Astronomers find 12 new moons orbiting Jupiter, including an "oddball"

"This is an unstable situation", said USA astronomer Dr Scott Sheppard from the Carnegie Institution for Science in Washington DC, who led the discovery team. This is in contrast to other Jupiter moons such as Ganymede - the biggest in the solar system with a diameter of 5,268km. Instead, scientists will have to wait for a future spacecraft, either flying past Jupiter or orbiting it.

Astronomers are still finding moons at Jupiter, 400 years after Galileo used his spyglass to spot the first ones. Most likely to be pieces of a once larger moon that was broken up in orbit, they take almost a year to complete a lap around Jupiter.

The Blanco 4-meter telescope Sheppard was using is uniquely suited to spotting potential new moons both because the camera installed on it can photograph a huge area of sky at once and because it's particularly good at blocking stray light from bright objects nearby - say, Jupiter - that might wash out fainter ones.

The small moon rotates at the distance of the retrograde moons, but it's traveling in the other direction.

These are images of one of the new moons, named Valetudo.

The orbits of the 9 newly discovered retrograde moons of Jupiter are shown here. Those moons orbit close to Jupiter and travel in the same direction as the planet spins. In 2017, the group reported two additional Jovian moons. The lost moons were initially sighted in 2003, but scientists could not define their exact orbits and lost track of them. "Cars are coming right at you, and it's very likely you're going to have some head-on collision".

Sheppard and his colleagues have proposed naming the oddball moon Valetudo, after a minor goddess and great-granddaughter of the Roman god Jupiter. The nine moons are thought to be the fragments of three larger bodies that collided with asteroids, comets or other moons.

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Given their small size, if the moons had existed in the early days of the solar system, the gas and dust that surrounded the Sun at that time would have exerted a strong drag on them, causing them to lose speed and spiral in to crash into Jupiter.

Astronomers suspect that the retrograde moons may be the remains of larger moons that were destroyed in head-on collisions with prograde objects. That is tiny compared to Jupiter's diameter of 88,846 miles (142,984 km). Sheppard, who is broadly interested in the formation of solar systems and has been involved in the discovery of 48 of Jupiter's known moons, realized this was the ideal opportunity to advance two separate research goals with the same telescope data. These moons orbit Jupiter in the opposite direction.

But one moon, tentatively dubbed "Valetudo" (green), is an oddball, orbiting with Jupiter's spin but within the retrograde cluster.

During its full opposition the planet was brightest in Britain on May between 9:30pm and 4:30am BST while in the U.S. it peaked on May 9 between 1:10am and 6:20am ET.

For example, the discovery that the smallest moons in Jupiter's various orbital groups are still abundant suggests the collisions that created them occurred after the era of planet formation.

The newfound moons are small, between about 1 and 3 kilometers across. Since they are still around, now, that means they formed after that gas and dust had been swept away by the solar wind.