Astronomers searching for signs of a hypothetical “Planet Nine” have instead come up with 12 new moons of Jupiter, including one that hints at a cosmic crack-up.
The discoveries were made more than a year ago, and the orbits of two of the moons were confirmed soon after they were found. It took much longer for the other 10 to have their orbits verified.
“It takes several observations to confirm an object actually orbits around Jupiter,” Gareth Williams of the International Astronomical Union’s Minor Planet Center explained in a news release. “So, the whole process took a year.”
The Minor Planet Center published the remaining 10-pack’s orbital parameters today, marking their formal acceptance as Jovian moons. That brings Jupiter’s total tally to 79 moons, easily besting runner-up Saturn’s count of 62.
None of the dozen moons is more than a couple of miles across. Two of them have relatively close-in orbits, going in the same direction as Jupiter’s spin. Nine of them orbit farther out, in a retrograde direction — that is, opposite to the direction of the giant planet’s rotation.
“Our other discovery is a real oddball and has an orbit like no other known Jovian moon,” said Scott Sheppard of the Carnegie Institution for Science, who led the discovery team. “It’s also likely Jupiter’s smallest known moon, being less than 1 kilometer in diameter.”
The oddball moon crosses the orbits of the outer retrograde moons.
“This is an unstable situation,” said Sheppard. “Head-on collisions would quickly break apart and grind the objects down to dust.”
The oddball could be the last remaining remnant of a once-larger moon that gave rise to the retrograde retinue during previous smash-ups. The circumstances of the moons’ orbits lend further support to the view that they were formed long after Saturn and its larger moons coalesced from a primordial cloud of gas and dust.
Sheppard and his colleagues spotted most of the moons using the Blanco 4-meter telescope at the Cerro Tololo Inter-American Observatory in Chile, during a campaign aimed at tracking down evidence for the so-called Planet Nine or Planet X.
Back in 2016, astronomers reported that the unusual orbits of some objects on the edge of the solar system, far beyond Pluto, could best be explained by the existence of an undetected planet several times more massive than Earth. Astronomers around the world, including Sheppard, have been looking for Planet Nine ever since.
“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 said.
Over the months that followed, several other telescopes provided additional data to support the discoveries.
The moons are now known by their numerical designations, such as S/2016 J2. Eventually it’ll be up to the discoverers to propose formal names to the IAU.
Sheppard and his team have already suggested a moniker for the oddball moon: Valetudo, the mythological great-granddaughter of the Roman god Jupiter and the goddess of health and hygiene.