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If you recall the acronym "My very excellent mother just served us nine pizzas," you can remember the nine planets of our solar system in their order from the Sun. But there's another way to categorize the planets: by composition. Mercury, Venus, Earth, and Mars are known as the terrestrial planets because they are composed primarily of rock and metal. Jupiter, Saturn, Uranus, and Neptune are known as gas giants due to their size and the fact that they are composed primarily of hydrogen and helium. Pluto, the planet farthest from the Sun, is a frozen planet of ice and belongs to a category all its own.
One of the main differences between the first two groups of planets is their number of moons. While the smaller terrestrial planets only have three total moons, one for Earth and two for Mars, the gas giants have more than 100 combined. But if these planets have so many moons, why does Earth have only one?
According to astronomers, the answer lies in the formation of the solar system. The solar system formed when a giant, swirling cloud of gas collapsed under its own gravity. As the gases heated up and rotated faster, they formed a disk of rotating hot gas with a mass concentrated at the center. The mass became the Sun, and the planets formed out of the remaining disk.
In order to form planets, the gas in the disk pulled particles together with its gravity. The planets in the solar system vary by the kind of particles that were available in the different parts of the disk.
Near the Sun, it was too hot for planets to form. A little farther away, there were metal flakes and small pieces of rock that stuck together when they collided. These formations grew so big that they eventually collided, breaking apart. Only the largest formations survived to become Mercury, Venus, Earth, and Mars—the terrestrial planets.
Farther away from the Sun, it was colder, so small pieces of ice added to the metal flakes and chunks of rock. As formations grew quickly, their gravity was able to grab the surrounding hydrogen and helium in the disk. These masses captured so much gas, that they formed four mini solar systems. In each case, the gases rotated, heated up, collapsed, and formed a mass concentration at the center. The masses of these mini solar systems became the planets Jupiter, Saturn, Uranus, and Neptune, and the formation of many icy satellites became their moons. This gives new meaning to the saying "many moons ago"!
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