Science & Tech

21 moons ‘swarm’ planet Uranus

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Discovery of three new Uranian moons focused attention on ‘captured’ comets

In 1999 three new moons were discovered orbiting Uranus, a great gasball of a planet about 2 billion miles from Earth. The discovery raised the number of Uranian moons to 21, the most, as far as is known, in the skies of any planet. Researchers believe the moons were “captured” billions of years ago. “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.