Here's some background: Pluto was discovered by astronomer Clyde Tombaugh in 1930. Even though it was known to be relatively small and very different in many other ways from the giant planets of the outer solar system, it was still dubbed the 9th planet. For a long time the astronomy books—including the ones I read in school—treated Pluto like an outpost at the edge of the solar system. It became a special place in many people's minds—a frontier, of sorts.

Recently, however, using bigger and better telescopes, astronomers have started discovering many other planet-sized worlds even farther away than Pluto. Thousands of these "Kuiper Belt Objects" (named for the astronomer Gerard Kuiper, who predicted their existence) have now been discovered beyond the orbit of Neptune. While none of them yet found are bigger than the Earth's moon, a few of these worlds may be larger than Pluto. So that begs the question: Does the solar system now have dozens—or hundreds—of more planets? Do those "other Plutos" count, too? Do kids have to memorize all their names in school?

Such a large number of "planets" bothered some astronomers. So much so that many of my colleagues sought to re-classify objects in the solar system to try to better make sense of the large numbers of worlds out there. One way to draw the boundaries was to call a world a planet only if it is fairly alone in its neighborhood, and if it isn't orbiting another world. It's a limited definition that only takes into account where a world is and how it moves, not what it's actually like. By that definition, the solar system would have just 8 planets, a bunch of moons, and a bunch of "dwarf planets" like Pluto and some of the largest asteroids. The International Astronomical Union took a vote on this new definition, and Pluto was demoted. I'm a member of the IAU, but I wasn't allowed to vote because I couldn't attend that meeting in person.

I was bummed. I would have voted against it. Indeed, personally I have a rather different definition of a planet. I think a planet is any world that has had or still has an active, dynamic interior history—an interesting "geologic life," if you will. Why should Jupiter's moon Ganymede, larger than the planet Mercury and just as interesting, not be a planet? And why not Pluto, with three moons of its own? By my reckoning, the solar system has about 30 to 35 known planets--and perhaps more, depending on what else is discovered beyond Neptune. I think that all of these diverse and exotic neighboring worlds of ours are planets, and—I apologize kids—we should take pride in memorizing their names and learning about their histories.

Some of us are trying to get the IAU to reconsider their definition. The case is not closed—Pluto may once again be a planet!