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  • The universe contains about 100 billion galaxies.

  • Each of those galaxies contains about 100 billion stars.

  • Many of those stars have planets orbiting them.

  • So how do we look for life in all that immensity?

  • It's like searching for a needle in trillions of haystacks.

  • We might want to focus our search on planets that we know can support life as we know it --

  • what we call habitable worlds.

  • What do such planets look like?

  • To answer that question, we don't look out there.

  • Instead, we look at ourselves. At Earth.

  • Because this is the one planet in the universe that we know for certain is habitable.

  • When we look at Earth from space, we see a blue, watery world.

  • It's no coincidence that three quarters of the surface is covered by oceans.

  • Because of its unique chemical and physical properties,

  • water is absolutely essential for all life as we know it.

  • And so we get especially excited about other worlds on which water is abundant.

  • Fortunately, water is very common in the universe.

  • But life needs water in the form of liquid, not ice, and not vapor,

  • and that's a little bit less common.

  • For a planet to have liquid water at its surface, three things are important.

  • First, the planet needs to be large enough that the force of gravity

  • keeps the water molecules from flying off into space.

  • For example, Mars is smaller than Earth, and so has less gravity,

  • and that's one important reason that Mars has a very thin atmosphere,

  • and no oceans at its surface.

  • Second, the planet needs to have an atmosphere. Why?

  • Because without an atmosphere, the planet is in a vacuum,

  • and liquid water isn't stable in a vacuum.

  • For example, our moon has no atmosphere, and so if you spill some water on the moon,

  • it will either boil away as vapor, or freeze solid to make ice.

  • Without the pressure of an atmosphere, liquid water can't survive.

  • Third, the planet needs to be at the right distance from its star.

  • Too close, and the surface temperature will exceed the boiling point of water,

  • and oceans will turn to vapor.

  • Too far, and the surface temperature will fall below the freezing point of water,

  • causing the oceans to turn to ice.

  • Fire or ice. For life as we know it, neither will suffice.

  • You can imagine that the perfect zone where water stays liquid looks kind of like a belt around a star.

  • We call that belt the habitable zone.

  • So when we search for habitable worlds, we definitely want to look for planets in the habitable zones around their stars.

  • Those regions are the best bets to find planets like Earth.

  • But while habitable zones are a pretty good place to begin the search for planets with life,

  • there are a couple of complications.

  • First, a planet isn't necessarily habitable just because it's in the habitable zone.

  • Consider the planet Venus in our solar system.

  • If you were an alien astronomer, you'd think Venus is a pretty good bet for life.

  • It's the right size, it has an atmosphere, and it's in the habitable zone of our sun.

  • An alien astronomer might see it as Earth's twin.

  • But Venus is not habitable, at least not at its surface.

  • Not by life as we know it. It's too hot.

  • That's because Venus' atmosphere is full of carbon dioxide, an important greenhouse gas.

  • In fact, its atmosphere is almost entirely carbon dioxide,

  • and is almost 100 times thicker than our own.

  • As a result, the temperature on Venus is hot enough to melt lead,

  • and the planet is dry as a bone.

  • So finding planets of the right size and distance from their stars is only a beginning.

  • We also want to know about the makeup of their atmospheres.

  • The second complication emerges when we look a little more deeply at planet Earth.

  • In the last 30 years, we've discovered microbes living in all sorts of extreme environments.

  • We find them in fissures of rock miles beneath our feet,

  • in boiling waters of the ocean floor,

  • in acidic waters of thermal springs,

  • and in cloud droplets miles above our heads.

  • These so-called extremophiles aren't rare.

  • Some scientists estimate that the mass of microbes living deep underground

  • equals the mass of all the life at Earth's surface.

  • These subterranean microbes don't need oceans or sunshine.

  • These discoveries suggest that Earth-like planets may be only the tip of the astrobiological iceberg.

  • It's possible that life might persist in aquifers beneath the surface of Mars.

  • Microbes may thrive on Jupiter's moon Europa,

  • where liquid water ocean probably lies beneath the icy crust.

  • Another ocean beneath the surface of Saturn's moon Enceladus is the source of geysers erupting into space.

  • Could these geysers be raining microbes?

  • Could we fly through them to find out?

  • And what about life as we don't know it, using a liquid other than water?

  • Maybe we are the crazy creatures living in an unusual and extreme environment.

  • Maybe the real habitable zone is so large

  • that there are billions of needles in those trillions of haystacks.

  • Maybe in the big scheme of things, Earth is only one of many different kinds of habitable worlds.

  • The only way to find out is to go out and explore.

The universe contains about 100 billion galaxies.

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