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.