Planet Nine search finds 12 new Jovian moons
New discoveries include an oddball and the most distant known object in the solar system. Richard A Lovett reports.
The discovery brings the official count of Jupiter’s moons to 79.
The moons came to light during a survey scanning the sky for objects in a part of the solar system called the Kuiper Belt, far out beyond Pluto. The orbits of some of these planetoids, says Scott Sheppard, an astronomer at Carnegie Institution for Science in Washington DC, US, provide clues to the existence of a giant planet the size of Neptune, which many astronomers believe lies undiscovered somewhere in the Kuiper Belt.
One of the planetoids discovered by the survey, named 2012 VP-113, is the most distant known object in the solar system, Sheppard adds.
Last year, Sheppard’s team realised that Jupiter would be in their telescope’s field of view during one of their viewing runs. “So we looked at Jupiter for new moons at the same time we were looking for very distant things,” he says.
Distinguishing them from Kuiper Belt objects, Sheppard says, was easy because they would have quite different motions against the backdrop of stars. “A distant thing moves really slowly,” he says, “whereas any moon of Jupiter would move at a similar rate to Jupiter.”
That said, it required the assistance of with telescopes to confirm that the dozen new objects were indeed moons, and not just asteroids that happened to be in its vicinity. “It takes several observations to confirm an object actually orbits around Jupiter,” says Gareth Williams of the International Astronomical Union’s Minor Planet Centre. “So, the whole process took a year.”
All 12 panned out, however, and are now officially listed on the Minor Planet Center’s website.
All of them are “outer” moons, meaning that they lie as far as 10 to 20 million kilometres from Jupiter. That’s dozens of times farther out than the distance from the Earth to our own moon: testimony to the power of Jupiter’s giant gravity field.
“Jupiter is 320 times as massive as the Earth,” Sheppard notes.
Nine of the moons, which probably measure between one and four kilometres in size, are part of swarm of outer moons that have “retrograde” orbits, meaning that they travel in the opposite direction of Jupiter’s spin.
More interestingly, each of the nine has an orbit matching those of one of three families of other outer satellites — suggesting that they are also parts of these family groupings.
The families, Sheppard says, are thought to be remnants of three 100-kilometre diameter moons that broke apart in long-ago collisions, leaving remnants as large as 60 kilometres across. “Each of these three groups has one large member and a bunch of fragments,” he says.
Two of the other new moons are part of a closer-in group that orbits in the opposite direction, or “prograde”. They too are thought to be fragments of a larger body now broken apart.
The twelfth, Valetudo, is different.
For the others, the families of retrograde and prograde orbits are distantly enough separated that they don’t really interact with each other, Sheppard says. You could think of the prograde and retrograde moons as cars on opposite sides of a freeway, separated by a median.
Valetudo, however, is a fish swimming upstream.
It lies in the band of moons with retrograde orbits, but is itself in a prograde orbit. “Valetudo is going down the highway the wrong way,” Sheppard says.
“It’s also likely [to be] Jupiter’s smallest known moon, being less than one kilometre in diameter,” he says.
He thinks that it, too, was once much larger, but has now been whittled away to a tiny remnant by repeated collisions with objects going in the opposite direction.
“We think it was pulverised mostly to dust [until] all that’s left is this one object,” he says.
As is fitting for a moon orbiting far out from Jupiter, Valetudo is named for a relative of the Roman god Jupiter — but not a particularly close one. In Greco-Roman mythology, Valetudo, the goddess of health and hygiene, was one of Jupiter’s great-granddaughters.
But in an intriguing coincidence, vale tudo is a Brazilian form of mixed martial arts.
And that makes the name even more fitting for a moon whose entire history may have been written in a string of violent collisions.