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The search for
planets beyond our solar system

has brought a discovery
of historic significance:

a planet slightly larger than Earth

orbiting a red dwarf star
called Proxima Centauri.

At just over 40 trillion
kilometers from the sun,

Proxima Centauri is our
closest stellar neighbor.

In the coming years, astronomers
will probe its dim light

to understand the tiny
world that circles it.

Does this planet have a climate,

flowing water, even life?

What will it take to get a closer look,

to launch our first interstellar journey

to the pale red dot?

On a clear night, far from city lights,

you can see around 4,500 individual stars.

These bright stars are
spread throughout our region

of the galaxy to distances

many thousands of light
years away from Earth.

134 of them lie within
50 light years of Earth,

within the neighborhood of our sun.

Nestled among those bright stars

are some 2,000 others that are so dim,

you need a high-powered
telescope to see them.

They all sit within the local bubble,

a region probably cleared of
gas by an ancient supernova.

Our little corner of this bubble

is occupied by a handful
of well-known stars.

They include 61 Cygmi, 11.4
light years from the sun,

Epsilon Eridani, 10.5 light years away,

Sirius, the dog star,
at 8.6 light years away,

then there's our nearest
neighbor, the triple star system

Alpha Centauri, 4.4 light years away.

Alpha Centauri A and B are
a pair of sun-like stars.

Proxima Centauri is
gravitationally bound to this pair.

At 4.2 light years away,
this red dwarf star

is the closest star to our sun.

Astronomers have been carefully monitoring

the Alpha Centauri system as
part of an intensive campaign

to find planets in the solar neighborhood.

The study began at the
European Southern Observatory

in the mountains of Chile in 1996,

when astronomers began looking

for a slight change in
the color of nearby stars.

Called a doppler shift,
it's similar to the way

the sound of a train whistle
distorts as it races past us.

The same effect can be seen

in the wavelength of a star's light

if a planet tugs it slightly
toward or away from us.

This wobbling motion
causes the star's light

to stretch towards the
blue part of the spectrum,

then the red.

The instruments that measure this wobble

are so sensitive that they
can detect a star's movement

down to speeds of just
a few meters per second.

The search seemed to bear fruit in 2012,

when astronomers reported
tantalizing hints

of an Earth-sized planet
circling Alpha Centauri B.

Excitement spread.

Unfortunately, the data did not hold up

to further observation.

Still, it fed the hope that we'll one day

stumble upon a world like
Earth in our own backyard

with oceans, a robust
magnetic field to shelter it

from solar winds, a moon
to stabilize its orbit,

and even creatures who
reveal their presence

by beaming electronic signals into space.

In 2016, Alpha Centauri
was back in the news.

Astronomers announced they
had found evidence of a planet

orbiting Proxima Centauri.

They confirmed this discovery with data

from a network of
telescopes around the world.

The newly discovered planet, they said,

is at least 1.3 times the mass of Earth.

It occupies a close orbit,

swinging around its parent
star every 11.2 days.

Surprisingly, that puts it
within the star's habitable zone.

That's the region around a star,

in which temperatures
are in the right range

for liquid water to exist
on the planet's surface.

If the planet is much closer,
the heat from the star

would boil the water away.

If further out, the planet's
water would freeze solid.

In our solar system, Earth sits squarely

within the habitable
zone, which extends out

toward the orbit of Mars
and in toward Venus.

Our star is roughly 10 times
the radius of Proxima Centauri.

Because Proxima has only 2%
the luminosity of the sun,

its habitable zone sits well
inside the orbit of Mercury.

While our sun's light spans
the electromagnetic spectrum,

Proxima's light is concentrated

in the long wavelength infrared portion.

That means that if you
were to visit the planet,

your eyes would see a world
draped in green and red.

There is enough solar
heat striking the planet

to allow liquid water to flow.

However, it's still a matter of debate

whether Proxima B and
other planets like it

could ever support a
climate, much less life.

And yet, small dim stars
like Proxima Centauri

may offer our best chances
of finding a habitable planet

in the solar neighborhood.

The Kepler Space Telescope
was launched in the year 2009

with a major goal of
finding small rocky planets

that could resemble Earth.

Pointed at a patch of sky in the direction

of the Cygnus constellation,

the telescope was used
to measure the brightness

of 150,000 stars every 30 minutes.

Kepler uses what's known
as the transit method.

When a planet passes in front of a star,

it causes the star's
light to dip slightly.

The initial survey picked up planets

that had the most pronounced
effect on the star's light,

large, heavy worlds that whip
around their parent stars

every five to 50 days.

Some are large rocky
worlds called super-Earths.

They most likely formed
within warm dust clouds

left over from the formation
of their parent stars.

Some of these are so close
to their parent stars

that they're most likely
emblazoned with lava.

Others are gas giants that migrated in

from the outer solar systems,

drawn by the gravity of their star.

In the closest orbits, solar
heat and particle winds

are slowly stripping them
of their thick atmospheres.

In a more recent survey,
scientists used Kepler

to focus on 42,000 sun-like stars.

In a statistical study
based on this sample,

they found that 22% of these stars

have planets about the size of Earth,

with an orbit in the right
range to support liquid water.

If this estimate is right, then our galaxy

potentially harbors around
nine billion such planets.

Statistically speaking,
we can expect to find

at least one within 12
light years of Earth.

That doesn't mean it will support life.

Venus sits in the inner
edge of our habitable zone.

It's enshrouded in a thick atmosphere

dominated by a runaway greenhouse effect.

There is no liquid water
or life on its surface.

Mars, at the outer edge,

has lost nearly its entire atmosphere

and today, the planet is
cold, dry, and barren.

To improve their chances
of turning up an Earth 2.0,

scientists are intensely monitoring

a much larger population of dim stars,

like Proxima Centauri, that are nestled

among their brighter cousins.

These so-called M stars, or red dwarfs,

range from 12 to 120th
the mass of our sun

and make up 76% of all
the stars in our galaxy.

Interest in these stars
was spurred, in part,

by studies of the star Gliese
581, 20 light years away,

in the southern constellation of Libra.

The planet hunters of the
European Southern Observatory

first deduced the
presence of Gliese 581 b,

a planet 16 times the mass of Earth.

At a distance of only six
million kilometers from its sun,

it's well inside the habitable zone.

It's flanked by two
others, planet c and e.

Planet d, still unconfirmed,

is at the outer edge
of the habitable zone,

about 33 million kilometers from its sun.

It receives only 30% of the light

that Earth gets from our sun.

It may be an icy world that migrated in

from the outer solar system.

Gliese 581 g, also unconfirmed,

is thought to lie at the inner
edge of the habitable zone.

At twice the mass of Earth,
it might have a chance

of supporting liquid water

but this planet is nothing like Earth.

Our Earth spins on its
axis, 366.25 rotations

for every turn around the sun.

With the exception of polar
areas in opposing seasons,

every part of the Earth
sees the sun every day.

That helps distribute solar heating,

while generating the
winds and ocean currents

that define our global climate.

Earth is far enough from the sun

that the gravity of the star
doesn't exert much influence

on its spinning motion.

In much closer orbits, Gliese
581 g, like its cousin,

Proxima b, will likely have fallen

into what's known as gravitational lock.

That means that only one
side ever faces the sun,

while the other side faces out
into the darkness of space.

The traditional thinking is that

if there is water on this planet,

it has most likely evaporated
from the light side

and become impounded as
ice on the dark side.

That view began to
change in the late 1990s,

when NASA scientists
simulated the atmosphere

of a locked planet.

They found that global wind patterns

could develop and transport
heat to the dark side.

Scientists at the California
Institute of Technology

sought to test this finding on our planet.

Working with a supercomputer program

normally used to predict our climate,

they simply turned off Earth's rotation.

The team placed the sub-solar point,

the region facing the sun,

just off the coast of South America.

Evaporation on the sunny side

sent air rising up in storms

and pushing back over the
poles to the dark side.

As a result of this poleward flow,

Alaska experienced summer all year round.

Land masses on the dark side,

including Europe, Asia,
and Africa, went cold.

Remarkably, over the five
decades of the simulation,

the oceans retained enough
heat to stay clear of ice.

On the light side, large parts
of the Western Hemisphere

dried out, turning to desert.

But if you pitched your
tent at the sub-solar point,

you'd find a hot and rainy climate

that resembles this place,
the island of Kauai in Hawaii.

Its highest point is Mount Waialeale,

that's Hawaiian for rippling
or overflowing water.

Known as the wettest place on Earth,

the summit averages 11
meters of rain per year.

How would life adapt to
sunlight all the time?

Plants depend on light
to drive photosynthesis,

a process that combines CO2 with water

to produce sugars needed for growth.

They use the daynight cycle
to maximize this process,

to tune their growth rates,

and to take in more carbon dioxide.

In polar regions, some
plants are known to thrive

in periods of extended sunlight in summer.

So, if you take away the night,

there is a chance that life could adapt.

On Earth, its ability to do so

draws upon hundreds of
millions of years of evolution

in a diverse global biosphere,
dependable daynight cycles,

and moderate temperatures.

The question is whether a climate

could develop in the first
place on a locked planet.

A team from the University of Chicago

sought to answer this question

by simulating the different
states such a planet could take.

The most promising scenario,

assuming abundant water
and carbon dioxide,

is a planetary type
called an eyeball Earth.

This planet is gravitationally locked

with an icy dark side
and a sunny light side.

In between is a transition zone,

where the sun is always low in the sky.

Here, the ice gradually fades

into a warmer, sun-filled realm.

In this representation,
the sub-solar point

is flanked by a continent
and areas of open ocean.

What could tip the balance
toward habitability,

according to one recent study,

is the planet's ability
to maintain a cloud cover.

If warmth from the light
side flows to the dark side,

it can moderate temperatures

and allow ice to melt
along the transition zone.

As water flows back to the light side,

it evaporates, fueling more clouds.

Clouds further moderate temperatures

by reflecting solar
radiation back to space,

while also contributing
to a greenhouse effect

that holds heat in.

But even if Proxima b has all
the right characteristics,

it would still face formidable
obstacles to nurturing life.

Proxima Centauri, for example,

is known as an active M5 star

that currently emits intense
flares every 10 to 30 hours.

For a planet in such a close
orbit, blasts of radiation

can split water molecules
in the atmosphere.

That allows hydrogen to waft into space

and oxygen to bind to
rocks on the surface.

This process can strip
the planet of water.

To find out what state Proxima b

and other nearby planets are
in, astronomers will turn

to a series of powerful new telescopes

coming online over the next decade.

The long awaited James
Webb Space Telescope,

slated for launch in 2018,
represents the next generation

of great space observatories.

With a segmented primary mirror

that is almost three times
larger than that of Hubble,

the James Webb will capture
the trickle of photons

coming from the early universe

and allow scientists

to finally parse the
light of nearby stars.

The WFIRST Telescope is slated
to launch in the mid-2020s.

With the resolution of Hubble,
WFIRST will take in light

from a region 100 times larger.

Its main targets include
distant exploding stars

used to measure the
expansion of the universe,

along with planets interacting with stars

in nearby solar systems.

WFIRST will bring a powerful new technique

to planet hunting.

If a star passes in front of another star,

it acts as a lens, brightening and dimming

in ways that can betray
the presence of planets.

These new instruments
promise an unprecedented view

into the light of nearby stars.

By deploying coronagraphs

to block the light of parent stars,

they'll be able to pick up
light reflected by planets.

This technique will allow scientists

to measure surface temperatures,
as well as distinguish

between a planet's light and dark sides.

They can also use a spectrograph
to detect atmospheres

and to look for chemical
signatures of life.

There will be no shortage of targets.

The Belgian Trappist Telescope

turned up a surprising cache of planets

circling what may seem an unlikely place.

At 40 light years away, TRAPPIST-1

is a cross between a large
Jupiter-like planet and a star.

This ultra cool dwarf
emits only a tiny fraction

of the light of our sun.

Astronomers use the transit method,

using their telescope
to detect slight dips

in the light of the star
as planets passed in front.

Then, they followed up
with an intensive program

of observations from
telescopes on the ground

and in space, including NASA's
Spitzer Infrared Observatory.

They found seven planets bunched together

in orbits close enough
to be tidally locked.

At least three of them
are at the right distance

and temperature for liquid water.

Planet F is the most
likely to harbor life.

It's 1.3 times the mass of Earth

with an orbit lasting 9.21 Earth days.

Gracing its salmon-colored skies

are at least six sister planets,

all as large or larger than our full moon.

Here, you get 200 times less
visible light than on Earth.

Instead, the star bathes the
planet in infrared light.

It delivers enough energy to
potentially power a climate.

A broad initiative has begun to follow up

on the flood of discoveries

in this golden age of planet finding.

Ground-based telescopes
are now being outfitted

with a new generation of imaging devices.

At the European Very Large
Telescope in Chile, for example,

scientists are installing

a new infrared imager inspectograph

specifically with
Proxima Centauri in mind.

This effort is being done in collaboration

with a whole new player
in the space sciences.

Breakthrough Initiatives
is a privately funded,

10 year, 100 million dollar plan

to search for
extraterrestrial intelligence.

It hopes to lay the foundation
for spacecraft missions

out into our solar neighborhood.

Any interstellar mission must confront

the ultimate obstacles
to space exploration,

distance and time.

The New Horizons Probe recently made

the 4.8 billion kilometer
journey to Pluto,

arriving in 9.5 years.

At its top speed, 84,000
kilometers per hour,

it would take 54,400 years

to travel the 40 trillion
kilometers to Proxima Centauri.

The spacecraft, Voyager
II, would take even longer,

70,000 years, traveling at
69,000 kilometers per hour.

In a sense, though, Voyager paves the way

for future interstellar treks.

Voyager's designers imagined
these twin spacecraft

would far outlast human civilization.

With their current momentum,
they could wander the galaxy

for millions, even tens
of millions of years.

What if intelligent beings

detected and captured one of them?

Its handlers famously placed
a trove of information aboard

to educate these alien captors.

That includes information
about Planet Earth,

its people and culture,

including images and sound recordings.

It also offers a way to find us

in case anyone decides to follow up.

This symbol is a system of
celestial GPS coordinates.

It places Earth at the
junction of radio beacons

emitted in this part of
the galaxy by pulsars.

They are a type of ultra-dense star

that spins rapidly, giving
off pulses of radio energy.

Like sailors who once used a sexton

to navigate by the stars,
a future spacecraft

could use pulsars to orient itself

in the featureless expanse
of interstellar space

and navigate to a distant target.

NASA engineers are now designing
a prototype space sexton

to be tested aboard the
International Space Station.

As the station orbits the Earth,

the instrument locks on to
the light of distant pulsars.

In 1990, as it sped
toward the outer regions

of our solar system,
the Voyager I spacecraft

turned its camera back toward Earth.

From six billion kilometers
away, it sent back an image

of the pale blue dot we call Earth.

It was a humbling reminder of
the rare and precious world

that has nurtured us.

Today, we're looking out
beyond our solar system

to a planetary destination
illuminated by the dim light

of its parent star, Proxima Centauri

but can we realistically get anywhere near

this pale red dot over
four light years away?

Interstellar flight has long been a staple

of science fiction.

Humans from technologically
advanced future civilizations

routinely board starships for
remote corners of the galaxy.

For some real-world scientists,

the challenge of crossing
the immense voids of time

and space has led to whole
new lines of investigation.

Propulsion engineers
have submitted proposals

for the development of
advanced spacecraft,

powered by nuclear fusion, anti-matter,

the gravity of tiny,
artificial black holes,

the strange physics of warp drive,

and specially designed sails

to be pushed along by solar winds,

or by giant lasers.

This is a surprisingly old idea.

The 17th century
astronomer, Johannes Kepler,

observed comet tails being blown

by what he thought must be a solar breeze.

He served up an imaginary
scene to his friend, Galileo,

of ships powered by sails
adapted to the heavenly breezes.

In 1873, James Clerk Maxwell showed

that light exerts pressure,

an idea that motivated
modern space sail designs.

NASA engineers were able to
put this idea to the test

during the Mariner 10
mission to Mercury in 1974.

They used the pressure of
sunlight on its solar panels

to stabilize its flight around the planet.

Modern mission designers
have warmed to sails

because they don't require
enormous amounts of fuel.

Recent advances in the
miniaturization of electronics

and strong, light, new materials

are giving the idea a new currency.

And now, the discovery of
planets in our solar neighborhood

offers a range of potential
interstellar targets.

A new plan announced by
Breakthrough Initiatives,

called Operation Starshot, will use sails

to target the closest of
these targets, Alpha Centauri.

The breakthrough plan calls
for an array of powerful lasers

installed on a high plain
or desert in South America.

In these dry, high altitude locations,

the laser light is less
likely to be distorted

by moisture in the atmosphere.

The lasers would draw power

from a large, dedicated generating plant.

The idea is to deploy a
fleet of ultra-thin sails

in Earth orbit, ready for launch.

Each one is outfitted with
a miniature command center,

two centimeters wide and
weighing only a gram.

This tiny chip contains sensors,

shielding against cosmic radiation,

and a radioactive power supply.

Add to that mini-thrusters,

a communication system, and a camera.

This light sail is relatively
small, four meters wide,

and perhaps only a few
hundred atoms thick.

It would have to be tough enough

to withstand the impact
of a high-powered laser.

The launch array locks on to its target

on a line of sight to Alpha Centauri.

Ignition,

and liftoff.

The next day, with the charge restored,

the array is ready to fire again.

The spacecraft quickly reaches
20% the speed of light,

passing Saturn within hours

and Pluto in just a day or two.

For most of the journey,

the spacecraft go dormant
to preserve power.

Periodically, the lead craft wakes

to perform guidance maneuvers

or to send information back
to the next spacecraft in line

for relay back to Earth.

Finally, in two decades time,

the fleet approaches its
target, Proxima Centauri.

The craft may be able to
use the wind of particles

coming from the star to slow down.

As each flies by, they turn
to face the planet, Proxima b.

The camera captures the scene.

Perhaps as it turns around,

we'll see our sun, a brilliant white dot

framed by the galactic landscape.

As for the pale red dot and
its planetary companion,

what will we learn from
the morsels of information

that make it back to Earth?

With luck, we'll find that
Proxima b is having its day

in the sun, with the
development of a global climate

that perhaps has nurtured life.

For some on our own planet,

the news that a spacecraft
has arrived at an alien world

will no doubt pass overhead

in the ongoing daily rush of technology,

crisis, war,

consuming, and escape.

For others, that day will be filled

with a sense of humility, fascination,

and wonder.