In a curveball of cosmic proportions, astronomers went looking for a hypothetical planet on the far fringes of our solar system, and instead found 12 new moons dancing around Jupiter.
To be clear, these moons are no Ganymede, the largest moon in the solar system. They’re tiny, some barely a mile across, and they are tracing all kinds of weird paths around the giant world.
The team found the new moons almost by accident, thanks to a serendipitous alignment of Jupiter with their initial research project and some recently installed telescope technology.
“Fascinating,” says planetary scientist Doug Hemingway of the University of California, Berkeley. “This is a great reminder that when you build up the capability to study one thing, you never know what else you might discover along the way.”
Normally, spotting new moons around Jupiter is quite difficult. Anything small enough to still be hiding from our view is quite faint, and tracking those dim dots requires powerful telescopes that often have too small a field of view to capture the entire Jovian system. To make matters worse, Jupiter is quite bright, and its glare can obscure tiny moons.
But last year, the Carnegie Institution for Science’s Scott Sheppard and his colleagues were hunting a faraway planet rumored to orbit beyond Pluto—a planet so large its gravitational heft rearranges the orbits of smaller, distant objects. So, the team aimed a telescope at Chile’s Cerro Tololo Inter-American Observatory at the star fields surrounding our cosmic neighborhood, looking for faraway pinpricks of light moving in solar orbits.
Along the way, Sheppard and his colleagues realized that Jupiter was in the telescope’s crosshairs—and they decided to take advantage of the opportunity.
“We’re using a new camera that was only put on the telescope a few years ago,” Sheppard says. “It can search large areas much quicker, so we only had to use four images to cover the whole area around Jupiter.”
The first hints of 10 new moons appeared in March 2017 (two had been spotted earlier), and the signals were seen again the next month. But confirming orbits takes a while, and it wasn’t until the team re-observed Jupiter in May that all the moons were confirmed, bringing the planet’s total to 79.
“These small outer moons are by far the most numerous type of moon in the Jovian system,” says Bonnie Buratti of NASA’s Jet Propulsion Laboratory. “It may seem like an ordinary discovery to add these small moons to an already large collection. Not by any means.”
The dozen new moons range in size from roughly one to three miles across. Two of the moons are clustered relatively near the planet, and they orbit in the same direction that Jupiter spins. They’re likely the remnants of a much larger moon that has been broken into pieces over the billions of years since the birth of the solar system.
But nine of the moons, grouped into three more distant clusters, move backward, orbiting in a fashion known as retrograde. Like the inner cluster, these are likely clumps of moony debris left over from collisions that destroyed much larger parent moons that were each perhaps several hundred miles across.
“What did they collide with? Some comet? A rogue asteroid? Other moons in the Jupiter system?” Sheppard asks.
In another fun twist, those parent bodies are unlikely to be native to the Jovian system, and instead were captured by the giant planet’s gravity in the very early days of the solar system.
That might sound odd, but it’s not unusual for large planets to have snared smaller bodies over time, particularly during an early epoch when the giant planets migrated and perturbed the orbits of much less massive worlds.
One of Saturn’s best-known satellites, Phoebe, was captured from the icy debris field known as the Kuiper belt, which lives beyond the orbit of Neptune. And perhaps not coincidentally, Neptune’s most famous moon Triton is also a captured Kuiper belt object.
“Dynamicists believe the orbits of these outer moons, at least the ones that were captured early, were disrupted by the migration of the gas giants,” Buratti says.
And then there’s the tiny weirdo Valetudo, the only one of the dozen to have earned a publicly proposed name. Following astronomy naming conventions, the suggested moniker for this moon comes from mythology—the Roman goddess of health and hygiene.
Like the inner two moonlets, Valetudo orbits in the same direction as Jupiter’s spin. But its orbit is tilted relative to the planet, and it moves out near the three retrograde clusters. Because of that, Sheppard suspects that collisions with Valetudo are the culprits behind those busted up clusters, and that the strange moonlet is all that remains of a much larger moon.
“It’s like if you’re driving down the highway the wrong way,” he says. “Collisions are very likely to happen.”
Sheppard took a look at the other giant planets, too, and didn’t find any new moons orbiting Uranus and Neptune. That kind of bummed him out.
“Uranus is the best one to find moons around,” he says, “because you get to name things after Shakespearean characters.”
Editor's Note: This story was updated at 12:55 p.m. ET.