Astronomers are still discovering new natural satellites of Saturn and Jupiter, with the latest update seeing Saturn claim the crown for the solar system's most moons.
Saturn officially has the most moons of any planet in the solar system, usurping Jupiter. The International Astronomical Union (IAU) has announced the addition of 28 natural satellites for the ringed planet, bringing its total to 117 versus Jupiter’s 95.
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| Saturn is the second-largest planet in the solar system SCIEPRO/SCIENCE PHOTO LIBRARY/Getty Images |
As the solar system’s most massive planets, Saturn and Jupiter have enough gravitational pull to pluck space rocks from their orbit around the sun. These captured satellites, termed irregular moons, often have strange, widely looping orbits and astronomers are still discovering new ones.
In 2021, Edward Ashton at the University of British Columbia in Canada and his colleagues looked around Saturn for every object larger than 3 kilometres, and found around 150 objects that appeared to be irregular moons.
They also found that many of these moons orbit in the opposite direction to Saturn’s rotation and are clumped together in both size and their distance from each other, suggesting that many come from a recent collision, or collisions, from some of Saturn’s larger moons.
Ashton and his team submitted their findings to the IAU’s Minor Planet Center (MPC), which has the last word on orbiting bodies. Since 5 May, it has confirmed 28 new natural satellites around Saturn, which brings the total from 89 to 117, besting Jupiter’s 95. Even more are expected to be announced in the coming days, says Ashton.
But exactly what counts as a moon can be a bone of contention. The MPC differentiates between natural satellites that have only been observed once and those that have been seen more often, giving the latter an extra seal of approval with a Roman numeral, such as Saturn LX. While Saturn’s newest moons don’t currently have these numbers, they should do shortly as many have been observed fully travelling around the planet, says Gareth Williams, who recently retired from the MPC.
Some astronomers also take issue with the IAU lumping together very large bodies, like Jupiter’s moon Ganymede – which at 5000 kilometres wide is larger than the planet Mercury – with objects that are just a couple of kilometres in diameter.
It is estimated that Jupiter and Saturn could have many hundreds of as-yet undiscovered moons below this low threshold, so it is possible that Jupiter could reclaim the title, only for Saturn to overturn it again.
“Jupiter and Saturn are so dominant, mass-wise, that it feels like it’s the kind of thing where we’re just going to keep finding new moons as telescopes get better and software gets better,” says David Brown at the University of Warwick, UK.