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Why Does Jupiter Have 79 Moons, But Earth Only Has One?

VO: Noah Baum WRITTEN BY: Nathan Sharp
The moon is always there. Lighting up the night sky, controlling our tides and inspiring philosophers, thinkers and writers. But why does Earth have just one moon? Some of the other planets have lots of them. Jupiter has 79! And some have none at all. It's all to do with the history of our Solar System, and it all started around 4.5 billion years ago...

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Why Does Jupiter Have 79 Moons, But Earth Only Has One?

Our poor, lonely Earth, spinning with just one celestial neighbor while other planets get dozens. Well, actually, Mercury, Venus, Earth, and Mars have all but three moons between them, but the outer giants… They have at least 172, and possibly many more. So, why does Earth only get one when Jupiter and the outer planets have dozens each? And for that matter, why are there so few moons between the first four planets and so many between those distant worlds?

It rests on the difference between terrestrial and Jovian planets. The terrestrial planets are Mercury, Venus, Earth, and Mars. They’re small, compact, and rocky planets that are close to the sun and much warmer than their outer counterparts. The Jovian planets consist of those beyond the asteroid belt – Jupiter, Saturn, Uranus, and Neptune. These are comparatively massive planets comprised of gas, and they’re extremely cold. They each have many, many moons.

It all dates back to the formation of our solar system, roughly four and a half billion years ago. Everything started out as a massive collection of gas and space dust. Upon congregation though, that massive cloud of gas and dust began to collapse due to the force of gravity, and while it was collapsing, it began to heat up and rotate, forming what’s known as the protoplanetary disk. The terrestrial planets were subsequently created when small chunks of metal and rock collided together but remained intact thanks to gravity. However, many of these chunks - AKA planetesimals – were destroyed upon impact, which is why the terrestrial planets are so relatively tiny. Only the most massive (though still reasonably small) of planetesimals could survive the collisions and form the planets we know today.

However, the further out you go in the protoplanetary disk, the colder it becomes. And it’s out here that small pieces of ice floated around along with the metal and rock. When the metal, rock, and ice collided, they also created planetesimals, but because collisions weren’t as frequent, they were able to grow much larger than the terrestrial planets. These massive collections of rock, metal, and ice then enacted their own gravitational pull, which attracted massive clouds of gas floating around in the disk, forming the gas and ice giants. While all of this was happening, the metal, rock, and ice continued to collide together to form other, smaller planetesimals, which were sucked into the orbit of these giant planets. In a cosmic nutshell, that’s why they have so many moons and we don’t. We were simply too small (and our environment too chaotic) to collect them.

There are many further theories on the creation and collection of moons, though. The Capture Theory says that a large planet such as Jupiter is so massive that it collects passing satellites and hooks them into orbit due to the intense force of its gravity. There needn’t be massive crashes and high-speed deflections, just something formed elsewhere that becomes attracted to any given planet. This in part explains why Mars has two moons. Mars is close to the asteroid belt – closer than any other terrestrial planet – so its gravitational pull could snag two passing satellites, now known as the moons Phobos and Deimos.

But, there’s something else holding us back, too. We – as in, the Earth – also have to contend with the sun. The sun is so massive that it can easily attract and suck in passing satellites, which is probably why the planets closest to it – Mercury and Venus – have no moons… They can’t exert any sort of gravitational pull to rival that of the solar centrepiece. However, the sun’s gravitational force weakens the further out you go, which explains why Jupiter and the Jovian planets were able to collect so many moons. Not only is the sun’s force weaker ‘out there’, but the giants’ gravity is much stronger than ours, and this combination allows them to snag many passing satellites, collecting them into their orbits.

In terms of the cold, hard numbers, our celestial neighbors are laughing at us. Jupiter has 79 known moons, largely due to its incredible size. Its biggest moon, Ganymede, is the ninth largest object in the solar system. In fact, it’s diameter is larger than Mercury’s, so it could have been its own planet had it not been sucked into the orbit of Jupiter. The other gas giant, Saturn, has as many as 62 moons, and its largest, Titan, is also bigger than Mercury. Titan is also the only moon to have its own atmosphere and evidence of surface liquid. Again, it may have been its own planet, similar to Earth, had it not been sucked into Saturn’s orbit. The ice giants don’t have quite as many moons, but it’s still an incredible collection when compared to ours. Uranus has 27 known moons, while Neptune has 13 confirmed.

So, why do we have a moon at all? As with a lot of other things – namely the capability to harbour life – Earth is in a bit of a Goldilocks zone. We’re not so far away that we can collect satellites from the asteroid belt, but we’re also far enough away from the sun, and we’re a large enough planet to exert our own significant gravitational pull.

The giant-impact hypothesis proposes a particularly popular theory on exactly how our moon came to be. Way back when the Earth was first forming, back when the protoplanetary disk was chaotically causing chunks of rock and metal to collide into each other, a nearby object slammed into the early stages of what is now our planet. This object was another planetesimal about the size of Mars – it’s now remembered as Theia. When this Mars-sized protoplanet collided with the early-Earth, it caused a fantastic explosion 100 million times larger than that which was triggered by the asteroid that wiped out the dinosaurs – according to NASA. And, this incredible event sent countless particles off of early-Earth into space. Over time – as in, over millions of years – these particles were bound together by gravity to eventually form the moon. Among many other things, it explains why the moon is light in comparison to Earth – it was formed from the remnants of crust rather than the core. It also explains why the compositions of Earth and the moon are so similar – because they were part of the same collision.

So, Earth’s singular moon is mostly down to our planet’s interesting and unique Solar System location. The outer planets – especially Jupiter – were able to grow to an incredible size due to the decreased density of dust and rock. Ultimately, they became so massive that they could exert their own gravitational force on passing objects. Meanwhile, the terrestrial planets are either far too small (the result of constant collisions), or far too close to the sun to generate a significant gravitational pull. The Earth was able to collect our only moon essentially because one massive collision threw large amounts of loose material into space, near our orbit.

Ultimately, the moon is made the way it is, and located where it is, thanks to a single (but momentous) chance event. If Theia had missed us, we’d in all likelihood have no moon to speak of. But, instead, Theia hit us, and we were able to attract the remnants of the crash. Observing the moon often feels a fairly peaceful pursuit, but its creation was anything but.

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