It’s been 400 years since Galileo discovered the first of Jupiter’s moons. But now the number of moons astronomers have found orbiting the planet is a total of 79.
The team of astronomers originally wasn’t even looking for moons when they discovered 12 new moons. Scott Sheppard of the Carnegie Institution for Science says he and his colleagues had been trying to track down a giant planet they think may be lurking at the outer reaches of our solar system.
As a tool for their research, Sheppard was using a 4-meter Victor Blanco Telescope in Chile in March of last year. He realized that Jupiter was right near the part of the solar system he wanted to research.
“So we could also search for Jupiter moons while looking for things that are well beyond Pluto,” Sheppard says.
The team had a large camera attached to the Blanco telescope. “[That camera] allows us to search the whole area around Jupiter in a very few images,” he says. They hit the jackpot — 12 new moons appeared in the images.
They reported their find on Tuesday in an online notice from International Astronomical Union. And all 12 moons have now been confirmed by other telescopes.
Nine of the new moons are in previously discovered clusters of moons that are in what astronomers call a retrograde orbit. “They’re going around the planet in the opposite direction that Jupiter rotates,” Sheppard says.
Astronomers believe retrograde moons have a different origin story from prograde moons, which move in the same direction that their planet rotates.
Retrograde moons were probably objects that once were wandering around the solar system and got snared by Jupiter’s gravity.
“They didn’t form with Jupiter,” he says. “We think Jupiter captured them as these objects got too close to Jupiter in the past.”
There is one of Jupiter’s moons Sheppard and his colleagues call “oddball.”
Instead of orbiting with the other prograde moons, its orbit takes it out as far as the retrograde moons.
The oddball is also the smallest of the moons that Sheppard and his colleagues found, just 1 kilometer across. Sheppard thinks it may be all that’s left of a larger moon that crashed into one or more of the retrograde moons sometime in the past.
Sheppard and his colleagues have suggested naming the oddball moon Valetudo, after a minor goddess and great-granddaughter of the Roman god Jupiter. He also expects that more searching will turn up even more moons. Maybe 100 or more of the really small ones.