Is Pluto a Planet?

Astronomy ^ | December 29, 2023 | David J. Eicher

[nickcarraway]

A planet is a planet wherever it resides, right? Dave Eicher examines the case of the icy world.

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 Pluto’s 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.

I’m glad that Clyde wasn’t around to see the demotion. He was a wonderful guy — a funny guy — who I got to know pretty well during the last two decades of his life. He was a joy to be around, and a huge fan of puns. “What do you call an angry crow?, he would blurt out as you sat under the stars, say, at the Texas Star Party. “A raven maniac.” Oh, Clyde. All the group could do was moan and then giggle at his stream of jokes and puns.

But back to Pluto. For many years prior to Pluto’s reclassification, astronomers had come to realize that the Kuiper Belt, the cloud of small bodies in the outer solar system, holds countless thousands of icy objects, and some of them are relatively large. Would astronomers eventually have a situation in which they had many more planets to add to the equation? Nerves began to be rattled. Pluto, some reasoned, could be the tip of the iceberg of an entirely new class of countless objects.

The rules for planets The key moment came at the 2006 meeting of the IAU in Prague. The IAU put forth three important criteria for planetary status: 1. A planet orbits the Sun. 2. A planet is massive enough to exist in hydrostatic equilibrium. That’s a fancy way of saying that it is spherical. And 3. A planet has “cleared its neighborhood” of smaller bodies within its orbit. Pluto meets the requirements for the first two categories, astronomers reasoned, but it has not cleared its orbit of smaller bodies. Thus, in late 2006 the IAU gave a present to Pluto — a reduced importance and the designation 134340 Pluto, designating it a dwarf planet (with an asteroidal nomenclature).

The world’s press writers had a field day with poor Pluto. They went wild. The story seemingly wouldn’t go away. Prominent Caltech astronomer Mike Brown, leader of a team that had discovered many asteroids, wrote a book titled How I Killed Pluto and Why It Had It Coming. In New York, Neil Tyson at the Hayden Planetarium moved the Pluto model away from the planets and grouped it with the small bodies of the solar system. In an enormous reaction, schoolchildren everywhere took up the fight and shed many a tear.

The issue again came to prominence when the historic New Horizons mission was underway and, in 2015, commenced its flyby of the distant world, discovering moons and a system that is far more active, on Pluto and its largest moon Charon, than anyone had suspected.

The goalposts were moved, perhaps unfairly

The logic behind the IAU decision came under intense scrutiny once again. Pluto clearly orbits the Sun and is spherical. It is the third criterion, the “clearing of bodies,” where the confusion lies. This qualification is ambiguous at best, and perhaps flawed. Many planetary scientists remain skeptical of this idea because it biases against objects of smaller size with increasing distance from the Sun. A house is a house, whether it’s in a city or in the countryside. Shouldn’t a planet be a planet wherever it resides? At the Pluto-like distance of 40 Astronomical Units from the Sun, Earth would not clear its orbit either. So would Earth then not be called a planet were it much farther from the Sun? Some astronomers consider large moons like Ganymede equal to planets. If stars can orbit stars, why couldn’t planets orbit planets?

The whole thing became even more outlandish in 2010, when astronomers discovered the first Earth Trojan asteroid, 2010 TK7. This tiny asteroid spans 300 meters and moves along with Earth, separated by 60° in our orbit. You know what else doesn’t clear its orbit, then? Earth! And we are still a planet, right?

So the whole thing is a bit silly. Whatever you call it, Pluto is still out there, an intriguing, distant body in our solar system and the last significant body out there to be robotically explored. We will no doubt discover more icy objects in the Kuiper Belt in the future. But none will have that special history that does Pluto, that keeps it a favorite of planetary scientists and schoolchildren alike. Wherever you are, Clyde, you can keep right on smiling.

David J. Eicher is Editor of Astronomy, author of 26 books on science and history, and a board member of the Starmus Festival and of Lowell Observatory.

[rellic]

I worked at the Keck Observatory for almost ten years and interacted daily with the people that voted on the question
of Pluto.
Make no mistake, they are very passionate about their hobby,
astronomy, but most everything they study is old history and of no significance to our modern world.
And it is a HOBBY they made into an industry!
The photon that strikes your face and warms you in the day took over a million years to get here.
Mathematics is your friend.
The Photons striking you all day and night long may have taken billions of years to get to you.
Does that solve our immediate survival problems?
No.
Only place I worry about is Earth.
That is where we live and can exist,
universe is nasty out there.
Go to Mars if you wish, but I’m sure Our Biology
won’t stand up to the rigors of alien worlds.
That being said I’m not some ECO “don’t touch or use anything” nut case like most “Ecologists”.
Conservation and let things grow, harvest when it is safe.
Aloha.

[metmom]

Yes. I never gave up on Pluto.

[algore]

It just proves that if you can control the language and claim to have Science on your side you can reclassify anything

I mean they never would ever change the definition of a vaccine would they ?

I had a list of words where the definitions had undergone massive changes but cannot find it.

[TermLimits4All]

I was taught in school, some decades ago, that Pluto is a planet. It’ll remain a planet for me.

[Mr Rogers]

[Some Fat Guy in L.A.]

Looks like a planet to me. I’ll just stick with that.

[zeugma]

I think arguments about definitions are inane, and changing the meaning of a word well-established in the language is insidious. All they really accomplished by reclassifying Pluto is to annoy a lot of people.

Exactly. It's also absurd for a tiny group of people who aren't even specialists in the object being reclassified to do so, and the rest of us go along with it. As far as I'm concerned, the solar system has 9 planets.

[isom35]

this video answers the question once and for all https://www.ign.com/videos/talkshow-with-spike-feresten-tv-pluto-is-no-longer-a-planet

[telescope115]

It looks like a planet to me….

[Tacrolimus1mg]

I’m flashing back to an episode of Rick and Morty.

[Svartalfiar]

Obviously Earth isn’t a planet either by their convoluted logic, these too clever by half nitwits did not take into consideration that most of life was wiped out including the dinosaur era by an asteroid some 65+ million years ago. This asteroid could have been crossing Earth’s orbit for a billion years and obviously Earth never cleared it until that fatal day.

You are right, but you're wrong.

That asteroid was not in Earth's orbit, it simply crossed it, the same way many comets and other bodies, at some point, cross the orbits of all the planets. The definition of "clearing its orbit" refers to any other bodies in the relatively same orbit, orbiting the same object. Which, every single planet has, except for Mercury (so far). Look up what "Trojan" or "Greek" asteroids are.

[Whatever Works]

I remember when Pluto was demoted. They waited for half of the voting body to leave (those in favor of leaving it a planet) and voted to change it. Controversial? I’ll say! I waited to see if the new designation would stand and got yelled at by a parent who’d just heard the news and DEMANDED that I teach it that way (We were studying the solar system at the time). I explained it had happened just 24 hours earlier and wanted to wait, given the circumstances and she went BALLISTIC on me. I hate clueless parents. I still think Pluto is a planet. They were the early wokesters. They changed the definition to create the answer they wanted ... After removing any opposing voices.

[chuckee]

Pluto is more of an exurb to our galaxy than it is a planet. Rentals are cheap there because the commute is a beast.

[Bikkuri]

I now use C.E. as "Christian Era" just to tick them off 😉

[poconopundit]

Pluto is a planette.

* * *

Ha, that’s clever, FatherofFive! You win the thread.

Seriously, this story is excellent: it lays out the interesting science and controversy in a pleasing and reader-conscious way.

Nice post, nick.

[Gaffer]

In my time (born 1950) it was a planet. Seems to me that ‘scientists’ like to redefine parameters. Is this to get ‘noticed?’ or published?

[quantim]

Whatever.

[SunkenCiv]

Thanks DoodleBob.

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The demotion of Pluto was a political act by a supposedly scientific org, was a gratuitous and unnecessary action, carried out to belittle the US. IMHO of course. I'll stick with David Levy's view:

"To Pluto And Far Beyond" By David H. Levy, Parade, January 15, 2006 -- We don't have a dictionary definition yet that includes all the contingencies. In the wake of the new discovery, however, the International Astronomical Union has set up a group to develop a workable definition of planet. For our part, in consultation with several experienced planetary astronomers, Parade offers this definition: A planet is a body large enough that, when it formed, it condensed under its own gravity to be shaped like a sphere. It orbits a star directly and is not a moon of another planet.

[Reynoldo]

Is Pluto a planet?

Yes.

[nickcarraway]

Saying Pluto isn’t a planet is insurrection!