I'm really glad to be here.

I'm glad you're here,

because that would be a little weird.

I'm glad we're all here.

And by here, I don't mean here.

We're here.

But here.

I mean Earth.

And by we, I don't mean
those of us in this auditorium,

but life,

all life on Earth --

(Laughter) --

from complex to single-celled,

from mold to mushrooms

to flying bears.

(Laughter)

And the interesting thing is that Earth

is the only place we know of
that has life,

8.7 million species.

We've looked other places, maybe not
as hard as we should or we could,

but we've looked and we haven't found any.

Earth is the only place
we know of with life.

Is Earth special?

This is a question that I've wanted
to know the answer to

since I was a small child, and I suspect
80 percent of this auditorium

has thought the same thing

and also wanted to know the answer,

and to understand whether there are
any planets out there

in our solar system or beyond
that can support life,

the first step is to understand
what life here requires.

And it turns out, of all of those
8.7 million species,

life only needs three things.

On one side, all life
on Earth needs energy.

Complex life like us derives
our energy from the Sun,

but life deep underground can get
its energy from things

like chemical reactions.

There are a number of different energy
sources available on all planets.

On the other side, all life needs food
or nourishment.

And this seems like a tall order,
especially if you want a succulent tomato.

(Laughter)

However, all life on Earth derives
its nourishment from only six

chemical elements, and these elements can
be found on any planetary body

in our Solar System.

So that leaves the thing in the middle
as the tall pole,

the thing that's hardest to achieve,

not moose, but water.

(Laughter)

Although moose would be pretty cool.

And not frozen water, and not water
in a gaseous state, but liquid water.

This is what life needs
to survive, all life.

And many Solar System bodies
don't have liquid water,

and so we don't look there.

And other Solar System bodies
might have abundant liquid water,

even more than Earth,

but it's trapped beneath an icy shell,

and so it's hard to access,
it's hard to get to,

it's hard to even find out
if there's any life there.

So that leaves a few bodies
that we should think about.

And so let's make the problem
simpler for ourselves.

Let's think only about liquid water
on the surface of a planet.

There are only three bodies
to think about in our Solar System

with regard to liquid water
on the surface of a planet,

and in order of distance from the Sun,

it's Venus, Earth, and Mars.

And you want to have an atmosphere
for water to be liquid,

and you have to be very careful
with that atmosphere.

You can't have too much atmosphere,
too thick or too warm an atmosphere,

because then you end up too hot

like Venus, and you can't
have liquid water.

But if you have too little atmosphere
and it's too thin and too cold,

you end up like Mars, too cold.

So Venus is too hot, Mars is too cold,

and Earth is just right.

You can look at these images behind me
and you can see automatically

where life can survive
in our Solar System.

It's a Goldilocks-type problem,
and it's so simple that a child

could understand it.

However,

I'd like to remind you of two things

from the Goldilocks story that we may not
think about so often,

but that I think are really relevant here.

Number one: if Mama Bear's bowl

is too cold

when Goldilocks walks into the room