20 More Moons Found Orbiting Jupiter, Bringing Total to 60

Two astronomers surveying the region around Jupiter have detected 20 new moons, bringing the giant planet’s total to 60.

Although Galileo Galilei detected the planet’s four largest moons in 1610, the discovery of dozens of smaller satellites has occurred only recently. The feat has required digital cameras capable of imaging huge swaths of sky and computer programs that can pick out orbiting objects as they slowly cross the night sky.

Since they began their systematic search in late 2000, the two astronomers -- David C. Jewitt, a professor of physics and astronomy at the University of Hawaii, and graduate student Scott S. Sheppard -- have found 43 of the planet’s 60 moons. (Four were discovered by Galileo, four were discovered during a flyby of the Voyager spacecraft and nine were discovered during the 20th century by other astronomers who painstakingly analyzed photographic plates.)

Their latest discovery of 20 moons was made in February, although the findings are just being published today in the journal Nature. “It’s new technology that’s made this possible,” Sheppard said.

The team discovered the moons using the 3.6-meter Canada-France-Hawaii telescope atop Hawaii’s Mauna Kea, which let them see moons as small as 1 mile in diameter. They hope to use the neighboring 8.3-meter Subaru telescope to see moons just one-fourth of a mile across. “They’re getting smaller and smaller,” Sheppard said.

The team estimates that Jupiter has a total of 100 moons that are at least 1 mile in diameter. Jupiter’s moons are named after mythological characters. With the flood of discoveries, though, “we’re starting to run out of names,” Sheppard said.

The new findings add to what one astronomer dubs Jupiter’s “moonopoly.” Jupiter now swamps its closest rival, Saturn, which has 31 moons and was long regarded as the planet with the most satellites.

“Saturn was on top for 230 years -- a big affront to Jupiter,” said Douglas P. Hamilton, an associate professor of astronomy at the University of Maryland. Even Uranus, a diminutive world compared with mighty Jupiter, was briefly thought to have the most moons after three new ones were found circling it in 1999, for a total of 22.

All of the giant planets serve as moon magnets, perhaps because they were able to capture passing asteroids with their gravitational pull and once-robust atmospheres that create friction and drag on objects.

The large number of moons detected around Jupiter is due in part to the fact that Jupiter’s moons appear brighter than those of distant planets because they are closer to both the sun, which illuminates them, and to Earth, from where astronomers view them.

Sheppard said Jupiter’s moons appear to fall into six distinct “families.” The families each have distinctive orbits and include one large moon and several smaller ones, he said.

This is a major clue as to how the moons could have formed. Sheppard and Jewitt believe each family may contain progeny from a larger satellite that shattered when it was hit by a passing comet or collided with another moon.

“We believe we now have Jupiter really figured out,” Sheppard said. Next stop: Saturn.

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Many moons

Researchers recently discovered 20 more moons orbiting Jupiter.

*--* Planet Number of moons Earth 1 Mars 2 Jupiter 60 Saturn 31 Uranus 22 Neptune 11 Pluto 1

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Source: Nature, D. Hamilton