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We are finding
more and more exoplanets,

more and more locations for liquid water,

and the possibility
for life as we know it.

But, where are the alien civilizations?

Where are the technologically
advanced creatures?

Why can't we find signs
of other sentient beings?

Where are their signals out
there, amongst the stars?

Perhaps the answer to
that age old question,

are we alone in the galaxy, is a yes.

American astronomer and
astrophysicist Frank Drake

established the Search for
Extraterrestrial Intelligence,

or SETI back in 1960.

Listening to the stars
and the noisy galaxy,

trying to find signals
that are deliberate,

messages from other planets,

where life has evolved an intelligence,

a technological civilization like our own,

able to comprehend the vastness of space.

Like us, looking for the answer
to that age old question,

are we alone?

There are 3,700 known exoplanets so far.

NASA's tests is going to find lots more.

The European CHEOPS
mission will specifically

study the known exoplanets

and look for more around our
brighter stellar neighbors.

Multiple planetary
systems and terrestrial,

Earth-like planets are coming up trumps

and we've only been looking for 15 years.

We have been looking and listening

for nearly 60 years
now, with zero results.

No radio signals,

no laser or other light
flashes in our direction,

no attempts at communication
that we can detect.

So, where are all the aliens?

Well, it's a complicated question.

Stars that can harbor Earth-like planets?

Viable exoplanets that
can sustain liquid water?

As for life, how long does
it take to generate life?

Then, how long to evolve
into intelligent life?

Will that intelligent
life become technological?

How long will an advanced
civilization survive?

Tough questions when you only
have Earth as your example.

Then, there is the time factor.

The universe is 13.8 billion years old.

Our star, the Sun, is
4.6 billion years old,

and probably a 3rd or 4th generation star.

Earth is 4.543 billion years old.

Our moon is just the right size

to stabilize Earth's rotation

and provide ocean tides and sea currents.

Life on Earth is at least
3.5 billion years old.

Several major climactic
and asteroid interventions

steered the evolutionary
cart, leading to us.

Homo sapiens, 200,000 years.

Our civilization began around 6,000 years.

The Industrial Age is just over 200 years.

The ability to send signals
into space, maybe 100 years.

Powerful radio signals,
television signals,

radar, and laser light, say, 50 years.

Being generous, our
signals have traveled out

in a sphere with a radius
of 100 light years.

That's about 1/1000th of
the width of the Milky Way,

which is 105,700 light years across.

The nearest galaxy is Andromeda,

at 220,000 light years away.

How long will we continue to broadcast?

In other words, how long

will our technological society last?

Some think not so very long.

We have to be realists.

There will not be too many
terrestrial-like planets

with liquid water and an
atmosphere to support life,

nurture that life to evolve
over billions of years

into a sentient species that
may just develop technology

and an understanding of the cosmos,

to want to listen out and
respond to a signal from space.

There is, most likely, life out there,

but it's not talking to us yet.

Let's crunch some numbers.

We'll take the arbitrary
figure of 100 light years,

where a sufficiently advanced society

could detect our radio signals.

They then work out where
the signal came from,

and dispatch a response.

This could take a few years to achieve.

Then that signal will take up to 100 years

to get back to us.

100 light years in
kilometers roughly equals

nine plus 14 zeros after it.

What stars are within that reach?

We're only looking at main sequence stars,

ones that are stable and long lived,

moving through various stages

of their lifetime of billions of years.

So, we have 3,868 stars to choose from,

plus a few more very dim
ones we haven't found yet.

Exoplanet hunters have
been targeting the dimmer,

smaller stars called red
dwarfs, or, M-type stars,

because it is much easier to
detect planets around them.

There are 2,026 stars of
that class in our region.

They have also been looking
at the slightly larger,

orange K-type main sequence stars,

because of their spectral
and luminosity qualities,

astronomers favor them for having

habitable goldilocks regions.

There are 947 and counting,

though there are only 155
within 50 light years.

Then there's the G-type stars.

Our sun is a G-type,

so they are highly regarded
for habitable zones.

There are 512 of them.

F-types are slightly larger than our sun,

303 are nearby, and
several have known planets.

A-type, which are young stars,

a mere 100 million years old, and again,

have been found to harbor
Jupiter-like gas planets; 76.

B-type are giant blue stars,
and rare, only 4 in our region.

And, supergiant O-types are hot,

and hostile, and nowhere near us.

So, how many exoplanets?

3,700 confirmed exoplanets so far.

Five Mercury-sized rocky worlds,

72 Mars-sized,

701 Earth-sized, terrestrials,

982 called Super Earths,

799 Neptune-type gaseous worlds,

and 1,217 Jupiter-sized gas planets.

So, that gives a current total of

1,683 Earth-like, rocky planets.

How many of these rocky worlds

could harbor liquid
water on their surface?

Well, there is the habitable,
or goldilocks zone,

around these stars,
where the planets receive

the right amount of solar
flux, where water is liquid.

Too close and the water would boil away,

too far and it will freeze,

making life more difficult to evolve.

So, scientists have estimated there are

17 definitely within this zone,

but three are outside our
distance limit of 100 light years,

so that leaves 14.

Plus, another 13 terrestrials
that could seed life,

but be very harsh and unforgiving to it.

That makes 27 candidates

capable of harboring life as we know it.

- Life could be incredibly common,

because there are lots
of planets out there,

or, it could be an incredibly rare event

that only happens on one in a
billion planets in a galaxy.

We really just don't know, so,

whenever I get asked that
question, my answer is,

I don't have an opinion yet,
I'll need some more evidence.

TRAPPIST-1,
an M-type dwarf star

at 39.6 light years distance,

contains a solar system of
seven terrestrial planets,

and it a major contender
for harboring liquid water;

they're all Earth-sized candidates.

Other multi-planet systems in
our region include HR 8832,

55 Cancri, and 61 Virginis.

The names just roll of the tongue,

each with at least three
planets orbiting it.

- Well, looking at exoplanets,

these planets around other stars,

is a bit like detective work.

They're so small next to a bright star,

that it's hard to tease out information

unless you do it incrementally.

So, what we can tell right now is that

these planets are, in fact,
similar to Earth in size,

and we can tell that some of them

may be similar to Earth
in its composition,

doing crude studies of the
density of these planets.

We're also using the Hubble
Space Telescope right now

to do followup observations
of the atmospheres

of some of these planets to see

the composition of the
atmosphere and wondering,

and trying to find out
if maybe there might be

water in the atmospheres,
to see if they are

similar to our own Earth's atmospheres.

With names like
TRAPPIST, YZ Ceti, Gliese 876,

HD 219134,

and Wolf-1060,

these exo-worlds sound
ponderous and uninteresting.

But, one day they may be
given more appealing names,

because these are worlds
with great potential.

Elements such as hydrogen,
helium, carbon and oxygen

abound throughout the universe,

so we might assume that they
are available to such planets

and therefore water would be plentiful.

Even worlds completely
covered in water are feasible.

These basic elements also make up

the active ingredients for life.

Elements and amino acids are commonplace,

so microbial life could emerge

on all these target planets quite readily.

- Now, we strongly suspect,
based on an example of one,

our Earth, that the laws of biology

work the same everywhere too.

And by that I mean, the laws

that Charles Darwin discovered for us.

We think that based on these laws

that evolution can drive
life to greater complexity

and ultimately to intelligence.

It's the smart thing to do.

As with Earth, it could take

billions of years for evolution to create

ever greater complexity, and
eventually, intelligence.

On Earth, most highly
intelligent creatures

are of the mammalian family,

but the octopus developed its intelligence

through an entirely separate route.

This would indicate that intelligence

has many chances of
evolving on these worlds.

So, why are there no similar

technological societies among
these 27 candidate planets?

What could be stopping
intelligent creatures

from developing technology,
radios, and space ships?

There are several possible answers.

Earth has had major climactic disasters

that nearly wiped out life completely.

Perhaps a similar disaster was successful

on one of these other world and killed it.

But, even with these events

befalling some of our candidate planets,

there should still be 25 or
more potential civilizations.

- So, you'd think there'd
be plenty of opportunities

for life to evolve somewhere else,

and maybe swing by the Earth
or at least call on the radio.

But we haven't found any alien monoliths,

or beer cans,

or cigarette ends, and
we have not heard them

tweeting on the radio either.

So, where are they?

There are lots of theories about that,

but I'm gonna concentrate
on the more plausible ones.

First of all, there's the water trap.

Maybe the worlds that have
water on them are all ocean,

for the most part, and if that's the case,

you can't discover combustion,
you can't make metals,

so you can't make a radio or a spaceship.

If our dolphin friends
lived on an ocean planet,

they would be stuck where they are,

in the stone age forever.

Or, maybe on a planet that
has dry land but no metals,

or very few metals.

Same problem.

You can't develop a technology.

And, how about the difficulty
of interstellar travel?

Maybe it's just too hard.

It looks like a real challenge for us.

It could be a couple of hundred
years before we try that.

Maybe it's just to hard.

And then, there's another theory,

which is, that we could be the first.

We could be the first
intelligence to evolve

in this part of the galaxy.

Someone has to be.

We could be the elder race.

So, here's a time history of Earth.

When you look at all the
time that life has been here,

nearly 4 billion years,
humans have only been around

for a couple of thousand years,

civilization for about 6,000 years,

depending how you count it,

and our technical era only for 200 years.

When you look at this
picture, it is obvious

that the most likely first alien lifeforms

that we discover will not be intelligent.

They'll be somewhere back here,
equivalent to life on Earth

during the first three
billion years of evolution.

Similarly, nearby life on an exoplanet

is probably plodding its way
up the evolutionary ladder.

Remember how long it took
us to get where we are,

where we could even think
about life elsewhere.

We Earthlings
have the prime real estate

in our solar system.

We may well be alone for the moment

in our section of the galaxy,

the only ones with space travel.

If so, we need to take care of our planet

and our civilization and
stick around long enough

to see other intelligences
and civilizations emerge

over the coming millions of years,

on those 25 nearby planets.