why do the outer planets have so many moons

My name's Eric Loberg, with the Taylor Planetarium at the Museum of the Rockies. And I was going to explain why some planets have more moons than others do. Some of it is just a feature of where the planets are located at and how they form. Most planets form when the little chunks of rock in the solar system started to come together and form larger and larger planets. If these planets were close to asteroid belts, you may get more moons because those asteroids will be captured by the planets. So, planets like Mars who has two moons, or Pluto who has five, often those are near asteroid belts and they have few more moons than some of the smaller ones. Small planets like Mercury or even Venus and Earth were closer to the Sun. The Sun absorbed most of that material that was leftover. And so, we don't have very many moons. The Earth's moon was probably a Mars size object that came from outside our solar system. Some of these objects came in, in an early bombardment period and smashed into the Earth. And that's what formed our moon. Why some planets have many moons is basically because of their size. The planets in our solar system that have more moons are the large planets, Jupiter, Saturn, Uranus and Neptune. They're so big that they can collect a lot of moon.


If comets swing by them, they can sometimes absorb comets, they can grab asteroids. And there's a lot more material as it starts to condense into that big planet. There is more left behind that swirled around the planet and it left more moons. We can look at the number of moons in the solar system. Mercury has zero moons and is close to the Sun, Venus has zero moons, also close to the Sun. Earth has our one, which was probably a large impact that came from farther out. And Mars has two moons. All of these planets were very close to that great, big Sun and the Sun absorbed most of those objects. Mars probably has two because there's a big asteroid belt between Mars and Jupiter. Jupiter has 50, the largest planet, Saturn, second largest planet, has 53. Uranus also very large, 27, Neptune, farthest away has 14. And then, there's another kind of rocky asteroid belt called the Kuiper belt, Pluto out there has five. Just because it was near those little rocky leftovers, so it could grab some of them. so, why do some planets have more moons than others? Mostly because of their size, the larger planets have more moons. I'm Eric Loberg, with the Museum of the Rockies, Taylor Planetarium.
Within the solar nebula, the slow process of accretion began.


It was first led by electrostatic forces, which caused tiny bits of matter to cling together. Eventually they grew into bodies of sufficient masses to gravitationally attract one another. This is when things were really set into motion. When electrostatic forces ran the show, the particles were traveling in the same direction and at close to the same speed. Their orbits were pretty stable, even as they were being gently drawn toward one another. As they built up and gravity became an increasingly stronger participant, everything grew more chaotic. Things started slamming into each other, which altered the bodies' orbits and made them more likely to experience further collisions. These bodies collided with one another to build up larger and larger pieces of material, similar to using a piece of Play Doh to pick up other pieces (creating a larger and larger mass all the while--though sometimes the collisions resulted in fragmentation, instead of accretion). The material continued to accrete to form planetesimals, or pre-planetary bodies. They eventually gained enough mass to clear out their orbits of most of the remaining debris. The matter closer to the proto-SunБwhere it was warmerБwas composed primarily of metal and rock (particularly silicates), whereas the material farther away consisted of some rock and metal but predominantly ice.


The metal and rock could form both near the Sun and far from it, but ice obviously couldn't exist too close to the Sun because it would vaporize. So the metal and rock that existed close to the forming Sun accreted to form the inner planets. The ice and other materials found farther away accreted to form the outer planets. This does explain part of the compositional differences between the inner and outer planets, but some dissimilarities still remain unexplained. Why are the outer planets so large and gaseous? To understand this, we must first understand what is often called the Бfrost lineБ of our solar system. This is the imaginary line which divides the solar system between where it is warm enough to harbor liquid volatiles (such as water) and cold enough for them to freeze; it is the point away from the Sun beyond which volatiles cannot remain in their liquid state, and could be thought of as the dividing line between the inner and outer planets (Ingersoll 2015). The planets beyond the frost line were perfectly capable of harboring rock and metal, but they also could sustain ice.

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