Discoveries boost Jupiter’s retinue to 79 moons - including a wrong-way oddball

Muriel Hammond
July 21, 2018

On July 17, 2018, astronomers announced they've discovered even more moons orbiting Jupiter - 10 additional moons, in fact, bringing the known total of Jupiter's moons now to 79.

The Carnegie team led by Dr Sheppard first spotted the moons previous year while they were searching for a giant planet orbiting the sun beyond Pluto.

The new find boosts Jupiter's moon count to 79, easily making it the most populous place for moons in our solar system. Previously believed to hold just 61 objects, this group circles the planet in the opposite direction of Jupiter's rotation and is even more distant than the prograde moons.

Eleven are "normal" moons, with nine of them part of a distant outer swarm that orbit in the retrograde, meaning they move in the opposite direction to Jupiter's spin.

Two of the new moons orbit Jupiter closer in, and are prograde.

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. "Head-on collisions would quickly break apart and grind the objects down to dust". Many of Jupiter's outer moons were likely formed by collisions between larger retrograde moons and oddball prograde satellites.

It took a year for their orbits to be confirmed with a series of other telescopes in the United States and Chile. Sheppard's team speculates Valetudo could be a remnant of a collision between one or more moons.


Jupiter's moons are arranged in a specific pattern that the giant planet has worked out over time. He explained that the institution had named the moon Valetudo, and while that may sound like Latin for Mommy's Special Little Boy, it's really the Roman god Jupiter's great-granddaughter. "Most of the small objects that helped build the planets we see today were incorporated into the planets themselves, and these moons are all that remains", Sheppard wrote.

Scott Sheppard: "We believe these objects were probably captured by Jupiter a long time ago and they are grouped in their orbits".

Jupiter's moons range in size from shrimpy satellites to whopping space hulks. The two inner moons take less than a year to orbit Jupiter.

This, as you can imagine, has the potential to end poorly for our oddball friend and at least one of the other moons that are heading in the opposite direction.

Jupiter might yet have more undiscovered moons, Sheppard told Nature.

Not only that, but when the orbital characteristics (shape, tilt, and so on) are compared, these nine retrograde moons seem to fall into three groups; that implies that each group used to be a single moon that got smashed somehow, possibly a collision with another moon-sized body.

"It takes several observations to confirm an object actually orbits around Jupiter", Gareth Williams at the International Astronomical Union's Minor Planet Center, said in a statement. They are also part of a larger group of small moons thought to be the left-over remnants of a once larger moon. Nothing against Earth's handsome Luna, but according to The Verge the number of confirmed moons around Jupiter is now up to 79, thanks to ten - or 12, depending on who's counting and how - recent confirmations by the International Astronomical Union.

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