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Jupiter's moon count reaches 79, including tiny 'oddball'

18 July 2018

This brings Jupiter's total number of known moons to a whopping 79-the most of any planet in our Solar System.

Because of how many observations it takes to determine an object in space is actually in orbit around Jupiter, it took about a year to confirm that these were, indeed, new Jovian moons.

Researchers in the U.S. stumbled upon the new moons while hunting for a mysterious ninth planet that is postulated to lurk far beyond the orbit of Neptune, the most distant planet in the solar system.

In March 2017, Jupiter was in the flawless location to be observed using the Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile, which has the Dark Energy Camera and can survey the sky for faint objects.

Because Jupiter moves across the sky at a known speed, anything nearby moving at the same speed in the same direction becomes a candidate for a moon - but confirmation is a time-consuming process, Sheppard explained to ScienceAlert. Those moons orbit close to Jupiter and travel in the same direction as the planet spins.

Gareth Williams, an astronomer at the Harvard-Smithsonian Center for Astrophysics and director at the International Astronomical Union's Minor Planet Center, predicted that "there aren't any bigger objects undiscovered out there" around Jupiter.

The scientists say the moons weren't seen before because they are tiny - the biggest ones only about two miles across.

Nine of the new moons are part of a distant outer swarm of moons that orbit in the opposite direction of Jupiter's spin.

"This is an unstable situation", said Sheppard. They are thought to be the remnants of larger parent bodies that broke apart as a result of a collision with other bodies like asteroids, comets or other moons. "What these other objects were has been a mystery".

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Europa, one of Jupiter's larger moons, casts a shadow on the planet's surface.

Two of them are pretty straightforward.

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.

But the last moon is a weird one. Researchers have proposed naming the "oddball" Valetudo, after the Roman goddess of health and hygiene.

Astronomers suspect that the retrograde moons may be the remains of larger moons that were destroyed in head-on collisions with prograde objects. Galileo discovered the first four of Jupiter's moons, all huge, in 1610.

In the spring of 2017, Jupiter happened to be in an area of sky the team wanted to search for Planet Nine. Two of the discoveries are part of an inner group in the prograde that orbit in the same direction as the planet's rotation.

"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. This tells us something about the timing of the formation of these moon families, which, in turn, tells us something new about the formation of the Solar System. 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. It was probably right in the middle of the planets we know so well, Sheppard said.

"A full paper will likely be written after these simulations are done in a few months". Lines point to Valetudo, the newly discovered "oddball" moon.

Jupiter's moon count reaches 79, including tiny 'oddball'