Captured moons of the giant planets

This article originally appeared in the February 2011 issue of Astronomy magazine.

For more than a century, astronomers have known that natural satellites in the solar system don’t only orbit in close, nearly circular tracks in the same plane as their host worlds. Sometimes objects have distant and highly elliptical orbits, and they are called irregular satellites. Often their trajectories are tilted to the planet’s plane. Their orbits also tend to precess, meaning they trace out more of a loop-de-loop pattern instead of a simple ellipse. And most irregular satellites orbit in the direction opposite their planets’ rotations, called retrograde motion.

Astronomers believe ordinary satellites formed with their planets. However, the characteristics of irregular satellites imply they did not form in this way, but instead were captured. But when, and how?

In the late 20th and early 21st centuries, astronomers discovered a surprising number of irregular satellites. They had known about Saturn’s oddball Phoebe and Jupiter’s Himalia since the early 1900s. But then the tally jumped from 10 to 107 in less than 10 years thanks to technological advancements. The count has slowed recently while scientists put more of their efforts into characterizing the bodies they know of and learning where they came from.

Where are they?

The irregular-satellite haul geared up as astronomers attached wide-field CCD cameras to large telescopes. In 1997, Brett Gladman, now at the University of British Columbia, and colleagues used the Hale Telescope at Palomar Observatory to discover two satellites orbiting Uranus. While this team focused on Uranus, Neptune, and Saturn, David Jewitt, now at the University of California, Los Angeles, and Scott Sheppard, now at the Carnegie Institution of Washington, discovered the majority of the objects around Jupiter. They used both the Canada-France-Hawaii and Subaru telescopes atop Mauna Kea for their searches.

The teams looking for irregular satellites surveyed regions called “Hill spheres” around each of the solar system’s giant planets — Jupiter, Saturn, Uranus, and Neptune. Within such an region, the planet’s gravitational pull is greater than the Sun’s. The astronomers searched each planet’s Hill sphere because if an object passes through this region at a slow pace relative to the planet’s orbital speed, the gravitational pull could capture the incoming object as a satellite. 

But this doesn’t mean that any region within the Hill sphere provides stable orbits. The sphere’s outer half is mostly unstable, as solar tides can tear away any satellites not firmly gravitationally attached to the planet. Thus, astronomers have found most irregular satellites within the inner half of the giant planets’ Hill spheres.

Astronomers also haven’t found irregular satellites orbiting the poles of the planets. This region is gravitationally unstable. The “Kozai resonance” is to blame for the lack of orbiting objects here. Gravity perturbs the orbit, which becomes more elliptical to compensate. Eventually, the orbit will be so stretched out that the planet can lose control of the satellite as it reaches its path’s greatest distance, or the object can veer into the planet at its orbit’s closest approach.

The irregular satellites have outlasted many gravitational encounters during their lifetimes, but how did they get to their locations in the first place? And what do astronomers know about these objects? First astronomers have to learn as much as possible about the satellites before determining where they came from.

Observing these oddities

In the past few years, scientists have stopped looking for irregular satellites and instead are analyzing those currently known. “Even if we push it to 200, it probably won’t change the picture that much,” says Jewitt.

An object’s spectrum can tell a researcher about its composition and motion. Unfortunately, most irregular satellites are too faint for astronomers to gather such spectra. (They have detailed information for only three — Phoebe, Himalia, and Neptune’s Triton.) So instead, observational efforts measure colors and “show how color is a proxy for composition,” says Jewitt. His focus has shifted to this research using sensitive optical telescopes.

Astronomers have collected colors for about two dozen irregular satellites that orbit Jupiter and Saturn — they’re closer and thus brighter than those around Uranus and Neptune. When astronomers analyze the colors of the irregular satellites, they find that most of the objects look similar, which implies a common source.

But even color observations are reaching a technological limit. “A 30-meter telescope will help, or one needs to send more spacecraft to the outer solar system,” says David Nesvorny of the Southwest Research Institute in Boulder, Colorado.

“Close-up, good physical observations are not likely to happen anytime soon,” says Sheppard. “Probably not within our lifetimes.” This means astronomers are doing what they can, now, to answer the questions about irregular satellites.

Jewitt adds: “What’s happened in the last few years is that there’s been this burst of dynamical excitement motivated by the burst of new observations and characterizations, and so maybe that will take us somewhere to understanding these bodies.”

Capturing bodies

So where did irregular satellites come from? In the 1970s, three theories emerged: two related to the protoplanetary gas and dust disk in the early solar system, and the other requiring chance flybys.

In our system’s youth, many small icy bodies (called planetesimals) orbited the Sun. The “gas drag” model says that occasionally one of these objects dove into the disk and slowed due to friction. At that point, the planet’s gravity could capture this object.

The “pull down” method proposes that as the gas giant rapidly accreted a large amount of surrounding gas, its Hill sphere increased quickly. This trapped any nearby small bodies. However, both of these capture methods pertain to the gas giants’ formation and not to how the ice giants (Uranus and Neptune) formed. So neither provides a satisfying answer to the question of how irregular satellites ended up at the farthest planets.

The third theory — “three-body interactions” — could partly explain the capture of these bodies. This posits that a planet and a binary planetesimal, or two planets and one small icy body, could result in an object becoming gravitationally trapped. 

The model implies that for each of the giant planets, the chances of capturing bodies relates to its mass — and thus the size of its Hill sphere — after the planets reach their “established” size. But more importantly, the three-body interactions don’t relate to only the planets’ formation; the model applies to later periods, too.

Model citizens

Even if the giant planets trapped a plethora of irregular satellites as they formed, the solar system may have undergone an upheaveal period. Any loosely bound objects would have been torn away from their parent worlds.

In 2005, astronomers published a hypothesis regarding the giant planets’ evolution. The Nice model (pronounced “niece” and named after the city in France) suggests that the outer planets likely formed nearer to each other and closer to the Sun than they are now. The simulations imply that after the protoplanetary disk evaporated, the giant planets originally existed between 5 and 17 astronomical units (AU, the Earth-Sun distance) from the Sun. Just outside the farthest ice giant sat a disk of planetesimals that extended to about 35 AU.

According to the model, the orbits slowly “wiggled” over time. Then, about 600 million years into the model, Jupiter and Saturn passed through orbits that coincided with a 2:1 resonance (meaning, Jupiter made two revolutions for every one that Saturn made). The combined gravitational effect forced Jupiter, Saturn, Uranus, and Neptune to scatter from each other. As the planets interacted, they tweaked the orbits of any small objects in their paths.

Uranus and Neptune both moved into the planetesimal disk. Thus, small icy bodies scattered everywhere: Some escaped the solar system, some moved toward the inner solar system and collided with planets, others remained in this disk, and the giant planets grabbed some as “new” irregular satellites.

So what’s the proof of this early solar system upheaval? “This model makes predictions about the solar system that match observations,” explains Nesvorny. Its simulated orbits coincide with those of Jupiter, Saturn, Uranus, and Neptune. The icy bodies that remained in the disk match up with some of the objects in the Kuiper Belt — the disk of icy bodies beyond Neptune. Those that moved to the inner solar system could explain the “late heavy bombardment,” which corresponds to craters seen on our Moon’s surface.

According to Nesvorny: “Irregular satellites were sort of a puzzle when it was realized that their loosely bound orbits do not withstand the epoch of planetary encounters.” So he and colleagues performed simulations building off the Nice model to see where the gas giants’ current irregular satellites came from. “I got an idea in 2007 that while the existing populations of irregular satellites go away, new populations can be captured from the transplanetary disk. This is because planetary encounters typically happen in a region that is rather densely populated by planetesimals.”

During the upheaval period, our solar system was a chaotic place with frequent collisions and near misses. If a binary asteroid or binary Kuiper Belt object (KBO) came close to a massive planet, one of those objects might have been gravitationally trapped while its companion was flung away. 

Nesvorny’s team’s first paper showed how Neptune, Uranus, and Saturn got their irregular satellites. However, it couldn’t explain those at Jupiter. So it went back to their simulation and made a few changes. “They have succeeded in tweaking their model such that it does give irregular satellites at Jupiter as well,” explains Jewitt.

Extend the model to the belt

According to Nesvorny, the Nice model also accounts for the structures scientists see in the Kuiper Belt and its different groups of objects. The “classical” objects are named as such because their orbits are what astronomers originally expected KBOs would follow. Scientists divide them into dynamically “cold” classical and “hot” classical because the objects have slightly different characteristics. The orbits of the cold ones aren’t tilted as much as those of the hot classical KBOs. The cold bodies also appear redder and are brighter than the hot objects.

The Kuiper Belt also contains two other groups of objects: those in resonances with Neptune (Pluto is one example); and the “scattered disk,” which occasionally pass close enough to Neptune to gravitationally interact.

When looking at the Kuiper Belt, scientists find that while some KBOs look redder than the irregular satellites, other object groupings have colors similar to the irregular satellites. Is this a coincidence? Or were the objects from the same place?

Perhaps the planetesimals scattered during the reshuffling event moved into the Kuiper Belt as the hot classical, resonant, and scattered disk KBOs. These were the objects that, according to the Nice model, started out between about 17 and 35 AU. Nesvorny’s team argues that the cold classical KBOs formed right where they currently are, at 45 AU. The fact that the cold and hot ones came from different populations explains why they have different characteristics.

Collisions abound

Not all scientists agrees that the Nice model describes where the irregular satellites come from, or that these objects are the same as KBOs. “I think the Kuiper Belt model is plausible,” says Jewitt. “I think it’s very flexible, and it can fit various measurements, which is a good thing. But I don’t see any killer evidence that that’s what happened as opposed to some other thing.”

But that’s the whole point of testing new models — to try to disprove or prove competing ideas in order to figure out the best-fitting one. The study of irregular satellites has exploded in the past decade. “The fact that people are thinking about this is what’s exciting,” says Jewitt.

One concept scientists do agree on is that the planets must have captured their irregular satellites, and it had to happen in the solar system’s younger days. That’s when there was a lot more “stuff” flying around. And if more objects were getting in each other’s way, collisions must have been more common. Astronomers see evidence of such collisions on the most studied irregular satellite. Observations of Jupiter’s Hill sphere show a dust density about 10 times higher than expected, which suggests a large number of collisions also probably occurred within that region.

The quest to understand irregular satellites around the solar system’s giant planets begins with the most important questions. “Where from, and when, and how are still not really known,” says Jewitt.
Because technology has limited the observers, this work is currently performed with computer simulations. Different groups tackle these questions from different angles. But until astronomers have their 80-foot-wide optical telescopes, such modeling is the best way they’ll get their answers.