Jupiter has recently been stripped of its title of having the most known moons at a count of 79, when astronomers discovered 20 new moons orbiting the planet Saturn. This brings Saturn’s known moon count from 62 up to 82, taking the top spot as the planet with the most moons of our Solar System! Most of these new moons are very small and range in size from only two kilometres to five kilometres in diameter, with many taking years to orbit the planet. Saturn’s moon count may stay at 82 for some time to come, and it’s likely that astronomers will need much larger telescopes to resolve any moons smaller than the newly discovered ones.

Moons have always played an important role in the history of astronomy. In 1610, Galileo Galilei famously observed Jupiter’s four large moons orbiting the planet, which defied the then current understanding that Earth was the centre of the universe. Since then, hundreds of moons have been observed orbiting other planets, especially the gas giants located in the outer Solar System.  Even asteroids that have their own little moons have been discovered! Titan, the first of Saturn’s moons to reveal itself, was found in 1655 by Christiaan Huygens. This moon is fascinating enough to be a world in its own right, as it’s larger than the planet Mercury and has an atmosphere that allows the existence of liquid methane on its surface. Many more moons were discovered in the coming years by Earth-based telescopes. It was not until spacecraft started visiting the planet, starting with Pioneer and Voyager missions in the 1980s, that Saturn’s moon-count began to jump sharply upwards. By 2009 Saturn had a total of 62 known moons discovered by both the visiting spacecraft and astronomers who had been observing the gas giant here on Earth. With Earth at a count of only one, 62 was an impressive number of moons but Jupiter still managed to take the lead with 79 as of 2018. However, with this new, exciting discovery, Saturn inched ahead by a count of three now sitting at a total of 82!

These newly discovered moons fit into three main categories that share properties with already-known moons. Astronomers call these the Inuit group, the Gallic group, and the Norse group.

The first group, the Inuits, are small irregular bodies that orbit Saturn in the same direction as the planet’s spin, at an inclination (the tilt of an object’s orbit) of around 40-50°. Two of the new moons fit into this group. The second is known as the Gallic group, which also orbit Saturn in the same direction, but at a tilt of 35-40°. Only one of the new moons belongs in this group. The remaining 17 have been put into the Norse group. These small bodies have a retrograde orbit meaning they go around Saturn in the opposite direction. They also have much higher orbital inclination at around 136-175°.  All of the new moons are far too small in size to have surface features resolved from current ground-based telescopes. Future missions to Saturn would be needed to get a closer look.

Scott Sheppard, from the Carnegie Institution for Science, led the team who discovered these moons using the Subaru Telescope in Hawaii. They were found by going through old data and images taken between 2004 to 2007. It took Sheppard and his team years to confirm that many of the tiny objects they saw slowly moving over time were indeed new moons. There is now an open competition to name all these moons but be warned: there are a few rules! Each of the moons must be named in relation to their corresponding group. The two moons in the Inuit group must be named after giants in Inuit mythology. The seventeen who belong to the Norse group must be named after the giants on Norse mythology, and the single moon in the Gallic group must be named after a giant in Gallic mythology.

You can learn more about the naming competition and take part here.

Josh Kirkley, Astronomy Educator

Front tile image: Saturn as taken by Cassini. Credit: NASA. 

Above left image: Five of Saturn’s many moons seen from the Cassini spacecraft along with Saturn’s outer ring. From left to right: Janus, Pandora, Enceladus, Mimas, and Rhea.

Above right image: Most of the newly discovered Saturnian moons orbit the planet backwards, belonging to the Norse group. Credit: Carnegie Institution for Science.