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. The poster depicted Pluto as “this crazy and very eccentric planet,” he says. “It was everyone’s favorite crazy planet.” Brown, now an astronomer at Caltech, still recalls the mnemonic he learned for the names of the planets: Martha visits every Monday and (the a was 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.”
Brown’s childhood insight now sounds like a premonition. In August 2006 the International Astronomical Union officially demoted Pluto, putting it into the new category of “dwarf planet,” a sun-orbiting object big enough to be forced into a spherical shape by gravity but not big enough to clear its own orbit. The IAU more recently deemed Pluto and two newfound bodies to be “plutoids,” bright dwarf planets that circulate mostly outside Neptune’s orbit. The decision was controversial and continues to generate protest, fueled by new findings from this icy realm.
These remote celestial bodies are the largest known members of the Kuiper belt, a band of rocky, icy objects that orbit the sun in a swath stretching from beyond Neptune to a distance of nearly 5 billion miles. Whatever you call them, one point is beyond debate: Pluto is no longer 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 an uncount-able number 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, since astronomers are only on the edge of discovering this vast new terrain.
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 of these objects became short-period comets, with orbits of 200 years or less, that blasted in toward the sun, crossing the paths 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 1,200 have been detected in the 2-billion-mile-wide Edgeworth-Kuiper belt (commonly truncated to Kuiper belt), including burly Eris, even larger than Pluto. Almost all of the biggest have been found by Brown and his colleagues. More than 100,000 objects at least 30 miles across may occupy the belt.
But our solar system doesn’t end there. Far beyond the Kuiper belt lies the mysterious Oort cloud, a spherical shell that stretches to the boundaries of interstellar space and blasts its own dark ice balls toward the sun. Trillions more bodies may lurk there. A few may be as big as Mercury or Mars. Imprinted in those far-flung worlds, scientists say, is the history of the solar system before planets came to be. Every Kuiper belt object and Oort cloud entity is a geologic fossil, preserved at low temperatures, largely unaltered by time, and made up of the material from which the solar system formed. Understanding their compositions—and why they are where they are today—will help scientists reconstruct the nascent moments of our planetary neighborhood and our sun’s younger days, when it was just one of a cluster of stars.
A reconstructed 1910 image of Halley's comet.
Image courtesy of Lowell Observatory/NOAO/AURA/NSF
Name That Celestial Object
Scientific nomenclature can barely keep up with the range of objects that astronomers are discovering in the solar system these days. Future discoveries will no doubt suggest new categories and further blur the boundaries of old ones. In the meantime, here’s a field guide to the known residents.
Planets are defined by the IAU as bodies orbiting the sun that have been made spherical by their own gravity and have cleared their orbit of other objects. Not every astronomer accepts this designation.
Dwarf Planets are similar to planets but are too small to clear their own orbits. Most lie beyond the orbit of Neptune, making them plutoids, a subcategory named for the most famous member of their group.
Asteroids are rocky, metallic, or carbonaceous bodies whose solar orbit takes them beyond Mars into the so-called asteroid belt. They typically lack sufficient gravity to retain an atmosphere. The latest count of asteroids in the inner solar system, inside Jupiter, is between 1.1 million and 1.9 million.
Comets are icy bodies that follow elliptical orbits. Those originating in the Kuiper belt have orbital periods shorter than 200 years. Longer-period comets originate much farther out in the Oort cloud. Known comets number in the thousands, but trillions might be out there.
Classical Kuiper Belt Objects orbit the sun at 3.9 billion to 4.5 billion miles out. They are sometimes called cubewanos, after QB1, the first Kuiper belt object discovered (1992).
Resonant Kuiper Belt Objects orbit in synchrony with Neptune. Pluto is the prototype, orbiting twice around the sun for every three solar circuits made by Neptune. The majority of known RKBOs are thought to orbit with a similar 2:3 resonance and thus are called plutinos. About 100 other objects with resonant orbits have been detected.
Scattered Kuiper Belt Objects have very eccentric and tilted orbits that carry them from around 3.3 billion miles from the sun to almost 100 billion miles out. They may have been kicked into their far-flung orbits by Neptune’s gravity.
