Science & Technology

Saturn Is the New Moon Champion of the Solar System, With 20 Newly Discovered Worlds

Scientists have just found 20 — we'll say that again — 20 new moons orbiting Saturn. We've been observing the ringed planet since ancient times, yet all the while it was being orbited by tiny moons that were so small, they escaped previous notice by telescopes and past space missions. This new set of discoveries makes Saturn the planet with the most moons in the solar system, at 82.

Hints of a Violent History

Scientists spotted these tiny worlds using the Subaru telescope on top of Mauna Kea in Hawaii. They were even able to figure out a bit of history behind these newly discovered worlds.

The moons are all very tiny, at only 3 miles (5 kilometers) in diameter. Of the 20-strong group, 17 of these moons orbit the planet in a retrograde direction — that is, backward relative to Saturn's rotation. Their orbits match up with other retrograde moons circling Saturn, suggesting that they all likely came from the same parent world. Scientists think that long ago, a larger retrograde-orbiting moon shattered and broke up into all the other worlds we see today.

The other three moons rotate in a prograde direction, or in the same direction as Saturn. Two of these moons match orbits with the "Inuit group" of moons already seen around Saturn. The Inuit group likely all came from another large moon that orbited in the same direction as Saturn's rotation before breaking apart. And as for that last prograde moon? It's probably an outlier, seeing as it's so far away from Saturn compared to any other discovered prograde moons.

"This kind of grouping of outer moons is also seen around Jupiter, indicating violent collisions occurred between moons in the Saturnian system, or with outside objects such as passing asteroids or comets," said principal investigator Scott Sheppard, an astronomer at the Carnegie Institution for Science, in a statement.

The discovery images for the newly found very distant prograde moon of Saturn. They were taken on the Subaru telescope with about one hour between each image. The background stars and galaxies do not move, while the newly discovered Saturnian moon, highlighted with an orange bar, shows motion between the two images.

Move Over, Jupiter

Last year, Sheppard and a team of astronomers found 12 new moons orbiting Jupiter, but only one of those worlds was circling backward. It's in such a dangerous orbit, in fact, that it's possible that sometime in the solar system's future, this rogue moon could end up in a head-on collision with some other moon among Jupiter's group of 79.

The astronomers were initially looking for a large object beyond Pluto that might be disturbing asteroids in the region, called Planet Nine. Planet Nine is supposed to be 10 times the mass of the Earth and 20 times further away from the sun than Neptune. A multi-year search of this purported object hasn't come up with anything yet, but it did reveal more moons around Jupiter — and now we have even more new moons to explore near Saturn.

"Using some of the largest telescopes in the world, we are now completing the inventory of small moons around the giant planets," Sheppard added. "They play a crucial role in helping us determine how our solar system's planets formed and evolved."

Carnegie held a contest a few months ago to name some of Jupiter's new moons, and it proved so popular that the institution plans another one for Saturn. You can enter right here — the only catch is that you have to suggest names that come from giants of Norse, Gallic, or Inuit mythology. Good luck!

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Learn more about the residents of our solar system in "Solar System: An Exploration of the Bodies that Orbit the Sun" by Marcus Chown. We handpick reading recommendations we think you may like. If you choose to make a purchase, Curiosity will get a share of the sale.

Written by Elizabeth Howell October 7, 2019

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