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- [Narrator] We inhabit a
little blue planet called Earth.

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Earth is habitable because
it had the right conditions

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and ingredients to
enable life as we know it

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to evolve and flourish.

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And one of the most important factors

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in making our blue planet habitable

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is the wet stuff itself, water.

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It's kind of extremely important for life.

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When it comes to the search
for habitable exoplanets,

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planets outside our solar system,

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we think the ability to have liquid water

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on a planet's surface is just
as important for those worlds.

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There's a handy concept astronomers use

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to help them figure out where
to look around any given star

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for exoplanets that just might
have the right stuff light.

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It's called the habitable zone.

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It's the distance from a
star where a rocky planet

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with an atmosphere could have
liquid water on its surface.

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In our solar system, the
habitable zone extends

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from around the orbit of Venus
to around the orbit of Mars.

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Earth happens to be nice
and comfy in the middle.

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But it takes more than
just a good location.

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Orbiting in the habitable zone
does not guarantee a planet

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will be habitable.

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After all, the Moon is right here with us,

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but it's certainly not habitable.

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Sorry, Moon.

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The location of the habitable zone depends

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on how big and bright a star is.

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The nearest star to our
sun, Proxima Centauri,

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has at least one planet
in the habitable zone.

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But because Proxima is much
smaller and dimmer than our sun,

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its habitable zone is way
smaller and closer to the star.

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So every star has a habitable zone,

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but that doesn't mean there
will be planets there.

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For example, the star known
as Kepler-90 is similar

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to our sun and it too has eight planets,

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but they're all huddled
super close to the star,

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well inside of the habitable zone.

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But the longer we look,

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the more likely it becomes that one day,

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we'll find another little
blue water-covered planet

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right in its own comfy habitable zone.

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And that would be a planet
worth a closer look, indeed.

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(playful music)