A team of astronomers led by Dr. Scott Sheppard of the Carnegie Institution for Science has discovered twelve new moons orbiting Jupiter: eleven ‘normal’ outer moons and one that they’re calling an ‘oddball.’
Various groupings of Jupiter’s moons with the newly-discovered ones shown in bold; Valetudo has a prograde orbit that crosses the retrograde orbits. Image credit: Roberto Molar-Candanosa / Carnegie Institution for Science.
Dr. Sheppard and colleagues first spotted the moons in the spring of 2017 while they were looking for distant Solar System objects as part of their hunt for a hypothetical Planet Nine.
“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,” Dr. Sheppard explained.
Dr. Gareth Williams of the International Astronomical Union’s Minor Planet Center used the team’s observations to calculate orbits for the new Jovian moons.
“It takes several observations to confirm an object actually orbits around Jupiter. So, the whole process took a year,” Dr. Williams said.
This discovery brings the total number of known Jovian moons to 79 — the most of any planet in 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,” the astronomers said.
“These distant retrograde moons are grouped into at least three distinct orbital groupings and are thought to be the remnants of three once-larger parent bodies that broke apart during collisions with asteroids, comets, or other moons.”
“The newly discovered retrograde moons take about two years to orbit Jupiter.”
“Two of the new discoveries are part of a closer, inner group of moons that orbit in the prograde, or same direction as the planet’s rotation,” they added.
“These inner prograde moons all have similar orbital distances and angles of inclinations around Jupiter and so are thought to also be fragments of a larger moon that was broken apart.”
“These two newly discovered moons take a little less than a year to travel around Jupiter.”
Recovery image of Valetudo from the Magellan telescope in May 2018; Jupiter is not in the field but off to the upper left. Image credit: Carnegie Institution for Science.
“Our other discovery — named Valetudo — is a real oddball and has an orbit like no other known Jovian moon,” Dr. Sheppard said.
“It’s also likely Jupiter’s smallest known moon, being less than one kilometer in diameter.”
Valetudo is more distant and more inclined than the prograde group of moons and takes about 1.5 years to orbit Jupiter.
So, unlike the closer-in prograde group of moons, this prograde moon has an orbit that crosses the outer retrograde moons.
As a result, head-on collisions are much more likely to occur between Valetudo and the retrograde moons, which are moving in opposite directions.
“This is an unstable situation. Head-on collisions would quickly break apart and grind the objects down to dust,” Dr. Sheppard said.
“It’s possible the various orbital moon groupings we see today were formed in the distant past through this exact mechanism.”
The astronomers think Valetudo could be the last-remaining remnant of a once-larger prograde-orbiting moon that formed some of the retrograde moon groupings during past head-on collisions.
