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Tour the solar system: Earth

In this installment of the "Tour the solar system" series, Assistant Editor Bill Andrews explores what makes Earth unique and how space travel has led to a better understanding of our home planet.
earth
True-color image of Earth.
Image courtesy of NASA/GSFC

Despite the beautiful sights found elsewhere in the solar system, Earth remains the favorite planet for many. If nothing else, it's home to all our favorite people, places, and burger joints; unless you're an astronaut, everything you've ever heard with your own ears and touched with your own fingers has been on this relatively tiny planet. Earth's diameter spans 7,926 miles (12,760 kilometers) at the equator; it takes 24 hours to rotate once, and about 365 days to orbit the Sun once. These last two, of course, are better known as a day and a year.

At an average distance from the Sun of about 93 million miles (150 million km), Earth inhabits a special place called the habitable zone. This is the area around a star where a planet can maintain liquid water and thus life. Neither too hot like the inner planets nor too cold like the distant giant planets, temperate Earth is the only place we know of that harbors life.

But Earth is unique for many other reasons. For one thing, it's the only planet in our solar system with a single moon. Earth is also one of only four bodies in the solar system known to have a substantial atmosphere — the other three are Venus, Mars, and Saturn's moon Titan. Earth's atmosphere is made up of many layers, with the troposphere at the surface, and the uninhabitable thermosphere enclosing the International Space Station's orbit far above it.

The planet itself is layered as well. Deep in Earth's interior lies a solid inner core of iron and nickel and a liquid outer core made of the same, plus small amounts of other, lighter elements. Surrounding the core is Earth's mantle, where a super-hot, gooey mix of molten iron and rock slowly circulates beneath the crust, or surface. Volcanoes, earthquakes, and even the shape of the continents all result from various interactions between the crust and the mantle.

Using various dating methods, scientists have determined Earth is about 4.5 billion years old, just barely younger than the solar system itself. Life, in its simplest forms, showed up just a billion or so years later. Over time, as Earth's environment changed, life-forms grew more complex.  Homo sapiens appeared about 200,000 years ago in Africa, whose influence on the planet and other life-forms is unprecedented. Just think: Every human action in history has taken place in less than half a percent of Earth's existence, and look at how significantly we've impacted the planet.

In the past 50 years, mankind has begun exploring the universe beyond our home planet. While this has granted us amazing access to study the Moon, other planets, and beyond in amazing detail, it's also put Earth in a new perspective, allowing us to look back on it from the outside. NASA may be most famous for sending astronauts and robots to alien landscapes, but such exploration has proven just as important for improving our understanding of Earth.

Our understanding of Earth has continuously evolved over hundreds of thousands of years, so odds are we're not done yet. As we keep learning about space and the cosmos, we'll inevitably learn more about our home world, and ourselves.

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