A dozen new moons have been discovered around Jupiter. This brings the total number of known moons around the planet to 79 – the most anywhere in our Solar System. While 11 of them appear to be part of previously known groups of moons, one is unlike anything observed before.
The moons were discovered by a team of astronomers looking for Planet 9, a large object theorised to exist in the distant Solar System beyond Pluto.
“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 on the fringes of our Solar System,” says Scott S Sheppard from the Carnegie Institution for Science, who led the discovery.
The moons’ discovery was announced on 17 July 2018, however the moons were first spotted in 2017.
It has taken the last year to collect the observations needed to calculate the object’s orbits, proving they are moons of Jupiter.
Nine of the moons are part of a distant swarm moving in retrograde, meaning that they orbit the opposite direction to Jupiter’s spin.
Two are closer to Jupiter and have a prograde motion. All 11 are thought to be the remnants of larger moons that broke up in Jupiter’s early history.
“Our other discovery is a real oddball and has an orbit like no other known Jovian moon, says Sheppard. “It’s also likely Jupiter’s smallest known moon, being less than one kilometre in diameter.”
This peculiar moon, which the team call Valetudo, moves in prograde but its orbit takes it out to the same distance as the retrograde moons.
It’s thought that the ‘oddball’ is the remnant of a larger prograde moon which collided head on with something, creating the retrograde moon swarms.
By studying the interplay between these objects, astronomers can understand the conditions around early Jupiter.
For instance, the gas and dust surrounding Jupiter during its formation must have dissipated by the time the moons were created, or else the drag would have slowed their orbit causing them to crash into the planet.
Iain Todd is BBC Sky at Night Magazine's Staff Writer. He fell in love with the night sky when he caught his first glimpse of Orion, aged 10.