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By Root 215 0

of the sky and comparing them to see if anything had changed. On February 18, 1930, he found a faint object that moved from one night to the next: a new wanderer! Unlike the myriad asteroids (hundreds were known by then), Pluto was not between Mars and Jupiter; it was well beyond Neptune, where a real ninth planet should be. Still, it was a bit odd. It was found to go around the sun in an elongated, rather than circular, orbit, and while all of the planets orbit the sun in a flat disk, Pluto was found to be tilted by almost twenty degrees away from the rest. Pluto also looked different. It was so small that you couldn’t tell it was a planet at all. In fact, it appeared starlike. Some astronomers didn’t want to call Pluto a planet. Shouldn’t it just be called an asteroid instead? By then, though, the word asteroid had lost its literal meaning of “starlike” and instead referred specifically to that belt of objects between Mars and Jupiter. Should it be called a comet? Comets can have elongated and tilted orbits like Pluto’s, but none had ever been seen so far away, and the word comet (from coma, Latin for “hair”) specifically refers to the fuzzy appearance of comets in the sky. Pluto was not fuzzy; it looked like a star, albeit one that moved. Though it looked and behaved like no other planet known, there was no other way to classify it, so it became accepted as the ninth planet, had the element plutonium named for it, and remained unchallenged for almost seventy years as the tiny lonely oddball at the edge of the solar system, the planet with the ice spires, the planet with the orbit so extreme that it couldn’t even fit on my poster on the wall, the incongruous period at the end of the solar system.

What I didn’t immediately grasp when Jane Luu joined me on the roof overlooking the San Francisco Bay at the Berkeley astronomy department in 1992 was that the discovery of the Kuiper belt gave Pluto a context. It took me and most other astronomers a few more years to realize that Pluto is neither lonely nor an oddball, but rather part of this vast new population called the Kuiper belt. Just as the explosion of asteroid discoveries 150 years earlier had forced astronomers to reconsider the status of Ceres, Pallas, Juno, and Vesta and change them from full-fledged planets to simply the largest of the collection of asteroids, the new discovery of the Kuiper belt would certainly force astronomers to reconsider the status of Pluto. It was becoming more and more clear that if the asteroids were the schools of minnows swimming among the pod of whales, then Pluto and the Kuiper belt objects were simply a previously overlooked collection of sardines swimming in a faraway sea. If Ceres was to be thought of as just the largest of the vast collection of asteroids and thus not a planet, why should Pluto not suffer the same fate? What, after all, was a planet?

Chapter Three

THE MOON IS MY NEMESIS

When I first started looking for planets, I lived in a little cabin in the mountains above Pasadena. I have a feeling I was the only professor at Caltech at the time who lacked indoor plumbing and instead used an outhouse on a daily (and nightly) basis. I worked long hours, and it was almost always dark, often past midnight, when I made my way back into the mountains to go home for the night. To get to my cabin, I had to drive up the windy mountain road into the forest, past the national forest parking lot, and down to the end of a dirt road, and finally walk along a poorly maintained trail by the side of a seasonal creek. For some time after I first moved in, I tried to remember to bring a flashlight with me to light my way, but more often than not I forgot. On those nights, I had to navigate the trail by whatever light was available or, sometimes, by no light whatsoever.

The time it took to get from the top of the trail to the bottom, where my cabin was, depended almost entirely on the phase of the moon. When the moon was full, it felt almost like walking in daylight, and I practically skipped down the trail. The darker quarter moon