The Carnegie team has made its second game-changing discovery: 12 “new” moons in Jupiter’s orbit. The moons, which are yet unnamed, bring Jupiter’s total moon count up to 92. Saturn is once again in second place with 83.
The previously unspotted moons were recorded late last month in the circulars of the International Astronomical Union’s Minor Planet Center, which maintains a record of the Solar System’s natural satellites. Like Jupiter’s other moons, nine are retrograde, meaning they’re orbiting “backward,” or in opposition to Jupiter’s orbit. They’re also among Jupiter’s outermost Jovian moons. The remaining three hug Jupiter a little more closely but maintain the planet’s orbital direction.
Jupiter’s moons’ orbits, not including the 12 newly-discovered moons. (Image: Scott Sheppard)
Carnegie astronomer Scott Sheppard and his team first noticed the 12 moons in 2021 and 2022, but at the time, they were only “potential” moons; confirming them would take time, which is why we’re only hearing about them now. Astronomers have to track a potential moon for a full orbit to confirm that it actually belongs to a given planet, and Jupiter’s nine retrograde moons take 550 days to complete a full orbit. This lengthy period also gives astronomers data with which to determine moons’ histories. Sheppard and his colleagues believe Jupiter’s new retrograde moons are fragments of up to three larger bodies that were once in Jovian orbit before colliding and breaking apart.
But if we already knew about 80 of Jupiter’s moons, why’d it take so long for these 12 to be discovered? To start, they’re pretty small: only five of the nine retrograde moons are larger than 8 kilometers. And while the three prograde moons orbit closer to Jupiter, that tight proximity makes them harder to see. Jupiter’s massive size and reflectivity obscure objects close to it, forcing astronomers to use incredibly high-powered telescopes to even earn a chance at finding them.
Sheppard’s team accidentally located a different set of moons orbiting Jupiter back in 2018. At the time, he and his colleagues had been looking for the fabled Planet Nine, the Solar System’s hypothetical ninth planet. Jupiter “just happened” to be near the search fields the team had focused on, leading to the discovery of 12 new planets. Like this time, most were distant and retrograde, while a couple followed Jupiter’s orbital direction closer by. One “oddball” prograde moon is believed to be the product of a collision between retrograde and prograde bodies.
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