Move over Jupiter – Saturn has more moons

Move aside Jupiter. Saturn now has the largest number of known moons of any planet in the Solar System.

On 7 October, the International Astronomical Union’s Minor Planet Center announced that astronomers have found 20 new moons around the ringed planet, bringing its total to 82. That surpasses Jupiter, which has a mere 79.

And there may be even more waiting to be found. 

Currently, says the discovery team’s leader, Scott Sheppard of the Carnegie Institution for Science, Washington, D.C., the vast distance to Saturn means that moons smaller than about five kilometres in diameter are too faint to be visible.

Even finding the five kilometre ones required using one of the largest telescopes in the world, the Subaru Telescope atop Mauna Kea, Hawaii.

“We believe there are about 100 moons around Saturn bigger than one mile [1.6 kilometres] in size,” Sheppard says, “but future, larger, telescopes will be needed to find [them].”

The new moons are on far-flung orbits 16-27 million kilometres out from Saturn – more than 40 times the distance from the Earth to the Moon. That’s close to the limit at which even Saturn’s enormous gravity can keep them from being dislodged by distant tugs from other planets, such as Jupiter or Uranus. 

The moons fall into three groups whose larger members had been previously discovered.

Each of these groups has different orbital properties, suggesting that its members share similar origins, most probably as fragments of older moons that were shattered by incoming asteroids or comets.

Moons in the Inuit group (so termed because its moons are named for giants in Inuit mythology), for example, are on orbits inclined by about 46 degrees from the main plane of the Saturn System. Those in the Gallic group are slightly less inclined, while those in the Norse group orbit in the opposite direction from the rest of the Saturn system.

The existence, locations, and orbits of these moon groups, Sheppard says, suggests they were formed from 50-kilometre-sized protoplanets captured by Saturn just after the planet-formation process ended.

“These moons are remnants of the objects that helped form the planets,” he says, “so by studying them, we are learning about what the planets formed from.”

That’s important enough, he says, that a future mission to Saturn might be designed to fly by one of them en route to a primary destination closer to the planet itself.

The immediate task, however, is to give these moons names, making sure they fit nicely within their groups’ mythology.

To facilitate, Sheppard’s team is conducting a Twitter campaign to solicit suggestions—something he and his colleagues did earlier this year for five new moons of Jupiter discovered in July 2018.

The contest is open now, and will remain open for two months.


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