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Newfound Moons Tell Secrets of Solar System

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This article is posted on the New York Times website. You have to be registered at the site to view articles so, to save some time I've posted the article in its entirety here:


Not too long ago, it was easy for an armchair astronomer to keep up to speed on the moons of the solar system. There was the Moon, of course, and the four Jovian satellites spotted by Galileo, those two around Mars, and some odd ones here and there — that weird fractured cue ball orbiting Uranus, for instance.

These days, though, it is tough to tell the moons without a scorecard. In the past six years, dozens of satellites have been discovered around the giant planets, more than doubling the total in the solar system. Jupiter is the current leader, with 61, followed by Saturn, Uranus and Neptune. The tally for these four planets is 124 (the other five planets have only four among them), but that number is sure to change in the next year or two.

"They're all over the place," said Dr. Brett Gladman, an associate professor at the University of British Columbia who has been involved in the discoveries since 1997.

The new moons are very small, many just a couple of miles in diameter, and carve distant, eccentric paths around their planets. Many orbit in retrograde fashion, in a direction opposite to their planets' rotations.

They have little in common with the large moons in the solar system — none of the volcanoes of Jupiter's Io, or the atmosphere of Saturn's Titan, or the massive rifts of Uranus's Miranda. Many don't even have names yet. They are little more than orbiting rocks.

Yet scientists say these moons offer some of the only clues to the early years of the solar system. They are a window into the past, some 4.5 billion years ago, when the planets formed from a swirling nebular disk of gas and dust.

"Before this we had very little information" about those early times, Dr. Gladman said. "Now we have lots of different tidbits to look at."

Those tidbits have been discovered thanks to improved technology, particularly the development of larger charge-coupled device cameras. Used with some of the world's largest telescopes, these C.C.D. cameras, which use the same basic technology as consumer digital cameras, but are much bigger and more precise, complex and expensive, have enabled astronomers to track fainter objects in wider swaths of the sky.

"What's happened with satellites is what's happened all through astronomy," said Dr. David Jewitt, a professor at the University of Hawaii. "We are much more efficient than anyone before could be."

With a graduate student, Scott S. Sheppard, and other colleagues, Dr. Jewitt has discovered most of the new moons of Jupiter, including 21 this year, using two large telescopes on Mauna Kea in Hawaii that have the world's largest C.C.D. cameras.

Like Dr. Gladman and his colleagues, co-discoverers of nine of the new Jovian moons as well as others around Uranus, Neptune and Saturn, they aim the telescopes at parts of the sky corresponding to the region of space where Jupiter's gravity is dominant, an area called the Hill sphere.

Jupiter is massive, with a surface gravity two and a half times that of Earth, so its Hill sphere is very large. "You have to search a good fraction of the sky to find these satellites," Mr. Sheppard said. "We can go very deep and cover a big area at the same time."

By taking multiple images of the same area over time and using computers to analyze them for anything that moves, the astronomers come up with potential moons. Follow-up observations and more data crunching enable an object's orbit and size to be calculated, distinguishing a satellite from, say, an Earth-crossing asteroid in orbit around the Sun.

It is tedious to find objects that are among the faintest in the night sky; the satellites' light comes from reflected sunlight, which diminishes greatly on its way from the Sun and even more on its way back to observers on Earth. But the goal is not numbers for numbers' sake, to put Jupiter in the record book as the Planet With the Most Moons.

"Irregular satellites have been known as a class for basically a whole century," said Dr. Jewitt said. Around 1900, astronomers discovered larger irregulars.

"But we don't know why planets have them," Dr. Jewitt continued. "The reason is, the sample is too small."

By finding more satellites, astronomers hope to be able to make some generalizations about their origin and about the early solar system.

"I really do think the answer has to be an observational answer," Dr. Jewitt added. "We're really groping in the dark theoretically."

One major clue is already apparent. The fact that most of the satellites' orbits are retrograde and eccentric speaks volumes about their origins: They had to have come from elsewhere, and been captured by the planets at some point. If they formed at the same time as the planets, from the spinning nebular disk, their orbits would be nearly circular and in the same direction as the planets' rotation, like the "regular" moons.

Capture, in turn, implies something else about these objects: They had to have become satellites early on. The capture mechanism requires conditions that do not exist now.

"Objects don't just come dawdling by and get captured by a planet," Dr. Gladman said. "They have to have some way to dissipate energy."

In the case of the irregular satellites, they could not have shifted from an orbit around the Sun to an orbit around one of the giant planets without slowing down — through friction in an atmosphere, perhaps; the influence of gravity; or a collision with another object. But Jupiter's current atmosphere is too compact for this to happen, and its gravity, though strong, is not strong enough. And there are not enough objects flying through the solar system to make a recent collision likely.

"We don't have any current model for dissipation," Dr. Jewitt said. "The suggestion is that capture must have happened at a much different time."

That different time is in the first few million years after the formation of the planets, according to several theories as to the conditions that could have led to capture.

Perhaps the leading theory, at least for Jupiter and Saturn, is the "gas drag" idea, that the planets' atmospheres were at one point bloated, extending millions of miles beyond their current envelopes. This gas would have created enough friction to slow down some planetesimals — the rocky objects in the solar system that were the precursors to the planets — so that they would go into orbit around the planetary core.

A bloated atmosphere may in turn have been a function of the way these planets formed, said Dr. David Nesvorny, a research scientist at the Southwest Research Institute in Boulder, Colo. If, for example, Jupiter's core itself was formed by the accretion of many planetesimals, then at some point it became massive enough to start attracting gas from the spiraling nebula disk.

"Basically, Jupiter was sucking gas from this disk," Dr. Nesvorny said, and these streams of gas could have been the atmosphere that would have slowed other planetesimals.

But there are two other possibilities for capture, Dr. Nesvorny said. One is that rapid growth of the core led to a corresponding increase in gravity, enough to pull down a nearby object. The other is that captured objects were a result of a collision between two planetesimals, the force of the collision being enough to dissipate the energy of at least one of them.

Either of these two theories may be a more likely explanation for the satellites of Uranus and Neptune, which formed differently from Jupiter and Saturn, without the large amounts of gas. (Earth and the other inner, terrestrial planets had neither massive cores or gas, which is one reason they have so few moons.)

If gas drag was the mechanism for Jupiter, then scientists would expect to see a certain size range among the planet's irregular satellites. Small objects would have slowed too much and spiraled into the core. Large objects would not have slowed enough and would have continued along their way around the Sun. But an object that was just right — perhaps 10 to 100 miles in diameter — would have been slowed enough to bind it to the planet in permanent orbit.

"One issue is what's the size distribution of these satellites," Dr. Jewitt said. "There may be a cut-off. We hope to address that observationally."

The issue is complicated by the fact that most of the satellites that have recently been discovered are in fact just fragments of larger objects.

"Each of these giant planets captured a few relatively large satellites, then they got smashed up" through collisions with other objects, Dr. Gladman said. That, too, would have happened early in the history of the planets, he added, because there were far more rogue objects flying through space than there are now.

Despite the difficulties in interpreting the discoveries, astronomers are still trying to find more moons. Dr. Jewitt and Mr. Sheppard, for instance, are trying to find more Jovian satellites — Mr. Sheppard thinks there are probably 100 of a kilometer in diameter or larger — and want to search for even fainter, and thus smaller, objects.

But Dr. Gladman theorizes that the days of discovering large numbers of new moons is probably over, that the major technological advancements that improved observing ability have already been achieved.

"The easy part's been done," he said. "There's not going to be a repeat of this thing."

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