Jupiter has been found to have a dozen more moons orbiting it and they have been idenrified for the first time. Of the 12, 11 moons are "normal" outer moons, and one "oddball" moon. That makes Jupiter the planet with the highest number of moons with a grand total of 79 natural satellites—the highest of any planet in the solar system.
The team of astronomers were led by Scott S. Sheppard, of Carnegie. The moons were first spotted in the spring of 2017, says a release put out by the researchers. This discovery was made while on the lookout for distant solar system objects far beyond Pluto. They were looking for the mysterious Planet 9, which is believed to be a massive object that is actually affecting Pluto's orbit and even possibly firing comets at Earth.
A few years back in 2014, this same team found an object with the most-distant orbit in the solar system. They were also the first to realise that there just might be a massive planet out there in the outer reaches of the solar system. This "Planet 9" is also sometimes called Planet X.
"Jupiter just happened to be in the sky near the search fields where we were looking for extremely distant solar system objects," said Sheppard.
So we were serendipitously able to look for new moons around Jupiter while at the same time looking for planets at the fringes of our solar system," he added.
Using the team's observations, Gareth Williams at the International Astronomical Union's Minor Planet Center was able to measure out the orbits for these newly found moons each about one to three kilometres in size.
As to why the announcement comes so long after it was first observed since the spring of 2017, Williams said that, "It takes several observations to confirm an object actually orbits around Jupiter."
"So, the whole process took a year."
Nine of the 12 newly discovered moons are reported to be a part of Jupiter's distant outer swarm of moons that orbit it in the retrograde—opposite direction of the planet's spin. These distant retrograde moons are now grouped into at least groups that are thought to be the remnants of three once-larger parent bodies that broke apart because of collisions with asteroids, comets, and other moons near Jupiter. The retrograde moons take nearly two years to completely orbit Jupiter.
Of the remaining 3, 2 moons are part of a close, inner group that orbit in Jupiter's prograde--same direction as planet rotation. They also have similar orbital distances from the gas giant and their angles of inclination around Jupiter point out them also once being part of a much larger boody that has since broken down. These two moons, being a lot closer to the planet take less than a year to travel around Jupiter, say the researchers.
"Our other discovery is a real oddball and has an orbit like no other known Jovian moon," Sheppard explained. "It's also likely Jupiter's smallest known moon, being less than one kilometer in diameter."
The one "oddball" moon lies between the two groups of moons and its orbit is more inclined than the prograde group. This one takes about one and a half years to orbit Jupiter, say the researchers. This moon also has a prograde orbit, but its path crosses the outer retrograde moons. Chances of the oddball moon crashing into the outer retrograde moons moving in the opposite direction are very real, say the astronomers, who have proposed the oddball be named "Valetudo" after the Roman god Jupiter's great-granddaughter, the goddess of health and hygiene.
Sheppard called this situation unstable and explained that, "Head-on collisions would quickly break apart and grind the objects down to dust."