Did we discover 10 or 12 new moons of Jupiter?

Did we discover 10 or 12 new moons of Jupiter?

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I saw multiple news sites reporting that a team discovered 12 new Jupiter's moons:

While some other news sites claim that they found only 10 new moons:

Why does the number of moons differ in the different articles? Did the team now found 10 or 12 moons?

Per the Carnegie Science article that Magic Octopus Urn linked from NASA in the comments, a Carnegie Science team led by Scott S. Sheppard noticed something new in spring of 2017 (though some observations occurred as early as 2016). It took a year to confirm the discovery of the new moons.

Ten of the moons orbit in the outer swarm of moons, which is one of the ways to divide the groups of moons. Nine of these follow the pattern of the other moons which orbit Jupiter in retrograde (the opposite direction of the planet's rotation). One they call "oddball" because it orbits prograde (the same direction as the planet's rotation) and has a more inclined orbit than the inner prograde moons.

Two of the moons orbit in the inner group, which is what brings the total number of discoveries up to twelve, as referenced in the first set of articles in your post. Like the other inner moons, these orbit prograde.

One of the retrograde moons and the "oddball" were first noticed in 2016. One of the inner group and the rest of the retrograde moons were discovered in 2017. The second inner group moon was discovered in 2018, then most were announced together on July 17, 2018. (Dates collected from Wikipedia.)

The retrograde moon discovered in 2016 and one of the retrograde moons discovered in 2017 were announced in 2017, this means only ten moons were announced on July 17, 2018, which is why the Nature article refers to ten new moons. The EarthSky article talks some about the ten and the twelve, and it does clarify both groups (divided by inner/outer and year of announcement).

Jupiter Officially Has 12 New Moons

When Galileo first peered through his telescope at Jupiter in 1610, he was shocked to see that the planet was not alone—it was orbited by four moons, a fact that upended then-current theories of astronomy. Imagine how surprised he would be today to know that the Solar System's largest planet is now known to have 79 satellites.

Ian Sample at The Guardian reports the discovery of 12 new moons orbiting the gas giant, including one oddball flying in the wrong direction.

The new moons, reported in The International Astronomical Union’s Minor Planet Center’s Electronic Circular, were first found by a team of U.S. astronomers in March 2017 while they searched the skies for signs of Planet 9, a hypothesized ninth planet orbiting the sun far beyond Neptune. Kenneth Chang at The New York Times reports that Jupiter was scheduled to pass through their search area, so astronomer Scott S. Sheppard of the Carenegie Institution for Science and his team thought the gas giant was worth a quick look, training the Magellan Telescope at the Las Campanas Observatory in Chile on the area. What they found were 12 new moons orbiting the planet.

According to a press release, confirming the new celestial bodies took some time. “It takes several observations to confirm an object actually orbits around Jupiter,” says Gareth Williams of the Harvard-Smithsonian Center for Astrophysics and director of the International Astronomer’s Union’s Minor Planet Center, who calculated the orbits of the new moons. “So, the whole process took a year.”

The new moons are small, reports Ben Guarino at The Washington Post, most less than two miles in diameter, a fraction of the size of Jupiter’s four inner Galilean moons, one of which is larger than the planet Mercury. Nine of the new moons are clustered in the outer swarm of Jupiter’s moons and have roughly 2-year-long retrograde orbits, meaning they’re going the opposite direction of the planet's rotation. It’s believed these moons are the remnants of larger space rocks that broke apart during collisions with asteroids, comets or other moons. Two other moons orbit much closer to Jupiter in a group of inner prograde moons, which orbit in the same direction as Jupiter’s rotation, taking about one Earth year to make an orbit. Those moons are also believed to be remnants of a larger moon that was smashed to pieces.

The twelfth moon has already earned a proposed name, despite being only about two-thirds of a mile in diameter. That’s because Valetudo, named for the Roman goddess of health and hygiene, is a true oddball. Though it has a prograde orbit, it lives among a group of retrograde moons, meaning it careens across their orbits every once in awhile. “Valetudo is like driving down the highway on the wrong side of the road,” Sheppard tells Sample. “It is moving prograde while all the other objects at a similar distance from Jupiter are moving retrograde. Thus head-on collisions are likely," though still relatively rare, happening once every billion years or so. If Valetudo does smash into one of her neighbors, however, the impact will be large enough to detect on Earth.

The other moons don’t have names yet. Sheppard tells Chang the team may invite the public to offer suggestions, though he’s already vetoing “Planet McPlanetFace.”

Sheppard says that there are likely more chunks of rock left over from collisions around Jupiter, some of which could be moons. Which opens a whole astronomical can of worms, since there is no clear definition of what constitutes a moon. Some argue that, technically, every bit of dust orbiting a planet (which also has a contentious definition) can be considered a moon. Others suggest we should come up with criteria for what qualifies as a moon. Williams tells Chang that it’s a debate we can hold off having for a while. “We are nowhere near being able to image individual ring particles, let alone get enough observations for orbit determination, even from spacecraft,” Williams said. “I think that is a question for a future generation. Currently, it is too hypothetical.”

While the new moons are important just for helping us map our Solar System, Guarino reports that they may have deeper scientific value as well. The gravity from our largest planets—Jupiter, Saturn, Neptune and Uranus—have Hoovered up most of the small chunks of rock and debris left over from the formation of our Solar System. But these new moons may be chunks of that pre-planetary solar system, suspended in Jupiter’s orbit, and could tell us what the planets we know today are made of.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

Galileo Would Be Stunned: Jupiter Now Has 79 Moons

The planet Jupiter now has a total of 79 identified moons.

QAI Publishing/UIG via Getty Images

More than 400 years after Galileo Galilei discovered the first of Jupiter's moons, astronomers have found a dozen more — including one they've dubbed "oddball" — orbiting the planet. That brings the total number of Jovian moons to 79.

The team of astronomers originally wasn't even looking for the 12 new moons. Scott Sheppard of the Carnegie Institution for Science says he and his colleagues had been trying to track down a giant planet they think may be lurking at the outer reaches of our solar system.

As part of that search, Sheppard was using the 4-meter Víctor Blanco Telescope in Chile in March of last year and realized that Jupiter was right near the part of the sky he wanted to search.

"So we could also search for Jupiter moons while looking for things that are well beyond Pluto," Sheppard says.

One thing that helped was the especially large camera attached to the Blanco telescope. "[That camera] allows us to search the whole area around Jupiter in a very few images," he says.

They quickly hit the jackpot — 12 new moons appeared in the images. As they reported Tuesday in an online notice from International Astronomical Union, all 12 have now been confirmed by other telescopes.

Nine of them are in previously discovered clusters of moons that are in what astronomers call a retrograde orbit. "They're going around the planet in the opposite direction that Jupiter rotates," Sheppard says.

An illustration shows how the orbits of the 12 newly discovered moons (bold) fit into the known orbital groupings of the Jovian moons (not bold). The "oddball" with the proposed name Valetudo orbits in the prograde, but crosses the orbits of the planet's outer retrograde moons. Image by Roberto Molar Candanosa, courtesy of Carnegie Institution for Science hide caption

An illustration shows how the orbits of the 12 newly discovered moons (bold) fit into the known orbital groupings of the Jovian moons (not bold). The "oddball" with the proposed name Valetudo orbits in the prograde, but crosses the orbits of the planet's outer retrograde moons.

Image by Roberto Molar Candanosa, courtesy of Carnegie Institution for Science

Astronomers think retrograde moons have a different origin story from prograde moons, which travel in the same direction that their planet rotates.

Sheppard says Jupiter's prograde moons probably formed from the same spinning disk of stuff that eventually coalesced to form the planet. By contrast, retrograde moons were probably objects that once were wandering around the solar system and got snared by Jupiter's gravity.

"They didn't form with Jupiter," he says. "We think Jupiter captured them as these objects got too close to Jupiter in the past."

Most of the prograde moons orbit much closer to the planet than the retrograde moons do. But the moon Sheppard and his colleagues call "oddball" is different — instead of orbiting with the other prograde moons, its orbit takes it out as far as the retrograde moons.

The oddball is also the smallest of the moons that Sheppard and his colleagues found, just 1 kilometer across. 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.

Sheppard and his colleagues have proposed naming the oddball moon Valetudo, after a minor goddess and great-granddaughter of the Roman god Jupiter. Even though a dozen new moons is a pretty good haul, Sheppard expects that more searching will turn up even more moons. Maybe 100 or more of the really small ones.

And that raises a question: Does an object less than a mile across deserve to be called a moon?

Sheppard agrees it's problematic. "Maybe there will have to be a new definition for the smaller moons. Maybe a dwarf moon for anything that's 1 kilometer in size or smaller," he says.

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

Astronomers searching for signs of a large planet far beyond Pluto have stumbled across 12 previously undetected moons orbiting Jupiter , pushing the giant planet's total to a record 79, the Carnegie Institution reported Tuesday.

"Jupiter just happened to be in the sky near the search fields where we were looking for extremely distant solar system objects," Scott Sheppard, leader of the Carnegie team, said in a statement. "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."

Nine of the newly discovered moons are part of an outer group that orbit in the opposite, or retrograde, direction of Jupiter's spin, taking about two years to complete one trip around the planet. The new moons were found in three distinct clusters and are thought to be debris left over when three larger bodies were broken apart in earlier collisions.

Two previously undetected moons were found closer in, part of a group that orbits in the same "prograde" direction as Jupiter's rotation. With orbital periods of about one year, they also are thought to be the result of earlier collisions.

One of the 12 newly discovered moons orbiting Jupiter, dubbed Valetudo, someday could be obliterated in a head-on collision with a another moon thanks to its oddball orbit. Carnegie Institution

Sheppard described the 12th new moon as an "oddball" with "an orbit like no other known Jovian moon. It's also likely Jupiter's smallest known moon, being less than one kilometre (0.62 miles) in diameter."

Dubbed Valetudo after the Roman god Jupiter's great-granddaughter, the tiny moon orbits in the prograde direction but at a greater distance from Jupiter, taking about a year and a half to complete one trip around the planet. As such, the orbit crosses those of the outer retrograde moons, raising the possibility of a possible head-on collision at some point in the future.

Space & Astronomy

"This is an unstable situation," said Sheppard. "Head-on collisions would quickly break apart and grind the objects down to dust."

One of a dozen new moons discovered around Jupiter is circling the planet on a suicide orbit that will inevitably lead to its violent destruction, astronomers say.

Valetudo (one of Jupiter's moons) is driving down the highway on the wrong side of the road. - Scott Sheppard, Carnegie Institution

Researchers in the US 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.

The team first glimpsed the moons in March last year from the Cerro Tololo Inter-American Observatory in Chile, but needed more than a year to confirm that the bodies were locked in orbit around the gas giant. &ldquoIt was a long process,&rdquo said Scott Sheppard, who led the effort at the Carnegie Institution for Science in Washington DC.

Jupiter, the largest planet in the solar system, was hardly short of moons before the latest findings. The fresh haul of natural satellites brings the total number of Jovian moons to 79, more than are known to circle any other planet in our cosmic neighbourhood.

Astronomers have discovered twelve new moons orbiting Jupiter, bringing the total number of Jovian moons to 79.

Nine of the new moons belong to an outer group that orbit Jupiter in retrograde, meaning they travel in the opposite direction to the planet&rsquos spin. They are thought to be the remnants of larger parent bodies that were broken apart in collisions with asteroids, comets and other moons. Each takes about two years to circle the planet.

Two more of the moons are in a group that circle much closer to the planet in prograde orbits which travel in the same direction as Jupiter&rsquos spin. Most likely to be pieces of a once larger moon that was broken up in orbit, they take nearly a year to complete a lap around Jupiter. Which direction the moons swing around the planet depends on how they were first captured by Jupiter&rsquos gravitational field.

Astronomers describe the twelfth new Jovian moon as an &ldquooddball&rdquo. Less than a kilometre wide, the tiny body circles Jupiter on a prograde orbit but at a distance that means it crosses the path of other moons hurtling towards it. Scientists have named the new moon Valetudo after the Roman god Jupiter&rsquos great-granddaughter, the goddess of health and hygiene. But given the impending violence, it may be more than coincidence that Vale Tudo, which translates from Portuguese as &ldquoanything goes&rdquo, is an early form of full-contact mixed martial arts.

&ldquoValetudo is like driving down the highway on the wrong side of the road,&rdquo said Sheppard. &ldquoIt is moving prograde while all the other objects at a similar distance from Jupiter are moving retrograde. Thus head-on collisions are likely.&rdquo

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.

Which raises the question of how long the tiny moon has left. &ldquoCollisions don&rsquot happen all that frequently, every billion years or so,&rdquo said Sheppard. &ldquoIf one did happen, we would be able to detect it from Earth, but it is unlikely to happen anytime soon.&rdquo

Astronomers discover 12 more moons of Jupiter, including an oddity

Washington: Our solar system's oldest and biggest planet, Jupiter, has many moons. And astronomers have just announced the discovery of a dozen more.

On Tuesday, the International Astronomical Union's Minor Planet Centre published the orbits for 12 newfound Jovian moons - bringing Jupiter's total to 79 moons, said Scott Sheppard, a scientist at the Carnegie Institution for Science in Washington, DC.

Jupiter, in an image provided by NASA's Hubble Space Telescope. Credit: NASA via AP

Sheppard did not set out to detect new moons. His team at Carnegie, along with collaborators at the University of Hawaii and Northern Arizona University, were hunting for objects far beyond Pluto.

"We're looking for new possible planets and dwarf planets in our solar system, just seeing what is out there," Sheppard said.

But cosmic serendipity placed the moons in front of their telescope. During their survey in March 2017, the astronomers realised that Jupiter had moved into their field of view. The Blanco four-metre telescope, at the Cerro Tololo Inter-American Observatory in Chile, is equipped with a camera able to spot very faint objects. This proved to be quite helpful, as the unknown moons around Jupiter are small and dim.

These images show the movement of Jupiter's moon dubbed Valetudo (marked in yellow) relative to background stars. Credit: NASA

"We were able to go a little bit fainter than anyone has been able to go in the past," Sheppard said, "and that's why we were able to find these new moons."

Jupiter's moons range in size from shrimpy satellites to whopping space hulks. Galileo discovered the first four of Jupiter's moons, all huge, in 1610. The largest Galilean moon, Ganymede, is bigger than the planet Mercury. Those moons orbit close to Jupiter and travel in the same direction as the planet spins.

The moons Sheppard spied are further-flung and tiny, each little more than three kilometres in diameter. One moon detected by Sheppard and his colleagues is the smallest Jovian moon ever discovered. They named it Valetudo, after a daughter of Jupiter and the Roman goddess of hygiene and personal health.

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

But he said he expected astronomers would discover more tiny moons in the coming decades. As for the number of satellites that orbit Jupiter, Williams said he dreaded "to think how many objects are in the hundred-metre range."

A NASA image of Jupiter's moon Europa, with the giant planet in the background. Credit: NASA

Size isn't a requirement to be a moon - there's no such thing as a dwarf moon. 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.

Astronomers group Jupiter's moons by their distance from the planet as well as their orbital direction. If a moon circles in the same direction as a planet's rotation, that moon's orbit is called prograde. If moon circles a planet in the opposite direction of a rotating planet, that orbit is retrograde. (A small subset of Jupiter's 79 moons do not have well-known orbits.)

Most moons, including Earth's, have prograde orbits. Two of the newly discovered moons, the ones closest to Jupiter, have prograde orbits, too.

The other nine moons, grouped in clusters of three, have retrograde orbits. Those moons were probably once part of much larger moons that splintered into smaller objects, Sheppard said. The astronomers are running computer simulations to determine how the ancient moons fragmented.

This composite includes the four largest moons of Jupiter which are known as the Galilean satellites. The Galilean satellites were first seen by the Italian astronomer Galileo Galilei in 1610. Shown from left to right in order of increasing distance from Jupiter, Io is closest, followed by Europa, Ganymede and Callisto. Credit: NASA

Valetudo is something of an oddball. It has a prograde orbit at a distance where the rest of Jupiter's moons have retrograde orbits. What's more, those orbits intersect. The stage is set, in other words, for possible moon-moon collisions.

The astronomers do not know the composition of the dozen newly identified moons. They could be rock, ice or a mixture. "The only thing that we know at the moment are the orbits and the approximate size," Williams said. "We know nothing, really, more than that."

The realm of the giant planets - between Jupiter, Saturn, Uranus and Neptune - is largely devoid of small objects. When those planets formed, their gravity vacuumed up nearby gas, rocks and other debris. This matter became part of the planets themselves.

But Sheppard suspects these moons could be holdouts, the "last remnants" of early solar system objects.

"By looking at these outer moons," he said, "we can get an insight into what the objects were like that ended up forming the planets we see today."

The Discoveries of Galileo – Part 1: Jupiter

We amateur astronomers should all be familiar with who Galileo di Vincenzo Bonaulti de Galilei aka Galileo Galilei (1564-1642) was. He has been called the father of observational astronomy. I happened upon a book the Austin Public Library, Discoveries and Opinions of Galileo, first published in 1957 by Stillman Drake and I was fascinated by the writings of Galileo. Drake was a Canadian historian of science best known for his complete works on Galileo. In this book Drake has translated many of Galileo’s letters and other publications. In the 1600’s the language of the educated class was Latin. Galileo was a revolutionary in that he wrote about his philosophical i.e. scientific observations in Latin and in the local Italian dialect. He wanted the masses to read his writings, not just the elite educated class!

The painting in figure 1 by Giuseppe Bertini shows Galileo courting the reigning Medicini family of Florence for financial support. He was employed as the tutor of mathematics to one son, Cosimo II, in the summer of 1605. Five years later the young Cosimo would become the Grand Duke of Tuscany. It is believed that the Duke, after looking through the telescope, rewarded Galileo by appointing him as the chief mathematician and philosopher of Tuscany.

Galileo first heard of an invention out of Holland in the year 1608 called a “spyglass” that magnified distant objects. The invention of the spyglass is attributed to Hans Lipperhey who was a spectacle maker. He developed one of the first refracting telescopes "for seeing things far away as if they were nearby." Lipperhy’s spyglass could magnify distant objects to about 3-4x.

Galileo wrote, “About ten months ago a report reached my ears that a certain Fleming had constructed a spyglass by means of which visible objects, though very distant from the eye of the observer, were distinctly seen as if nearby.” So, Galileo used his talents to make his own spyglass in the summer of 1609.

Figure 2 shows a replica of one of his first spyglasses. It’s about 39” long (980mm) with a tube diameter of about 1.5” (37mm) and it gave him a magnification of

Galileo pointed his spyglass to the night sky and was one of the first humans to see the beauty and majesty of God’s creation up close. Galileo made both simple and detailed drawings of his celestial observations depending on how the resolution he could see in his spyglass.

I want the focus on this article to be Galileo’s observation of that wandering star known to everyone in the 17 th century and today as Jupiter. And I want to compare Galileo’s nightly drawings of that celestial object to today’s advanced night sky astronomy software.

On a clear, cold night in January 1610 Galileo turned his spyglass toward a bright wandering star. Until this point in human history people didn’t know that the five visible “wandering stars” were planets and not stars. This image from the Let There Be Night website (figure 3) gives an approximate idea of the magnification and a narrow field of view of what Jupiter may have looked like to Galileo. But even with that narrow view Galileo was able to note some “stars” next to Jupiter. Galileo of course was astonished! He wrote,

“But what exceeds all wonders, I have discovered four new planets and observed their proper and particular motions, different among themselves and from the motions of all the other stars and these new planets move about another very large star [Jupiter] like Venus and Mercury, and perchance the other known planets, move about the Sun.”

Imagine you were in Galileo’s shoes four hundred and ten years ago and you would probably think the tiny points of light around the bright planet were stars too! Galileo noticed that the smaller companion stars changed position from night to night. He made detailed sketches of Jupiter and its “stars” from January 7 to January 15, 1609 (figure 4, left panel). I used Stellarium astronomy software to validate the position of Jupiter’s moons on the same dates as his observations (figure 4, right panel).

Note his first drawing on January 7, 1610 in figure 4. See the position of the “stars” he drew and compare them to the Stellarium software. I have put a ”J” over the planet Jupiter in both images. I am amazed at how well the algorithms from the Stellarium software can reach back into time and match up with the first drawings of Jupiter magnified through a telescope!

Galileo drew the position of the “stars” on seven nights to track their movement. You can see the movement of the companion stars in the software and compare them to his drawings. Note that because of the limited magnification of the optics of Galileo’s primitive lenses that he sometimes could not resolve an individual “star” if it was too close to another star or Jupiter i.e. his January 10 drawing. I was stunned at how accurate our modern software is in going back in time to display the position of celestial objects.

On a personal note, I purchased my first telescope, a Meade DS-2130 5” GoTo reflector, in 2009. I remember being out at a dark location one summer night in Lago Vista, Texas in 2012 and looking at Jupiter. I could only see three of the Galilean moons much like the January 8 Stellarium image. I kept asking myself over and over, “Where is the fourth moon?” Then about 30 minutes later I noticed a small white pimple on the right side of Jupiter. As I watched in awe, I could see in real time as the pimple grew and then completely separated from Jupiter as the fourth moon. And I remember thinking to myself, “This must have been the same awe Galileo felt when he first observed this same thing for the first time!”

Back to Galileo in 1610, it wasn’t long before Galileo realized these were not stars but what he called planets revolving around the “star” named Jupiter. It was a stunning revelation to Galileo: small planets are moving around a “star” just like Venus and Mercury revolves around our Sun!

This was definitely a “Eureka!” moment for Galileo! He honored the Medicini family and Cosimo II, who became Grand Duke of Tuscany in 1609, by naming these four “planets” the Medicea Sidera (Medicean stars) in his 1610 pamphlet Sidereus Nuncius, “Starry Messenger” (figure 5).

In the next installments of this series I will cover Galileo’s observations of sunspots, the Moon, Venus, and the Milky Way. A little preview: his writings on sunspots in 1613 are what initially got him in trouble with the Roman Catholic Church!

Ed LaBelle has been a member of Austin Astronomical Society since 2010. He has worked as an engineer in the semiconductor industry for 30 years. Ed is the founder of the Psalm 19 Astronomy Society, a sidewalk astronomy ministry, based in Austin, Texas USA.

Saturn beats Jupiter with 82 moons, including 20 just discovered

Saturn has broken a new solar system record with the discovery of 20 new moons circling it.

The ringed planet now has 82 known moons, smashing the previous record held by Jupiter, which has 79, reported the Carnegie Institution for Science Monday.

In fact, Jupiter has held the "moon king" title for decades, and got a dozen new moons added to its tally just last year, says Scott Sheppard, the Carnegie astronomer who led the new discoveries.

"Saturn has just surpassed it for the first time today," he added. "I think these moons are exciting because they tell us about the planet formation process."

In fact, astronomers believe that Saturn's smaller moons are the "last remnants" of the objects that originally came together to form the giant planets.

The new moons are all about five kilometres in diameter. They were first spotted in 2004 using the Subaru telescope on Mauna Kea, a dormant volcano in Hawaii that's home to many large telescopes. But it took 10 years of improvements to computer power and algorithms to derive their orbits from the data and link their positions from year to year.

The data and calculations were submitted to the Minor Planet Centre of the International Astronomical Union, the scientific organization that assigns names to celestial bodies, which confirmed they are new moons.

It invited the public to propose official names for the moons, which are currently identified by a series of letters and numbers. Suggestions can be tweeted to @SaturnLunacy with the hashtag #NameSaturnsMoons. Nominators are invited to explain why they picked the name, and "photos, artwork, and videos are strongly encouraged."

Hi friends! I'm back with a new handle, because we've got a whopping 20 newly discovered moons of Saturn to name. Please help us! <a href=""></a>

[email protected]

Two of the moons are part of a group of moons called the Inuit group, whose original members were discovered by Canadian astronomers and named by team member John Kavelaars, currently at the Dominion Astrophysical Observatory in Victoria, and Inuit storyteller and children's author Michael Arvaarluk Kusugak. The two new moons will be given names from Inuit mythology. All the moons in the group (the ones named already are Ijiraq, Kiviuq, Paaliaq, Siarnaq and Tarqeq) orbit at an angle to Saturn's orbit and are thought to have originally been part of a single object about 40 kilometres wide that broke into fragments.

A third moon is part of a similar group called the Norse group that will be given names from Norse mythology.

The other 17 new moons are retrograde moons — unlike the Inuit and Norse moons, they travel in the opposite direction to Saturn's rotation.

A number of other potential Saturn moons spotted in 2004 have not been confirmed as moons yet.

Sheppard says he thinks Saturn probably has 100 moons that are a kilometre in size or bigger, but astronomers may need to wait for the next generation of bigger, better telescopes to confirm them all. On average, Saturn's known moons are bigger than Jupiter's, he added. "But since Saturn's further away, it's harder to detect its moons."

Jupiter has 10 more moons we didn't know about — and they're weird

Astronomers have discovered 10 small moons orbiting Jupiter, bringing its total to 79 — by far the most moons known around any planet. One of the finds is an oddball that moves in the opposite direction from its neighbours.

Together, the moons help to illuminate the Solar System’s early history. The existence of so many small satellites suggests that they arose from cosmic collisions after Jupiter itself formed, more than 4 billion years ago.

“They did not form with the planet, but were likely captured by the planet during or just after the planet-formation epoch,” says Scott Sheppard, an astronomer at the Carnegie Institution for Science in Washington DC. He and his colleagues announced the discovery on 17 July.

Sheppard’s team typically hunts for objects in the very distant Solar System, out beyond Pluto, and sometimes spots planetary moons during these searches. Last year, the group reported two additional Jovian moons. In this case, the scientists were looking for a putative unseen massive planet popularly known as Planet Nine. Jupiter was in the same part of the sky, so they were able to hunt for moons as well.

The researchers discover new Solar System bodies, and calculate their orbits, by photographing the same part of the sky weeks or months apart. They then look for objects that shift position between the two images, relative to the background stars. The team first spotted most of the new Jovian moons using the Blanco 4-metre telescope at the Cerro Tololo Inter-American Observatory in Chile, and followed up with further observations at other telescopes.

These images show the movement of the Jovian moon dubbed Valetudo (labelled in yellow) relative to the background stars. Credit: NASA

All the newfound moons are small, between about 1 and 3 kilometres across. Seven of them travel in remote orbits more than 20 million kilometres away from Jupiter, and in the opposite direction from the planet’s rotation. That puts them in the category known as retrograde moons.

The eighth moon stands out because it travels in the same region of space as the retrograde moons, but in the opposite direction (that is, in the same direction as Jupiter’s spin). Its orbit is also tilted with respect to those of the retrograde moons. That means it could easily smash into the retrograde moons, pulverizing itself into oblivion. It may be the leftovers of a bigger cosmic collision in the past, Sheppard says.

Jupiter’s moons are named after gods with connections to the mythological Jupiter or Zeus. Sheppard has proposed naming the oddball Valetudo, after one of Jupiter’s descendants, the Roman goddess of hygiene and health.

The ninth and tenth newfound moons orbit closer to Jupiter, moving in the same direction as the planet.

Had all these small moons formed at the same time as Jupiter, they probably would have been captured by the gas and dust still swirling around the newborn planet, and have been engulfed. Their existence suggests that they are leftovers of later collisions between space rocks that left the debris encircling Jupiter.

If astronomers can work out the history of these collisions, they could also determine the sizes of satellites pulled into the orbit of a young Jupiter. “That's the big question, and that's what makes these ten new moons interesting," says Douglas Hamilton, an astronomer at the University of Maryland in College Park. “How can we link all this to how planets formed?”

Sheppard says there might still be a few more moons of Jupiter to discover — as yet unseen because they were hiding in the Sun’s glare when the scientists were looking. Saturn, the runner-up to Jupiter in the moon competition, has 62 known satellites.

The Dance of Jupiter's Moons

Four hundred years ago, the astronomer Galileo Galilei announced his discovery of four moons orbiting around the planet Jupiter, each seen as a distinct white dot through his telescope. However, only in the span of the last four decades have astronomers been able to study the Jovian moons in detail to reveal that the four—Io, Europa, Ganymede, and Callisto—are fascinating worlds of their own.

Though they are all of similar sizes—about one fourth of Earth's radius—the four moons are diverse: Io is violently volcanic, Europa is encrusted in ice, Ganymede has a magnetic field, and Callisto is pockmarked with ancient craters. Moreover, icy Europa is considered a strong candidate for hosting life in the solar system.

One open question still puzzles planetary scientists: How did the Jovian satellites form?

Now, Caltech professor of planetary science Konstantin Batygin (MS ✐, PhD ✒) and his collaborator Alessandro Morbidelli of Observatoire de la Côte dɺzur in France have proposed an answer to this longstanding question. Using analytical calculations and large-scale computer simulations, they propose a new theory of the Jovian satellites' origins. The research is described in a paper appearing in the May 18 issue of The Astrophysical Journal.

During the first few million years of our sun's lifetime, it was surrounded by a protoplanetary disk made up of gas and dust. Jupiter coalesced from this disk and became encircled by its own disk of satellite-building material. This so-called circum-Jovian disk was fed by material from the protoplanetary disk that rained down on Jupiter at the planet's poles and flowed back out of Jupiter's sphere of gravitational influence along the planet's equatorial plane. But this is where things get tricky for satellite formation how did this ever-changing disk accumulate enough material to form moons?

Batygin and Morbidelli's new model addresses this by incorporating the physics of interactions between dust and gas in the circum-Jovian disk. In particular, the researchers demonstrate that for icy dust grains of a specific size-range, the force dragging them toward Jupiter and the force (entrainment) carrying them in the outward flow of the gas cancel each other perfectly, allowing the disk to act like a giant dust trap. Batygin says the inspiration for the idea came when he was out for a run.

"I was running up a hill, and saw that there was a bottle on the ground that was not rolling down the hill because wind coming from behind me was pushing it upward and holding it in equilibrium with gravity," he says. "A simple analogy came to mind: if a beer bottle rolling down an inclined plane is akin to orbital decay of solid grains due to hydrodynamic drag, then particles of a certain size-range must find an equivalent balance in orbit of Jupiter!"

The researchers' model proposes that, due to this balance between inward drag and outward entrainment, the disk around Jupiter became rich in icy dust grains, each about one millimeter in size. Eventually, this ring of dust became so massive that it collapsed under its own weight into thousands of "satellitesimals"—icy asteroid-like objects about 100 km across. Over thousands of years, satellitesimals coalesced into moons, one at a time.

According to the model, as the first moon (Io) formed and its mass reached a certain threshold, its gravitational influence began to raise waves in the gaseous disk of material that surrounded Jupiter. By interacting with these waves, the moon migrated toward Jupiter until it reached the inner edge of the circum-Jovian disk, close to its present orbit. The process then began again with the next moon.

This sequential process of formation and inward migration led Io, Europa, and Ganymede to lock into an orbital resonance—a configuration where for every four times Io goes around Jupiter, Europa goes around twice and Ganymede goes around once. This so-called Laplace resonance is one of the most striking and well-known features of the moons' orbits.

Finally, the model suggests that radiation from the sun eventually blew away the remaining gas in the disk around Jupiter, leaving behind the residual satellitesimals that then formed the fourth and final major moon, Callisto. However, with no gas left to drive long-range migration, Callisto could not join the other moons in resonance, and was left stranded to revolve around Jupiter every two weeks.

"The process we described for the formation of the satellites of Jupiter may be a general one," says Morbidelli. "We now have observations of the disk around one extrasolar giant planet, PDS70c, and it looks extraordinarily rich in dust, like we envision for the disk of Jupiter before the formation of its satellites."

There is still much to discover about the Jovian moons. NASA's Europa Clipper mission, launching in 2024, will visit Europa with the aim of discovering whether or not it has conditions amenable to life. The European Space Agency also plans to send a mission, called JUpiter ICy moons Explorer (JUICE), focusing on Ganymede, the largest of the Jovian moons.

The paper is titled "Formation of Giant Planet Satellites." The research was supported by the David and Lucile Packard Foundation, the Alfred P. Sloan Foundation, and Observatoire de la Côte dɺzur.