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About 5 billion years ago our Solar System did not exist at all. Instead there was in its place a large cloud of gas and dust called a nebula. Over many millions of years the immense gravity of this large cloud caused the dust and gas to slowly fall inward towards its center.

As matter in the cloud fell towards the center it began to spin. The basic laws of motion cause this spinning. Objects in space do not speed up, or slow down unless their speed is changed by something else. Also all objects move in the same direction until their path is changed by something else. As the dust and gas fell into the center of the cloud each particle resisted slowing down, or changing directions. However the gravity of the growing matter in the center tried to pull the particles directly to the center. The strength of the gravity was not enough to pull the particles directly in, but it was strong enough to bend their paths around into a circle. As the cloud began to swirl it also flattened out, much like spinning a lump of dough on your hand causes it to flatten out into a Pizza crust.

Now we have a flat spinning cloud of dust and gas.

The center continued to collect more and more matter growing larger and larger. At the same time smaller clumps of matter began to form throughout the disk. These smaller clumps would eventually become planets, moons, asteroids, and comets.

As matter collects into clumps it heats up. The more matter, which collects the hotter, an object becomes. The Earth is still very hot in its core; this heat is left over from when the Earth originally formed. Eventually the Sun became so hot in its core that it ignited, turning hydrogen into helium. Once the Sun ignited the formation of the Solar System quickly ended. The new stars intense radiation and solar winds blew away the remaining dust and gas in the cloud so that the Sun and its planets could not grow any larger.

The planets closest to the Sun (Mercury, Venus, Earth, and Mars) are called the terrestrial planets. They are small dense rocky planets. The outer planets (Jupiter, Saturn, Uranus, and Neptune) are called the Jovian Planets. They are large and made up of gas. Pluto is closer to a comet than a planet.

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7y ago

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