(Image: Illustration of Jupiter and its moon Io.)
Jupiter has had its share of visitors. The Pioneer 10 and 11 spacecraft, as well as Voyagers 1 and 2, flew by the gas giant in the 1970s. Ulysses, Cassini, and New Horizons all passed by the planet for gravity assists to get to their ultimate destinations. The Galileo probe orbited Jupiter from 1995 to 2003, and Juno has been there since 2016.
It may come as a surprise, then, that astronomers still find new moons orbiting the largest planet in the solar system. But 12 more moons were just discovered, bringing the total to 79, the most of any planet that orbits the sun. These small moons were not discovered by a spacecraft, but rather by powerful telescopes on Earth—and by a team, led by Carnegie Institution for Science astronomer Scott Sheppard, that didn't even set out to look for them.
"These moons are the last remnants of the objects that the planets were built from," Sheppard tells Popular Mechanics via email. "Most of the small objects that helped build the planets we see today were incorporated into the planets themselves, and these moons are all that remains."
11 New Moons and an Oddball
Two of the new moons were actually announced last year, but all of them were spotted at roughly the same time around 2017.
"It takes several observations to confirm an object actually orbits around Jupiter, so the whole process took a year," says Gareth Williams of the International Astronomical Union Minor Planet Center, who calculated the orbits of the new moons, in a press release.
The majority of the new moons, which range from about one to three kilometers in diameter (0.6 to 1.8 miles), were initially discovered with the Blanco 4-meter telescope in Chile. Additional telescopes in Chile, Arizona, and Hawaii were used to confirm their existence and determine their orbits.
The new moons orbit in two separate groups. Two of the small satellites were found orbiting in the "prograde group," a collection of moons that hang out just outside the four large Galilean moons (Io, Europa, Ganymede, and Callisto). The moons are so named because they orbit in prograde, or the same direction that Jupiter spins on its axis, completing an orbit in a little less than a year. The 10 remaining moons orbit in the farther-out "retrograde group"—a group of moons that orbit in the opposite direction of Jupiter's rotation, circling the planet about every two years.
There is one mysterious moon among the retrograde group that goes against the tide, traveling in the prograde direction. This oddball—nicknamed Valetudo for the Roman goddess of health and hygiene, one of Jupiter's great-granddaughters—is possibly Jupiter's smallest moon, less than a kilometer in diameter. It is thought to be the last surviving remnant of a larger moon that broke apart during a series of collisions as the planetary system was forming.
"There appear to be groups of moons with very similar orbital properties, which suggests there were once bigger moons that broke apart and we are now seeing the fragments of once bigger moons," Sheppard says. "We think there were originally three retrograde moons each around 100 km in size, but each of these parent moons were broken apart by collisions in the past with something."
That something could have been Valetudo—or the former large moon that it came from.
"We believe the oddball moon, which we are calling Valetudo, was captured in the opposite direction than the retrograde moons," Sheppard says. "Valetudo is like driving down the highway on the wrong side of the road. ... Valetudo likely has collided with some of the retrograde moons in the past, and what we see today is the last remnant of a once much larger moon that has now been ground to dust."
While the original, larger Valetudo was smashed in violent collisions, the smaller remnant that exists today is probably safe—for now. "Valetudo... is likely to collide with a retrograde object head-on every billion years or so," Sheppard says, which means it could have happened multiple times in the 4.5-billion-year history of the solar system, but is highly unlikely to occur again anytime soon.
The two small moons discovered in the prograde group also likely came from a common original body that was destroyed in collisions. It just goes to show how chaotic and violent planetary formation can be.
Distant Planetoids and Moons Closer to Home
Sheppard and his team were not even searching for Jovian moons at first, but hunting for a hypothetical giant planet in the distant realms often called Planet Nine or Planet X. In 2014, Sheppard's team announced the discovery of the celestial body with the most distant known perihelion, or closest approach to the sun. The likely dwarf planet 2012 VP113 "never gets closer to the sun than 80 AU [Astronomical Units]," he says.
While looking for these extremely distant objects, Jupiter happened to wander into the team's field of view. "We knew Jupiter was near our observations for distant planets and dwarf planets that are beyond Pluto, so we specifically designed our observations to be able to search for new moons of Jupiter while also searching for much more distant solar system objects," Sheppard says.
Fortuitously, the same technique for finding far-flung objects orbiting the sun works to search for small objects orbiting Jupiter. "Both surveys take two images of the same sky location separated by a few hours," Sheppard says. "We then look for objects that have moved between the two images." While distant stars will not move in a few hours, objects beyond Pluto and moons orbiting Jupiter will.
Seventy-nine is an impressive number of Jovian moons, but Sheppard says there are likely more waiting to be discovered. "We think there are some 100 moons larger than 1 km around Jupiter... so a few more left to find."