Sheppard says Jupiter's prograde moons probably formed from the same spinning disc of stuff that eventually coalesced to form the planet. These two newly discovered moons take a little less than a year to travel around Jupiter.
One moon in particular caught the researchers' attention. "It's also likely Jupiter's smallest-known moon, being less than 1km (0.6 of a mile) in diameter".
The 12th new moon is a bit of an oddball, Sheppard said, with "an orbit like no other known Jovian moon". Those eleven moons are probably remnants of larger bodies that got broken up in collisions.
"Jupiter just happened to be in the sky near the search fields where we were looking for extremely distant Solar System objects", Sheppard said.
Nine of the new moons are part of a distant swarm of moons that orbit Jupiter in the opposite direction of its spin rotation.
This twelfth moon has a wide, 1.5-Earth-year orbit around Jupiter and travels among the retrograde moons.
It has a prograde orbit but is more distant and at a different incline. Some of the outer moons, on the other hand, are retrograde moons, which orbit in the opposite direction. Nine of them have retrograde orbits, going in the opposite direction to Jupiter's spin.
"Jupiter is like a big vacuum cleaner because it's so massive", Sheppard said.More news: Putin: Trump was 'of no interest' to Russian Federation before 2016 election
A head-on collision between two moons would "grind the objects down to dust", he added.
Our solar system's oldest and biggest planet, Jupiter, has many moons.
Astronomers suspect that the retrograde moons may be the remains of larger moons that were destroyed in head-on collisions with prograde objects.
Valetudo, as the team calls this oddball moon, is named after the Roman goddess of health, cleanliness and hygiene.
The finds were announced Tuesday in a bulletin from the International Astronomical Union's Minor Planet Center at the Smithsonian Astrophysical Observatory in Cambridge, Mass., which serves as the global clearinghouse for the study of moons, asteroids and comets.
Using the Blanco four-metre telescope at Cerro Tololo Inter-American in Chile - which had been recently fitted with a new and highly sensitive instrument called the "Dark Energy Camera", which is about the size of a small vehicle - they detected objects that seemed to be moving against the background stars.
The moon orbits in the prograde direction, but at a greater distance from Jupiter with an orbital period of about a year and a half. That means the revolve around Jupiter opposite from the planet's rotation.
While the planet was visible to the naked eye during its peak opposition on May 8, for the next few weeks star-gazers should be able to spot it through binoculars or a telescope. "We think these moons are the last remnants of the material that formed the giant planets". Many scientists are on the same quest, believing that a ghostly gas giant out past the orbit of Neptune could explain a number of orbital quirks in other objects. We're not just talking about one or two stragglers, either.
This was at a time when the Sun was still surrounded by a rotating disc of gas and dust from which the planets were born. They also include a cluster of moons beyond Callisto, shown in blue in the image above. So they were likely formed after they had dissipated.