Saturn has more moons than Jupiter – but why are we only finding out about them now?


This Hubble Space Telescope image of Saturn and a few of its moons shows how hard it can be to spot the gas giant’s tiny orbiting companions.
NASA / ESA / Hubble

Lucyna Kedziora-Chudczer, Swinburne University of Technology

With the discovery of 20 more moons orbiting Saturn, the ringed planet has overtaken Jupiter as host to the most moons in the Solar system. Saturn now has 82 known moons, whereas Jupiter has a paltry 79.

Announced at the International Astronomical Union’s Minor Planet Centre by a team of astronomers from the Carnegie Institute for Science led by Scott S. Sheppard, the discovery is the latest advance in the 400-year history of our understanding of the satellites of our neighbouring planets.

As technology has improved, we have observed more and more of these tiny, distant worlds – and we can be reasonably confident there are still plenty waiting to be discovered.

How do we even know Saturn has moons?

Although most planets of the Solar system are visible to the naked eye and have been known to humans since antiquity, it wasn’t until Galileo Galilei turned a telescope on Jupiter in 1610 that we discovered Earth was not alone in having an orbiting companion.

Galileo saw Jupiter’s four largest moons and could make out what we now know are Saturn’s rings. Decades later, with better telescopes, Christian Huygens and Giovanni Domenico Cassini observed Saturn’s moons.




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It became clear that the giant planets are surrounded by multitudes of satellites, resembling smaller versions of the Solar system. By the middle of the 19th century, telescopes had improved enough that the first eight moons of Saturn – including Titan, the largest – had been viewed directly.

The introduction of photographic plates, which enabled the detection of fainter objects with long-exposure observations, helped astronomers increase their count of Saturn’s moons to 14.

Closer inspections

It was a long journey (literally) to the next big improvement in our view of Saturn’s moons. Many of the smaller moons were not discovered until the Voyager fly-by missions in the 1980s and the more recent 13-year stopover of the Cassini spacecraft in Saturn’s orbit.

Until these closer visits, we knew little about the moons aside from the fact that they existed.

One of Cassini’s goals was to explore Titan, which is the only moon in the Solar system with a thick, smoggy atmosphere. Another was to take a look at Saturn’s other mid-sized moons, including frozen Enceladus, which may hold an ocean of liquid water beneath its icy crust.




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Cassini also discovered much smaller moons, so-called “shepherd moons” that interact with Saturn’s rings by carving gaps and wavy patterns as they pass through a rubble of rocks and snowballs.

Bigger telescopes, more moons

These close-up observations from space advanced our understanding of individual moons that stay near to Saturn. Recently, many more moons have been found in orbits much further from the planet.

These more distant moons could only be detected with large optical telescopes such as the Subaru telescope at Mauna Kea in Hawaii. The telescope is equipped with sensitive cameras that can detect some of the faint objects separated by millions of kilometres from Saturn.

The new moons were discovered by comparing photos like this pair taken about an hour apart. While the background stars stay fixed, the moon – highlighted with orange bars – moves between frames.
Scott Sheppard

To confirm that these objects are indeed associated with Saturn, astronomers have to observe them over days or even months to reconstruct the shape and size of the moon’s orbit.

Many small moons are fragments of shattered large moons

Such observations revealed a population of moons that are often described as “irregular” moons. They are split into three distinct groups: Inuit, Gallic, and Norse. They all have large, elliptical orbits at an angle to those of moons closer to the planet.

Each group is thought to have formed from a collision or fragmentation of a larger moon. The Norse group consists of some of the most distant moons of Saturn, which orbit in the opposite direction to the rotation of the planet. This suggests they could have formed elsewhere and were later captured by the gravitational force of Saturn.

Of the 20 new moons, 17 belong to the Norse group including the furthest known moon from the planet. Their estimated sizes are of the order of 5km in diameter.

Most of the newly discovered moons have retrograde orbits, going in the opposite direction to Saturn’s spin.
Carnegie Institution for Science

Have we found all the moons now?

Are we likely to find even more moons around Saturn? Absolutely.

Some of the newly discovered moons are very faint and at the limit of detection with currently available instruments. New, bigger telescopes such as Giant Magellan Telescope will allow us to observe even fainter objects.

In the meantime, the 20 new moons need names. Carnegie Science has invited everyone to help.The Conversation

Lucyna Kedziora-Chudczer, Program Manager / Adjunct Research Fellow, Swinburne University of Technology

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Jupiter’s new moons: an irregular bunch with an extra oddball that’s the smallest discovered so far



File 20180723 189335 bt90kx.jpg?ixlib=rb 1.1
A moon shadow on Jupiter, the red planet now has a dozen more moons added to the list or such orbiting bodies.
NASA/JPL-Caltech/SwRI/MSSS

Jonti Horner, University of Southern Queensland and Christopher C.E. Tylor, University of Southern Queensland

Jupiter is the largest planet in the Solar system and has been studied intensively for hundreds of years, so you might think there would be little left to find.

But earlier this month, researchers announced that another 12 moons have been added to the number of such bodies orbiting the giant planet.




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That brings the tally for Jupiter to a whopping 79, the most moons for any known planet. But where did these newly discovered moons come from, and what do they tell us about Jupiter and its place in the Solar system?

Moons: regular and irregular

The Solar system’s giant planets have two types of moon: regular and irregular.

Regular moons orbit close to their host, follow nearly circular paths, and move in the same plane as the planet’s equator. In some ways, these moons resemble miniature planetary systems, and we think that they formed in much the same manner as the planets around the Sun.

As the giant planets gathered material from the disk of gas and dust that surrounded the young Sun – a process known as accretion – they were surrounded by their own miniature disks. Within those disks, the regular moons grew, all in the planet’s equatorial plane.

Artist’s impression of a protoplanetary disk – a place where planets are born. Around young giant planets, similar disks give birth to regular moons.
ESO/L. Calçada

But the irregular moons are another story.

Their orbits are highly eccentric (elliptical) and inclined relative to the plane of their host planet’s equator. Many even move on retrograde orbits, travelling in the opposite direction to the spin and orbital motion of their hosts. And they are located much farther from their planet than their regular cousins.

Where do the irregulars come from?

Because of their wild orbits, the irregular moons cannot have formed in the same way as their regular cousins. Instead, they are thought to have been captured by their host planets as the process of planet formation came to an end.

We think that each giant planet captured just a handful of irregular moons – a number far smaller than we see today. Over the billions of years since, those moons were pummelled and destroyed by passing asteroids and comets, and collisions with other members of their swarm.

The shattered fragments of those ancient satellites form families of smaller moons – the irregulars we see today. For example, among Jupiter’s satellites we see at least four distinct families of irregular moons, each named after their largest member.

The motion of Jupiter’s irregular moons around the giant planet. The main plot (bottom, left) shows the orbits looking top-down, while the other (right and top) plots show the movement out of the plane of the system. Moons of the same colour are members of the same family.
Christopher Tylor

What does the new discovery add to our understanding?

If we consider Jupiter’s moons in terms of their orbital distance, and the direction in which they move, we can break them into three distinct groups.

The first consists of the inner eight moons, including the famous Galilean moons Io, Europa, Ganymede and Callisto, whose orbits lie in the plane of Jupiter’s equator, at distances less than 2 million kilometres.

The second group lies significantly farther from the planet, and move on orbits tilted by between 25° and 56° relative to Jupiter’s equator. These are the prograde irregulars – ten moons orbiting at distances between 7 million and 19 million km. Two of the new discoveries are members of this group.

The final and most populous group is the retrograde irregulars – 60 moons located between 19 million and 29 million kilometres from Jupiter, all moving on orbits inclined by between about 140° and 170° to Jupiter’s equator.

In other words, they orbit backwards, in the opposite direction to everything else. Nine of the new discoveries fall into this category.

Plot showing the three groups of moons orbiting Jupiter.
Carnegie Institution for Science/Roberto Molar-Candamosa

So that covers 11 of our new moons. What of the 12th? Well, it turns out that the most exciting of the new moons is an oddball – an object that does not fit into any of the groups mentioned above.

The oddball: Valetudo

The 12th new moon has tentatively been named Valetudo, after Jupiter’s mythological great granddaughter.

Valetudo is the dimmest of the newly discovered moons. At just a kilometre in diameter (or less), it is the smallest Jovian moon found to date.

The yellow lines point to the tiny moving speck of light, the newly discovered moon Valetudo.
Carnegie Institute for Science

In terms of its orbital distance, Valetudo lies bang in the middle of the retrograde irregulars – some 24 million kilometres from the giant planet. But its orbit is prograde – meaning that it moves in the direction of Jupiter’s rotation, and in the opposite direction to all other satellites in its vicinity.

Valetudo’s size and unusual orbit pose interesting questions.

How did something so small survive in the celestial firing range around Jupiter?

Could Valetudo be the final surviving remnant of a previously uncharted family, whittled to nothing by aeons of headlong flight into the retrograde irregulars?

Are there are other members of the Valetudo family out there, awaiting discovery?




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Beyond these questions, Valetudo’s small size offers an important clue to the origin of the Jovian satellite system. Had Valetudo been on its current orbit while Jupiter was still accreting, it would have been too small to resist the drag of the inflowing gas. Like a ping pong ball in a gale, it would have been dragged inwards, to be devoured by the giant planet.

In other words, tiny Valetudo tells us that the process that created the irregular satellite families continued long after the formation of Jupiter was complete. In fact, that process likely continues even now, with occasional collisions tearing moons asunder, to birth new families of irregular worlds.

The ConversationWho knows? The next such collision might come when Valetudo runs into one of the retrograde irregulars. Given that their orbits cross, it may only be a matter of time.

Jonti Horner, Professor (Astrophysics), University of Southern Queensland and Christopher C.E. Tylor, PhD Candidate, Adjunct Lecturer, Assistant Examiner, University of Southern Queensland

This article was originally published on The Conversation. Read the original article.