Jupiter now has a total known satellite count of 79, more than any other planet in our system. Ten more moons have been confirmed to orbit around the gas giant. Astronomers at Carnegie Institution for Science were looking for incredibly small distant objects in March of 2017, and said to themselves, hey, what the heck, while we’re at it, let’s look at Jupiter while we’re at it.
The moons they found have now been observed multiple times, and they’ve worked out the orbits and submitted them to the International Astronomical Union, which officially recognizes celestial bodies. What they found out was that there’s an inner zone wherein satellites are swinging around Jupiter in a prograde direction, and there’s one more in the busy main satellite zone that’s also moving prograde. “It’s basically driving down the highway in the wrong direction,” said Scott Sheppard, an astronomer at Carnegie who led the discovery teame. “That’s a very unstable situation. Head-on collisions are likely to happen in that situation.”
The newly discovered moons are all very small, two miles wide at most with some under a mile in diameter. They break down into three different types. Two orbit closer to Jupiter, moving in the same direction that the planet spins. Farther out, about 15.5 million miles from the surface, there are nine moving in the opposite direction, or retrograde. That’s where the one oddball is too. It’s called Valetudo, and it’s moving with Jupiter’s spin, like the two inner moons.
Another moon of Jupiter called Carpo also orbits far out from Jupiter, moving in the opposite direction of many other moons in the area. However, Valetudo orbits much farther away than Carpo, and it may actually be the smallest moon Jupiter has. Astronomers think it’s good evidence that moon-on-moon collisions have happened in Jupiter’s past, and these are responsible for the lunar landscape around the planet today. “Valetudo, at just 1 kilometer across, is probably the last remnant of a much larger moon that’s been ground down into dust over time,” says Sheppard.
Finding moons around Jupiter can be tough. As the biggest planet in our Solar System, it has a very large area of influence, so there’s a lot of space where moons could potentially be. It’s difficult to search that area in a timely manner with a telescope. “It’s like looking through a straw, and you’re just covering as many points around Jupiter as you can looking for these things,” says Sheppard. And since Jupiter is so large, it reflects a whole lot of light. That means there can be a lot of glare when searching for super faint moons around the planet.
The Blanco 4-meter telescope used to find the moons was perfectly suited to the task. It’s got the biggest camera of any large-class telescope out there, and it allowed the astronomers to cover a big area of space around Jupiter in a shorter amount of time, allowing them to get the job done while requiring far fewer photographs. Additionally, Blanco’s camera is well-shaded, according to Sheppard, which helped reduce the glare and scattered light from Jupiter.
Sheppard believes this new crop of moons tells a big story about Jupiter’s past. The astronomers argue that those nine moons, all moving in the same direction far out from Jupiter, may actually be pieces of a bigger moon that existed long ago. Some of them share specific traits with each other, like the same orbital angles, which makes the scientists think that these moons are actually fragments of three larger moons. “We think, originally, there were three parent bodies, and, somehow, each of those parent bodies got broken apart. And a big question is: what broke those objects apart?” says Sheppard. That’s where Valetudo comes in. With a moon like that nearby, it’s possible that numerous head-on collisions occurred, reducing these objects to the small sizes we see today.
The newly discovered moons are all likely from the earliest days of the Solar System. This large cache of moons orbiting far out from Jupiter is thought to be made up of the same material that served as the building blocks for the planets when the system was first forming.
There may be even more tiny moons out there, waiting to be discovered. “What we’re seeing is just the tip of the iceberg,” says Hamilton. “The smaller we look, the more moons we find.”