WASHINGTON -- Astronomers found a dozen new moons orbiting Jupiter, 11 normal outer moons and one "oddball", adding the planet's total number of known moons to 79, the largest of any planet in the Solar System.

A team led by Carnegie Institution for Science's Scott S. Sheppard first spotted the moons in the spring of 2017 and Gareth Williams at the International Astronomical Union's Minor Planet Center used the team's observations to calculate orbits for the newly found moons.

"It takes several observations to confirm an object actually orbits around Jupiter," Williams said. "So, the whole process took a year."

According to Carnegie Institution for Science's news release on Tuesday, nine of the new moons are part of a distant outer swarm of moons that orbit the planet in the retrograde, or opposite direction of its spin.

These distant retrograde moons are grouped into at least three distinct orbital groupings and are thought to be the remnants of three once-larger parent bodies that broke apart during collisions with asteroids, comets, or other moons.

The newly-discovered retrograde moons take about two years to orbit Jupiter.

Other two of those new discoveries are part of a closer, inner group of moons that orbit in the prograde, or same direction as the planet's rotation.

These inner prograde moons all have similar orbital distances and angles of inclinations around Jupiter and so are thought to also be fragments of a larger moon that was broken apart.

These two newly-discovered moons take a little less than a year to travel around Jupiter.

"Our other discovery is a real oddball and has an orbit like no other known Jovian moon," said Sheppard. "It's also likely Jupiter's smallest known moon, being less than 1 km. in diameter."

The new "oddball" moon is more distant and more inclined than the prograde group of moons and takes about one-and-a-half years to orbit Jupiter.

Unlike the closer-in prograde group of moons, this new oddball prograde moon has an orbit that crosses the outer retrograde moons, according to the study.

As a result, head-on collisions are much more likely to occur between the "oddball" prograde and the retrograde moons, which are moving in opposite directions.

"This is an unstable situation," said Sheppard. "Head-on collisions would quickly break apart and grind the objects down to dust."

The team named the oddball "Valetudo" after the Roman god Jupiter's great-granddaughter, and suggested that it could be the last-remaining remnant of a once-larger prograde-orbiting moon that formed some of the retrograde moon groupings during past head-on collisions.

Because of their sizes, only 1 km. to 3 km., these moons are more influenced by surrounding gas and dust.

Therefore, if these raw materials had still been present when Jupiter's first generation of moons collided to form its current clustered groupings of moons, the drag exerted by any remaining gas and dust on the smaller moons would have been sufficient to cause them to spiral inwards toward Jupiter.

Their existence showed that they were likely formed after the gas and dust dissipated, the researchers said. (Xinhua)