That brings its total known satellites to 79, the most in the solar system.
Nine of the dozen moons are well away from Jupiter and have retrograde orbits, meaning they go around the planet in what we'd think of as the "wrong direction". All of these moons are thought to be the remnants of three much larger bodies that were destroyed by collisions with other moons, asteroids or comets.
Nine of the 12 moons are part of an outer group of satellites that orbit Jupiter in retrograde, which means that they orbit in the opposite direction of the planet's rotation.
The most interesting of the new moons is Valetudo (pronounced val-eh-TOO-doh), named after the ancient Roman god Jupiter's great-granddaughter, the goddess of health and hygiene.
The oddball Jovian moon, known as Valetudo, crosses the orbits of moons that move in the opposite direction. Most likely to be pieces of a once larger moon that was broken up in orbit, they take almost a year to complete a lap around Jupiter.
Outside the orbits of the prograde moons is the largest group, the retrograde moons, which orbit around Jupiter in the opposite direction to the planet's spin. "This is an unstable situation", continued Sheppard. Nine of those 10 moons are what the astronomers are calling normal, but they've labeled one as a real oddball.
Which raises the question of how long the tiny moon has left.
The researchers were searching for the proposed "Planet X" or "Planet 9", which astronomers believe exists and could account for the way distant objects orbit in a similar manner.
But what's particularly wild about these newly discovered moons is that researchers weren't even looking for them.
When Galileo first peered through his telescope at Jupiter in 1610, he was shocked to see that the planet was not alone-it was orbited by four moons, a fact that upended then-current theories of astronomy.
Back in 2016, astronomers reported that the unusual orbits of some objects on the edge of the solar system, far beyond Pluto, could best be explained by the existence of an undetected planet several times more massive than Earth. They also include a cluster of moons beyond Callisto, shown in blue in the image above.
Astronomers made the discoveries using the Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile and the Subaru telescope on Mauna Kea, Hawaii.
"What astonishes me about these moons is that they're the remnants of what the planet formed from", he said.
These building blocks of planets can provide a window into the early years of the solar system. They're thought to have formed after the gas and dust from the earliest stages of planetary formation had dissipated.
"If we do find this planet in the next few years, it would be a pretty fantastic discovery for astronomy". That makes it a powerful tool for surveying the night sky in search of faint objects.