Opinion How Saturn beat Jupiter to become the Moon King

Discovering moons or planets is by no means an easy task. A single snapshot wouldn’t suffice: One would have to take sequential pictures and then stack them by appropriately shifting the images to account for the movement of the objects. It is necessary to stack images so that faint objects become discernible. To give you an idea of the faintness of these moons: Detecting one is comparable to finding the light of a bunch of candles on the Moon from the Earth! All these mean a patient and meticulous study that can take years.

The newly found Saturn’s satellites are certainly not like our Moon. They are tiny in comparison, perhaps a few kilometres across, compared to the Moon’s radius of 1,737 km. Such an abundance of mini-moons says a lot about the dangerous traffic in the region around Saturn, or even Jupiter, which has 95 moons at the last count. These small objects are likely the result of collisions between bigger objects, probably tens of kilometres across in size, which moved in chaotic orbits.

In fact, many of the newly discovered moons have been found to be clustered in groups, hinting at their common origin. One can even hazard a guess regarding the epoch of these collisions because if it had occurred too long ago, then the small ones would have been smashed into smithereens by now. These newly found moons were, therefore, produced rather recently, say around a hundred million years ago. In the yardstick of the time that the solar system has been around, for roughly 4.65 billion years, these events are, indeed, recent.

It is also possible that the collisions took place somewhere else in the solar system and parts of the debris have been gobbled up on their paths by Saturn. Located farther away than Jupiter, Saturn’s orbit is closer to the outer parts of the solar system, which is teeming with icy objects, small and large. It is, therefore, likely that these newly found moons of Saturn are icy objects that have been collected by the giant planet. The fact that icy objects are easily fragmented in collisions, compared to the rocky satellites of Jupiter, may also help in explaining why Saturn appears to have a larger share of satellites than Jupiter.

It could also be that astronomers have been lucky with observing the region around Saturn. Jupiter is closer to us, so its sphere of gravitational influence appears to be bigger than the Earth. Scanning such a large portion of the sky around Jupiter is certainly more difficult than carrying out a similar study on Saturn.

The path of Saturn has also helped astronomers during their observations. Moving in its orbit, Saturn has recently shifted to a location, which, from the Earth’s perspective, has a background that is not crowded with stars. At the beginning of the recent campaign, around 2019, Saturn moved out of the portion of the sky that looks towards the centre of the Milky Way. That has certainly helped the scientists in their examination of the field. Looking for faint moons against a background of star clusters would have, otherwise, been incredibly difficult.

It may be difficult for Jupiter to catch up with the 274 moons that Saturn has. Still, one cannot rule out the possibility. Like any other interesting scientific result, this observation is certainly going to push astronomers further. While theorists are going to be busy thinking about collisions in the outer solar system, how the orbits of giant planets may have evolved since their origin, and how they would have collected souvenirs as their moons, observers will continue to shift and scan the sky for fainter objects that appear to revolve around the giant planets.

The writer is an astrophysicist at the Raman Research Institute, Bangalore