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By Root 205 0

deemed large and important enough to name an element, neptunium, in its honor, but almost no one can recall the names of the other fifteen. It was a confusing time. What counted? What didn’t? On the wall in my office at Caltech I have a collection of maps of the solar system dating from about 1850 to 1900. Each map labels the solar system differently. A page from a sky atlas drawn in 1857 clearly shows Ceres, Pallas, Juno, and Vesta as “small planets,” while dozens of other asteroids are generally shown in the “zone of asteroids” between Mars and Jupiter. A German map from a year earlier lists all of the known Asteroiden by date of discovery, with no references to their being planets at all. Even by 1896, the solar system map from the Rand McNally Atlas explicitly states that the solar system contains only the sun, planets, and comets—asteroids are not mentioned at all—and that planets are either primary (what we would call planets today) or secondary (what we would call moons). In the margins of my Rand McNally map are drawings of how big the sun would look from the planets. At the top of the margin, the sun, seen from Mercury, is huge. At the bottom, the view from Neptune shows a tiny, distant disk. In the middle are the views from Ceres, Pallas, Juno, and Vesta, still tenuously holding on to their claims to be planets. The sun looks exactly the same from each of these four since they are all the same distance away.

By the turn of the century, though, somehow all of the confusion about what was and wasn’t a planet had settled. I cannot find anything written or drawn in this period that doesn’t separate the asteroids from the planets. What was their offense that they were cast down from the pantheon? In the end, their major sin seems to have been that there were too many of them all in the same place. The big planets go around the sun in orbits far from one another with no overlap, but the hundreds of known asteroids had crossing and overlapping orbits and were all one big jumble. How many is too many? When there were only four and the solar system appeared stable at eleven planets—which it did for forty years—no one (except the chemists, who couldn’t discover elements fast enough) seems to have complained. But the prospect of a never-ending parade of smaller and smaller planets all in essentially the same orbit around the sun was too much. There was no official vote or pronouncement, but by the early 1900s it became conventionally agreed that the solar system had only eight planets. Planet Ceres, which had held on for a century, along with all of its smaller neighbors, was demoted, with no outcry from the citizens of planet Earth.

By recognizing that Ceres and the swarm of other new bodies were fundamentally different from planets and should be classified differently, astronomers had—perhaps inadvertently, but certainly profoundly—changed the scientific meaning of the word planet. The word no longer simply meant anything that moved around the sun and wandered around the sky. Asteroids wandered, but they wandered in a swarm; they were the schools of minnows swimming among the pod of whales. Planets were the whales of the solar system.

As a kid I knew asteroids, too. On my poster on the wall they looked like tiny pebbles strewn in a vast band between Mars and Jupiter. They were the things—the meteors—that sometimes hit the moon and made those giant craters. I had seen shooting stars, which I knew were tiny fragments of these asteroids burning up in the earth’s atmosphere. Maybe I didn’t know their individual names or anything specific about them, and perhaps as individuals they were indistinguishable. But from what I knew by the time of my 1970s childhood, the difference between a planet and an asteroid was as obvious as the difference between a boulder and a handful of sand.

After the uncertainty and confusion about planets had been settled for a few decades and textbooks were clear that there were eight and only eight planets, the ninth was finally discovered. Clyde Tombaugh found Pluto by taking repeated pictures