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Saturn Adds 62 More Moons to Its Count

If all the objects are recognized by scientific authorities, the ringed giant world will have 145 moons in its orbit.

These two giant worlds are late in their bout for satellite-based supremacy. But now the fight over which planet has the most moons in its orbit has swung decisively in Saturn’s favor.

“They both have many, many moons,” said Scott Sheppard, an astronomer from the Carnegie Institution for Science in Washington, D.C. But Saturn “appears to have significantly more,” he said, for reasons that are not entirely understood.

The newly discovered moons of Saturn are nothing like the bright object in Earth’s night sky. They are irregularly shaped, like potatoes, and no more than one or two miles across. They orbit far from the planet too, between six million and 18 million miles, compared with larger moons, like Titan, that mostly orbit within a million miles of Saturn. Yet these small irregular moons are fascinating in their own right. They are mostly clumped together in groups, and they may be remnants of larger moons that shattered while orbiting Saturn.

Pan shines in the Encke Gap in Saturn’s A ring. The dozens of moons recently discovered by astronomers are even smaller than Pan.

The growing number of moons also highlights potential debates over what constitutes a moon.

“The simple definition of a moon is that it’s an object that orbits a planet,” Dr. Sheppard said. An object’s size, for the moment, doesn’t matter.

The new moons were discovered by two groups, one led by Dr. Sheppard and the other more recently by Edward Ashton of the Academia Sinica Institute of Astronomy and Astrophysics in Taiwan. Dr. Sheppard’s group, in the mid-2000s, used the Subaru Telescope in Hawaii to hunt for more moons around Saturn.

Most of Saturn’s irregular moons orbit the planet in what astronomers call the Inuit, Norse and Gallic groups. Each group’s objects may be the remains of larger moons, up to 150 miles across, that once orbited Saturn but were destroyed by impacts from asteroids or comets, or collisions between two moons. “It shows there’s a big collision history around these planets,” Dr. Sheppard said.

Those original moons may have been captured by Saturn “very early on in the solar system,” Dr. Ashton said, perhaps in the first few hundred million years after its formation 4.5 billion years ago. Not all orbit in these groups, however, with a few rogue moons orbiting in a retrograde direction — that is, opposite to the orbits of the other moons.

“We don’t know what’s happening with those retrograde moons,” Dr. Sheppard said. Dr. Ashton suspects they may be remnants of a more recent collision.

Learning more about the new moons is difficult owing to their small size and remote orbits. They appear to be a special class of object, different from asteroids that formed in the inner solar system and comets in the outer solar system. But not much more is known.

“These objects might be unique,” Dr. Sheppard said. “They might be the last remnants of what formed in the giant planet region, likely very icy-rich objects.”

Closer observations of Saturn’s tiny moons could give scientists a window into a tumultuous time in the early solar system. During that period, collisions were more common and the planets jostled for position, with Jupiter thought to have migrated from nearer the sun farther out to its current orbit. “That gives you additional information on the formation of the solar system,” Dr. Denk said.

Yet the irregular moons we are seeing so far may only be the beginning. “We estimated that there are potentially thousands,” around Saturn and Jupiter, Dr. Ashton said. Uranus and Neptune, too, may have many such irregular moons, but their vast distance from the sun makes them difficult to discover.

Saturn, despite being smaller than Jupiter, appears to have many more irregular moons. It may have three times as many as Jupiter, down to about two miles in size. The reason is unclear, Dr. Ashton said.

Jupiter’s original moons may have tended to be larger, and less likely to shatter. Or Saturn may have captured more objects into its orbit than Jupiter. Or Saturn’s moons may have been on orbits that were more likely to overlap and collide, producing smaller, irregular moons.

Whatever the reason, the outcome is clear. Jupiter is on the ropes, and it is unlikely to recover its title as the planet with the most moons. As astronomers’ capabilities to find smaller and smaller satellites improve, “Saturn will win by miles,” Dr. Alexandersen said. “I don’t think it’s a contest any more.”

NASA’s Cassini spacecraft burned up in Saturn’s atmosphere on Friday, after 20 years in space.

Category: Science

Source: NYTimes Science

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