts break it down. – Astronomy Magazine

A NASA image of Pluto shows its "heart-shaped" portion at bottom right. Credit: NASA.

Astronomy.com: We were going to write something new about Pluto here. But Astronomy Magazine editor Dave Eicher has already done the heavy lifting. Before we get to the experts heres how Dave described the discovery and death of Pluto as a planet in a December 2023 column:

In 1930 a young astronomer from Kansas, employed as an observer at Lowell Observatory in Arizona, discovered Pluto. It was the first planet in the solar system to have been discovered since 1846, when astronomers in Germany detected Neptune. Clyde Tombaugh, just 24 at the time, was hailed as a hero, Disney named a cartoon dog after the new planet, and for 76 years the solar system was a happy place.

And then, in 2006, the International Astronomical Union (IAU) reconsidered Plutos status. In a controversial vote, astronomers not planetary scientists demoted Pluto to the status of being classified as a dwarf planet, taking away one major planet and reducing the number in our solar system to eight. Astronomers suddenly took sides, seeing various sides in the logic, and schoolchildren all around the world were heartbroken, having been enamored with the story of the most distant and mysterious planet that was discovered by a young, self-educated researcher, and having that status heartlessly yanked away.

Astronomy.com: OK, the scene is set. Were working in a shared document and Alison will go first. Alison, is Pluto a planet?

Alison Klesman, senior editor, Astronomy: To me, Pluto is not a planet and thats a sign of progress. Classification and nomenclature in science is admittedly often arbitrary, but the idea is to at least group like with like so we can better understand how the universe works. By grouping like with like and separating things that are dissimilar, we can get an idea of how a certain group of, say, objects (moons or dwarf planets or centaurs or major planets) formed and evolved that sets them apart from the others. Sometimes those differences are big, and sometimes not. But every little clue helps, and to me, thats where creating those separate categories becomes most important.

I often try to think of the demotion of Pluto like this: Say you are studying four-legged animals. All youve come across so far are mammals (though you dont know that yet): cats, dogs, deer, badgers, raccoons. Then you come across something strange. Its cold-blooded, not warm-blooded. It doesnt have fur but it has scales. It lays eggs instead of birthing live young. But its got four legs and a tail and breathes oxygen and does all the other things the animals youve seen so far do. So, you classify it exactly the same as the rest of them, thinking its just a weird outlier.

But then you keep searching, and you find more animals like this weird one. You find more and more until you realize its not a strange subset of mammal, its actually something else entirely, lets call it a reptile! They are similar, but not the same, and the first one you found, that seemed like such a weird outlier, fits nicely under the reptile umbrella when you make it a separate group.

Admittedly, I am not a biologist, so maybe this metaphor is a bit off, but hopefully you can see where Im going with it! Thats why Im OK calling Pluto something other than a planet. What we call Pluto doesnt affect what it is, only how we understand it within the larger context of the solar system. We now know that Pluto is not an outlier among the other planets, but fits squarely into a different group of objects Kuiper Belt objects, among other things that reveals a very different and hopefully more accurate story than we had when we were trying to make it fit in a group that just wasnt quite right.

Astronomy.com: Thats a wise answer and we do see where you are going with that metaphor. This part gets right to the heart of the question, too: What we call Pluto doesnt affect what it is, only how we understand it within the larger context of the solar system.

Mark, what do you think?

Mark Zastrow, senior editor, Astronomy: The cop-out real answer is that it depends on who you ask. And thats OK! Its totally fine for different scientific communities to use words differently. The word evolution means one thing to a biologist studying, say, the heritability of genetic traits and a very different thing to an astronomer studying how galaxies merge and grow. Pluto can be a planet to people who study planets and a not-planet to people who study not-planets, or who study planets in a different way.

Im generalizing here, but the debate around Plutos planethood can roughly be split into two camps people who study how solar system bodies move, and people who study solar system bodies geology.

To make an even more sweeping generalization, how something moves through the sky tends to fall into the domain of what wed call astronomy, while studying the geology of those bodies is planetary science.

This may seem like a trivial distinction, and in many ways, it is science is interdisciplinary. But the field of professional astronomy does break itself down along these lines in certain institutional ways. Whether you identify as an astronomer or a planetary scientist will likely determine whether you join, say, the American Astronomical Society or the American Geophysical Union, and perhaps whether you are faculty in an astronomy department or a geology department.

Im not saying that all astronomers agree with the IAUs definition, or that all planetary scientists disagree. (Many scientists simply dont care.) But the point Im trying to make is that the International Astronomical Union is the body of authority for astronomers, but not necessarily planetary scientists. And what constitutes progress for one group of scientists may make less sense for another.

The IAU definition that was passed in 2006 says a planet has to meet three criteria:

Its the last criterion that Pluto fails, as it is part of the Kuiper belt of icy objects and is not even the largest body in it. From the standpoint of someone who studies the dynamics of the solar system, excluding Pluto from the ranks of planethood makes sense, and could certainly be considered a more logical classification scheme.

But to people who study these objects as geological worlds, it also makes perfect sense to think of Pluto as a planet. In fact, since Plutos demotion, weve learned that Pluto is even more like the other planets than we thought back when it was formally considered a planet! The flyby of NASAs New Horizons mission in 2015 showed us that Pluto has an atmosphere and is geologically active, with mountains, volcanoes, and glaciers. If youre trying to group like with like, from a geophysical standpoint, Pluto is very much like the other planets, and is an active, living world unto itself.

Of course, you could argue, so are many of the solar systems moons! So if youre going to ignore the dynamics, wouldnt you have to call those planets, too? Well, historically, large moons were called planets, as a team of planetary scientists pointed out in Icarus in 2021.

Which gets to my last point maybe the IAU shouldnt have even tried to define the word planet in the first place. That was the opinion of the person who headed the IAUs planet-definition committee, Harvard astronomer and historian Owen Gingerich. In a 2014 debate at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Gingerich argued that planet is a culturally defined word that has changed its meaning over and over again and over the ages. And, he added: My feeling is that in retrospect, the IAU should not have attempted to define the word planet.

Instead, Gingerich said, the IAU should have stuck to defining technical classes classical planets, dwarf planets, minor planets, exoplanets, and so on and let everyone, scientists and non-scientists alike, decide how to use the word planet on their own. Which, despite the IAUs resolution, is pretty much where were at, anyway.

Astronomy.com: Again, you guys are making a lot of sense. Dave Eicher bats clean-up on this question. Dave, youre up.

Dave Eicher, editor in chief, Astronomy: First, let me say as someone who knew Clyde Tombaugh, that it is a little embarrassing how the astronomy community has handled this issue. But Plutos heritage aside, lets look at the facts. In 2006, at a meeting of the International Astronomical Union in Prague, astronomers voted to demote Pluto to dwarf planet status. These were astronomers at the meeting, not planetary scientists a significant distinction. Three criteria were cited for this decision. A planet needs to be independently orbiting the Sun, large enough to be spherical, and has to clear its orbit of smaller bodies. The first two were clearly met by Pluto, but not the last. However, substantial amounts of ink have been spilled since, pointing out the unstable basis for the conclusion. First, if Earth were 40 AU from the Sun, as is Pluto, it would not clear its orbit of smaller bodies. But I think we all agree that Earth is a planet. Should the definition of a planet be dependent on where it exists physically? A house is a house whether its in the city or the countryside. Ah well.

Further, since Plutos demotion, two asteroids have been discovered that share Earths orbit exactly what disqualified Pluto. They are 2010 TK and 2020 XL5. They are Trojans orbiting ahead and behind Earth, in our orbit. So does that disqualify Earth as a planet? The whole business is a bit silly. In the end, dwarf planets are planets too. Part of what drove this was the fear that discovering lots of larger bodies in the outer solar system the Kuiper belt would force the solar system to add lots of planets. So why not get rid of the largest Kuiper belt object, Pluto?

So I would say its fine to consider Pluto a planet, or not. Whatever makes you happy. The distinction in nomenclature wont upset Pluto in the least. Itll be just fine, as it always has been.

What do you think? Email us at astronomyeditorial@astronomy.com

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Is Pluto a planet? The experts break it down. - Astronomy Magazine