The discovery of a dozen new moons for Jupiter makes the king of planets the king of moons, too — at least for now.
The biggest planet in the solar system now has the largest family of moons. Since December 20th, the Minor Planet Center (MPC) has published orbits for 12 previously unreported moons of Jupiter. More publications are expected, says Scott Sheppard (Carnegie Institution for Science), who recently submitted observations of the Jovian system taken between 2021 and 2022. The discoveries bring the list of Jovian moons to 92, a hefty 15% increase from the previous tally of 80.
The MPC's orbital calculations confirm the new objects are in orbit around Jupiter. Other data from Sheppard’s observations even enabled recovery of the last “missing” Jovian moon, S/2003 J 10; the newest observations extended the track of its orbit to 18 years.
Jupiter vs. Saturn
The new finds put Jupiter’s lunar family count well ahead of Saturn’s 83 confirmed moons. However, while Jupiter may have the most moons for now, Saturn might catch up. A search for objects with sizes down to about 3 kilometers across that are moving along with the gas giants found three times more near Saturn than near Jupiter. The more numerous Saturnian objects might have come from a collision that disrupted a larger moon a few hundred million years ago. (The fragments have not been tracked carefully enough to count as moons yet, though.)
If we could count all moons measuring at least 3 kilometers across, “Saturn would have more moons than all the rest of the solar system,” says Brett Gladman (University of British Columbia, Canada), who helped identify the new Saturnian objects but was not involved in the Jovian observations.
All of the newly discovered moons are small and far out, taking more than 250 days to orbit Jupiter. Nine of the 12 are among the 71 outermost Jovian moons, whose orbits are more than 550 days. Jupiter probably captured these moons, as evidenced by their retrograde orbits, opposite in direction to the inner moons. Only five of all the retrograde moons are larger than 8 kilometers (5 miles); Sheppard says the smaller moons probably formed when collisions fragmented larger objects.
Three of the newly discovered moons are in among 13 others that orbit in a prograde direction and lie between the large, close-in Galilean moons and the far-out retrograde moons.
They’re harder to find than the more distant retrograde moons, though, says Sheppard. “The reason is that they are closer to Jupiter and the scattered light from the planet is tremendous,” he says. That light obscures them in the sky. Five were found before 2000, and only eight more have been discovered since then.
Besides the interest in their origins, these prograde moons could make suitable targets for a flyby from an upcoming mission. Three missions are in the works for the Jupiter system: the European Space Agency's Jupiter Icy Moon Explorer (JUICE), scheduled for launch in April; NASA’s Europa Clipper , set for launch late next year; and a Chinese mission being considered for the 2030s.
Beyond the Galilean Moons
The prograde objects outside the Galilean moons fall into two groups: the nine moons of the Himalia group orbit 11 to 12 million km from Jupiter, and the more distant duo in the Carpo group at 17 million km. The new discoveries added two of Himalia’s current tally of nine, and one of Carpo’s duo.
Searches for prograde moons outside these groups turned up nothing.
In the yawning gap between Himalia and the Galilean moons, there’s only one moon known: Themisto, a 9-kilometer object discovered by Elizabeth Roemer and Charles Kowal in 1975 but not recovered until 2000. It orbits 7.5 million kilometers (4.6 million miles) from Jupiter, roughly halfway between Callisto at 1.9 million km and the group of prograde moons starting at 11 million km.
That's a big hole. “We have searched very deeply for objects near Themisto, and have found nothing else to date,” says Sheppard. He says glare from Jupiter is so strong it would hide anything smaller than 3 kilometers across.
A single prograde moon, the 1-km Valetudo, orbits beyond the Carpo group, at 19 million km from Jupiter. After discovering it in 2018, Sheppard called Valetudo an “oddball” because its orbit crosses those of a few retrograde moons. This highly unstable situation is likely to lead to head-on collisions that would shatter one or both objects. Sheppard adds that Valetudo might be all that remains of a larger prograde moon that had suffered from earlier collisions. No other members have been found to date.
Discovering New Moons
Discoveries of small moons of Jupiter or Saturn are typically reported in Minor Planet Center Electronic Circulars. But those reports take time.
Analyzing observations and calculating trajectories is more complex for planetary moons than for asteroids or comets, because a moon’s path depends on both the gravity of its planet as well as the Sun. Observations must also track the moon for a full orbit to show it really orbits the planet, and the outer moons of Jupiter take about two years to orbit the planet. For asteroids and comets, on the other hand, a few weeks of observations may suffice to predict their course because their path depends only on the Sun.
We can expect more reports as Sheppard, Gladman, and others continue the hunt for new moons in the outer solar system.