A group of scientists searching for a theorized planet in the outer solar system instead discovered 12 new moons of Jupiter, bringing the giant planet’s total count of moons to 79. All are small, with diameters ranging from 0.6 miles (one km) to 1.87 miles (three km).
Led by Scott S. Sheppard of the Carnegie Institute for Science, the research team found the moons in the spring of 2017 while using the Blanco 4-meter Telescope at Cerro Tololo Observatory in Chile and the American National Optical Astronomical Observatory. Because several separate observations are required to confirm an individual object is actually in orbit around Jupiter, confirmation of all 12 took a year to complete.
“Jupiter just happened to be in the sky near the search fields where we were looking for extremely distant solar system objects, so we were serendipitously able to look for new moons around Jupiter while at the same time looking for planets at the fringes of our solar system,” Sheppard explained.
Nine of the newly-discovered moons are part of a larger swarm in distant, retrograde orbits around the giant planet, which take about two years to orbit Jupiter. Having retrograde orbits mean they circle Jupiter in the opposite direction of the planet’s rotation.
Within this distant swarm, moons are divided into three separate orbital groups, each of which is theorized by scientists to be the result of an impact with a larger parent body, likely a comet, asteroid, or additional Jovian moon.
Two of the newly-found moons have prograde or direct orbits and are located much closer to Jupiter, in an inner swarm of satellites. These moons orbit Jupiter in less than one year. All moons in this swarm, based on their orbital inclinations and distances from Jupiter, are believed to be remnants of a larger single moon that broke apart in an impact.
Of all the newly discovered objects, one has a more inclined orbit than those in the inner swarm and takes approximately a year-and-a-half to orbit Jupiter. While its orbit is prograde, its inclination leads it to cross into the region of the retrograde, outer swarm of moons.
This “oddball” is the smallest of all Jovian moons, with a diameter of less than 0.6 miles (one km). Because its unusual but prograde orbit takes it into the region of the outer moons that have retrograde orbits, the chances of collisions occurring between the objects moving in opposite directions are high.
“This is an unstable situation. Head-on collisions would quickly break apart and grind the objects down to dust,” Sheppard said.
The researchers believe that this “oddball” moon may also be a remnant of a much larger moon, possibly one that caused the outer swarm to take retrograde orbits.
Scientists think these newly-found moons formed after the solar system’s main period of planet formation. Had they formed earlier, gas and dust left over from the solar nebula, as well as larger moons, would have destroyed them by driving them into the giant planet.