As the most massive planet in the solar system by a wide margin, Jupiter has a lot of pull in this neighborhood. With dozens of moons whizzing around it, it makes sense that a few have slipped under the radar, but the latest discovery is still a surprisingly large haul. Astronomers have announced the detection of 12 new moons orbiting Jupiter, including one particularly reckless "oddball."
With the grand total now up to 79, astronomers categorize Jupiter's moons into several groups. The four largest – Europa, Io, Ganymede and Callisto – are known as the Galilean moons, after their famous discoverer. A bit further out is a "prograde" group that orbits in the same direction as Jupiter's spin, while a more distant swarm is a retrograde group, orbiting in the opposite direction.
Of the 12 newly discovered moons, two are in the prograde group. They take a bit under a year to orbit Jupiter, and given the similarities in distance and angle of their orbits, objects in this group overall are believed to be the shattered remains of a single larger moon.
Nine of the new moons are out in the retrograde group, where they orbit once every two years or so. Objects in this group are thought to come from three larger moons that were torn apart by collisions.
If you've done your math, you'll notice there's still one new moon unaccounted for. This so-called oddball doesn't really fit into any other group, but while it's off doing its own thing it might be putting itself in danger.
"Our other discovery is a real oddball and has an orbit like no other known Jovian moon," says Scott Sheppard, lead researcher on the project. "It's also likely Jupiter's smallest known moon, being less than 1 km (0.6 mi) in diameter."
This moon has a prograde orbit, but it isn't following the path of that group. Instead it's further out, so that its orbit actually crosses those of the retrograde moons. Basically, it's zooming around in the wrong lane, posing a risk of a head-on crash with oncoming traffic.
"This is an unstable situation," says Sheppard. "Head-on collisions would quickly break apart and grind the objects down to dust."
These kinds of smash-ups in the past, the team says, could be responsible for the current groupings of moons we see today.
The discovery of these 12 new moons was made in (Northern Hemisphere) spring 2017, when Jupiter just happened to photobomb the observations of a team of astronomers looking for the mythical Planet Nine, on the fringes of the solar system.
"Jupiter just happened to be in the sky near the search fields where we were looking for extremely distant solar system objects, so we were serendipitously able to look for new moons around Jupiter while at the same time looking for planets at the fringes of our solar system," says Sheppard.
While most of the new moons haven't yet been named, the name Valetudo has been proposed for the "oddball." In Roman mythology, Valetudo was Jupiter's great-gradndaughter, and was known as the goddess of health and hygiene.
The team describes the research in the video below.
Source: Carnegie Institution for Science
