Ten more moons have been confirmed to orbit around Jupiter, bringing the planet’s total known satellite count to 79. That’s the highest number of moons of any planet in the Solar System. And these newly discovered space rocks are giving astronomers insight as to why the Jupiter system looks like it does today.
Astronomers at Carnegie Institution for Science first found these moons in March 2017, along with two others that were already confirmed in June of last year. The team initially found all 12 moons using the Blanco 4-meter telescope in Chile, though finding these objects wasn’t their main goal. Instead, they were searching for incredibly distant small objects — or even planets — that might be lurking in our Solar System beyond Pluto. But as they searched for these fringe space rocks, they decided to take a peek at what might be lurking around Jupiter at the same time. Now, the moons they found have been observed multiple times, and their exact orbits have been submitted for approval from the International Astronomical Union, which officially recognizes celestial bodies.
These moons are all pretty tiny, ranging between less than a mile and nearly two miles wide. And they break down into three different types. Two orbit closer to Jupiter, moving in the same direction that the planet spins. Farther out from those, about 15.5 million miles from the planet, there are nine that rotate in the opposite direction, moving against Jupiter’s rotation. But in this same distant region, one strange moon that astronomers are calling Valetudo is moving with Jupiter’s spin, like the two inner moons. That means it’s going in the opposite direction of all the other moons in the same area. “It’s basically driving down the highway in the wrong direction,” Scott Sheppard, an astronomer at Carnegie who led the discovery team, tells The Verge. “That’s a very unstable situation. Head-on collisions are likely to happen in that situation.”
Valetudo isn’t the only moon of Jupiter that acts this way. Another moon 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. Now with this discovery, 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.
Fortunately, the Blanco 4-meter telescope the scientists were using was the perfect piece of equipment to find these moons. It has the largest 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. Additionally, Blanco’s camera is well-shaded, according to Sheppard, which helped reduce the glare and scattered light from Jupiter.
This also makes Blanco adept at finding incredibly distant, faint objects, which is why the Carnegie scientists have been using the telescope to do a massive survey of bodies beyond Pluto. While scanning the night sky in March of last year, the team noticed that Jupiter happened to be directly overhead in the sky. So they decided to multitask: they would search for objects moving at roughly the same rate as Jupiter — potential moons — as well as objects moving much more slowly in the distant Solar System. After identifying 12 possible Jupiter moons, they reobserved the planet a month later and then again in May of this year with different telescopes to confirm what they had seen.
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.
All of these new moons are exciting for astronomers because they add to an already large group of objects surrounding Jupiter that stem from the earliest days of the Solar System. Unlike the planet’s large inner moons, like Europa and Io, 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. These pieces of rock and dust were likely floating around the Sun as it was forming, and rather than form into other planets, they got captured by Jupiter’s gargantuan gravitational pull. How that happened, though, is still a bit of a mystery. “The question we want to get at if we want to understand how Jupiter formed is what was the environment like that allowed the capture of moons? And how many moons were captured?” says Douglas Hamilton, an astronomer at the University of Maryland who was not part of the discovery team.
But it’s possible that even more moons like this are lurking around Jupiter, waiting to be seen. And the more we locate, the more we learn about how Jupiter became the planet it is today. “What we’re seeing is just the tip of the iceberg,” says Hamilton. “The smaller we look, the more moons we find.”