And one of them is quite the oddball.
Using the 4-meter Víctor Blanco Telescope in Chile, the team really hit the moon jackpot. They realized they could observe Jupiter at the same time.
The realm of the giant planets - between Jupiter, Saturn, Uranus and Neptune - is largely devoid of small objects.
A few years back in 2014, this same team found an object with the most-distant orbit in the solar system.
Because Jupiter is also a bright planet, astronomers have had to deal with the issue of glare and scattered light affecting the space where moons can exist. The newly discovered retrograde moons take about two years to orbit Jupiter. It's more distant than Jupiter's prograde moons - those that orbit in the same direction as Jupiter's spin - and its orbit is much more inclined, crossing the orbits of the outer retrograde moons. Near those nine is one moon that likes to move to the beat of its own and, like the two closer to Jupiter, has a rotation that mimics its host planet.
Sheppard said Valetudo's composition was likely "half ice and half rock", adding that the tiny satellite is the "last remnant of a once much larger moon that has been ground to dust from collisions" with other Jovian moons. Two of the newly discovered moons are in the inner circle of moons and orbit in prograde, or in the same direction as Jupiter's rotation.
The researchers believe the oddball may be a leftover fragment from a larger prograde moon that split apart from repeated collisions.
Astronomers are still finding moons at Jupiter, 400 years after Galileo used his spyglass to spot the first ones. One of the moons, Valetudo (between orange markers), can be seen in these images. Researchers have proposed naming the "oddball" Valetudo, after the Roman goddess of health and hygiene.More news: N. Korea’s Kim lambasts officials during ‘field guidance’ visits
Most moons, including Earth's, have prograde orbits.
"It's basically driving down the highway in the wrong direction", Sheppard said. This outlier is like no other moon orbiting Jupiter. This hypothetical large prograde moon may also have formed some retrograde moon groupings.
Sheppard and his team hope to further explore what could've caused these moons to form in order to get a better understanding of how the planet itself formed - and ultimately, more about how the rest of our galaxy came to be. The planet is so absolutely massive that it has dozens of moons orbiting around it, by far the most of any of our neighboring planets.
Jupiter has several different types of moons. They found 12, two of which were announced past year.
These building blocks of planets can provide a window into the early years of the solar system.
"We're looking for new possible planets and dwarf planets in our solar system, just seeing what is out there", Sheppard said. This "Planet 9" is also sometimes called Planet X. This tells us something about the timing of the formation of these moon families, which, in turn, tells us something new about the formation of the Solar System. "It probably has collided with them over time", Sheppard said.
So why is this confirmation so hard? "The only thing that we know at the moment are the orbits and the approximate size", Williams said.
They did not find Planet Nine. If Valetudo does smash into one of her neighbors, however, the impact will be large enough to detect on Earth.