When Galileo first peered through his telescope at Jupiter in 1610, he was shocked to see that the planet was not alone—it was orbited by four moons, a fact that upended then-current theories of astronomy. Imagine how surprised he would be today to know that the Solar System's largest planet is now known to have 79 satellites.
Ian Sample at The Guardian reports the discovery of 12 new moons orbiting the gas giant, including one oddball flying in the wrong direction.
The new moons, reported in The International Astronomical Union’s Minor Planet Center’s Electronic Circular, were first found by a team of U.S. astronomers in March 2017 while they searched the skies for signs of Planet 9, a hypothesized ninth planet orbiting the sun far beyond Neptune. Kenneth Chang at The New York Times reports that Jupiter was scheduled to pass through their search area, so astronomer Scott S. Sheppard of the Carenegie Institution for Science and his team thought the gas giant was worth a quick look, training the Magellan Telescope at the Las Campanas Observatory in Chile on the area. What they found were 12 new moons orbiting the planet.
According to a press release, confirming the new celestial bodies took some time. “It takes several observations to confirm an object actually orbits around Jupiter,” says Gareth Williams of the Harvard-Smithsonian Center for Astrophysics and director of the International Astronomer’s Union’s Minor Planet Center, who calculated the orbits of the new moons. “So, the whole process took a year.”
The new moons are small, reports Ben Guarino at The Washington Post, most less than two miles in diameter, a fraction of the size of Jupiter’s four inner Galilean moons, one of which is larger than the planet Mercury. Nine of the new moons are clustered in the outer swarm of Jupiter’s moons and have roughly 2-year-long retrograde orbits, meaning they’re going the opposite direction of the planet's rotation. It’s believed these moons are the remnants of larger space rocks that broke apart during collisions with asteroids, comets or other moons. Two other moons orbit much closer to Jupiter in a group of inner prograde moons, which orbit in the same direction as Jupiter’s rotation, taking about one Earth year to make an orbit. Those moons are also believed to be remnants of a larger moon that was smashed to pieces.
The twelfth moon has already earned a proposed name, despite being only about two-thirds of a mile in diameter. That’s because Valetudo, named for the Roman goddess of health and hygiene, is a true oddball. Though it has a prograde orbit, it lives among a group of retrograde moons, meaning it careens across their orbits every once in awhile. “Valetudo is like driving down the highway on the wrong side of the road,” Sheppard tells Sample. “It is moving prograde while all the other objects at a similar distance from Jupiter are moving retrograde. Thus head-on collisions are likely," though still relatively rare, happening once every billion years or so. If Valetudo does smash into one of her neighbors, however, the impact will be large enough to detect on Earth.
The other moons don’t have names yet. Sheppard tells Chang the team may invite the public to offer suggestions, though he’s already vetoing “Planet McPlanetFace.”
Sheppard says that there are likely more chunks of rock left over from collisions around Jupiter, some of which could be moons. Which opens a whole astronomical can of worms, since there is no clear definition of what constitutes a moon. Some argue that, technically, every bit of dust orbiting a planet (which also has a contentious definition) can be considered a moon. Others suggest we should come up with criteria for what qualifies as a moon. Williams tells Chang that it’s a debate we can hold off having for a while. “We are nowhere near being able to image individual ring particles, let alone get enough observations for orbit determination, even from spacecraft,” Williams said. “I think that is a question for a future generation. Currently, it is too hypothetical.”
While the new moons are important just for helping us map our Solar System, Guarino reports that they may have deeper scientific value as well. The gravity from our largest planets—Jupiter, Saturn, Neptune and Uranus—have Hoovered up most of the small chunks of rock and debris left over from the formation of our Solar System. But these new moons may be chunks of that pre-planetary solar system, suspended in Jupiter’s orbit, and could tell us what the planets we know today are made of.