"Jupiter just happened to be in the sky near the search fields where we were looking for extremely distant Solar System objects", he explained. Instead, scientists will have to wait for a future spacecraft, either flying past Jupiter or orbiting it.
"With the discovery of Valetudo, it seems the collisions that broke apart the retrograde moons were between other prograde Jupiter moons, like Valetudo", Sheppard told ScienceAlert. This means it crosses paths with the outer retrograde moons and could collide with them. It takes them about two Earth years to orbit the planet.
In March 2017, Jupiter was in the flawless location to be observed using the Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile, which has the Dark Energy Camera and can survey the sky for faint objects.
Astronomers think retrograde moons have a different origin story from prograde moons, which travel in the same direction that their planet rotates.
This interest in finding new moons around Jupiter wasn't just a spur of the moment decision for Sheppard.
This brings the total of known Jovian moons to 79. Measuring less than a mile in diameter, it's the smallest of Jupiter's known moons, and Sheppard describes it as an "oddball".
The newly discovered Jupiter moons, with diameters of one to three kilometres (0.62 to 1.9 miles), required multiple observations to verify. These regular satellites consist of an inner group of four moons that orbit very closely to the planet and a main group of four Galilean moons that are Jupiter's largest moons.
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. Sheppard's girlfriend came up with a name for it: Valetudo, the great-granddaughter of the Roman god Jupiter.
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Given their small size, if the moons had existed in the early days of the solar system, the gas and dust that surrounded the Sun at that time would have exerted a strong drag on them, causing them to lose speed and spiral in to crash into Jupiter.
The researchers believe the oddball may be a leftover fragment from a larger prograde moon that split apart from repeated collisions.
Valetudo is in Jupiter's distant, outer swarm of moons that circles in the opposite direction of the planet's rotation.
Nine of the found moons are part of the distant retrograde group that orbit in the opposite direction of Jupiter's spin rotation, according to Carnegie Institution for Science. Not yet, anyway. "Right now the only definition of a moon is something that orbits the planet", Sheppard said, as long as it isn't human-made.
It's further away than the prograde moons, taking around one and a half years to orbit around the planet. "By looking at these outer moons", he said, "we can get an insight into what the objects were like that ended up forming the planets we see today". With orbital periods of about one year, they also are thought to be the result of earlier collisions.
Eleven of the twelve new moons follow these conventions, but Valetudo is the odd one out.
Based on the team's observations, Gareth Williams at the International Astronomical Union's Minor Planet Center was able to calculate the orbits of the moons. Their existence shows that they were likely formed after this gas and dust dissipated.
