Saturn now has more moons than all the other planets combined!

For decades, Jupiter and SaturnSaturn in turn take the lead in the race for the greatest number of known satellites. After the announcements of thewinterwinter last that had increased the counter of the orange giant to 95 known satellites, it is now the planet famous for its majestic rings that has seen its number of identified companions increase, and not just a little. Indeed, 62 new irregular satellites of Saturn were discovered, which more than doubled their number, which rose from 59 to 121. Adding to this the 24 known regular satellites, this doordoor the total number of known satellites of Saturn at 145, or 50 more than Jupiter! Saturn becomes on this occasion the first planet of which we know more than 100 companions.

Dozens of small satellites in the outer suburbs of Saturn

This remarkable result is the result of the work of an international team led by Edward Ashton, currently a postdoctoral researcher at the Institute of Astronomy andastrophysicsastrophysics from Academia Sinica, Taiwan. This team also includes Brett Gladman, a professor in the Department of physicalphysical and astronomy from the University of British Columbia (Canada), Mike Alexandersen, from Harvard Smithsonian Center for Astrophysics (United States), Jean-Marc Petit, from the Besançon Observatory (France), and Matthew Beaudoin, also from the University of British Columbia.

Over the past two decades, Saturn’s surroundings have been repeatedly scanned for small moons with ever-increasing sensitivity. In their latest study, Edward Ashton and his collaborators used a technique called “moving and stacking” (“ shift and stack in English) in order to find less bright and therefore smaller satellites. This technique had previously been used to search for the satellites of Neptune and Uranus, but never before for Saturn. Move a set of sequential images to the speedspeed movement of a moon in the sky increases the signal from this companion when the data is combined, allowing satellites that are too faint to be seen in individual images to show up in the “stacked” image . The team used data acquired with the Canada-France-Hawaii Telescope (CFHTCFHT) between 2019 and 2021. By moving and stacking numerous sequential images acquired over three-hour durations, the team was able to detect moons as small as 2.5 kilometers in diameter.

Finding an object near Saturn in the sky does not guarantee that it is a satellite: it could also be a asteroidasteroid which passes right by the planet, although it is unlikely. To certify that an object is indeed in orbit around the planet, it must be followed for several years. After painstakingly matching objects detected on different nights for two years, the team managed to track 63 objects confirmed to be new satellites. One of these moons, designated S/2019 S 1, was announced in 2021; the rest began to be announced in the last two weeks and should be finished this week. For some objects, observations dating back several years have been found, which at the time did not allow their orbit around Saturn to be determined.

” Tracking these moons reminds me of the dot to dot game I used to play as a kid, because we have to connect the different appearances of these moons in our data with a viable orbit, but with like 100 different games on the same page and without knowing which point belongs to which game “, explains Edward Ashton.

Groups of satellites resulting from collisions

All of these new companions are irregular satellites, which are believed to have been captured by the planet long ago. Irregular satellites are characterized by their wide, elliptical and inclined orbit, unlike regular satellites.

Irregular satellites tend to form groups whose members have an inclination orbitalorbital similar. In the case of Saturn, three groups are thus defined: the Gallic group (inclination of about 36° with respect to theeclipticecliptic), the Inuit group (about 48°) and the Nordic group (retrograde orbits, with an inclination typically between 140 and 180°). These three groups are named after the mythologies from which the names given to their respective members are taken. Of the 63 satellites discovered around Saturn since 2019, eight belong to the Inuit group, two to the Gallic group and fifty-three to the Nordic group. This last group is by far the most populated, gathering together 99 known satellites. The Inuit group now has fifteen identified members while the Gallic group has seven. All of these objects are located at distances between 11 and just over 25 million kilometers from Saturn.

These groups are thought to be the result of collisions: the current satellites of a given group would be the products of one or more collisions suffered by the satellites initially captured by the planet. A better understanding of the distribution of orbits thus gives us clues about the collisional history of Saturn’s irregular moon system. From their past study of these objects, the team suggested that the large number of small moons in retrograde orbits stems from relatively recent destruction (in astronomical terms, i.e. within the last 100 million ‘years) of an irregular satellite of intermediate size, now broken into numerous fragments which are cataloged in the Nordic group.

According to Jean-Marc Petit, “ the study of irregular moons tells us about the final phases of the formation of the solar system. But the benefits gained from observing closer planets (brighter moons) are offset by a larger region to observe, and shorter shots before the moon moves in the image, so there is no planet easier to study than another “.