(Narrator)
To have any hope of finding alien life,

we have to know what to look for.

But where do we begin?

How do we narrow down a
seemingly infinite set of possibilities?

There's one thing we know for sure:

nature will have to play by her own rules.

No matter how strange alien life might be,

it's going to be limited by the same
physical and chemical laws

that we are.

On top of this,

each alien environment will further limit

what kinds of lifeforms can evolve there.

Despite these natural boundaries,
the possibilities

are staggering to imagine.

Trillions of planets,

each a unique cauldron of chemicals

undergoing their own complex evolution.

To guide our thinking,

this museum of alien life
will be divided into two exhibits:

life as we know it,

home to beings
with biochemistries like ours;

and life as we don't know it,

home to beings that challenge our concept

of life itself.

Before we venture too far into
the unknown,

we have to ask ourselves:

what if alien life

is more like us than we think?

If there's one feature that unites us

with the other specimens in this museum,

it's carbon.

(Nick Lane)
Carbon is ubiquitous,

it's one of the most common
elements in the universe,

and it's very good at forming
large, stable molecules.

(Narrator)
Carbon has the rare ability to form

four-way bonds
with other elements,

and to bind to itself

in long, stable chains,

enabling the formation of
huge complex molecules.

This versatility makes carbon
the centerpiece

in the molecular machinery of life.

And the same carbon compounds
that we use

have been found far from Earth,

clinging to meteorites,

and floating in far-off clouds

of cosmic dust.

The building blocks of life,

drifting like snow through the universe.

And if alien life has selected

other carbon compounds for their
biochemistry,

they will have plenty to choose from.

Scientists recently identified

over a million possible alternatives
to DNA—

all carbon-based.

If we ever discover

other carbon-based lifeforms,

we would be fundamentally related.

They would be our cosmic brethren.

But would they look anything like us?

If they hail from Earth-like planets,

we could share even more in common

than just our biochemistry.

(Jonathan Losos)
What would life be like on other planets,

if it is evolved?

Would it be like

the world today here on Earth,

or would it be completely different?

There are those who argue that,

from the argument of convergent evolution,

if conditions on other planets are
similar to here,

then we would see very similar life forms—

animal- and plant-like organisms
that look very familiar.

(Narrator)
On Earth, certain features like

eyesight, echolocation, and flight

have evolved multiple times independently

in different species.

This process of convergent evolution

could extend to alien planets like Earth,

where creatures face similar environmental
pressures.

It's no guarantee,

but there could be

certain universalities of life.

The greatest hits of evolution
on repeat across the universe.

Each feature would be attuned
to its local environment.

Dimly lit planets

would produce huge eyes to suck in
extra light,

like nocturnal mammals.

(Jonathan Losos)
Some people have gone so far as to say

that human-type organisms,

humanoids,

will occur on other planets.

The existence of other

humanlike organisms seems unlikely,

given the long, convoluted
chain of events

that produced us.

But we can't rule it out.

If just one in every
hundred trillion Earth-like planets

produced a humanlike form,

there could still be

thousands of creatures like us out there.

Convergent evolution

is also rampant in plant life,

and C₄ photosynthesis has arisen
independently

over 40 times.

Would alien plants look like ours

or something else entirely?

On Earth, plants appear green

because they absorb the other wavelengths

in the Sun's light spectrum.

But stars come in many colors,

and alien plants would evolve
different pigments

to adapt to their sun's unique spectrum.

Plants feeding off hotter stars

could appear redder,

by absorbing their energy-rich

bluer light.

Around dim red dwarf stars,

vegetation could appear black,

adapted to absorb

all visible wavelengths of light.

Earth itself

may have once appeared purple,

due a pigment called retinal

that was an early precursor
to chlorophyll.

Some think that
retinal's molecular simplicity

could make it a more universal pigment.

If so,

we may find that purple

is life's favorite color.

But the color of alien vegetation

is more than just a curiosity—

it's chemical information

that can be seen from lightyears away.

Earth plants leave a signature "bump"

in the light reflected off our planet.

Finding a similar signal
from another world

could point the way to alien vegetation.

Perhaps this will be

our first glimpse at alien life:

a vibrant hue cast by a distant world.

(Caleb Scharf)
What happens when you change

the day length of a planet?

What happens when you change

the tilt of a planet?

What happens when you change

the shape of the orbit?

What happens when you change

the gravity of a planet?

(Narrator)
Planets with long, elliptical orbits

would see drastic seasons.

There could be worlds

that appear dead for thousands of years,

then suddenly spring to life.

Most of the rocky planets
discovered so far

have been massive "super-Earths".

How would life evolve on these worlds?

In the seas,

gravity may not matter much at all.

(Unnamed)
A high-gravity planet

isn't high-gravity all over.

If you're in the sea
and that's where all life starts,

there's very nearly no gravity

'cause you're much the same density
as the stuff around you.

It's when the animals come out on land,

that they feel the gravity.

(Narrator)
High g-forces would necessitate

large bones and muscle mass

in complex life on land.

They would also demand

a more robust circulatory system.

And plant life could be stunted

by the energy cost of carrying nutrients

under stronger gravity.

Low-gravity planets

would more easily
lose their atmospheres to space,

and lack a magnetic field

to protect from cosmic rays.

But smaller worlds could be home

to secret oases:

huge cave systems

that provide hideouts for life.

The smallest possible habitable planets

are estimated at 2.5%

Earth's mass.

If surface life does evolve
on these worlds,

it could be a sight to behold.

Plant life could grow to towering heights,

able to carry nutrients higher
in lesser gravity.

And without the need

for bulky skeletons and muscle mass,

animals could have body types

that boggle the mind.

Here on Earth,

it took three billion years for evolution

to produce complex plant and animal life.

Simpler organisms are hardier,

more adaptable,

and more widespread.

The largest collection

in the museum of alien life

would likely be the "Hall of Microbes".

Yet finding even the tiniest alien microbe

would be a profound discovery.

And bite-sized life

could leave a big footprint.

Like stromatolites on Earth,

layers of microbes could build up

into huge rock mounds over time,

leaving behind eerie structures.

And in big enough numbers,

some alien bacteria could leave

a distinct biosignature

by exhaling gases that wouldn't
coexist naturally,

like oxygen and methane.

(Shawn Domagal-Goldman)
There's ways to make

oxygen without life,
there's ways to make

methane without life,

but to have them in the atmosphere together

is almost impossible unless you've got

biology making those gases at the surface.

And it would have an imprint

on the planet's spectrum of colors.

(Narrator)
Next-generation space telescopes

could find a signal like this,

on a world not far from home.

(Chris Crowe)
The closest Sun-like star

with an Earth-like exoplanet
in the habitable zone

is probably only 20 light years away,

and can be seen with the naked eye.

Most brown dwarfs are too hot

to support life as we know it.

But some are just cold enough.

All the prime elements for life

have been detected
inside their atmospheres.

And within these clouds,

some layers would provide

ideal temperatures and pressures

for habitability.

There could be photosynthetic plankton
in these skies,

kept aloft by churning upwinds.

And with enough force,

these upwinds could even support

larger, more complex life.

Predators.

This raises a crucial question:

what if we've been looking in
all the wrong places?

What if nature has other ideas?

Most of the universe
is too cold or too hot

for liquid water and the biochemistry

that supports life as we know it.

But in case our biases are misleading,

we have to cast a wide net—

to search for life outside
the habitable zone,

in places that seem wildly hostile to us.

Exotic environments will demand

exotic biochemistries,

and while no element can match
carbon's versatility,

one contender is a frontrunner.

At first glance,

silicon seems similar to carbon.

It forms the same four-way bonds

and is also abundant in the universe.

But a closer look reveals that

these two elements are false twins.

Silicon bonds are weaker,

and less prone to forming
large, complex molecules.

Despite this,

they can withstand a wider range
of temperatures,

opening up intriguing possibilities.

(Carl Sagan)
Life based on the silicon atom,

instead of carbon,

would be more resistant to the
extreme cold,

providing a whole new range

of weird forms.

(Narrator)
But silicon has a problem:

in the presence of oxygen,

it binds into solid rock.

To avoid turning to stone,

silicon beings might be confined

to oxygen-free environments,

like Saturn's frigid moon,

Titan.

Its vast lakes of
liquid methane and ethane

could be an ideal medium

for silicon-based life,

or other radical biochemistries.

Without ample sunlight,

beings on worlds like Titan

would likely be chemosynthetic,

deriving their energy
by breaking down rocks.

Such life forms could have

ultra slow metabolisms,

and life cycles

measured in millions of years.

In high temperatures,

typically rigid silicon-oxygen bonds

become more flexible and reactive,

triggering more dynamic chemistry.

This has led to a truly bizarre proposal:

silicon-based lifeforms

that live inside molten silicate rock.

In theory,
these forms could even exist

deep beneath the Earth inside
magma chambers

as part of a shadow biosphere.

If so,

then the aliens are right under our noses.

Other shadow biospheres
have been proposed—

forms of life living alongside us

that we don't even know are here—

including tiny RNA-based life
small enough to go

undetected by existing instruments.

Clouds of dust and empty space

might seem like the last place
you'd expect

to find anything living.

But when cosmic dust

makes contact with plasma,
a type of ionized gas,

something strange happens.

In simulated conditions,

dust particles have been seen

spontaneously self-organizing into

helical structures that resemble DNA.

These plasma crystals

even begin to exhibit lifelike behavior:

replicating,

evolving into more stable forms,

and passing on information.

Plasma is the most common state of matter

in the universe.

If complex, evolving plasma crystals

really exist,

and if they can be considered life,

they could be its most common form.

When massive suns explode,

some collapse into ultra dense cores

called neutron stars.

Hulking masses of atomic nuclei,

crammed together like sardines.

Conditions on the surface are
mind-boggling—

gravity is a hundred billion times

stronger than Earth's.

But beneath their iron nuclei crust

lies something strange:

a hot dense sea

of neutrons and subatomic particles.

Stripped of their electron shells,

these nuclei would obey

entirely different laws of chemistry,

based not on the electromagnetic force,

but the strong nuclear force

which binds nuclei together.

In theory,

these particles could link up to form
larger macronuclei,

which could then combine into even bigger

"super nuclei".

If so,

then this bewildering environment

would mimic the basic conditions for life—

heavy nucleon molecules,

floating in a complex particle ocean.

Some scientists have proposed
the unimaginable:

exotic life forms

drifting through the strange particle sea,

living, evolving, and dying

on incomprehensibly fast time scales.

Life is not something

that has to evolve naturally.

It can be designed.

And once intelligence is introduced

into the evolutionary process,

a Pandora's box is opened.

Free from typical biological limitations,

synthetic and machine-based life

could be the most successful of all.

It could thrive almost anywhere,

including the vacuum of space,

opening up vast frontiers

unavailable to biological organisms.

And compared to the glacial pace
of natural selection,

technological evolution

allows exponentially faster growth,

adaptability, and resilience.

By some estimates,
autonomous self-replicating machines

could colonize an entire galaxy

in as little as a million years.

We can't predict how hyperintelligent life

would organize itself,

but in theory,

there could be convergent evolution
at play.

The electrical properties of silicon

might make it a universal basis

for machine intelligence,

a redemption for its biological
shortcomings.

As the universe ages,

perhaps machine intelligence will come
to dominate,

and naturally occurring biological life

will be viewed as a quaint starting point.

Perhaps we ourselves

will lead this transition,

and the great human experiment

would be merely a first link

in a sprawling intergalactic chain
of life.

Loren Eiseley has said

that one does not meet oneself

until one catches the reflection

from an eye other than human.

One day that eye may be

that of an intelligent alien.

And the sooner we eschew

our narrow view of evolution,

the sooner we can truly explore

our ultimate origins and destinations.

English captions by
vrgtics, ericksoares3, and KillerGhoul