A Flurry of Tiny Moons Around Jupiter

Four hundred years after Galileo discovered the planet’s biggest satellites, astronomers are still finding the smallest ones.

NASA / SwRI / MSSS / Gerald Eichstädt / Seán Doran

If you ranked the planets of the solar system by the number of their moons, Jupiter would reign.

Jupiter, the biggest planet in our cosmic neighborhood—and perhaps the most photogenic one—is circled by a surfeit of objects. Some of them are truly massive, veritable worlds of their own, like icy Europa, and volcanic Io, and sparkling Ganymede. Some are tiny things, measuring just several miles across. And even today, when our solar system feels downright familiar in comparison with the darkness beyond its edges, some of Jupiter’s moons are still being discovered.

The International Astronomical Union’s Minor Planet Center, the organization in charge of confirming discoveries of new objects in the solar system, on Tuesday added 10 new moons to Jupiter’s roster, bolstering its place at the top of the most-moons list.

The moons were discovered by Scott Sheppard, an astronomer at the Carnegie Institution for Science, in the spring of last year. Sheppard found them with the help of a ground-based telescope in Chile that had recently received an upgrade: a camera made for scanning the night sky for very faint objects. Sheppard was looking for Planet Nine, the planet some astronomers believe lurks somewhere at the edge of our solar system, jostling the orbits of other objects in strange ways. As the telescope gazed in the darkness way beyond Pluto, it ended up catching something much closer: a flurry of glinting, tiny objects near Jupiter, the smallest of which was about half a mile wide.

Sheppard couldn’t say whether these points of light were actually moons, at least not right away. To determine whether something is indeed a moon, astronomers must track the object for about a year to determine that, yes, its motions are governed by the gravitational tug of a planet. Sheppard says he couldn’t get excited about his findings in earnest until he observed the objects again a year later, this past spring, and his suspicions were confirmed.

Because of their small size, the moons’ composition is difficult to discern. To powerful telescopes on Earth, they look like white specks. Here’s one, bookended by orange dashes (the points of light around it are stars in the distance that remain unmoved between observations). You can see how, over months, the moon’s position changes with respect to the objects around it:


The new discovery brings the total number of known moons around Jupiter to 79. That figure includes both confirmed and waiting-to-be-confirmed moons, according to Gareth Williams, an astronomer at the Harvard-Smithsonian Center for Astrophysics and the International Astronomical Union’s Minor Planet Center, who checked Sheppard’s coordinates and observations about the moons.

Because the moons are so small, it’s not possible to say for sure what they are made of, but Sheppard says he suspects it’s a mix of rock and ice, a common composition in this region of the solar system.

Sheppard says Jupiter likely captured these moons in its orbit after its own turbulent birth; in that chapter of our solar system’s history, the space around Jupiter was swimming with objects left over from the planet’s creation. “In the giant-planet region—Jupiter, Saturn, Uranus, and Neptune—now that region is devoid of small objects, and that’s because the giant planets have swept up everything that was originally in that area,” Sheppard says. “These outer moons are the last remnants of the objects that went into forming the planets.”

Now that these new, diminutive moons have made their debut, the International Astronomical Union will consider potential names, which have historically come from figures associated with the Roman god Jupiter. Sheppard has proposed a name for one of the newly discovered moons: Valetudo, for the goddess of health and hygiene, who is also a descendant of Jupiter. (Sheppard picked Valetudo as a tribute to his girlfriend, whom he teases for taking multiple showers a day.)

Sheppard says Valetudo is an “oddball.” The smallest of the bunch, it orbits Jupiter in the opposite direction of the other newly discovered moons. This configuration means that the tiny Jovian moons could collide, Sheppard says. “I think Valetudo was probably a much bigger moon at one point, and it’s going around the wrong way and so it probably collided with these other moons going the other way,” he says.

If you came to this story expecting to find dazzling, close-up images of Jupiter’s newly discovered moons, we have some bad news: The era of discovering massive worlds around the gas planet ended more than 400 years ago, with Galileo. Like this latest batch, many of the moons astronomers have discovered around Jupiter in the past several decades have been smaller than cities. Their minuscule size has prompted some astronomers, including Sheppard and Williams, to wonder whether they should even bother giving them names. Williams says that discoverers of moons don’t have to name them if they don’t want to. Sheppard suggests that perhaps it’s time to add another layer to our definition of a moon. “The definition of a moon is just anything that orbits a planet, so maybe once you start getting down to a kilometer or so in size, maybe we should start calling these things dwarf moons,” he says.

Still, it’s quite thrilling that hundreds of years after Galileo laid eyes on Jupiter’s four largest moons, astronomers are still finding its smallest. And they don’t need to rely on spacecraft around Jupiter, like Juno, to do it. “Spacecraft like Juno are really good at looking close to a planet, but their field of view is like looking through a straw,” Sheppard says. “They’re not able to search the wider space much further out from Jupiter. That’s where we’re finding these moons. It’s like looking at the forest from above, versus looking at the forest when you’re inside.”

And there are many more trees yet to be found.

“I think there are hundreds out there,” Williams says.