Jupiter's Moon Total Hits 79

Mindy Sparks
July 17, 2018

At the time, the researchers were searching for a theorized massive planet far beyond Pluto, known as Planet X or Planet Nine, which is thought to be tugging on the orbits of distant solar system objects.

"Jupiter just happened to be in the sky near the search fields where we were looking for extremely distant Solar System objects, so we were serendipitously able to look for new moons around Jupiter while at the same time looking for planets at the fringes of our Solar System", said Sheppard. With this discovery, Jupiter now has a staggering 79 known orbiting moons - more than any other planet in the solar system.

One of the new moons in the outer group is particularly unique, according to astronomer Scott Sheppard at the Carnegie Institution for Science, who led the team that first spotted the moons a year ago. "This is an unstable situation", continued Sheppard. "They're going around the planet in the opposite direction that Jupiter rotates", Sheppard says. They found a dozen small moons.

The researchers, from the Carnegie Institution for Science in Washington, DC, picked out one of the 12 moons as an "oddball". As for the number of satellites that orbit Jupiter, Williams said he dreads "to think how many objects are in the hundred-meter range".

Gareth Williams at the International Astronomical Union's Minor Planet Center used the team's observations to calculate orbits for the newly found moons.

They also have a retrograde orbit, or the opposite direction to the spin of Jupiter on its axis.

"These moons are the last remnants of the building blocks of the giant planets as all other material in the giant planet region likely fell into the planets to help form them", Sheppard said. Nine of the new satellites orbit in a distant swarm of outer moons that are thought to be leftover from a series of collisions that might have involved what were once three larger bodies. "What these other objects were has been a mystery".

The initial discovery of most of the new moons were made on the Blanco 4-meter telescope at Cerro Tololo Inter-American in Chile and operated by the National Optical Astronomical Observatory of the United States. The "oddball" with the proposed name Valetudo orbits in the prograde, but crosses the orbits of the planet's outer retrograde moons.

Two of them are pretty straightforward.


The realm of the giant planets - between Jupiter, Saturn, Uranus and Neptune - is largely devoid of small objects. Astronomers believe they were once all part of a larger moon that broke apart.

An illustration showing how the orbits of the newly discovered moons (bold) fit into the known orbital groupings of the Jovian moons (not bold).

Astronomers describe the twelfth new Jovian moon as an "oddball". These are part of a group of prograde moons that orbit closer to Jupiter than the retrograde moons do.

Most of the prograde moons orbit much closer to the planet than the retrograde moons do. In fact, of Jupiter's 67 previously discovered moons, the 33 outermost moons all have retrograde orbits.

Due to their sizes - just 0.6 to 1.9 miles (1 to 3km) in diameter - these moons are more influenced by surrounding gas and dust.

Jupiter's moons range in size from shrimpy satellites to whopping space hulks. The irregular satellites didn't form around Jupiter in the same way that the planets formed around the Sun, or the regular satellites around Jupiter, from a flat disc in a prograde orbit.

"What's really cool for me here is what they're calling their oddball", Horner told ScienceAlert. Their existence shows that they were likely formed after this gas and dust dissipated.

Finding a moon that tiny, he noted, means it must have formed after all the gas and dust had cleared. "It's also likely Jupiter's smallest known moon, being less than one kilometer in diameter".

The team's results are not yet available in a peer-reviewed journal, as Sheppard's team is now running supercomputer simulations to try and figure out how often Valetudo might collide with a retrograde moon. Most of these prograde moons take less than a year to travel around the planet.

Other reports by Iphone Fresh

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