In May, astronomers Scott Sheppard and David Jewitt at the University of Hawaii announced the discovery of 20 new moons orbiting Jupiter. A month later, a team led by Brett Gladman of the University of British Columbia added one more, bringing Jupiter's satellite count to 61. The number of known moons in our solar system is skyrocketing because digital light detectors and computer programs are surveying the space around neighboring worlds in unprecedented detail. The total has grown from 64 to 129 over the past five years, and the search is far from over. "You could expect 100 satellites down to about half a mile in diameter around each giant planet," Sheppard says. Jupiter's new satellites follow irregular orbits, which implies that they were captured early in the solar system's history. If the outer gas giants, Uranus and Neptune, formed closer to the sun and migrated away, as one leading theory holds, this movement should have disrupted the capture of small moons. But Sheppard's observations might show otherwise. "So far, all the giant planets seem to have similar irregular satellite systems. If proved, this means the giant planets all had similar formation histories," he says.
Planet Known Moons Found 1999-2003
Mercury 0 0
Venus 0 0
Earth 1 0
Mars 2 0
Jupiter 61 45
Saturn 31 13
Uranus 22 4
Neptune 11 3
Pluto 1 0
Total 129 65
