"This is an unstable situation", continued Sheppard. Like the others, they, too, are thought to be fragments of a larger moon that slammed into something else.
For example, the discovery that the smallest moons in Jupiter's various orbital groups are still abundant suggests the collisions that created them occurred after the era of planet formation, when the Sun was still surrounded by a rotating disk of gas and dust from which the planets were born.
The latest count of 79 known moons of Jupiter includes eight that have not been seen for several years. Telescopes in Chile, Hawaii and Arizona were used for the discovery and confirmation.
Sheppard did not set out to detect new moons. To check whether this could have happened, the researchers are working on supercomputer simulations of these orbits to calculate how many times an object with Valetudo's orbit could have collided with the retrograde moons in the solar system's lifetime. Two of the newly discovered moons were found among these prograde moons, and take a little less than a year to go around in their orbit once. Given their distance and angle from Jupiter, they are also most likely pieces of a once-larger moon.
The so-called "oddball" has such a unique orbit that it is at risk of smashing into the other moons - a cosmic collision that could risk wiping the space rocks out. The name Valetudo has been proposed for it, after the Roman god Jupiter's great-granddaughter, the goddess of health and hygiene. The new moons were found in three distinct clusters and are thought to be debris left over when three larger bodies were broken apart in earlier collisions.
From Jupiter's entire collection of 79 moons, Sheppard has been involved in the discovery of 54 of them, including most of the known retrograde moons. Aside from the hulking Galilean moons that stretch thousands of miles in diameter, most of Jupiter's moons, including the new twelve, are between a mile and a few tens of miles across.
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Most of the prograde moons orbit much closer to the planet than the retrograde moons do. Even though a dozen new moons is a pretty good haul, Sheppard expects that more searching will turn up even more moons.
The same survey that discovered these 10 also resurveyed two other moons previously discovered by the researchers, who verified the moons' orbital paths.
Eleven of the newly discovered moons are considered normal, with diameters between 1 and 3 kilometres.
A head-on collision between two moons would "grind the objects down to dust", he added.
The 12th moon is being called an oddball because, while it orbits in the same direction as Jupiter's spin, it is much farther away than the others orbiting in that direction and it is comparatively tiny. "We could choose our field of observation to be very close to Jupiter, so we could look for things moving at Jupiter's rate-foreground objects, moving quite fast", while still on the hunt for relatively slower-moving objects in the fringes of the solar system, Sheppard says.
Once they finish running and analyzing the simulations, the team plans to publish the results in early 2019.