Jupiter has 12 more moons than we knew about — and one is a weirdo
Astronomers just found 12 more moons around Jupiter. One, though, is really weird. Eleven of the newly discovered moons orbit in the same direction as their nearest neighbors. But one doesn’t. And that may put it on a fatal collision course.
“It’s driving down the highway on the wrong side of the road,” says Scott Sheppard. He’s a planetary scientist at the Carnegie Institution for Science in Washington, D.C.
Sheppard and colleagues found the moons while looking for something else entirely. They were searching for a putative planet that could exist beyond the orbit of Neptune. It’s known colloquially as Planet Nine. The researchers were conducting a survey in 2017 of the most distant objects in the solar system using the Victor Blanco 4-meter telescope in Chile. Jupiter happened to be visible in the same area of sky that the team was searching during one of its observing runs. “Might as well kill two birds with one stone,” Sheppard thought.
The researchers found a dozen objects moving around the sun at the same rate as Jupiter. Follow-up observations confirmed the moons’ existence and orbits. Two inner moons orbit in the same direction that Jupiter spins. Nine outer moons orbit the planet in the opposite direction. And then there’s that oddball traveler.
For all but the last one, these motions are normal for Jovian moons, which now number a whopping 79. Scientists think the inner moons formed from a disk of gas and dust that orbited the giant planet in the solar system’s early days. That’s similar to how the planets formed around the sun. The outer moons were probably free-floating space rocks captured when they came too close. Their opposite orbit was set by the direction from which they approached Jupiter.
But one moon broke the mold. The team calls this rock Valetudo after the Roman goddess of health and hygiene. It’s tiny, only about a kilometer (0.6 mile) across. It orbits in the same direction as Jupiter’s spin, but alongside the farther-out moons. As a result, Valetudo is probably doomed to collide with one or more of the other moons someday. The researchers are still calculating when this might happen. But they expect it to occur sometime between 100 million and a billion years from now.
Valetudo may be the last remnant of a bigger object that has already withstood several collisions. Or it could be from a family of moons that has since been smashed to smithereens. “It’s probably the largest surviving member, if not the only one,” Sheppard says.
Such nonconformist satellites are not rare, notes David Jewitt. He’s a planetary scientist at the University of California, Los Angeles, who was not involved in the new work. “But they are very interesting, because we know that they have been captured by their host planets, but we don't know how, or from where,” he says. Figuring out what oddballs like Valetudo are made of could help nail those details down.
astronomy The area of science that deals with celestial objects, space and the physical universe. People who work in this field are called astronomers.
colleague Someone who works with another; a co-worker or team member.
hygiene Behaviors and practices that help to maintain health.
Jupiter (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium. This planet also releases more heat than it receives from the sun as gravity compresses its mass (and slowly shrinks the planet).
moon The natural satellite of any planet.
Neptune The furthest planet from the sun in our solar system. It is the fourth largest planet in the solar system.
orbit The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, the object must be big enough to have pulled neighboring objects into the planet itself or to have slung them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
remnant Something that is leftover — from another piece of something, from another time or even some features from an earlier species.
satellite A moon orbiting a planet or a vehicle or other manufactured object that orbits some celestial body in space.
solar system The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.
sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.