Astronomers have discovered twelve new moons orbiting Jupiter —11 “normal” outer moons, and one that they’re calling an “oddball.” This brings Jupiter’s total number of known moons to 79 which is the most of any planet in our Solar System.
A team led by the Carnegie Institution for Science’s Scott S. Sheppard first spotted the moons in the spring of 2017 while they were hunting for a possible massive planet far beyond Pluto using a telescope at Chile’s Cerro Tololo Inter-American Observatory.
“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,” said Sheppard in a news statement.
“It takes several observations to confirm an object actually orbits around Jupiter,” said Gareth Williams at the International Astronomical Union’s Minor Planet Center who was able to use the team’s observations to calculate orbits for the newly discovered moons, which range in size from one to three kilometers.
“So, the whole process took a year,” he added.
Two of the newly found moons are part of a closer, inner group of moons that orbit in the prograde, or same direction as Jupiter’s rotation. These moons all have similar orbital distances and angles of inclinations around the planet and take a little less than a year to travel around Jupiter. They are thought to be fragments of a larger moon that was broken apart in the millions of years following the birth of the solar system.
Nine of the new moons are part of a distant outer swarm of moons that orbit it in the retrograde, or opposite direction of Jupiter’s spin rotation. They take roughly two years to complete an orbit around the planet. They are grouped into at least three distinct clusters and are thought to be the remnants of three larger parent moons that broke apart during collisions with asteroids, comets, or even other moons.
“Our other discovery is a real oddball and has an orbit like no other known Jovian moon,” said Sheppard. “It’s also likely Jupiter’s smallest known moon, being less than one kilometer in diameter.”
Nicknamed Valetudo, after the Roman god Jupiter’s great-granddaughter — the goddess of health and hygiene, this new “oddball” moon takes roughly one and a half years to orbit around the planet. It is more distant and more inclined than the prograde group of moons but unlike them, it has an orbit that crosses the outer retrograde moons.
As a result, head-on collisions are much more likely to occur between the “oddball” moon and the retrograde moon as they are moving in opposite directions.
“This is an unstable situation,” explained Sheppard. “Head-on collisions would quickly break apart and grind the objects down to dust.”
Astronomers believe that the various moon groupings we see today were formed in the past through this same mechanism. They believe it’s possible the “oddball” moon is the last remaining remnant of a once larger moon that also formed some of the retrograde moon groupings during past head-on collisions.
According to scientists, the discovery suggests that these moon were created after the era of planet formation, when the Sun was still surrounded by a rotating disk of gas and dust from which the planets were born.