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We know our world as an abode for life. Earth is teeming with fish, mammals, birds, reptiles, insects, and even some oddball "extremophiles" -- life forms that live in extreme environments like deep inside ice at the poles or around hot springs at the bottom of the ocean. Today, the diversity of life on Earth, and its hardiness at adapting to extreme environments, prompt many scientists to think life could thrive elsewhere in our solar system, or on planets and moons around other stars in our Milky Way galaxy.


But it was only in the sixteenth century, around the time of Copernicus, that people began to see Earth as a planet at all -- akin to the "wanderers" that were seen crossing the night sky against the stable background of stars. Since then, telescopic and probe-based studies of other bodies in the solar system have put our remarkable home into a broader context.

We know that Earth is the largest of the "terrestrial" planets in our solar system -- the four rocky planets close to the Sun.

Earth is tilted on its axis at 23.5 degrees, which creates the seasons. When one hemisphere is tilted toward the Sun, it experiences summer. The hemisphere tilted away experiences winter. Spring and fall occur when a hemisphere is between these two extremes.

Earth is the only planet where the normal temperature range allows water to exist on its surface as a liquid. In fact, oceans cover 70 percent of Earth's surface.

This is an extremely important factor for life as we know it.

Earth's atmosphere is another factor that makes life possible. Consisting of 78 percent nitrogen, 21 percent oxygen, and traces of other gases, it provides protection from most of the Sun's harmful radiation. It also protects us from most of the meteors that head for the planet; they burn up in the atmosphere before they can hit the ground.

The solid portions of Earth form three layers. The top layer is the crust, and is made of silicates -- rocks like quartz, feldspar, and others. The mantle lies beneath the crust, and contains most of Earth's mass. The mantle is made primarily of silicon, magnesium, and oxygen. The mantle wraps around a two-layered core: a partially molten outer layer of iron and nickel and an inner layer of solid iron. As Earth rotates, these layers spin at different rates, which generates our planet's magnetic field.

Earth's seven continents sit on large tectonic plates that float atop the mantle. Eons ago, the continents were joined together into a single land mass scientists have coined Pangaea. Starting about 200 million years ago, the plates began to drift apart, causing the continents to separate. They are still moving today, albeit extremely slowly. Earthquakes occur where tectonic plates run into each other and one plate is forced to slide underneath another.

Earth's Moon

Earth's partner in its yearly trek around the Sun, the Moon, is geologically dead. Dried lava fields called "maria" -- Latin for seas -- cover its surface, along with impact craters. The maria formed about four billion years ago, when giant asteroids punched holes in the Moon's crust, allowing molten rock to bubble to the surface, where it cooled and hardened.

Earth and the Moon are more like a double planet than a planet and a moon. The Moon is quite large in comparison to Earth -- about one-quarter of Earth's diameter. The two gravitation