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?
Next page: Grading the Contenders