How To Be A Planet

If you’re anything like me, you took the news of Pluto’s demotion eight years ago with a great deal of confusion. How does something stop being a planet? How does discovering a new planet make existing planets not planets anymore? What do you call a planet that’s not a planet anymore?


In the due course of time these things were explained to us. Mostly. But the general public never quite reconciled themselves to the loss of the ninth planet. Recent talk has stirred up public sentiment even more, to the point where people have begun asking if there’s a possibility that science will relent, and allow Pluto to return to planetary status. (

But what makes a planet? In 2005 a tenth planet was discovered in orbit beyond Pluto. Named Eris, there was a great deal of debate about allowing it into the official planetary system. It was too far away, astronomers argued. It was too much like a rather large asteroid. But Eris was bigger than Pluto! So if Eris wasn’t a planet, said the supporters firmly, then Pluto shouldn’t be either.


So a committee was formed, and they took a good hard look at the issue. To understand properly why there was concern about admitting Eris we have to go back in time to the discovery of the asteroid belt in the early 19th century. When the first asteroid was discovered it was duly given a name and admitted as a planet. Then another one was located . . . and another. For months every new object located was named and cataloged, until it was discovered that there were far, far more of these space rocks then anyone could keep track of! Eventually Sir William Herschel proposed the term “asteroid” (Greek for “star planet) as their irregular shape caused them to resemble stars.

So getting back to Pluto. Way out on the outer edges of the solar system is a second asteroid belt—one we call the Kuiper Belt. Pluto’s eccentric orbit takes it through this belt, and Eris’ is almost entirely inside of it. The Kuiper Belt is very far away and difficult to observe, and the fear is that there might be even more planetoids lurking, waiting to be found, leading to an every growing list of planets that will eventually grow unwieldy and impractical. So it became necessary to form a good, solid definition of “planet” that could be used to gauge all future solar system admissions and keep the number of planets taught in schools to a manageable number.

So what is a planet?

A planet is a celestial body that matches the following criteria:

1. It must orbit the sun
2. It must be round
3. It must clear it’s own path

It’s this last criteria that struck out both Pluto and Eris as planets, and the most highly contested point of definition. Both planets spend some time floating around in the Kuiper belt, thus their orbit is not cleared. Celestial bodies that meet the first two criteria but not the third are referred to as dwarf planets, or planetoids.

On one hand, the demotion wouldn’t have been such a big deal if it weren’t so heavily publicized as a demotion. After all, we already have planets sorted into terrestrial planets and gas giants. Adding one more category just makes it easier to divvy up and study the solar system. But Pluto has been a fan favorite ever since it was named by an eleven-year-old girl, capturing the imagination of astronomers across the world. It’s the setting for many science fiction novels, most notably “Have Space Suit, Will Travel” and “World of Ptaws.” It’s been featured on Doctor Who. ( People identify with it for it’s small size, and distant, lonely location. And people don’t take kindly to their pet planets being treated as less important.


Pluto of Roman Mythology, aka Hades in Greek Mythology.

So what is the case for re-promoting Pluto? The definition has been set and it didn’t make the cut. Anything else is just hopeful thinking, right?

Well, perhaps not. There are a lot of people who disagree with the definition set down by the IAU and don’t consider it final. And there is a case to be made for Pluto’s popularity, as some scientists agree that planet is a culturally defined word, so if people want Pluto to be a planet then it is a planet. However, at this point there’s no official action being taken so it could be a while before we have a ninth planet again. In the meantime we have a century’s worth of school texts and astronomy books that teach that the solar system has nine planets, and entire cultures that insist it should never have been demoted in the first place. As long as we keep that knowledge alive in our hearts, Pluto was never truly gone.

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