Twenty years ago, the existence of a distant wilderness beyond Neptune—seeded with tiny planets, dormant comets, and bits of ice and rock —was mere conjecture. There was Pluto, discovered practically by luck in 1930, and that was it. Astronomers photographed squares of the night sky and compared the images to see if anything was moving, but either their technology was not good enough, or they were searching in the wrong place. Or there was nothing more to find.
Then in 1992, after surveying the heavens for seven years, first with cameras and then with progressively more advanced digital imagers, University of Hawaii astronomer David Jewitt, with Jane Luu, one of his former graduate students, identified 1992 QB1. It was an icy object 125 miles in diameter (less than one-tenth Pluto’s size), orbiting the sun a billion miles beyond Pluto. “We really had no idea whether there was anything there,” Jewitt recalls. “But it was obvious as soon as we saw it.” They tracked the object all night, and then the next, before reporting the find to the Minor Planets Center at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Long-time center director Brian Marsden was skeptical; he suggested they had found a wandering comet. Jewitt bet him $500 that, like Pluto, 1992 QB1 was an orbiting object beyond Neptune. Eventually, Marsden paid up.
Since then, scientists have found more than 1,300 objects in this remote, mysterious region, and researchers estimate there are 70,000 of them with diameters of at least 60 miles. Pluto, it turns out, was neither alone nor unique. Today the Kuiper Belt, a region in space between 2.8 billion and 4.6 billion miles from the sun, is one of the hottest topics in astronomy. Named for Dutch-born U.S. astronomer Gerard Kuiper (rhymes with “viper”), who theorized its existence back in the 1950s, the Kuiper Belt defied detection for decades until scientists realized that Pluto was the first signpost.
Kuiper Belt Objects are leftovers from the whirling gas and dust disk that formed the solar system 4.5 billion years ago. Computer models suggest that the end of that process was marked by the outward migration of the planets, and as Neptune moved into its present orbit, its gravity tugged the remaining smaller bits with it deeper into space. Some of the pieces combined to form larger bodies like Pluto, but something stopped the planet-making process. The models say that in order for larger bodies like Pluto to have formed, there had to have been at least a hundred times more material in the Kuiper Belt than there is now. Somehow that material disappeared, perhaps pulverized in collisions or flung into interstellar space by the gravity of the outer planets.
“Dwarf planets are embryos,” says Alan Stern, a planetary scientist at the Texas-based Southwest Research Institute and a former NASA associate administrator for science. “If you have a Pluto-sized object, it should grow to be an Earth-sized object, unless you remove Pluto from the food supply or remove the food supply from Pluto. It is not clear what happened here.”
Or where it started. As the conviction grows that Neptune is the reason why the Kuiper Belt is where it is, researchers have become interested in finding out more about the belt’s origin and how its chaotic birth prevented other planets from forming. The migration had to have occurred during the final phases of solar system formation and had to have started very far from the sun. Otherwise the ice in the Kuiper Belt’s comets would have evaporated, and Pluto’s meager atmosphere would have boiled off. But “we don’t know where the shipwreck took place,” says California Institute of Technology astronomer Michael Brown, who in 2003 discovered Eris, a Kuiper Belt dwarf planet bigger than Pluto. “Or what happened after that.” By the time the migration ended, however, gravitational interactions had brought many Kuiper Belt Objects, including Pluto, into orbital synchronization, with Neptune (Pluto orbits the sun twice in the time it takes Neptune to orbit three times). So whatever happened, Pluto witnessed it.
As the big picture of the Kuiper Belt comes into focus, a team of scientists led by Stern is preparing for a first close look. In 2006, NASA launched New Horizons, a piano-size spacecraft weighing 1,054 pounds, on a nine-and-a-half-year voyage to the solar system’s outback. So long is its transit and so new is the field of Kuiper Belt study that some of the objects it will examine have not yet been discovered.
In 2015, the spacecraft will awaken from hibernation for a flyby that will take it within 6,200 miles of Pluto. It will also study Pluto’s largest moon, Charon, and two other tiny, recently discovered satellites, Nix and Hydra. New Horizons will take photographs, study Pluto’s wispy atmosphere, and analyze its surface, geology, dust, and temperatures. Scientists should then be able to draw at least some general conclusions about the nature of Kuiper Belt Objects. “I’m sure we’ll over-interpret,” says Stern, but the flyby will provide information that telescopes cannot get. The impact craters on Pluto’s relatively young surface, for example, should give researchers a better idea of the size and number of the objects now in the Kuiper Belt. Because Pluto’s surface ices, composed mostly of nitrogen, turn to gas when the planet comes closer to the sun, then refreeze when it moves farther away, the terrain changes seasonally and offers a fresh record of impacts. The surface is relatively “young.” Charon, with little or no atmosphere, has a much older surface, so it should have many more craters, and should provide a record of the size distribution of objects in the original belt. By counting the number and size of craters per unit of surface, scientists can determine the craters’ ages.
In planetary science, the mix of targets is about as good as it gets. Pluto is one of the larger known bodies in the Kuiper Belt, while Nix and Hydra are minuscule. “We have the bookends,” says project scientist Hal Weaver of Johns Hopkins University’s Applied Physics Laboratory in Baltimore, Maryland, which built the spacecraft. “Here’s an opportunity to get the first look at a completely different class of objects.” Weaver, who came to the project because he was interested in “frontier scientific research,” describes himself as a “comet guy,” and Pluto, he notes, “is nothing more than a big comet.”
Imaging of Pluto and preparations for the flyby will start five months before New Horizons’ closest approach, but the key portion of the journey is only the 12 hours before arrival and the 12 hours afterward. Downloading all the data to Earth, with a one-way transmission time of four and a half hours, will take nine months. By that time, New Horizons will be on its way to another object. No one knows yet what that will be, but by 2015 there should be a number of candidates.