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- [Narrator] Our galaxy is made up of some 400 billion stars

and at least 100 billion planets.

How many are like Earth?

With an atmosphere,

complex geology,

flowing water,

and abundant life.

Astronomers are seeking answers

in countless photons racing past the Earth

and in whole new theories about how planets form and evolve.

What they are finding

has only sharpened one of the oldest debates in science,

is biology a powerful and inevitable byproduct

of cosmic evolution

or is life rare in a galaxy shaped by violence and mayhem?

For a millennia, we have locked our gaze on the heavens

hoping for a sign that life exists somewhere beyond Earth.

In modern times, the search for life-bearing planets

has become a central quest in a new age of astronomy.

Giant observatories

equipped with a new generation of precision instruments

now allow us to detect planets

in solar systems across the galaxy.

And there's a whole new way of seeing

that has vastly expanded the search.

Earth in the 21st century.

Computers increasingly shaped the way we communicate,

learn,

travel,

grow food,

buy and sell,

make things.

The amount of processing power we can put on a computer chip

doubles every two years.

This trend called Moore's law

has turbocharge the pace of scientific discovery.

Vast centralized supercomputing facilities

have sprung up to search for patterns, hidden signals

in the torrent of data now streaming in from telescopes.

Astronomers once consumed with taking photographs

and making measurements

in cold observatories across countless lonely nights

now write computer programs, analyze data,

and build models

that simulate the complexities of star and planet formation.

Far from settling the question, are we alone,

their discoveries have reignited

one of the oldest disputes in science.

To the Greek philosopher, Aristotle,

a perfect Planet Earth

sat firmly at the center of a small, orderly universe.

The universe beyond was utterly unpopulated,

at least by any creatures other than gods.

Not everyone agreed.

Epicurus of Samos and his followers

imagined an infinity of inhabited worlds among the stars.

An ever evolving cosmos

gave rise to a boundless diversity of creatures.

So too were the modern descendants of Epicurus,

our galaxy is filled with life-bearing worlds

and potentially alien civilizations,

we just have to find them.

Astronomers have already determined

that stars like our Sun are not uncommon,

that planets the size of Earth,

at the right distance from their parents stars

to support liquid water

could number in the billions.

Yet a very different view has also taken hold,

reminiscent of Aristotle.

Biology is so complex, requires such stable conditions,

and so much time that Earth may well be an anomaly,

a rare oasis of life in a desolate universe.

Who is right?

Back in the early 1960s, the astronomer Frank Drake,

a modern day epicurean,

theorized that signs of alien intelligence

may well be streaming by us in the form of radio signals.

Employing a radio telescope at Green Bank, West Virginia,

he targeted to nearby stars, Epsilon Eridani,

an orange star just 10.5 light years away,

and Tau Ceti, a yellow star

about three quarters the size and mass of our Sun.

Since Drake began his work,

the Search for Extraterrestrial Intelligence, SETI,

has expanded to hundreds of astronomers,

thousands of stars, and a wide range of radio frequencies.

Searchers have checked and rechecked

Epsilon Eridani, Tau Ceti, and many other nearby stars.

So far, after six decades of searching,

they found no repeating messages or anomalous signals.

But they haven't given up

because there are so many more places

and different new ways to search.

Even as Drake looked for signals,

other observers

began looking for signs of planets themselves.

The Dutch-American astronomer Piet van de Kamp

sought evidence in the light of stars.

After comparing photographic plates

taken over three decades,

he found that several nearby stars seem to wobble

as if tugged upon by unseen massive companions.

In particular, Barnard's Star,

a red dwarf in the neighborhood of our sun

showed enough movement for Van de Kamp to claim

he'd found an orbiting planetary system.

He spotted other nearby wobblers,

the Lalande 2185, 61 Cygni, and Epsilon Eridani,

the third closest visible star to us.

Van de Kamp's findings did not stand up to scrutiny,

an error in the telescope itself

doomed his results to wide rejection.

But his gut feeling that the stars in The Milky Way

post an abundance of planets took hold.

And yet, what astronomers began to see

is not what they expected.

In the winter of 1991,

engineers at the Arecibo radio telescope in Puerto Rico

had noticed structural flaws in the colossal instrument.

They'd ordered it frozen in position.

This limitation gave astronomers,

Alex Wolszczan and Dale Frail,

the time they needed to observe a type of collapse star,

a rapidly spinning pulsar.

Most pulsars spin very fast,

emitting a flash with each revolution.

One of their targets flashed about 6.2 times each second,

but its timing was not predictable.

It seemed to flash faster and slower in a complex pattern

best explained by the gravitational tugging

of two dense rocky planets.

A third, much smaller world would later reveal itself.

Three burned out zombie planets

orbiting the corpse of a dead star.

These are not the only pulsar planets out there,

the oldest known planet called Methuselah

rides above the Milky Way.

At more than two and a half times heavier

than the largest planet in our solar system, Jupiter,

Methuselah lumbers along in 100 year orbit

around a pair of dead stars, a white dwarf, and a pulsar.

These discoveries became part of a wider challenge

to theories about how solar systems form and evolve,

and how planets like Earth come to be.

The reigning idea goes back over two and a half centuries

called the solar nebula model.

This universal model is set in motion

by the explosion of a giant star.

Shock waves from the blast

cause a massive cloud of hydrogen gas to collapse.

This computer simulation

shows the remarkable events that follow.

Huge volumes of dust and gas

began to stream into the center of the cloud.

The tighter the cloud is packed, the more it heats up.

A storm erupts,

a raging hurricane of dust, gas and ice crystals.

In its eye, as temperatures rise, a star ignites.

As matter flows toward it,

the central region becomes a vast rotating disc.

This disc is shaped by fierce hot winds

from the newborn star

which push lighter gases to the outer zones

while leaving dust and rocks to dominate the inner zone.

As the winds die down, planet formation begins.

Beyond a certain distance called the frost line,

planets formed quickly,

growing large as they scoop up gas dust and ice.

Inside the frost line,

dust grains and rocks

coagulate into solid bodies called planetesimals,

most perhaps a kilometer wide, but some much larger.

As they collide or merge,

they form the planets we know today.

In time, eight emerge

to spin peacefully around the sun ever since.

If our solar system was any guide,

astronomers expected to find rocky planets

similar to Earth, Venus and Mars in the sooty inner zones

where iron and silicon would melt and flow together

to make solid planetary course.

Cool guess worlds like Jupiter and Saturn

would orbit farther out in the frosty zone

where ammonia, methane and water

condense into small planets that grow large

by gravitationally sweeping up lighter elements.

Even further out, icy titans like Neptune and Uranus

would crystallize after most of the gaseous hydrogen

had been captured by the older, bigger planets.

It didn't take long

for the data to explode this conventional understanding

as a new age of planet hunting unfolded.

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Looking into the southern constellation of Pictor,

astronomers examined an infant star

and attended an extra large newborn planet.

The star, Beta Pictoris, is twice the size of our sun,

50% hotter, and almost nine times more luminous.

Its dense solar disk

extends some four times beyond the orbit of our Pluto.

Passing stars have been tugging on the disk

producing waves of turbulence.

The planet in an orbit twice Jupiter's distance from the sun

is adding to the chaos.

A NASA simulation shows a top view,

the planet's gravity is hurling a stream of debris

on a spiral path toward the star.

A side view shows pressure waves

created by its slightly tilted orbit.

These waves ramp up collisions

between icy and rocky bodies all through the disk.

New models of solar system formation

would have to take into account

a host of factors at work in places like Beta Pictoris,

the size of the solar nebula,

the type of parents star,

external influences,

and the interaction of planetary bodies.

Astronomers saw this dynamism at work

as their planet discoveries piled up.

Several teams focused on detecting wobbles

in the light of stars caused by orbiting planets.

As the star moves toward us, then away,

it will change color,

shifting from blue to red and back to blue.

This can reveal planets orbiting very close to their stars,

even if they lie very far away from us.

In October 1995,

Swiss astronomers, Michel Mayor and Didier Queloz,

use this radial velocity method

to examine the star, 51 Pegasi, 50 light years from Earth.

51 Pegasi is like our sun,

but 23% wider and 11% more massive.

Wobbling motions in this star's light

implied the presence of a giant planet,

47% less massive than Jupiter,

but one and a half times its size.

Dimidium, as the planet is now called,

defied all expectations.

It sits in an orbit that is eight times closer to its star

than Mercury is to our sun.

This close orbiting gas giant

is hotter than Jupiter by at least 1500 degrees Celsius.

How did it get into such a perilous position?

The planet at 51 Pegasi

was only one in a string of oddball worlds

found patrolling their inner solar systems.

Large rocky planets lined with deep oceans.

Small gas planets.

Worlds covered in ice,

or lined with flowing lava.

With discoveries like these,

it was our solar system that began to seem like an anomaly.

To understand the full range of planet types

and how they formed within larger planetary systems,

astronomers needed to target entire populations of stars

and to discern details on a finer scale.

In 2009, astronomers developed

a new planet hunting strategy,

combining spacecraft, telescopes, and supercomputers.

Would it show just how unique Earth is

or would it tip the scales toward the Epicurean model

of a galaxy filled with inhabited worlds?

Astronomers pointed the remarkable Kepler telescope

at a group of about 500,000 stars

up to 3000 light years away.

Their aim, to detect the slight dimming of a star

when a planet sails across its face.

For the technique to work,

the planet must orbit along the line of sight

between star and telescope.

Using this transit technique,

Kepler astronomers

with the help of citizen scientists across the worldwide web

spotted nearly 2700 new planets.

Kepler's successor,

the Transiting Exoplanet Survey Satellite, TESS,

sweeps across almost all of the sky

looking at the brightest stars near Earth.

But the types of planets TESS is revealing

swings the pendulum back toward Aristotle.

Transit surveys like these

tend to find big planets orbiting close into their stars.

These missions,

working in concert with the Hubble Space Telescope

and ground based observatories,

have turned up still more hot Jupiters similar to 51 Pegasi.

Some facing powerful solar winds

are rapidly losing their atmospheres.

Based on the Kepler data,

scientists estimate Jupiter-sized planets and larger

make up just under a fifth of all planets.

The most prevalent type,

at just over 50% of all known planets, is Neptune-sized,

with rock or ice cores,

surrounded by atmospheres of water, ammonia and methane gas.

Some theories suggest that deep inside,

bolder-sized carbon diamonds

rain downward toward their rocky cores.

The ones detected so far sit close to their suns.

They may be former Jupiters that lost mass

or gas giants that were flung inward by larger cousins.

But in the data,

at a point smaller than about two Earth radii,

this class of mini Neptunes

gives way to dense rocky worlds that dwarf our planet.

These super Earths

make up 18% of exoplanets discovered so far,

among them, Kepler-22b,

is a super Earth

revolving in the habitable zone of its star.

This planet may be a waterworld with a deep global ocean

clinging to a massive rocky core.

Orbiting close to the star 55 Cancri

is a rocky planet

almost eight times more massive than Earth.

Its surface is a scorching 2000 degrees Celsius.

Kepler-452b has an orbit size close to that of Earth

and a parent star similar to the sun,

but the planet is some five times larger than Earth

so it's likely a cloudy world more like Venus.

Then there are the Earth-sized planets,

about 11% of the total.

Earth-size does not make them life bearing.

Considered these planets discovered by the TESS satellite.

HD 21749, 53 light years from Earth,

is an orange K-type star 70% the size of our sun.

It's planet C, maintains such a close orbit,

that its year is only eight Earth days long.

Its surface is rocky,

likely hovering around 400 degrees Celsius.

Neighboring planet B is a hot mini-Neptune

23 times the mass of Earth.

Within a deep layer of clouds,

temperatures average around 150 degrees Celsius.

This planet, 1.3 times the mass of Earth,

closely orbits the star LHS 3844.

With a year lasting only 11 Earth days,

its surface would be lined with pools of lava.

Finally, the sun-like star, Pi Mensae,

boasts a planet twice the size of Earth.

Its surface temperature is over 900 degrees Celsius.

Still, as observational

and computational techniques advance,

there are glimmers of hope for finding Earth-like planets.

Red dwarfs are the galaxy's most plentiful star type.

They are hard to see, lightweight, cool,

and much fainter than the sun.

Their planets orbit close in,

their years rushing by in just a few days.

Among them, Proxima Centauri, the closest star to our sun.

With barely 12% of the Sun's mass, it's only 2% as bright.

After extensively studying the star for more than 18 years,

astronomers have confirmed at least one Earth-sized planet,

Proxima Centauri b

completes an orbit in a little more than 11 days.

It revolves at the distance where water could be liquid

just like Earth.

Like many red dwarfs,

Proxima Centauri has a stormy temperament,

large flares blasting with deadly

ultraviolet, X ray and gamma radiation

likely eroding away any atmospheres its planets may have.

Meet Proxima Centauri b,

part of a breed of so called eyeball Earths.

These planets orbit so close to their parents stars

that they become gravitationally locked

with one side facing perpetually in

and the other out.

One side is always arid and scorched by solar flares,

the other in shrouded in thick ice.

Can a watery ring with fertile land and life forms

exist in the twilight zone between them?

Here is another candidate.

TRAPPIST-1 is an ultra cool dwarf star

that hosts at least seven planets,

all bunched into close orbits.

All are made of rock,

but with densities that suggest an abundance of water.

At just 11% the diameter of our sun,

this star puts out just 5/100 the sun's light.

Fortunately, there is another source of heat.

Like tussling siblings,

the planets jostle one another's orbits,

tidal friction causes them to heat up from within.

Those closest to the star are likely steamy and hot,

those farthest are icebound.

The fourth planet out, slightly smaller than Earth,

has the potential for liquid water.

You probably won't find an advanced civilization here.

In time, astronomers' using space telescopes

or ground-based instruments

will widen their discoveries

to include more planets farther from their stars.

What they have found to date

has already transformed theories

of how our solar system formed,

while dramatically altering the odds

that we will find other life-bearing planets.

Soon after our star ignites in this new view,

Jupiter forms in the icy gas-rich region

beyond the frost line.

It quickly cuts off the flow of matter

to the inner solar system,

slowing the formation of rocky worlds.

Scientists have used supercomputers

to project Jupiter's path

amid countless small objects

that fill the early solar system.

The rings show the orbits of myriad would be planets,

flashes marked collisions between them.

The blue ring tracks the orbit of Jupiter.

As it grows,

Jupiter increasingly responds

to the gravitational pull of the growing sun,

as well as the pushing and pulling forces from Saturn.

Its orbit begins to shrink.

Named for the turn a sailboat takes into the wind,

scientists called Jupiter's inward move the grand tack.

With the rising giant does not swallow,

it flings out of the solar system or pushes toward the sun.

Chaos reigns as one smooth orbits

turn into a disordered swarm.

The simulation

shows Jupiter reaching the orbit of present day Mars.

At this time, Saturn begins to pull it back out.

Jupiter's grand tack

would have dramatically reduced

the mass of the inner solar system.

If a super Earth had formed,

it was likely obliterated or driven into the sun.

From then on, the planets that formed or survived

were bound to be relatively small.

Ironically, Jupiter's destructive foray

may have sowed the seeds of life

by ferrying in large numbers of icy bodies.

Destructive impacts continued,

possibly bringing an end

to the life bearing chances of Mars.

Earth was not out of the woods.

Scientists theorize that early in its life,

our planet was hit by a wayward Mars-sized world

called Theia.

Instead of destroying the young Earth,

the angle of impact created a splash of debris

that would coalesce into the moon.

Over time, the moon's gravity

acted to slow Earth's orbit and steady its spin.

Tidal effects from the moon's gravity churned the seas,

helping to create conditions for life to emerge.

Jupiter continued to serve as a benefactor.

Working in tandem with Saturn,

the gas giant shoved Uranus and Neptune

off to the edge of interstellar space,

that disrupted the orbits of millions of comments,

flinging them toward the sun.

These ice projectiles

are thought to have delivered massive amounts of water

to our fledgling world.

Ironically, one of the very first planetary systems

to be examined by both Piet van de Kamp and Frank Drake

turns out to be arranged remarkably similar to our own.

Epsilon Eridani has a cloud of icy debris

leftover from the system's formation

in about the same place as our Oort cloud.

A second disk of rocky bodies,

the equivalent to our Kuiper belt,

and a gas giant planet

at the outer edge of a ring of asteroids

like our Jupiter guarding our main belt.

Astronomers don't yet know,

but there could be a world like ours.

The search for other Earths

has long focused on the habitable zones of stars

where Earth-sized planets might settle into stable orbits.

There, they'd have time to develop atmospheres,

ocean currents, volcanism,

and other factors that helped spawn life here.

But the tangled history of our solar system

shows that making a life-bearing planet

is a matter of rare circumstance and considerable luck.

Accordingly, some astronomers

are seeking to redefine the search,

focusing instead on finding the specific conditions

that could support life.

The most habitable planet, they say,

may not be exactly like Earth.

In a class all its own,

a super habitable planet

could be about one and a half times the size of Earth.

It orbits an orange colored K-dwarf star,

cooler and longer lived than the sun.

Set in the habitable zone of its star,

the planet is lined with abundant water,

a molten interior spins off a robust magnetic field.

Together with a thick atmosphere,

it deflects cosmic rays and solar radiation

that pose a danger to life forms.

Geological activity produces a patchwork of dry landforms

kept low by erosion.

With stable conditions over long periods of time,

the chances are good that chemistry and energy

can come together to spawn life

and generate a habitable climate.

From the thousands of extrasolar planets discovered so far,

scientists have distilled

a list of 24 Super habitable planets

with these general characteristics.

How will we know if there is life there

or on any other candidate?

Future space telescopes will be equipped with instruments

to search for oxygen, methane, and other markers of biology

as starlight filters through their filters.

Giant new ground-based observatories

may take actual pictures of planets,

opening a window into their rotation rates,

climate, seasons, oceans, even the presence of vegetation.

In the absence of evidence like this,

a skeptical follower of Aristotle

may conclude that our planet remains wholly unique.

But the new Epicureans with ever more powerful technologies

hold to a different vision.

They believe the sheer number of planets in our galaxy

means we must not be alone.

If our world developed intelligent life forms

curious about their place in the universe,

then so too did another.

Even now, its inhabitants may be examining images

of a yellow G-type main sequence star

somewhere in the Milky Way.

They see a tiny, pale blue dot

tucked into a remarkably circular orbit.

Will they wonder if they have found that other Earth,

perhaps with intelligent life

able to communicate across the voids of space,

or will they wonder after poring over their data

and running planetary formation models,

is it big enough or special enough to support life?

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