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why do the outer planets have more 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.
Many planets in our solar system have more than one moon. Mars has two moons, Jupiter has 67, Saturn 62, Uranus 27, Neptune 14. Those numbers keep changing, and you can see from NASAвs Jet Propulsion Laboratory. It makes sense that the outer worlds, with their stronger gravity, would have more moons. Meanwhile, our planet Earth has just one moon. Doesnвt it? Moons are defined as Earthвs natural satellites. They orbit around the Earth.

And, in fact, although Earth sometimes has more than one moon, some objects you might have heard called Earthвs second moon arenвt, really. Letвs talk about some non-moons first. Quasi-satellites are not second moons for Earth. A quasi-satellite is an object in a co-orbital configuration with Earth (or another planet). Scientists would say there is a 1:1 orbital resonance between Earth and this object. In other words, a quasi-satellite is orbiting the sun, just as Earth is. Its orbit around the sun takes exactly the same time as Earthвs orbit, but the shape of the orbit is slightly different. The most famous quasi-satellite in our time в and an object you might have heard called a second moon for Earth в is 3753 Cruithne. This object is five kilometers в about three miles в wide. Notice it has an asteroid name. Thatвs because it is an asteroid orbiting our sun, one of several thousand asteroids whose orbits cross Earthвs orbit. Astronomers discovered Cruithne in 1986, but it wasnвt until 1997 that they figured out its complex orbit. Itвs not a second moon for Earth; it doesnвt orbit Earth. But Cruithne is co-orbiting the sun with Earth. Like all quasi-satellites, Cruithne orbits the sun once for every orbit of Earth. Earthвs gravity affects Cruithne, in such a way that Earth and this asteroid return every year to nearly the same place in orbit relative to each other. However, Cruithne wonвt collide with Earth, because its orbit is very inclined with respect to ours.

It moves in and out of the plane of, or plane of Earthвs orbit around the sun. Orbits like that of Cruithne arenвt stable. Computer models indicate that Cruithne will spend only another 5,000 years or so in its current orbit. Thatвs a blink on the long timescale of our solar system. The asteroid might then move into true orbit around Earth for a time, at which time it would be a second moon в but not for long. Astronomers estimate that, after 3,000 years orbiting Earth, Cruithne would escape back into orbit around the sun. By the way, Cruithne isnвt the only quasi-satellite in a 1:1 resonance orbit with Earth. The objects and, among others, are also considered quasi-satellites to Earth. These objects are not second moons for Earth, although sometimes you might hear people mistakenly say they are. Does Earth ever have more than one moon? Surprisingly (or not), the answer is yes. Earth does sometimes have temporary moons. In March of 2012, astronomers at Cornell University published the result of a computer study, suggesting that asteroids orbiting the sun might temporarily become natural satellites of Earth. In fact, they said, Earth usually has more than one temporary moon, which they called. These astronomers said the minimoons would follow complicated paths around Earth for a time, as depicted in the images above and below. Eventually, they would break free of Earthвs gravity в only to be immediately recaptured into orbit around the sun, becoming an asteroid once more.

The little moons envisioned by these astronomers might typically be only a few feet across and might orbit our planet for less than a year before going back to orbit the sun as asteroids. Have astronomers detected any of these minimoons? Yes. Writing in the magazine Astronomy in December 2010, (Manager of NASAвs Near-Earth Object Program Office at NASAвs Jet Propulsion Laboratory) described an object discovered in 2006 that appears to fit that description. The object в now designated 2006 RH120 в is estimated to be 5 meters (about 15 feet) in diameter. Yeomans said that, when this object was discovered in a near-polar orbit around Earth, it was thought at first to be a third stage Saturn S-IVB booster from Apollo 12, but later determined to be an asteroid. began orbiting the sun again 13 months after its discovery, but itвs expected to sweep near Earth and be re-captured as a minimoon by Earthвs gravity later in this century. Bottom line: That asteroid called 3753 Cruithne is not a second moon for Earth, but its orbit around the sun is so strange that you still sometimes hear people say it is. Meanwhile, astronomers have suggested that Earth does frequently capture asteroids, which might orbit our world for about a year before breaking free of Earthвs gravity and orbiting the sun once more.

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