10 new moons discovered around Jupiter, but something odd is going on

Pat Wise
July 19, 2018

The team, led by astronomer Scott S. Sheppard of the Carnegie Institution for Science, was actually using ground-based telescopes to look for evidence of the mysterious Planet Nine, a proposed outer Solar System body. The moons were accidentally discovered while looking for massive planets beyond Pluto.

While the hunt for this planet continues, this major moon haul could help piece together more parts of the jigsaw that is the history of the solar system's earliest years, as elucidating the complex influences that shaped a moon's orbital history can help reveal this. The newly discovered retrograde moons take about two years to orbit Jupiter.

It's easy to understand why these 12 new additions had been missed so far. Maybe 100 or more of the really small ones.

That seems to be what happened to astronomers working at the Cerro Tololo Inter-American Observatory in Chile, where a planned survey of trans-neptunian objects was interrupted by Jupiter.

That brings the number of moons at Jupiter to 79, the most of any planet. They exist in retrograde - going the opposite direction of Jupiter's spin rotation.

Jupiter has several different types of moons.

Its four biggest moons - Io, Europa, Ganymede, and Callisto - are known as the Galilean moons because they were first spotted by Galileo Galilei in 1610.

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Beyond these is a group known as the "prograde" moons - more tiny, irregular satellites, all of which travel around Jupiter in the same direction that the planet rotates (counter-clockwise, in the view shown below).

The newly discovered Jupiter moons, with diameters of one to three kilometres (0.62 to 1.9 miles), required multiple observations to verify. In their recent observations, Sheppard's team documented nine of these (along with two prograde, closer-in moons). Thus, they orbit in the same direction as the planet. If moon circles a planet in the opposite direction of a rotating planet, that orbit is retrograde. Nine of the new discoveries are found in this group. They found a dozen small moons.

The last of the team's discoveries is the strangest of them all. Others including the oddball are "pro-grade" moons travelling with the planet's spin. Not only is it the smallest of Jupiter's 79 known moons-at less than a kilometer in diameter-but it has a very unusual orbit. Not yet, anyway. "Right now the only definition of a moon is something that orbits the planet", Sheppard said, as long as it isn't human-made.

Finally, there are retrograde moons.

Some have wondered if perhaps that shouldn't count as a moon. Its path weaving within the retrograde moons is unstable, they add; a collision, at some point, is likely. "Head-on collisions would quickly break apart and grind the objects down to dust". Sheppard and his team believe that these moons are remnants of three larger moons that broke apart when they collided with other moons, asteroids or comets.

The findings are another piece of the puzzle of the formation of our solar system.

Sheppard, whose report appears in the International Astronomical Union Minor Planet Electronic Circular, suspects that Valetudo is the final remnant of a once much larger moon that has been ground to dust by collisions in the past.

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