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Discoveries boost Jupiter’s retinue to 79 moons — including …

Jovian moons' orbits

The oddball Jovian moon, known as Valetudo, crosses the orbits of moons that move in the opposite direction. Click on the image for a larger version. (Carnegie Institution of Science / Roberto Molar Candanosa)

Astronomers searching for signs of a hypothetical “Planet Nine” have instead come up with 12 new moons of Jupiter, including one that hints at a cosmic crack-up.

The discoveries were made more than a year ago, and the orbits of two of the moons were confirmed soon after they were found. It took much longer for the other 10 to have their orbits verified.

“It takes several observations to confirm an object actually orbits around Jupiter,” Gareth Williams of the International Astronomical Union’s Minor Planet Center explained in a news release. “So, the whole process took a year.”

Moon discovery images

The Magellan telescope in Chile captured these recovery images of the oddball Jovian moon, known as Valetudo, in May. The moon can be seen moving relative to the background of distant stars. (Carnegie Science / Las Campanas Observatory)

The Minor Planet Center published the remaining 10-pack’s orbital parameters today, marking their formal acceptance as Jovian moons. That brings Jupiter’s total tally to 79 moons, easily besting runner-up Saturn’s count of 62.

None of the dozen moons is more than a couple of miles across. Two of them have relatively close-in orbits, going in the same direction as Jupiter’s spin. Nine of them orbit farther out, in a retrograde direction — that is, opposite to the direction of the giant planet’s rotation.

“Our other discovery is a real oddball and has an orbit like no other known Jovian moon,” said Scott Sheppard of the Carnegie Institution for Science, who led the discovery team. “It’s also likely Jupiter’s smallest known moon, being less than 1 kilometer in diameter.”

The oddball moon crosses the orbits of the outer retrograde moons.

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

The oddball could be the last remaining remnant of a once-larger moon that gave rise to the retrograde retinue during previous smash-ups. The circumstances of the moons’ orbits lend further support to the view that they were formed long after Saturn and its larger moons coalesced from a primordial cloud of gas and dust.