S/2019 S 1: One of the closest irregular moons of Saturn

Released: 30th June, 2022, Academia Sinica, Institute of Astronomy & Astrophysics (ASIAA), Taiwan

Banner Image Credit: NASA/JPL/Space Science Institute

"We found one NEW Saturnian moon! There are more to be found in the future!" said Edward Ashton.

The giant planets in the Solar System are host to two kinds of moons: regular and irregular. Regular moons are typically larger, orbit close to the planet, and were formed around their host planet. Irregular moons, on the other hand, are typically smaller, orbit further out (compared with regular moons) and are believed to be leftover planet building blocks that were captured by the giant planets early on in the Solar Systems history.

A team of astronomers led by Postdoc Fellow Edward Ashton in ASIAA used observations from Canada France Hawaii Telescope (CFHT ) situated on Mauna Kea, Hawaii to discover a new irregular moon of Saturn, designated as S/2019 S 1. With a size of about 5 km this moon is small but quite a bit larger than the 2 km size of the smallest known irregular moon around Saturn. This moon has a direct (or prograde) orbit, meaning it orbits is the same direction as the regular moons. This is rare as most known irregular moons have retrograde orbits, meaning they orbit in the opposite direction compared to regular moons. On average, S/2019 S 1 orbits about 11 million km away from Saturn, making it one of the smallest orbits among the known Saturnian irregular moon. The small orbit of S/2019 S 1 is not too unsurprising since moons with direct orbits have, on average, smaller orbits than moons with retrograde orbits. Since S/2019 S 1 has a small orbit, it spends a significant amount of time close, from our perspective, to Saturn. Which means that if S/2019 S 1 was smaller it would have been lost in the glare of Saturn.

Finding an object in the sky that is near a planet and moving in the sky at a similar motion as the planet is not enough evidence to say that the object is definitely a moon. The object has to be tracked for multiple months to a year before the object can be confirmed to be in orbit around the planet. Tracking can become tricky if there are multiple nearby moving objects and if there are too few tracking images, which was the case for discovering S/2019 S 1.

The fact that S/2019 S 1 is a fair bit larger than the smallest known irregular moon around Saturn and that moons with direct orbits are closer in to Saturn than moons with retrograde orbit means that there are likely many more undiscovered direct saturnian irregular moons hiding in the glare of the Saturn.

The offset of S/2019 S 1 relative to Saturn (red dot) as seen from Earth. Coloured circles represent a detection of the moon, with the dashed line showing the best fit orbit. Each colour represent a different calendar year, with the 2019 (purple) arc starting from 1st of July 2019. As a reference, the orbit of the outermost regular moon, Iapetus, is shown during the same time frame (grey dotted line). Credit: Ashton et al.

This research was presented in the paper: Discovery of the Closest Saturnian Irregular Moon, S/2019 S 1, and Implications for the Direct/Retrograde Satellite Ratio, in The Planetary Science Journal 3, 107

Media Contact:

Dr. Edward Ashton, Email: eashton@asiaa.sinica.edu.tw, Tel: +886 2 2366 5469