Not little green humanoids arriving in shiny UFOs,

although that would be nice.

But it's the search for planets orbiting stars far away.

Every star in our sky is a sun.

And if our sun has planets --

Mercury, Venus, Earth, Mars, etc.,

surely those other stars should have planets also,

and they do.

And in the last two decades,

astronomers have found thousands of exoplanets.

Our night sky is literally teeming with exoplanets.

We know, statistically speaking,

that every star has at least one planet.

And in the search for planets,

and in the future, planets that might be like Earth,

we're able to help address

some of the most amazing and mysterious questions

that have faced humankind for centuries.

Why are we here?

Why does our universe exist?

How did Earth form and evolve?

How and why did life originate and populate our planet?

The second question that we often think about is:

Are we alone?

Is there life out there?

Who is out there?

You know, this question has been around for thousands of years,

since at least the time of the Greek philosophers.

But I'm here to tell you just how close we're getting

to finding out the answer to this question.

It's the first time in human history that this really is within reach for us.

Now when I think about the possibilities for life out there,

I think of the fact that our sun is but one of many stars.

This is a photograph of a real galaxy,

we think our Milky Way looks like this galaxy.

It's a collection of bound stars.

But our [sun] is one of hundreds of billions of stars

and our galaxy is one of upwards of hundreds of billions of galaxies.

Knowing that small planets are very common,

you can just do the math.

And there are just so many stars and so many planets out there,

that surely, there must be life somewhere out there.

Well, the biologists get furious with me for saying that,

because we have absolutely no evidence for life beyond Earth yet.

Well, if we were able to look at our galaxy from the outside

and zoom in to where our sun is,

we see a real map of the stars.

And the highlighted stars are those with known exoplanets.

This is really just the tip of the iceberg.

Here, this animation is zooming in onto our solar system.

And you'll see here the planets

as well as some spacecraft that are also orbiting our sun.

Now if we can imagine going to the West Coast of North America,

and looking out at the night sky,

here's what we'd see on a spring night.

And you can see the constellations overlaid

and again, so many stars with planets.

There's a special patch of the sky where we have thousands of planets.

This is where the Kepler Space Telescope focused for many years.

Let's zoom in and look at one of the favorite exoplanets.

This star is called Kepler-186f.

It's a system of about five planets.

And by the way, most of these exoplanets, we don't know too much about.

We know their size, and their orbit and things like that.

But there's a very special planet here called Kepler-186f.

This planet is in a zone that is not too far from the star,

so that the temperature may be just right for life.

Here, the artist's conception is just zooming in

and showing you what that planet might be like.

So, many people have this romantic notion of astronomers

going to the telescope on a lonely mountaintop

and looking at the spectacular night sky through a big telescope.

But actually, we just work on our computers like everyone else,

and we get our data by email or downloading from a database.

So instead of coming here to tell you

about the somewhat tedious nature of the data and data analysis

and the complex computer models we make,

I have a different way to try to explain to you

some of the things that we're thinking about exoplanets.

Here's a travel poster:

"Kepler-186f:

Where the grass is always redder on the other side."

That's because Kepler-186f orbits a red star,

and we're just speculating that perhaps the plants there,

if there is vegetation that does photosynthesis,

it has different pigments and looks red.

"Enjoy the gravity on HD 40307g,

a Super-Earth."

This planet is more massive than Earth

and has a higher surface gravity.

"Relax on Kepler-16b,

where your shadow always has company."

(Laughter)

We know of a dozen planets that orbit two stars,

and there's likely many more out there.

If we could visit one of those planets,

you literally would see two sunsets

and have two shadows.

So actually, science fiction got some things right.

Tatooine from Star Wars.

And I have a couple of other favorite exoplanets

to tell you about.

This one is Kepler-10b,

it's a hot, hot planet.

It orbits over 50 times closer to its star

than our Earth does to our sun.

And actually, it's so hot,

we can't visit any of these planets, but if we could,

we would melt long before we got there.

We think the surface is hot enough to melt rock

and has liquid lava lakes.

Gliese 1214b.

This planet, we know the mass and the size

and it has a fairly low density.

It's somewhat warm.

We actually don't know really anything about this planet,

but one possibility is that it's a water world,

like a scaled-up version of one of Jupiter's icy moons

that might be 50 percent water by mass.

And in this case, it would have a thick steam atmosphere

overlaying an ocean,

not of liquid water,

but of an exotic form of water, a superfluid --

not quite a gas, not quite a liquid.

And under that wouldn't be rock,

but a form of high-pressure ice,

like ice IX.

So out of all these planets out there,

and the variety is just simply astonishing,

we mostly want to find the planets that are Goldilocks planets, we call them.

Not too big, not too small,

not too hot, not too cold --

but just right for life.

But to do that, we'd have to be able to look

at the planet's atmosphere,

because the atmosphere acts like a blanket trapping heat --

the greenhouse effect.

We have to be able to assess the greenhouse gases

on other planets.

Well, science fiction got some things wrong.

The Star Trek Enterprise

had to travel vast distances at incredible speeds

to orbit other planets

so that First Officer Spock could analyze the atmosphere

to see if the planet was habitable

or if there were lifeforms there.

Well, we don't need to travel at warp speeds

to see other planet atmospheres,

although I don't want to dissuade any budding engineers

from figuring out how to do that.

We actually can and do study planet atmospheres

from here, from Earth orbit.

This is a picture, a photograph of the Hubble Space Telescope

taken by the shuttle Atlantis as it was departing

after the last human space flight to Hubble.

They installed a new camera, actually,

that we use for exoplanet atmospheres.

And so far, we've been able to study dozens of exoplanet atmospheres,

about six of them in great detail.

But those are not small planets like Earth.

They're big, hot planets that are easy to see.

We're not ready,

we don't have the right technology yet to study small exoplanets.

But nevertheless,

I wanted to try to explain to you how we study exoplanet atmospheres.

I want you to imagine, for a moment, a rainbow.

And if we could look at this rainbow closely,

we would see that some dark lines are missing.

And here's our sun,

the white light of our sun split up,

not by raindrops, but by a spectrograph.

And you can see all these dark, vertical lines.

Some are very narrow, some are wide,

some are shaded at the edges.

And this is actually how astronomers have studied objects in the heavens,

literally, for over a century.

So here, each different atom and molecule

has a special set of lines,

a fingerprint, if you will.

And that's how we study exoplanet atmospheres.

And I'll just never forget when I started working

on exoplanet atmospheres 20 years ago,

how many people told me,

"This will never happen.

We'll never be able to study them. Why are you bothering?"

And that's why I'm pleased to tell you about all the atmospheres studied now,

and this is really a field of its own.

So when it comes to other planets, other Earths,

in the future when we can observe them,

what kind of gases would we be looking for?

Well, you know, our own Earth has oxygen in the atmosphere

to 20 percent by volume.

That's a lot of oxygen.

But without plants and photosynthetic life,

there would be no oxygen,

virtually no oxygen in our atmosphere.

So oxygen is here because of life.

And our goal then is to look for gases in other planet atmospheres,

gases that don't belong,

that we might be able to attribute to life.

But which molecules should we search for?

I actually told you how diverse exoplanets are.

We expect that to continue in the future

when we find other Earths.

And that's one of the main things I'm working on now,

I have a theory about this.

It reminds me that nearly every day,

I receive an email or emails

from someone with a crazy theory about physics of gravity

or cosmology or some such.

So, please don't email me one of your crazy theories.

(Laughter)

Well, I had my own crazy theory.

But, who does the MIT professor go to?

Well, I emailed a Nobel Laureate in Physiology or Medicine

and he said, "Sure, come and talk to me."

So I brought my two biochemistry friends

and we went to talk to him about our crazy theory.

And that theory was that life produces all small molecules,

so many molecules.

Like, everything I could think of, but not being a chemist.

Think about it:

carbon dioxide, carbon monoxide,

molecular hydrogen, molecular nitrogen,

methane, methyl chloride --

so many gases.

They also exist for other reasons,

but just life even produces ozone.

So we go to talk to him about this,

and immediately, he shot down the theory.

He found an example that didn't exist.

So, we went back to the drawing board

and we think we have found something very interesting in another field.

But back to exoplanets,

the point is that life produces so many different types of gases,

literally thousands of gases.

And so what we're doing now is just trying to figure out

on which types of exoplanets,

which gases could be attributed to life.

And so when it comes time when we find gases

in exoplanet atmospheres

that we won't know if they're being produced

by intelligent aliens or by trees,