While hunting for the proposed Planet Nine, a massive planet that some believe could lie beyond Pluto, a team of scientists, led by Scott Sheppard from the Carnegie Institution for Science, found the 12 moons orbiting Jupiter. With this discovery, Jupiter now has a staggering 79 known orbiting moons — more than any other planet in the solar system. Of the 12 newly discovered moons, 11 are “normal,” according to a statement from the Carnegie Institution for Science. The 12th moon, however, is described as “a real oddball,” because of its unique orbit and because it is also probably Jupiter’s smallest known moon, at less than 1 kilometer (0.6 miles) in diameter, Sheppard said in the statement.
In the spring of 2017, these researchers were searching for Planet Nine in the region past Pluto, and “Jupiter just happened to be in the sky near the search fields where we were looking,” Sheppard said. This gave the team a unique opportunity to search for new moons around Jupiter in addition to objects located past Pluto, according to the statement.
Nine of the newly discovered moons have retrograde orbits, meaning that they orbit in the opposite direction of the planet’s spin. These satellites are part of a large group of moons that orbit in retrograde far from Jupiter. In fact, of Jupiter’s 67 previously discovered moons, the 33 outermost moons all have retrograde orbits.
Two of the newly discovered moons orbit much closer to Jupiter and have a prograde orbit, meaning that they orbit in the same direction as the planet. These are part of a group of prograde moons that orbit closer to Jupiter than the retrograde moons do. Most of these prograde moons take less than a year to travel around the planet.
These two groups of prograde and retrograde moons consist of “irregular” satellites, or moons whose orbits have irregular, or noncircular, shapes.
In addition to these two groups, Jupiter has “regular” satellites, or moons with nearly circular orbits. These regular satellites consist of an inner group of four moons that orbit very closely to the planet and a main group of four Galilean moons that are Jupiter’s largest moons.
The newly discovered “oddball” moon has a prograde orbit, but it orbits farther from Jupiter than the other moons in the larger prograde group and it takes about one and a half Earth years to complete an orbit. The satellite’s oddness comes from its tiny size and the fact that, although it’s out in the realm of the retrograde moons, it’s orbiting in the opposite direction to them. Researchers have proposed naming the “oddball” Valetudo, after the Roman goddess of health and hygiene.
Valetudo is more than just the odd moon out; it’s also a serious collision hazard.
Because it’s orbiting in the opposite direction of the nine “new” retrograde moons, and across their paths, there is a high risk that it will hit one of them, according to the statement.