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How to Settle, Once and for All, the Whole "What's a Planet?" Debate

Defining things is a natural impulse, but it often doesn't fit the natural world.

By Phil Plait
Dec 8, 2010 6:00 AMDec 17, 2019 12:02 AM
Pluto - NASA
The dwarf planet Pluto, imaged by NASA's New Horizons mission. (Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute)


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When I was a kid, I knew exactly what a planet was: It was something big and round, and it orbited the sun. There were nine such beasts in the celestial menagerie. We knew Pluto was a misfit—smallish, distant, and orbiting on a weird elliptical path—but we had no doubt it was part of the family. The other planets certainly fit my description, and all was well.

I didn’t even consider Ceres, one of the solar system’s oddballs. But if I had, I’m sure I would have thought, “Ceres is an asteroid! It’s the largest one, sure, and maybe it’s even round, but it’s just the biggest of a bunch of rubble out there between Mars and Jupiter. A planet it ain’t.” As for objects past Pluto? There were no such things! Done and done.

Ah, the naïveté of youth. As an adult and as a scientist, I now see that the situation is far more complicated. Trying to rope the universe with our own definitions is like trying to put a spherical peg in a cubical hole. Why bother?

“This whole word planet is just magical,” says Mike Brown, a planetary astronomer at Caltech. “It is the one word that people understand about the solar system, and the solar system is the largest local geography that most people know. So this word really is special. It matters that we get it right.”

The word matters a lot to scientists, too, as Brown can well attest. He’s the man who recently stirred up a hornet’s nest by finding lots of new objects orbiting in the outer reaches of the solar system, one of which—Eris—is around 1,400 miles wide, about the same size as Pluto. Are these things planets? At the same time, other astronomers have been discovering Jupiter-mass or smaller bodies circling nearby stars. Are these things planets? The answer turns out to have a lot of implications for our understanding of how our solar system formed, how Earth evolved, and where to look for life elsewhere in the universe.

OK. Maybe we should bother.

Framing the Question

Perhaps my naive definition—big, round, and orbiting the sun—isn’t such a bad place to start. I imagine a lot of the public would give the same criteria. I asked my teenage daughter and she came up with something similar herself. So let’s begin there.

The last part of my youthful planet definition—something orbiting the sun—is the easiest to dismiss. In late 1995, when astronomers found the first planet-mass thing orbiting a star similar to the sun, they didn’t call it a “planet-mass thing.” It was an honest-to-goodness planet circling an honest-to-goodness star. Within a few months we found more, and more…and now, not so many years later, we’ve cataloged nearly 500 such planets. The list grows almost daily.

A planet, therefore, doesn’t need to orbit our sun, and we can already see our definition fraying around the edges. We’ll need to fix it. So let’s say a planet is something that is big and round and orbiting any star.

But wait! We know that planets don’t just orbit in a nice, neat pattern through all time. Astronomers are pretty sure that when the solar system was forming, things were fairly chaotic. Any protoplanetary object drifting too close to proto-Jupiter would have gone on a wild ride: The gravity of the mighty proto-Jupiter was capable of tossing the smaller newborn planet (pdf) completely out of the solar system. Computer models show that this kind of event was inevitable. Our solar system may have been born with dozens of planet-size objects that Jupiter ejected into interstellar space. Multiply this process by billions of other stars and the implications are unavoidable: Our galaxy is littered with ejected rogue planets, traversing the interstellar vacuum light-years from the warmth of a star.

OK, so a planet doesn’t need to orbit a star. But it’s still big and round, right? Right?

Grading the Contenders

It turns out that those two characteristics—big and round—are related. In the past few years, space probes, improved ground-based telescopes, and orbiting observatories have shown us close-up pictures of hundreds of objects in our solar system. What we have found is that a lot of them are lumpy and misshapen, while others are clearly round. It doesn’t take long to see the difference. Most smaller bodies are irregular; past a certain size they are all roughly spherical. Clearly, size matters.

The term planet shouldn’t be a definition; it should be a concept. And Brown proposes a decent one, which we were very close to earlier in this analysis: A planet is an object that is big and important in the solar system. By “important,” Brown is referring to objects that are the biggest in their neighborhood, able to bully smaller objects. “We are seeing order in the solar system, and part of the nature of understanding the planets is to see the order in things,” he writes. “I look at the bodies that are dominating the solar system. And they are much bigger than the next-biggest thing they’ve kicked around.” In that sense, we have a clear list of eight planets, and Pluto doesn’t count. At 1,400 miles across, it is just one of the biggest of the ice balls out past Neptune, barely able to make its influence felt among a population that may number in the hundreds of thousands.

Some people balk at using fuzzy words like big and important. But if things are a little hazy around the edges, it doesn’t matter, because we’re not defining anything. We’re just squinting a little and putting things in order. As Brown points out, concepts are for scientists and definitions are for lawyers.

Of course the public likes definitions as well. It is unsatisfying to have objects out there that aren’t really defined. But we may, as scientists, just have to deal with that. Changing public opinion, as any lawyer or politician will tell you, can be difficult. To me, we astronomers should tell the public that the question is not whether Pluto is a planet. In fact, that question shouldn’t even exist. We can’t really define what a planet is. Instead of teaching kids there are eight or nine or however many planets, we should instead teach them the concept of a planet: a biggish, roundish object that can affect things in its vicinity. Wherever you go, a planet is an object that absorbed many smaller objects to become the local bully. It is something that had a substantial impact on the environment around its star. If you want to call Pluto a planet, no galactic police will pay you a visit. If you don’t want to, that’s OK as well. Pluto doesn’t care what we call it. What a concept.

Is it a Planet?


If you want to become a planet, you cannot look like a peanut. That eliminates Itokawa and other small asteroids. The smooth round shape we associate with planets comes from their gravity, which tries to pull the mass toward the center. Little bodies like 1,755-foot-wide Itokawa do not possess enough heft for gravity to shape them. As a result, most asteroids—as well as comets and small moons— live out their days as static, irregularly shaped worlds. From a scientific perspective that is not such a bad thing. Since these asteroids and comets have roughly the same makeup today as when they formed some 4.6 billion years ago, they are fascinating time capsules of the early solar system.


When it comes to determining planethood, the International Astronomical Union has a credo: Location, location, location. Large moons like Saturn's 914-mile-wide, two toned Iapetus certainly resemble planets. They are round bodies; their gravitational energy drives dynamic geologic process; some even have active volcanoes or geysers. Yet they receive second billing because they orbit a planet and not the sun, and the IAU requires. Pluto, Eris, and Ceres—which the IAU designates as dwarf planets—also have positional misfortune. Although they circle the sun, they orbit in the regions that contain a lot of other stuff, making it impossible for them to clear their orbital paths. If Earth were located among the myriad objects out by Pluto and Eris, it too would fail to clear its neighborhood and so would not qualify as a planet.

Main Players

Saturn is the prototypical planet, immense and imposing (it is 74,900 miles across, 9.5 times as wide as Earth). But size alone is not the deciding factor. Mercury is smaller than two moons: Saturn’s Titan and Jupiter’s Ganymede. “I’m willing to let it go,” astronomer Mike Brown says of Mercury.

Despite their differences, Saturn, Mercury, and the other six planets satisfy the International Astronomical Union’s criteria; still, “the debate is going to go on for years,” says Mark Sykes of the Planetary Science Institute in Arizona. He hopes the question of what constitutes a planet will come closer to a resolution in 2015, when space probes arrive at Ceres and Pluto, perhaps revealing that they are dynamic enough to warrant a whole new definition of planet—and a new mnemonic device for schoolchildren to learn.

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