Printer Friendly - - science news articles online technology magazine articles Printer Friendly Beyond Pluto We are only beginning to discover how vast and strange our solar system truly is By Kathy A. Svitil Illustrations by Don Foley DISCOVER Vol. 25 No. 11 | November 2004 | Astronomy & Physics 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. THE NEXT THING Distance from sun: Unknown Diameter: Unknown Orbit: Unknown Features: Unknown file:///C|/quarantine/Discover%20Magazine/astronomy/Beyond%20Pluto.htm (1 of 11)10/4/2005 23:17:57 Printer Friendly - - science news articles online technology magazine articles Printer Friendly 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. 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 4.5 billion years ago. 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. Each of those objects has a tale to tell that is as lively as those that the nine—call it eight— planets have told so well. Let their stories begin. NAME THAT CELESTIAL OBJECT Scientific nomenclature can barely keep up with the range of odd 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 lack a standard definition. If a body orbits a sun and was made spherical by its own gravity, astronomers tend to call it a planet. Yet that definition would include at least four asteroids and dozens of Kuiper belt objects. ASTEROIDS (a.k.a. minor planets) 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 about 700,000. COMETS are icy bodies that follow elliptical orbits. Those that have orbital periods shorter than 200 years originate in the Kuiper belt. Longer-period comets originate much farther out in the Oort cloud. The known comets number in the thousands. 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). file:///C|/quarantine/Discover%20Magazine/astronomy/Beyond%20Pluto.htm (2 of 11)10/4/2005 23:17:57 Printer Friendly - - science news articles online technology magazine articles Printer Friendly RESONANT KUIPER BELT OBJECTS orbit in synchrony with Neptune. Pluto is the prototype: It orbits twice around the sun for every three solar orbits made by Neptune. About 20 percent of known RKBOs are thought to orbit with a similar 2:3 resonance and thus are called plutinos. Six other 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. A New Miniplanet Rises Detected last November by Brown and his colleagues, using the 48-inch Samuel Oschin Telescope, Sedna is the largest, farthest, coldest, and arguably weirdest world yet discovered in the distant solar system. It may be as large as 1,100 miles in diameter, and it has a mysterious deep red surface—neither particularly dark, like a typical rocky object, nor bright and icy like Pluto and other Kuiper belt objects. Its orbit is even more peculiar. It passes no closer to the sun than 7 billion miles, nearly twice as far away as Pluto ever gets, and it sails beyond the outer edge of the Kuiper belt, possibly into the Oort cloud. Is it a stray Kuiper belt object, a comet from the inner Oort cloud, or something different? Sedna’s eccentric orbit indicates that it must have formed at the edge of the same disk of gas and dust from which Earth and the other planets coalesced 4.5 billion years ago. Then it was somehow bumped far out into space and into an offbeat orbit. Although the details are debated, the most likely explanation involves both Neptune and a nearby star. Sedna may have begun its days in the SEDNA Kuiper belt. A chance close encounter with Neptune could have knocked it Discovered: November 14, 2003 off track, pushing the most distant Distance from the sun: 8 billion miles; point of its orbit farther and farther ranges from 7 billion to 90 billion miles away from the sun. Then a second Diameter: 800–1,100 miles event—the gravitational tug of a Orbital period: 10,500 years passing star?—could have yanked Features: It’s the coldest body Sedna away from Neptune’s influence in the solar system, with and even farther out. Mathematically, temperatures that approach Brown says, the ideal arrangement 400 degrees Fahrenheit. would have involved a star passing sufficiently close to ours that it would have shone “brighter than the full moon for something like 20,000 years.” Most stars are born in the company of others, as in the stellar nurseries of the Orion nebula. But until Sedna there had been no direct evidence that Images courtesy of NASA/Caltech (top) and NASA (bottom) file:///C|/quarantine/Discover%20Magazine/astronomy/Beyond%20Pluto.htm (3 of 11)10/4/2005 23:17:57 Printer Friendly - - science news articles online technology magazine articles Printer Friendly our sun had a social childhood; the nearest stars today are light-years away. “Sedna’s orbit seems to provide the first evidence that our sun formed in a dense cluster,” says Oort cloud expert Luke Dones of the Southwest Research Institute in Boulder, Colorado. If other objects with similar orbits are found, they might enable scientists to retrospectively piece together a group portrait of the sun’s nursery. “We basically have a fossil record of this early cluster of stars,” Brown says. “A year ago I would have never guessed that the record even existed. Now I’m convinced that it can tell us exactly how many stars there were, where they came from, and where they have all gone.” An Old Planet Sets Neither the Kuiper belt nor the Oort cloud had been predicted in February 1930, when astronomer Clyde W.