The battle for the best-known moons in the solar system rages on.
After losing its lead over Saturn in 2019, Jupiter has once again advanced. Astronomers have counted 12 previously unknown moons in orbit around our solar system’s largest planet, bringing the known total to 92 and leaving Saturn in the dust with its meager 83 count.
The orbits of the unnamed moons were published in The circular of the International Astronomical Union’s Minor Planet Center, which keeps records of all small bodies discovered in the solar system.
The observations were led by Carnegie Institution for Science astronomer Scott Sheppard, who spearheaded the discoveries after accidentally spotting a plethora of previously unknown moons of Jupiter during a hunt for the mysterious hypothesis planet nine in the outer solar system.
“Jupiter happened to be in the sky near the search fields where we were looking for extremely distant objects in the solar system,” said Sheppard in 2018after his team spotted a dozen new moons.
It’s actually not that strange that we’re just now discovering these objects. They’re pretty small and faint and hard to see, especially when Jupiter is right there as it’s quite big and big reflective.
However, in their search for evidence of Planet Nine, researchers used a more powerful telescope than ever before, zooming in at higher resolutions over a wider field of view than other observations in the past.
This allowed them to detect small moons that may not have been discovered before. Accordingly Sky & Telescopenine of the newly discovered moons are quite distant from Jupiter and orbit it in retrograde motion, the opposite direction to Jupiter’s rotation.
That’s not strange; Most of Jupiter’s moons are retrograde. This motion means they likely passed rocks that were caught by Jupiter’s gravity and stayed in orbit.
The other three moons are closer to the planet and orbit in the same direction as Jupiter’s rotation. These smaller, prograde moons are harder to see because Jupiter outshines them, but they likely formed in Jupiter’s orbit.
The moons were sighted in 2021 and 2022. Anything moving across the sky near Jupiter in the same direction and at the same speed is a potential moon, but it takes time to confirm them.
Follow-up observations must be made a month and then a year later to ensure the object is still there and still orbiting Jupiter. These follow-up observations can then also be used to map the object’s orbit.
This can tell us something about the history of Jupiter and its moons. For example, the retrograde moons are believed to be the remains of three larger bodies that were caught in orbit around Jupiter and then broke apart after colliding with other objects. But one of the moons discovered a few years ago, Valetudo, has a prograde orbit that intersects with the orbits of the retrograde moons.
This suggests that the retrograde moons may have formed through collisions with prograde moons with orbits like that of Valetudo. Finding more moons will help either confirm or refute this idea.
By now it’s very likely that there are many more moons around both Jupiter and Saturn that we have yet to find. And really, it doesn’t matter which planet has more. There is only one clear winner in this whole race: science.
