(air whooshes) - People have always dreamed of ways to be closer to the stars.

That's what brought us here to Mauna Kea in Hawaii.

From this spot, we can stand nearer to the sky and see farther and clearer than almost anywhere else on Earth to wonder what and perhaps even who is out there.

(chill ambient music) (air whooshing) (computer beeping) On Earth and in space, advanced telescopes have stared for weeks, even months into patches of sky, and they've seen that other stars are surrounded by planets of their own.

At least a planet for every star.

But what sort of planets are they?

Astronomers have learned that our galaxy is home to many kinds of planet/sun systems from hot Jupiters to warm Neptunes, even super-Earths of lava and diamond.

These planets have expanded our view of where life may be possible.

What drives astronomers to study them is to find an answer to that ultimate question.

Is life abundant, or are we unique?

We're standing in front of two of the most sensitive, precise, and advanced ground telescopes ever constructed, the Keck Observatory.

These instruments, and others that are being designed, will allow scientists, for the first time, to characterize these far-off exoplanets to paint a detailed picture of their sizes, their orbits, and even the chemicals in their atmospheres to understand where and how life might exist.

Combined with knowledge from biology, physics, and chemistry, we're learning a great deal about how life and planets coevolve.

We call it the science of astrobiology.

Decades before we discovered the first exoplanet, one scientist asked what we'd need to know in order to know whether another intelligent, technological civilization is, or was, or might one day be out there.

That scientist was a young radio astronomer named Frank Drake.

He gave us a way to estimate the number of technological civilizations that are out there.

N-star tells us how often stars are born.

It's now known to be around one star per year born in the Milky Way.

So we put a one there.

f sub p is the fraction of stars with planets, which we now believe is one, or at least one planet for every star.

Solar systems are the rule, not the exception.

n sub p is the estimate of how many planets orbit their stars at distances that allow for liquid water.

We think as many as one in five planets sit in these so-called habitable zones, or a value for n sub p of 0.2.

In all, there may be as many as 40 billion Earth-sized planets orbiting in habitable zones of Sun-like stars and red dwarfs in the Milky Way.

Now so far, our discoveries have filled nearly half of the equation and expanded what is possible, but the Drake Equation is still incomplete.

We don't yet know how many host life, f sub L, if any of that life is intelligent, f sub i, if it's built a civilization, f sub c, or how long that civilization might last so that we might find it.

When astronomers are searching for maybe that ultimate question of is life abundant or is it unique, what sorts of actual experiments are they doing here to try and get at that question?

- Well, the first thing you have to do is to find the planets, right?

And so that's one of the things that Keck does wonderfully well and many other telescope facilities, is we find them, either by transiting when the planet goes in front of its star and dips the light down a bit, or through the radial velocity method, or through direct imaging with Keck adaptive optics.

So you've gotta find the planets, that's step one, right?

Step two is are the planets at a distance from their host star where water could be liquid on the surface?

And then you wanna know something about the atmosphere of that planet.

And that's when things get really, really hard.

Because to be able to measure the atmosphere of that planet you either have to have an extremely precise measurement of the star before and during these eclipses, or you have to be able to measure the light that's bouncing off of that planet and measure the chemistry in its atmosphere.

And both of those things require extremely precise instrumentation, very, very large telescopes, and just sheer force of will to keep in the game.

- This telescope is amazing.

Each of its 36 hexagonal mirror segments is polished so smooth if they were the size of the Earth, their largest imperfection would only be three feet high.

And twice every second, these segment's positions are adjusted by an accuracy of 4 nanometers or 1/25,000th of a human hair.

The next phase of exoplanet exploration will be the search for biosignatures.

These are telltale chemical signs like oxygen or methane in those far-away atmospheres.

These will be detectable from future space telescopes and giant ground-based observatories planned on Earth.

- Then comes the big question.

How many of them actually show hints of life in their atmosphere?

And are we being fooled?

Just because you see ozone and methane and carbon dioxide and water vapor in the atmosphere, is that a slam dunk for life?

Not necessarily.

We want to be able to do that, and then we want to get at the essence of your question, which is if we look at a 100 planets and they're all in the habitable zone and we see nothing, then that's told you something statistically.

If you look at a 100 planets, and 50 of them, or 60 of them have something, that tells you something really amazing about the universe.

So we need to have the power and the precision to go after as many planets as possible, but at the same time just by exploring Earth we're finding out that life is thriving in places where we thought impossible.

So when you combine those two things and when you think about solar systems which are radically unlike ours, the mind really starts to stretch out and think that life could really be abundant out there in the universe and we should stop being so Earth-centric sometimes when we think about that.

- But if the cosmos is so vast and so full of so many places where life and intelligence may arise, then where are they?

Perhaps there's some great filter which prevents other life-bearing planets from reaching our level of civilization.

Maybe the appearance of even simple life on habitable worlds is so unlikely that biology itself is the great filter.

Or while life is common, maybe the emergence of even simple intelligence is rare.

But there is another option.

Maybe the great filter lies in technological civilizations themselves.

In the millennia since human civilization started, our most important discovery is the one that's enabled us to burn 100 million years of stored energy to power our technological growth, fossil fuels.

As a rule, it takes energy to build and grow a technological civilization, and harnessing massive amounts of energy has some impact on a civilization's environment.

Over there is the place where we measure the planet's atmospheric carbon dioxide concentration.

It's just crossed 415 parts per million for the first time since humans came into existence.

Now as evidenced by the measurements taken there, human activities are changing our planet's climate, and those changes may have dire consequences for us.

We're not the first life form to change the climate on the Earth.

Billions of years ago, ancient microbes breathed the first oxygen into the atmosphere making possible life as we know it today.

But the result of that shift was the death of massive amounts of Earth's early life to whom oxygen was poisonous.

It's an environmental shift that completely changed the course of how life unfolded on this planet.

Are we now about to shift the course of life on Earth again?

Is self-destruction in the process of harnessing energy an inherent risk in the development of all civilizations, human or alien?

Whether or not we are ever able to find another technological civilization might depend on the question of if civilizations can harness energy without destroying their own future.

So as we build ourselves up to be closer to the stars, we should at least ask will the same be true of us?

(chill ambient music) (air whooshing) (computer beeping)