Group C The Rocky Inner Planets

The Birth of the Solar System:

Our planet, the Earth, is part of a much larger community. This family of planets, moons, asteroids, comets, and one star probably began the same way many other planetary systems are forming, which we can observe through telescopes.

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. This spinning was caused by the basic laws of motion. 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 like 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 that 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 star's 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.

Do you see the pattern?

The inner planets are small and rocky while the outer planets are large and made of gas. Why do you think that might have happened? The inner planets are much closer to the Sun. As the Solar System formed, the dust and gas in the inner Solar System was pushed out much sooner than the gas and dust in the outer Solar System. The outer planets formed in an area which was richer in dust and gas. There was a lot more stuff for them to collect.

Note: Pluto was officially re-classified as a dwarf planet by the International Astronomical Union in 2006.

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