Why do you think saturn and jupiter have more moons than the other planets in our solar system?

Saturn and Jupiter have many moons for quite a few reasons, one of the main ones being that they have an absolutely immense gravitational pull. During the early stages of the formation of our solar system, there would of been many planet-like objects floating around which our gas giants would have attracted. Furthermore, these planets are so far out in the solar system water would if frozen (which explains Saturn's rings of ice). Infact, we can show that the ice can form moons by looking at some of the moons of Uranus, some of them are half made of ice!

A few of the outer moons of our planets are captured asteroids. Phoebe, which is a moon of Saturn, is believed to have been a captured asteroid.

I haven't heard anything about Saturn having more moons than jupiter.

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Why do the giant planets and their moons have compositions different from those of the terrestrial planets?

Astronomers divide the planets within our solar system into two categories. Mercury, Venus, Earth, and Mars are the so-called "terrestrial" or "inner" planets while Jupiter, Saturn, Uranus, and Neptune have been classified as "gas giants," also known as "outer planets."

The size gap between those factions is quite considerable; Although Uranus is the smallest outer planet, it's still 15 times more massive than Earth, the largest of the inner planets. None of the other planets can compete with Jupiter in terms of sheer bulk, however. You'd need more than 300 duplicates of our puny home world to equal Jupiter's colossal mass. It's an absolute monster.

Now, as Isaac Newton observed, there's a positive correlation between the mass of an object and the strength of its gravitational field. Because the gas giants are so massive, they're able to attract more satellites.

But that's not the only reason why planets like Jupiter have such large moon collections. Our solar system's gas giants are relatively far away from the sun. In contrast, some stars have massive, Jupiter-like planets called "hot Jupiters." Basically, these are gas giants which orbit in close proximity to their stars. (Imagine if Saturn switched places with Mercury.)

A 2010 paper by French astronomer Fathi Namouni argues that hot Jupiters have few, if any, moons. These planets are thought to originate in distant parts of their solar systems and then migrate inwards. Along the way, their moons get caught in a game of celestial tug of war. Gas giants may be big, but stars are much bigger. As such, they've got far stronger gravitational fields. So, when a hot Jupiter gets too close to its star, the star will eventually steal its moons.

Distance offsets this ability. The further you travel from the sun, the weaker its gravitational pull on you becomes. Therefore, if Namouni is correct, the real Jupiter has 79 moons and counting because it's an insanely massive planet that's far enough away from the sun to avoid lunar theft.

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.

Read more: Curious Kids: why does Saturn have rings?

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.

Read more: A look back at Cassini's incredible mission to Saturn before its final plunge into the planet

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.

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.

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.