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HARVARD GAZETTE ARCHIVES 21 Moons 'Swarm' Planet Uranus By William J. Cromie
Three new moons have been discovered orbiting Uranus, a great gasball of a planet about 2 billion miles from Earth. The discovery, last July, raises the number of Uranian moons to 21, the most, as far as is known, in the skies of any planet. "We were excited to find these newest satellites," says Matthew Holman, an astronomer at the Harvard-Smithsonian Center for Astrophysics. "We think they are cometlike objects captured by Uranus when the solar system was billions of years younger." The evidence for such capture is in the strange orbits of the moons. Most moons circle the mother planet in the same plane as planets circle the sun, called the ecliptic plane. Uranus boasts 16 moons that move this way. But the "newest" moons travel along irregular paths, at a steep angle to the ecliptic plane. Two such moons, called "irregulars" by astronomers, were found rotating around Uranus two years ago. The three new moons bring the total number of irregulars to five. They range between 6 and 12 miles in diameter and orbit about 10 million miles from Uranus. (Earth’s moon circles approximately 240,000 miles away.) Astronomers felt more relieved than surprised to find them because faint, reddish irregulars seem to be the rule rather than the exception in the outer solar system. Jupiter, the largest planet, has four irregulars as well as 12 regular moons. Saturn, the second largest, has at least one irregular among its 18 moons. It would be curious if Uranus, the third largest planet, didn’t have irregular companions. Neptune, smaller and farther away than Uranus, has at least one irregular in addition to eight regulars. Most astronomers believe the small unordered moons come from what is known as the Kuiper belt, a region of icy planetesimals circling between Neptune and Pluto, the farthest-out planet. In fact, some experts think Pluto, which is about the size of Earth’s moon, is a Kuiper object. Kuiper objects seem to be chunks of ice, dust, and gas left over from the beginning of the solar system, some 4 billion to 5 billion years ago. During the solar system’s first billion or so years, much more material whirled around the outer solar system than does there today. "By studying these irregular satellites, we should reach a better understanding of what the solar system was like at that time," Holman points out.Origin of Comets Today, some Kupier objects travel highly elliptical orbits that bring them close to the sun. When solar heat begins to melt them, they produce long, luminous tails, and we call them comets. Billions of years ago, such objects are thought to have swarmed like bees through the outer solar system. Uranus and its big companions consisted of rocky cores that attracted huge envelopes of gas around themselves. (Uranus is 4,000 times the diameter of Earth.) The gravity of such massive planets pulled in any Kuiper objects that wandered close enough. Meanwhile, in the inner solar system, there were less flying objects and gas. The smaller, rockier planets, like Earth and Mars, didn’t have enough gravitational pull to capture what gas and flying objects were around. (Many astronomers believe that the close approach of an object the size of Mars ripped the moon from Earth.) This is the story as Holman and others believed it happened. It fits with the evidence but is subject to changes as more observations are made. Goofy Discoveries Uranus today appears – through the lens of a good telescope – as a featureless blue-gray globe. It is strangely tilted on its side; that is, the north and south poles are in almost the same relative position as the Earth’s equator. Finding faint objects, less than 20 miles in diameter and orbiting Uranus about 1,700,000,000 miles from Earth, is enough to make an astronomer feel proud. The first two Uranian irregulars were discovered in 1997 with the Hale Telescope on Palomar Mountain in California, which has a lens 16 feet across. Brett Gladman and two companions had some spare time after a session of looking for Kuiper objects, so they decided to point the telescope in the neighborhoods of Uranus and Neptune. After finding two previously unknown moons around Uranus, they decided that a more thorough search would reveal even more satellites. Two years later, Gladman, Holman, J.J. Kavelaars of McMaster University in Canada, and Jean-Marc Petit of the Observatory of Nice, France, continued the search from atop a 14,000-foot extinct volcano on the island of Hawaii. "At that altitude, the lack of oxygen causes people to act a little goofy," Holman notes. "We didn’t get sick, but it took a lot longer to do anything." For four nights they stared intently at computer screens attached to the Canada-France-Hawaii Telescope on the mountain. Attached to the telescope was a powerful camera that could survey large areas in the vicinity of Uranus. The team took three images of each field of view, one hour apart. Then they used two different computer programs to spot any objects that had moved during that time. "Things got exciting when we finally saw something move at the right speed and in the right direction," Holman recalls. "Then it was time to shout ‘Victory!’" They shouted it twice during those four days. Later, Holman discovered a third moon while re-examining data from the telescope in his office in Cambridge, Mass. Thus, Uranus entered science and history books as the planet with the most moons. At the pace of astronomical discoveries, it may not stay there too long. To see what Holman and his colleagues saw on those nights, tune to http://cfa-www.harvard.edu/~mholman.
Copyright 1999 President and Fellows of Harvard College |
