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Question 3:
Why do you think NASA's Kepler spacecraft has been searching for an Earth-like planet that resides in the "habitable zone" of its parent star?
Earth's distance from the sun is "just right" for liquid water to exist on its surface. It is not too hot (as it would be if Earth orbited closer to the sun) and not too cold (as it would be if it orbited too far from the sun). This means that any extrasolar planet in the habitable zone of its parent star could have liquid water on its surface. Since all of life as we know it relies on liquid water, scientists think that a primary key to finding life on other worlds lies in detecting liquid water on them.

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