(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