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Our home planet is the third planet from the Sun, and the only place we know of so far that's inhabited by living things.While Earth is only the fifth largest planet in the solar system, it is the only world in our solar system with liquid water on the surface. Just slightly larger than nearby Venus, Earth is the biggest of the four planets closest to the Sun, all of which are made of rock and metal. The name Earth is at least 1,000 years old. All of the planets, except for Earth, were named after Greek and Roman gods and goddesses. However, the name Earth is a Germanic word, which simply means "the ground."Earth, along with the other planets, is believed to have been born 4.5 billion years ago as a solidified cloud of dust and gases left over from the creation of the Sun. For perhaps 500 million years, the interior of Earth stayed solid and relatively cool, perhaps 2,000F. The main ingredients, according to the best available evidence, were iron and silicates, with small amounts of other elements, some of them radioactive. As millions of years passed, energy released by radioactive decay?mostly of uranium, thorium, and potassium?gradually heated Earth, melting some of its constituents. The iron melted before the silicates, and, being heavier, sank toward the center. This forced up the silicates that it found there. After many years, the iron reached the center, almost 4,000 mi deep, and began to accumulate. No eyes were around at that time to view the turmoil that must have taken place on the face of Earth?gigantic heaves and bubblings on the surface, exploding volcanoes, and flowing lava covering everything in sight. Finally, the iron in the center accumulated as the core. Around it, a thin but fairly stable crust of solid rock formed as Earth cooled. Depressions in the crust were natural basins in which water, rising from the interior of the planet through volcanoes and fissures, collected to form the oceans. Slowly, Earth acquired its present appearance.

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