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The universe contains about 100 billion galaxies.
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Each of those galaxies contains about 100 billion stars.
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Many of those stars have planets orbiting them.
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So how do we look for life in all that immensity?
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It's like searching for a needle in trillions of haystacks.
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We might want to focus our search on planets that we know can support life as we know it --
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what we call habitable worlds.
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What do such planets look like?
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To answer that question, we don't look out there.
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Instead, we look at ourselves. At Earth.
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Because this is the one planet in the universe that we know for certain is habitable.
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When we look at Earth from space, we see a blue, watery world.
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It's no coincidence that three quarters of the surface is covered by oceans.
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Because of its unique chemical and physical properties,
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water is absolutely essential for all life as we know it.
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And so we get especially excited about other worlds on which water is abundant.
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Fortunately, water is very common in the universe.
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But life needs water in the form of liquid, not ice, and not vapor,
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and that's a little bit less common.
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For a planet to have liquid water at its surface, three things are important.
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First, the planet needs to be large enough that the force of gravity
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keeps the water molecules from flying off into space.
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For example, Mars is smaller than Earth, and so has less gravity,
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and that's one important reason that Mars has a very thin atmosphere,
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and no oceans at its surface.
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Second, the planet needs to have an atmosphere. Why?
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Because without an atmosphere, the planet is in a vacuum,
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and liquid water isn't stable in a vacuum.
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For example, our moon has no atmosphere, and so if you spill some water on the moon,
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it will either boil away as vapor, or freeze solid to make ice.
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Without the pressure of an atmosphere, liquid water can't survive.
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Third, the planet needs to be at the right distance from its star.
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Too close, and the surface temperature will exceed the boiling point of water,
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and oceans will turn to vapor.
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Too far, and the surface temperature will fall below the freezing point of water,
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causing the oceans to turn to ice.
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Fire or ice. For life as we know it, neither will suffice.
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You can imagine that the perfect zone where water stays liquid looks kind of like a belt around a star.
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We call that belt the habitable zone.
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So when we search for habitable worlds, we definitely want to look for planets in the habitable zones around their stars.
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Those regions are the best bets to find planets like Earth.
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But while habitable zones are a pretty good place to begin the search for planets with life,
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there are a couple of complications.
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First, a planet isn't necessarily habitable just because it's in the habitable zone.
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Consider the planet Venus in our solar system.
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If you were an alien astronomer, you'd think Venus is a pretty good bet for life.
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It's the right size, it has an atmosphere, and it's in the habitable zone of our sun.
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An alien astronomer might see it as Earth's twin.
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But Venus is not habitable, at least not at its surface.
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Not by life as we know it. It's too hot.
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