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I'm really glad to be here.
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I'm glad you're here,
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because that would be a little weird.
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I'm glad we're all here.
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And by here, I don't mean here.
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We're here.
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But here.
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I mean Earth.
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And by we, I don't mean
those of us in this auditorium,
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but life,
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all life on Earth --
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(Laughter) --
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from complex to single-celled,
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from mold to mushrooms
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to flying bears.
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(Laughter)
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And the interesting thing is that Earth
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is the only place we know of
that has life,
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8.7 million species.
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We've looked other places, maybe not
as hard as we should or we could,
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but we've looked and we haven't found any.
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Earth is the only place
we know of with life.
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Is Earth special?
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This is a question that I've wanted
to know the answer to
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since I was a small child, and I suspect
80 percent of this auditorium
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has thought the same thing
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and also wanted to know the answer,
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and to understand whether there are
any planets out there
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in our solar system or beyond
that can support life,
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the first step is to understand
what life here requires.
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And it turns out, of all of those
8.7 million species,
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life only needs three things.
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On one side, all life
on Earth needs energy.
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Complex life like us derives
our energy from the Sun,
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but life deep underground can get
its energy from things
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like chemical reactions.
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There are a number of different energy
sources available on all planets.
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On the other side, all life needs food
or nourishment.
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And this seems like a tall order,
especially if you want a succulent tomato.
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(Laughter)
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However, all life on Earth derives
its nourishment from only six
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chemical elements, and these elements can
be found on any planetary body
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in our Solar System.
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So that leaves the thing in the middle
as the tall pole,
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the thing that's hardest to achieve,
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not moose, but water.
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(Laughter)
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Although moose would be pretty cool.
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And not frozen water, and not water
in a gaseous state, but liquid water.
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This is what life needs
to survive, all life.
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And many Solar System bodies
don't have liquid water,
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and so we don't look there.
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And other Solar System bodies
might have abundant liquid water,
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even more than Earth,
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but it's trapped beneath an icy shell,
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and so it's hard to access,
it's hard to get to,
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it's hard to even find out
if there's any life there.
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So that leaves a few bodies
that we should think about.
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And so let's make the problem
simpler for ourselves.
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Let's think only about liquid water
on the surface of a planet.
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There are only three bodies
to think about in our Solar System
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with regard to liquid water
on the surface of a planet,
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and in order of distance from the Sun,
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it's Venus, Earth, and Mars.
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And you want to have an atmosphere
for water to be liquid,
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and you have to be very careful
with that atmosphere.
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You can't have too much atmosphere,
too thick or too warm an atmosphere,
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because then you end up too hot
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like Venus, and you can't
have liquid water.
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But if you have too little atmosphere
and it's too thin and too cold,
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you end up like Mars, too cold.
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So Venus is too hot, Mars is too cold,
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and Earth is just right.
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You can look at these images behind me
and you can see automatically
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where life can survive
in our Solar System.
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It's a Goldilocks-type problem,
and it's so simple that a child
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could understand it.
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However,
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I'd like to remind you of two things
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from the Goldilocks story that we may not
think about so often,
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but that I think are really relevant here.
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Number one: if Mama Bear's bowl
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is too cold
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when Goldilocks walks into the room