[narrator] Fifty years ago,
astronomers picked up this radio signal coming from deep space.
The signal repeated so regularly,
it kept time better than an atomic clock.
[imitates signal] What could that be? That couldn't be natural.
[narrator] They thought it might be an alien transmission,
so they nicknamed the signal LGM-1, for Little Green Men.
It turned out to be a pulsar,
radio waves from a neutron star collapsing 5.5 million years ago.
A lot of us put aliens in the same category as ghosts
or the Loch Ness Monster, a subject for science fiction.
Or left to the cranks, kooks and conspiracy theorists.
[man] Immediately, I was just lifted from the ground,
to about the height that they were off the ground.
That's when I first saw this thing coming straight down, just like an elevator.
This is a center for the distribution of information
coming through me telepathically from the space people.
[narrator] But time and again,
serious scientists have thought they've found evidence
of extraterrestrial life.
[man] Are these really canals on Mars? Are the polar caps frozen water?
[narrator] As recently as 2016,
astronomers proposed that never-before-seen dimming patterns
from a star could be evidence of gigantic structures built
by an advanced civilization to harness the star's energy.
It turned out to be dust.
Scientists feel confident that there is biology beyond Earth.
Not because we've found it, we haven't found it.
The reason that we think that they're out there is simply, if not,
then Earth is some sort of miracle.
[narrator] For most scientists who study the universe,
searching for aliens isn't crazy.
What's crazy is that we haven't found them.
In a universe so vast, where is everybody?
[Stephen Hawking] It is important to us to know if we are alone in the dark.
Unidentified objects that sound warning klaxons around the world.
[man] Oh, my gosh, look at that thing. It's resting!
[Ronald Reagan] How quickly our differences would vanish
if we were facing an alien threat from outside this world.
Out there is a million other civilizations.
They all look fabulously ugly.
And they're all a lot smarter than us.
[theme music playing]
Trash cans been vanishing from city sidewalks in alarming numbers.
[narrator] Stolen trash cans are a time-honored public nuisance.
You may have seen local news reports about it.
The New Yorker magazine even published a cartoon about it back in 1950,
blaming mischievous aliens.
That silly joke inspired one of the most profound insights
in modern scientific history.
Because the physicist Enrico Fermi saw that cartoon,
and the story goes, blurted out, "Where is everybody?"
The fact that we haven't found any evidence of aliens
became known as the Fermi Paradox.
There are about ten to the power of 22 total stars.
That's about 10,000 stars for every grain of sand on Earth.
A conservative scientific estimate says 5%
of those stars are similar to our sun,
which means 500 billion billion suns in the universe.
Many scientists are more confident than ever that aliens exist
because of some game changing discoveries in the last few decades.
Nobody could say for sure if there were any planets
outside of our solar system, until the 1990s.
Now, scientists think one in five sun-like stars is a planet...
similar to our own.
When I get asked what are the chances there's life out there,
I always answer 100%.
Just because there's so many stars and planets...
we think pretty much every star has planets.
[narrator] We've also discovered life on Earth
in environments where nobody expected to find it.
We see life all the way deep in the sub-surface of the planet,
miles down, in like, gold mines.
We see life near volcanic calderas.
We see life on nuclear reactors.
We see life in the most extremes.
That actually gives us lot of hope for the search for life elsewhere,
'cause we can't necessarily expect that all planets
will have just the same conditions as Earth.
[narrator] Estimates of how many Earth-like planets will develop life vary.
So, let's say even with this new scientific confidence,
it's just one out of every thousand.
That means every tenth grain of sand on Earth
represents a planet with life on it.
And if just one out of every thousand of those planets develop intelligent life,
that's a quadrillion intelligent alien civilizations in the universe.
10,000 just in our galaxy.
Extraterrestrial life also has time on its side.
Earth is only about a third as old as the universe.
And so there's been plenty of time
for life to evolve to advanced civilizations and for these civilizations to spread across the galaxy.
[narrator] With all that time and space, the math seems pretty clear.
We should've found aliens by now, or they should have found us.
There's one popular explanation for why we haven't found evidence of aliens.
We have found it.
Governments have just covered it up.
I believe that the flying saucers seen by veteran airline and Air Force pilots
are objects from another planet.
Our critics continually charge
that the United States Air Force is withholding information
from the general public on this subject.
This is absolutely untrue.
[narrator] Every so often,
something comes out that gives this theory new life,
like the revelation in 2017 that the US government had spent millions
on a secret program to investigate UFO sightings.
[narrator] But as almost any scientist will tell you,
looking into UFOs isn't the same as searching for extraterrestrial life.
The word UFO is "unidentified flying object."
It's unidentified.
So, by definition, we have to leave it open.
It doesn't mean it's been identified as an alien spacecraft.
[narrator] Scientists have their own favorite theories about
why we haven't found aliens.
It could be that they came here,
didn't like what they found and moved on.
Imagine for a moment, you get an infestation of ants in your house.
It happens.
Now let's say you wanna have a conversation with those ants.
Say, "Excuse me, can you please leave?" How would you even do that?
I like that theory, that we're just so dumb right now.
We're not even at the level where if they wanted to talk to us,
these so-called intelligent creatures out there could even communicate with us.
It could be that they've got better things to do
than just waft around the galaxy.
They've seen our planet,
they just don't wanna interfere with us
until we get to this point of technological or societal advancement
where we're ready to be interacted with.
Maybe the galaxy is colonized,
maybe it's heavily colonized,
but just not where we are.
In other words, the fact that we seem to be alone
may be only that we're in a backwater.
[narrator] But it's important to remember that when Fermi calculated the odds
that alien life is out there,
it was just an educated guess.
With so many stars and planets,
he bet at least some of them will develop life,
which would evolve and spread out.
The trouble with this bet is there's a lot we don't know about life.
The great filter theory helps us think about what we don't know.
Imagine the evolution of life as a series of hurdles.
First, molecules start replicating themselves,
which evolves into single-cell life, then multi-cell life
and then animals with large brains that can use tools,
and then smarter animals that create even better tools. That's us.
And finally, animals that can figure out how to colonize the galaxy.
Given the size and age of the universe,
it seems like a lot of alien species should have beat us to that last stage,
unless one of those stages is much harder than we think.
[Paul Davies] The view seems to be that
given the right conditions, life will obligingly pop up,
but the truth is, nobody has a clue.
We have no idea how non-life turns into life.
We know how life structures itself, but our gaps are in the major transitions.
[narrator] We know what the major hurdles are in the evolution of life.
Just not how hard they are to get past.
Another example of life would help us understand life better.
But so far, no aliens have contacted us.
So, it's up to us to find them.
[man] Listen to the sound of the sun and the stars.
[Jill Tarter] I've spent my career at the SETI Institute, S-E-T-I.
Search for Extraterrestrial Intelligence, that's the acronym.
But in fact, we don't know how to detect intelligence directly.
[narrator] When scientists look for intelligent aliens,
they look for what are called technosignatures...
evidence of alien technology.
We want to find extraterrestrial intelligence,
by finding something that's engineered, something that's artificial,
something that nature can't produce.
If I find technology...
I'm going to presume, at least at some time...
the existence of an intelligent technologist.
[narrator] From up close,
Earth has technosignatures in the form of city lights.
From further out, aliens might notice the satellites
and space stations orbiting our planet.
From even further, they might pick up radio signals
or stumble across the Voyager probes
that are hurtling across interstellar space.
Sometimes the things we think are technosignatures
turn out to be natural phenomena, like that pulsar.
But since radio signals are still our most promising leads,
scientists do a lot of listening to the sky.
[Tarter] If you've seen the movie Contact,
there's Jodie Foster on the hood of the car with the earphones on.
It's a bit ridiculous. Because in fact,
the computer back in the observatory control room
is doing the signal processing.
They're analyzing the equivalent of the Encyclopedia Britannica every second.
[narrator] Jill Tarter would know. Jodie Foster's character was based on her.
Holy shit!
She gets to say, "Holy shit!," you know?
We hope to some day have that moment.
[narrator] But that's just one way to search for extraterrestrial life.
There are also biosignatures.
Biosignatures are indications
that life existed or once did exist in any given environment.
[narrator] If aliens came to Earth looking for life after we were long gone,
they would find biosignatures in the form of fossils
and chemical evidence of life processes.
My favorite line to say with kids is that all life poops.
So we know that all life takes in energy and releases waste products.
[narrator] If aliens were observing us from afar,
they would see biosignatures in the form of water
and the gases in our atmosphere.
Oxygen is so reactive
that it can only be in our atmosphere if it's being continuously produced.
Without life, Earth's atmosphere would have no oxygen,
so we're trying to look for gases that don't belong,
that might be attributed to life, and we call them biosignature gases.
[narrator] Searching for biosignatures on other planets is really hard.
We can't even see planets outside our solar system. Stars are so much brighter than planets.
It's like trying to see a firefly in a spotlight.
Today, we have a planet finding technique called the transit technique.
When a planet goes in front of its star, the starlight drops by a tiny amount.
[narrator] These drops in light give scientists clues
about whether a planet might have life on it.
Like the distance from its star.
We call the "Goldilocks zone" the distance from the star,
where the planet, as heated by the star, is not too hot,
not too cold, but just right for life.
[narrator] Researchers have been able to surmise some amazing things about planets
just from these light patterns.
Scientists think they found a super Earth with really intense gravity.
A planetary system with seven planets all crammed into the Goldilocks zone.
And even a planet that could have red vegetation,
from the different wavelengths of light it receives.
We now know of over 3,500 planets outside of our solar system.
Most of them were discovered in just the last five years.
And tools are only getting better.
The next generation of space telescopes will be able to see more distant galaxies.
A newly launched satellite will survey the entire sky for possible planets,
rather than just small sections.
And astronomers are developing new technologies
that would let them see distant planets directly.
The line between what is considered completely crazy
and what is mainstream is constantly shifting.
[narrator] For all the exciting new ways to search for life in deep space,
scientists are also searching a lot closer to home.
-[man] We have landed. -[man 2] Roger.
[narrator] In the 1970s, we sent two landers to Mars
to test the soil for evidence of life.
The first and only time we've ever tried.
One of the experiments came back negative.
But another came back positive for evidence of a process
that we only associate with living things.
When some of the experiments came back positive,
and the others came back negative, it was controversial,
because it was ambiguous.
So it was hard to say, did we actually really find life?
[narrator] The contradiction could mean an unknown chemical reaction occurred
that only looked like a living thing consuming energy.
But since the '70s, we've learned that life in extreme environments
uses energy differently and leaves different markers on its environment.
The experiments that were designed
were designed based on life as we knew it back then,
which was a very limited view of life just here on Earth.
We need to go back to Mars and do the experiment again.
[narrator] The Mars 2020 mission is our next shot.
Unlike the Viking experiments, it won't test for currently living things,
but it will look for signs that life once did exist in certain Martian environments.
A mission is also in the works to look for biosignatures
in the frozen oceans of Jupiter's moon Europa.
In the search for intelligent life,
scientists are also trying to expand their thinking and their search.
Now, the only example we have of intelligent life is indeed us.
You know... [laughs]
in Star Trek, I guess it was the doctor on board, Bones,
who'd occasionally say, "It's life, Jim, but not as we know it."
[narrator] Actually, this is a common misquote.
The line is a lyric in the song "Star Trekkin" by The Firm
and was never said in the show.
It's life, Jim, but not as we know it. [narrator] Spock, however, said something similar
in Season One, episode 29.
It's not life as we know or understand it.
It is obviously alive. It exists.
[narrator] Advances in our own technology give us new ideas
about what intelligent aliens might be like.
One thing that we're doing in this century
and certainly in the first half of this century, it seems,
is to develop artificial intelligence that does more
than just play a good game of chess.
Humans are the best known reference for intelligence.
What a great standard to try to live up to.
We think of the aliens as being like a soft and squishy biology,
whereas in fact the majority of the intelligence in the universe
could very well be synthetic intelligence.
One good thing in terms of helping us to think about what we don't know
is to read science fiction.
Actually, Arrival was one of my favorite movies.
Just because of the concept that the aliens could be so different
from intelligent humanoids.
I really think that's how it's gonna end up being.
[narrator] Science fiction has shaped our space programs
from the very beginning.
[Kennda Lynch] The Martian Chronicles, War of the Worlds, many senior scientists
were inspired by those early, early novels,
and they've actually created the science reality of Mars exploration
that we have today.
[Sara Seager] It's a multi-generational search.
We're just starting now.
We're just kind of planting the seeds for a really long endeavor.
[man] Velocity build up in feet per second.
[man 2] Okay.
[Tarter] Consider the volume of all the Earth's oceans.
All right, and let's say, that's the volume of search space,
where we might find a signal.
Well, in 50 years, how much of that ocean have we searched?
It's a pretty disappointing one glass of water.
[narrator] Whether we find extraterrestrial life
or learn that we are alone,
it will tell us a lot about our civilization
and what our future might be.
Think back to that great filter theory.
It could be even life rarely gets started
or that the universe is teeming with life, but none of it...
as smart as us.
That is good news for the future of our civilization.
It means that we are maybe the only planet in the galaxy
that got as far as intelligent life,
and there's no reason we can't be set fair
for thousands or millions of years in the future.
[narrator] Or maybe the hardest stage is ahead of us.
And some unknown challenge awaits humanity.
If life on Earth is typical and we are typical,
but the typical thing is you don't survive very long then,
that doesn't say much about our future.
The importance for the search for life elsewhere in the universe
is kind of the search in understanding ourselves.
It's important to understanding how did we as a planet come here.
And how rare are we or how rare are we not?
And for humans, it's an understanding of, you know,
what's the next big step for us.