As a child, Mike Brown had all the trappings of an astronomer-in-the-making, with space books, rocket drawings, and a poster of the planets on his bedroom wall. On it, Pluto was depicted as “this crazy and very eccentric planet,” he says. “It was everyone’s favorite crazy planet.” Brown still recalls the mnemonic he learned for the names of the planets: Martha visits every Monday and—a for asteroids—just stays until noon, period. “The ‘period,’ for Pluto, was always suspicious,” Brown says with a laugh. “It didn’t seem to fit. So maybe that was when I first got the idea that Pluto didn’t belong.”
Nowadays Brown, a planetary astronomer at Caltech, has no doubt about Pluto’s place in the solar system: “Pluto is not a planet. There is no logical reason to call Pluto a planet.” Like a growing number of his colleagues, Brown believes Pluto is best understood as the largest known member of the Kuiper belt, a band of rocky, icy miniplanets that orbit the sun in a swath stretching from beyond Neptune to a distance of nearly 5 billion miles. “I don’t think it denigrates Pluto at all to say that it is not a planet. I think Pluto is a fascinating and interesting world, and being the largest Kuiper belt object is an honorable thing to be.”
No longer is Pluto a lonely outpost in an otherwise empty frontier. A string of discoveries has revealed that it is merely the entry point to a vast and still mysterious wilderness that teems with uncountable numbers of unusual objects. They come in a variety of shapes, colors, and sizes, many with their own moons, some in peculiar orbits that have been pushed by Neptune or pulled by passing stars. Stranger objects are likely to be found. Astronomers are only on the edge of discovering this vast new world.
In the 1940s and 1950s, astronomers Kenneth Edgeworth and Gerard Kuiper independently predicted that a reservoir of icy rocks lay beyond the orbit of Neptune. Many became short-period comets, with orbits of 200 years or less, that blasted in toward the sun, crossing the orbits of most planets. Excluding Pluto (discovered in 1930), the first official Kuiper belt object was not found until 1992, by astronomers Jane Luu and David Jewitt. Since then, in excess of 800 have been detected in the 2-billion-mile-wide Edgeworth-Kuiper belt (commonly truncated to Kuiper belt), including a few huge objects that are as much as three-quarters the size of Pluto (which is 1,430 miles wide). The two biggest have been found by Brown and his colleagues. At least 100,000 objects 30 miles wide may occupy the belt. Some of them, say researchers, are almost certainly the size of Pluto—if not larger.
