Jupiter is a world of many moons – and now we have found a dozen more, bringing the total count to 79. These new discoveries are tiny, all under 3 kilometres in diameter, and one is really weird.
Scott Sheppard at the Carnegie Institution for Science in Washington DC and his colleagues found these moons during a search for a more elusive object, the theorised Planet Nine that may reside in the outer reaches of the solar system. “We were observing and we realised that Jupiter was high up in the sky,” says Sheppard. “We figured we’d kill two birds with one stone.”
They first spotted the moons in 2017, but they needed to take several observations over the course of a year to show that these small rocks were orbiting Jupiter and not just passing through.
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New moon haul
All the new moons are far from Jupiter, with the closest taking a little less than an Earth year to circle the planet. Spacecraft like Juno, which is circling the gas giant, can only find moons that are near to their own orbits around the planet.
The new moons comprise two main groups: a pair of moons that orbit about 11.5 million kilometres from the planet’s surface, and nine that orbit in the opposite direction to Jupiter’s rotation, about twice as far from the planet.
There is also an odd moon out, unofficially named Valetudo. It is less than a kilometre across and orbits among the outer moons, but in the same direction Jupiter rotates. That means this tiny moon – the smallest ever found around Jupiter – is in danger.
“It’s travelling down the highway in the wrong direction,” says Sheppard. “It’s going to collide with something at some point.” In fact, he says, it has almost certainly been in a collision before: all these tiny moons are likely to be shards of much larger rocks that smashed each other up in orbit.
These larger rocks were probably captured by Jupiter’s gravity as they passed by, rather than directly forming in its orbit. The fact that the fragments didn’t spiral into Jupiter means that the larger rocks must have been smashed up after the planet and its other moons formed, when there was no longer that much gas around to slow the fragments and send them plummeting.
“All the giant planets are like vacuum cleaners: they’ve swept up all the smaller bodies around them,” says Sheppard. Moons such as these new ones that were captured but survived may be the last remnants of the objects that formed the planets, he says, so could help us figure out how our solar system was built.
“If you want to know how Jupiter formed, you have to work backwards and find out what were the original moons that Jupiter started with,” says Douglas Hamilton at the University of Maryland. “The more pieces of those original objects you have, the more likely you are to complete the puzzle.”
This article appeared in print under the headline “A dozen new moons for Jupiter”
