The solar system's outer planets host a multitude of irregular satellites

Not long ago, the solar system seemed to be a simple, orderly place. Anyone taking a trip to the local planetarium would have heard that the planets have 60 or so moons orbiting them on neat, nearly circular paths. That picture has now become a lot messier. Over the past 6 years, astronomers have discovered a passel of new moons around Jupiter, Saturn, Uranus, and Neptune, more than doubling the known number.

MOONS BY JOVE. Orbits of all the outlying moons of Jupiter relative to the orbit of Callisto, the outermost of the planet’s regular moons. A trove of 44 recently discovered irregular moons is shown in red. Inset: Motion of a faint body (circle) in this sequence of images reveals it’s an irregular satellite of Jupiter. Sheppard, et al./U. Hawaii
CAPTURED OBJECT. Caliban (arrow) is one of the nine known irregular moons of Uranus. Gladman, et al.
SATURNIAN SWARM. The loopy orbits of the irregular moons around Saturn. The planet has 14 known irregulars. B.J. Gray, Planet Pluto

Compared with Earth’s moon and most of the other satellites that planetary scientists have studied throughout the solar system, these recently discovered bodies appear downright unruly. They swoop in and out of the plane in which the planets orbit the sun and have highly elongated, rather than circular, paths. Some even orbit in the direction opposite to the rotation of their host planets. Because of these strange features, these objects are known as irregular satellites.

Most irregulars are tiny–less than 100 kilometers in diameter, or smaller than one-thirtieth the size of Earth’s moon–and can barely be detected from Earth. Many reside so far from their planetary chaperones that gravity barely holds them in place.

Besides revealing our solar system to be far more cluttered than astronomers had suspected, these piffling objects are providing new clues about what conditions were like during the system’s infancy. In particular, these moons may reveal details about a critical, last step in the formation of the outer planets.

For studying planet formation, irregular satellites are “one of the last [unexplored] frontiers,” says Matthew J. Holman of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Satellite revolution

Until recently, discoveries of irregular satellites were nearly as irregular as are the satellites themselves. For more than a century after researchers spotted the first small irregular satellite, Neptune’s Triton in 1846, only a handful had been found. Then, in 1997, a windfall began.

At observatories around the world, exquisitely sensitive solid-state light detectors, known as charge-coupled devices (CCD), had superseded photographic film, enabling astronomers to record objects hundreds of times fainter than ever before. Moreover, using large-format cameras consisting of millions of CCD pixels, researchers could search for the faint objects over large patches of sky.

Those were just the right tools for finding irregular moons, Philip D. Nicholson of Cornell University realized in September 1997. Nicholson and Brett Gladman, now at the University of British Columbia in Vancouver, were traveling to the medium-size Hale Telescope at Mount Palomar in California to search for objects in the Kuiper belt, the reservoir of comets that lies beyond the orbit of Neptune. But while on the airplane en route to Mount Palomar, Nicholson calculated that Uranus would be in the same field of view. He and Gladman decided that during their two nights at the Hale telescope, they and their colleagues would devote any spare time to a search for outlying moons around that planet.

The team succeeded. Gladman and his collaborators discovered the first two irregulars known to orbit Uranus.

A flurry of discoveries followed. Since Gladman’s finding, his team and another have spotted 70 more of the irregular satellites. Astronomers announced the latest find, an irregular satellite of Uranus, in an Oct. 9 circular of the International Astronomical Union.

The number of moons added to the roster since 1997 is “stunning,” says Joseph A. Burns of Cornell.

Finding irregulars isn’t just a matter of scouring the sky with a sensitive detector. In a single snapshot of the heavens, a tiny moon can look just like a galaxy billions of light-years distant. But there’s one distinguishing feature: The motions of moons are discernible as each inches across the sky in synchrony with the planet it orbits. In contrast, distant galaxies appear to remain still.

Astronomers use two strategies to identify distant moons. One method, recently employed by Holman, J.J. Kavelaars of the National Research Council of Canada in Victoria, and Gladman uses a medium-size telescope to take a dozen or so precisely timed images of the same patch of sky during a single night.

In each image, they then shift all objects back to the position they would have had, if they truly were moons, at the time when the astronomers took their first exposure of the night. Finally, they combine the images.

In this so-called shift-and-add technique, the objects that are satellites end up in exactly the same position in each superimposed image, producing a bright, easy-to-spot point of light.

The other strategy, adopted by David C. Jewitt and Scott S. Sheppard of the University of Hawaii in Honolulu, uses a large telescope capable of finding extremely faint bodies in individual images. The astronomers take three sequential images of a patch of sky. A computer scans the trio for any object that has changed position from one image to the next. Starting with that information on the object’s location, the team can then track the candidate satellite with a smaller telescope to discern its orbit and motion.

With these moon-spotting techniques, both teams have vastly enlarged the satellite retinue. Jupiter is the undisputed king, with 53 irregular satellites. This is followed by Saturn, which has 14; Uranus, with 9; and Neptune, with 7.

By finding large numbers of irregulars and measuring their dynamic and physical properties, researchers hope to uncover new insights about planet formation, says Jewitt.

Although astronomers continue to scan the distant reaches around the four outer planets for more irregular satellites, Gladman says observers have nearly reached the limit of what they can do with current telescopes and CCD cameras. Now, he says, “it’s time to figure out what it all means.”

All about moons

According to theorists, all the planets formed billions of years ago from a disk of gas, dust, and ice particles that surrounded the young sun. At first, this disk, or protoplanetary nebula, consisted of tiny particles only micrometers to centimeters in diameter. But the particles eventually coalesced into boulder-size bodies, some of which ultimately merged to make planet-size embryos.

The standard picture of planet formation has the solid cores of rock and ice in Jupiter and Saturn providing the gravitational muscle to snare enormous amounts of hydrogen and helium gas from the protoplanetary disk. Over about 10 million years, these planets became the gas giants they are today.

In contrast, Uranus and Neptune currently don’t have massive, gaseous envelopes, and it’s unclear whether they ever did so. These planets are akin to the heavy, icy cores of Jupiter and Saturn.

The so-called regular moons have several features in common with the planets they orbit. This suggests that each retinue of regular moons was born around the planet that it circles. The moons coalesced from miniature versions of the protoplanetary nebula that surrounded their fledgling planets.

However, irregular moons almost certainly weren’t formed where they now reside. Rather, astronomers suspect that irregulars, like asteroids and comets, are debris left over from the planet-making process. These small objects never stuck together to make planet-size bodies but instead zipped through the solar system at some 5 to 8 kilometers per second. As they roamed the young solar system, some of these bits of detritus were gravitationally captured by newly formed planets.

Before being captured, these small bodies had to be slowed down. Today, it would be nearly impossible for a planet to trap such a high-speed body. But some theorists have proposed that in their youth, Jupiter and Saturn had bloated atmospheres that extended far above the current extent of their cloud tops. A body whizzing through this extended atmosphere would encounter friction, slowing it down enough to fall under the influence of the planet’s gravity.

“If this is right, we should see the irregulars as survivors of gas giant-planet formation,” notes Jewitt. “The captured satellites in this model would consist of exactly the same type of matter that went into growing the ice-rock cores of the outer planets.” He views the distant moons as “bits that were neither incorporated into the planets nor thrown out of the system.”

The most exciting thing about the irregulars, says Jewitt, is that they haven’t been altered by the heat and countless collisions that merged other bodies into planets. The irregulars appear to be pristine relics of planet formation, refugees from the era 4.5 billion years ago when the solar system emerged.

Irregular influences

Things get more complicated in a competing model of planet formation known as the instability model. In that scenario, proposed by Alan P. Boss of the Carnegie Institution of Washington (D.C.), Jupiter and Saturn didn’t grow bit by bit.

In his view, they never had a solid core that then could gradually annex a massive envelope of gas. Instead, he argues, these giant planets arose rapidly when a huge, puffy cloud of material within the protoplanetary nebula suddenly collapsed.

In this fast-paced scenario, as in the previous one, friction from gas surrounding the planets would slow down passing objects, permitting them to be captured as irregulars, notes Boss.

Boss’ model presupposes that the solid bodies destined to become irregulars were already roaming the solar system when the outer planets appeared, says Jewitt. “Some models of giant-planet formation involve very short timescales, just thousands to tens of thousands of years,” he notes. “It’s not obvious that . . . the largest irregulars would have formed on such short timescales.”

Boss says that although the outer planets may have formed quickly, they probably didn’t begin forming until after the infant solar system was a few hundred thousand years old. So, they didn’t reach maturity until a multitude of potential irregular satellites had been produced.

Boss’ theory about the formation of Uranus and Neptune may be at odds with the presence of the irregulars around these planets, according to Jewitt. In Boss’ model, the young Uranus and Neptune had huge envelopes of gas, just as Jupiter and Saturn do. But because Uranus and Neptune reside farther out in the solar system, they were more vulnerable to the ultraviolet light from a passing star. Boss proposes that such radiation would have stripped away the gas that had once enveloped Uranus and Neptune, leaving behind giant cores of ice–the planets as we know them today.

Jewitt takes issue with this model. He suggests that as the vast shrouds of gas around Uranus and Neptune were ablated, the planets would suddenly have had a lot less gravity and any irregulars would have escaped. But if Uranus and Neptune captured satellites in a different manner than Jupiter and Saturn did, then the similarity of all of these satellite systems begs an explanation.

Several teams have recently found that some of the irregulars orbiting each planet belong to the same family, an indication of a shared origin. For instance, Jupiter’s Himalia and few other irregulars orbiting the giant planet have colors and orbital characteristics similar to those of the Hilda group of asteroids, which lies in the outer part of the asteroid belt. That suggests these Jovian moons are captured members of the Hilda asteroid group. Burns and Matija Cuk of Cornell describe their analysis in an upcoming Icarus.

The resemblances among some irregulars strongly suggest that members of each group derive from one large body that was fragmented–either by another satellite or a passing comet. For such collisions to have been commonplace, there must have been “much larger initial satellite populations or a much larger initial population of comets and asteroids than scientists now observe, says Jewitt. “Both seem plausible,” he adds.

The collision theory will soon be put to the test. When the Cassini spacecraft enters orbit around Saturn this summer, it will pass near the planet’s largest irregular satellite, Phoebe. The spacecraft’s images should reveal whether Phoebe has the large gouge that would be expected if it had suffered an ancient collision that gave rise to a host of smaller irregulars orbiting Saturn.

“In the last 10 years or so . . . we have made observations that give a new and different view of our solar system,” says Jewitt. “Where this will lead is still unclear, but few people would doubt that the end result will be a much improved understanding of the complex process of planet formation.”

Puzzling Pair

The split personality of Mars’ moons

While scientists analyze the spate of irregular moons recently found around the outer planets, they continue to puzzle over two more familiar objects–Phobos and Deimos, the two moons that orbit Mars. “These two satellites are still a mystery,” says Scott S. Sheppard of the University of Hawaii in Honolulu. “Their orbits are not like the irregular satellites of the giant planets,” since they’re close to Mars and have nearly circular orbits with nearly zero inclination, he notes. “It is hard to obtain orbits like the current Martian satellites have from captured bodies.”

On the other hand, he notes, their color resembles that of C asteroids, a common type of asteroid found in the outer part of the asteroid belt. So “based on their physical characteristics, they may be captured asteroids and should be classified as irregulars,” says Sheppard. “It is still a debate if they were captured or not.”


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