Nobody wants to get rid of Pluto, and if you say that Pluto's not a planet—that it's just a crazy small thing out on the edge of the solar system—people look at you like you're a big cosmic bully.
The word planet is a word that lives in people's minds and imaginations. Pluto is a planet because for 75 years everybody's known that Pluto is a planet, not because there's a scientific reason.
Astronomers don't like that because they think they own the word planet, but the word has been around for 2,000 years, and it's never been a scientific word, and it doesn't need to be.
The analogy I use is the word continent. There's no scientific definition for the word continent. If you could come up with a scientific definition that encompasses the seven continents, I'd be impressed. Go ahead and try. Madagascar might be a continent because it's on a separate geological plate. Europe? No, definitely not—why is it separate from Asia? I have no idea. Australia? Sure. Maybe we'd make New Zealand one. New Zealand's got two plates. Anyway, you'd never hear geologists argue about this because they don't care. In geology, it doesn't matter whether society calls these things continents or not. They talk about what's there: continental crusts, continental dynamics, and so forth. I think astronomers should take that cue and say, let's be realistic—society wants labels on a small number of things we know about.
As scientists, of course, we need to know the difference between the eight things that are, let's say, major planets versus the minor planets. That way, we'll have good scientific names, just like geologists. And if people want to call Pluto a planet, let them call Pluto a planet.
But then you have to think about calling Xena a planet because it's bigger. So if the International Astronomical Union wants to call Xena the 10th planet and give it a formal name, that'll be fine by me too.
What do I do next? Well, if I'm still doing this down the road it means I'm stuck in a rut. Sometimes people can just go on forever doing something that gets increasingly less interesting. But I couldn't, even if I wanted to, because we've run out of sky.
There's a natural end to this survey. By this time next year—barring really bad weather—we'll basically have covered the entire sky that we can see. We'd like to do the other hemisphere, too, and that'll take another couple of years. But that'll be it. So that's a question I actually discuss with my students. What do we do next?
A lot of people have this five-year plan—what they want to do and where they want to be. I have no idea.
Maybe I'm just waiting for somebody to tell me about some telescope that nobody happens to be using. . . .
| WHAT'S AT THE EDGE OF THE SOLAR SYSTEM? Astronomers must grope in the dark for clues to the shape and makeup of the vast exturbs beyond Pluto
The true outer limit is at least 500 times farther out, about 5 trillion miles beyond Pluto, where the sun looks like nothing more than a bright star and temperatures hover just a few degrees above absolute zero. Astronomers believe this region, called the Oort cloud, contains a vast collection of icy debris left over from material that came together to form the sun, Earth, and the other major planets 4.6 billion years ago. Nothing in the Oort cloud is visible directly through telescopes, but astronomers infer its existence because it occasionally spits out objects that plunge toward the sun, where they sprout long, vaporous tails and become comets. About 10 comets from the farthest reaches of the solar system show up each year. Using mathematical models of the subtle forces that knock them loose—the tug of passing stars, interstellar gas clouds, and especially the gravitational fields of the galaxy itself—Harold Levison of the Southwest Research Institute in Boulder, Colorado, has estimated how many other objects populate the Oort cloud. "The total is about a trillion. It's a huge number, even bigger than our country's deficits," he says. Most of these hidden objects are presumably irregular chunks of rock, ice, and frozen gases less than a mile across, Levison says. "But I'm sure there are also things Pluto-size, even moon-size or Mars-size out there." All the same, everything in the Oort cloud added together might not outweigh Earth. The most notable aspect of the region is its breathtaking emptiness. It is so vast that the average distance between objects is about as great as the span from Earth to the sun. The only dramatic action happens as passing stars stir things up every few million years or so, puffing the Oort cloud up like a bag of Jiffy Pop and stripping off its outer layer. Thus its population steadily declines over the eons. The inner Oort cloud, which stretches from a few tens of billions of miles to a few hundreds of billions of miles from the sun, is much more stable than the edge of the region. If any objects orbit there—nobody knows—they would stay in place indefinitely. Brian Marsden of the Harvard-Smithsonian Center for Astrophysics speculates that there could be full-fledged Earth-size planets in this zone, circling unseen in the dark. "We would not have detected them observationally or dynamically," he says. Today's best telescopes can penetrate only to the nearest part of the solar system's outer regions, known as the Kuiper belt. During the solar system's formative years, astronomers theorize, the region around Uranus and Neptune was full of icy bits of debris. As those giant outer planets grew, their powerful gravity flung much of that debris outward. The bits that migrated farthest scattered hither and yon, forming the Oort cloud. Material that stayed closer to the sun gave rise to the Kuiper belt. The existence of the Kuiper belt was confirmed by direct observation in the early 1990s, when astronomers began to use high-sensitivity digital cameras and motion-sensing computer software to survey large areas of the sky. David Jewitt and Jane Luu of the University of Hawaii uncovered the first Kuiper belt object—a 100-mile-wide frozen ball known as QB1—in 1992. Then the floodgates opened: Astronomers quickly found more than 1,000 similar bodies, most of them about 4 billion miles from the sun, though a few orbit four or five times farther out. The best estimate is that the region contains 100 million objects at least a mile wide. Once again, the small fry dominate. Levison and his colleagues reckon that the combined mass of all the Kuiper belt objects is significantly less than that of the Oort cloud, perhaps a 10th or a 100th the weight of Earth. Astronomers once pictured the Kuiper belt as a giant ring-shaped collection of bodies along a disklike plane, called the ecliptic, in which Earth and all the other major planets orbit. But wide-field surveys of the sky have yielded surprises. Most notably, Xena sits at a rakish 45-degree angle to the main planets, making a mockery of the old idea that the Kuiper belt is literally a belt. According to the latest thinking, Uranus and Neptune moved around as they formed, and smaller but still substantial bodies may have further stirred up the outer solar system in ways astronomers are still struggling to understand. For now, Brown and his fellow planet hunters have learned that they need to put old ideas aside and search in all directions for new worlds. That ever-broadening search is about to get a huge boost from Pan-STARRS—the Panoramic Survey Telescope and Rapid Response System—a prototype of which will start up in Hawaii later this year. It uses four sets of optics and the world's largest digital camera to watch the entire sky for anything that moves, ranging from asteroids that stray dangerously close to Earth to unseen bodies in the Kuiper belt and beyond. Pan-STARRS is a precursor to the much grander Large Synoptic Survey Telescope slated to begin operating in either Mexico or Chile in 2012. Using a mirror 28 feet wide—five times as big as the Pan-STARRS telescopes—and a camera the size of a pickup truck, it will be able to survey the entire sky in three days. Every 30 seconds it will gather 36 gigabytes of imagery, enough to fill 50 CDs. This digital cornucopia will most likely include a wealth of data about many thousands of Kuiper belt objects orbiting up to tens of billions of miles from the sun and almost certainly will include objects that once again contradict ideas about where the edge of the solar system is. "We've been wrong so many times before," Marsden says. Meanwhile, astronomers will get close-up views of the outer solar system in July 2015, when the New Horizons spacecraft flies past Pluto and sends back detailed images of the once most-distant planet and its three moons. If all goes well, New Horizons will then continue on into the Kuiper belt. John Spencer, an astronomer at the Southwest Research Institute who is working on the New Horizons mission plan, has not even picked his targets yet; he is waiting for Pan-STARRS to give him a better road map. What New Horizons sees may be the missing links that tie the solar system together, from here to the very edge. If Levison is right, the same scattering that created the Kuiper belt and built up the Oort cloud also bombarded the young Earth with comets, reshaping its surface and delivering the ices that helped create its oceans and atmosphere. When you take a big enough look at the solar system, Levison says, "it's all connected." -Corey S. Powell
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Xena, the "is/isn't" planet discovered by astronomer Mike Brown and his team, is the farthest object orbiting the sun that anyone has managed to find—roughly 10 billion miles out, more than 7 billion miles beyond Pluto. Yet Xena is nowhere close to the edge of our solar system. 
