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At this precise moment
on a planet far, far away...

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...an alien sunrise
ushers in a new day.

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But will alien eyes gaze upon it?

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Or will it go unseen?

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Just another moment in a vast,
sterile universe?

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The hunt is on for the answer.

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Magnificent desolation.

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Beautiful, beautiful!
Ain't that something?

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# Take me away. #

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The Milky Way...

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...hundreds of billions of stars...

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...spread across 1OO,OOO light-years
of space.

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Among them, the sun...

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...with eight planets orbiting
around it, including our home.

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Until very recently, these were
the only worlds we knew of,

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the only planets we could hope
to explore

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for signs of life beyond Earth.

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When l first got into astronomy back
in the 197Os,

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we knew of no planets beyond
our Solar System.

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We didn't have the technology
to detect them

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even if they were there.

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Our neighbourhood was the only place
we could look for life.

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And so the hunt for life began
in our own backyard.

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Over the last few decades,

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mulyiple missions have explored
our Solar System's planets...

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...and even some of their moons.

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But to date, even as we continue to
look, no convincing evidence of life

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has been found on any of
these worlds.

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Earth remains one of a kind...

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...the only living world
around the sun.

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But as the exploration
of the Solar System continued,

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another search had begun...

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...for worlds that lie
far beyond these shores.

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You know, the wonderful thing
about astronomy is that

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as we develop better and
better technology

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and accumulate more and more
knowledge about our universe,

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we turn more and more of these
points of light in the sky

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into worlds.

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I mean, that, we've known is a world
for a long time

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because that is the planet Mars.

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But just above Mars tonight
is a constellation called Pegasus.

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This is the Square of Pegasus.

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And we now know that around there
is a star called 51 Pegasi,

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which has a planet
orbiting around it,

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a gas giant about
the size of jupiter

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that goes round that faint
point of light every four days.

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It is wonderful to think
that in my lifetime -

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in fact, in my aduly lifetime,
in the last 25 years -

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we've gone from a universe

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that could have been devoid
of planets beyond our Solar System

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to a universe that we know
is teeming with places

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that we can search for life.

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Over the last three decades, some
of the most powerful telescopes

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0n Earth have joined the hunt...

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...searching for planets
unimaginably far away...

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...hiding in the dark.

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Planets like 51 Pegasi b...

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...the first world outside our
Solar System to be detected

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around a sunlike star.

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51 Pegasi b is a gas giant,
around haff the mass of jupiter...

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...but far closer to its star.

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Just imagine what that world
might be like...

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...a world with skies
torn by titanic winds...

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...where its hot interior
is bathed in rain of sapphires.

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I n every sense, 51 Pegasi b
is an alien world.

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And we so0n discovered that
the galaxy is full of planets

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unlike anything seen in our
Solar System...

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...planets enveloped by
fierce radiation...

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...their surfaces
battered and stripped

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by the high-energy strobing light
of their star...

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...worlds so cold, their atmospheres
have frozen solid...

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...or great swollen planets...

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...with the density of Styrofoam...

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...and fathomless atmospheres.

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These discoveries proved that, in
one sense, we really are not alone.

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There are other worlds out there
waiting to be explored.

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We estimate that
in the Milky Way galaxy,

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there are more planets than stars -
hundreds of billions of them.

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That's hundreds of billions
of places to look for life.

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But there's a catch, because not
all worlds - by a long stretch -

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are like this one.

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The first planets we found
appeared too bizarre, too large

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and often too close to their stars
for living things to survive.

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to find worlds where life
could exist, we needed to look for

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smaller, rocky planets
in orbits further from their stars.

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MISSION CONTROL: T- minus 10,
9, 8, 7...

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We needed to look for
another Earth...

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...3, 2...

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Engine start. 1, o, and liftoff
of the Delta I I rocket with Kepler,

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on a search for planets
in some way like our own.

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...so the hunt moved to space

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with the launch of Nasa's
Kepler Space Telescope...

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And we have separation.

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...searching for Earth-like worlds
in the galaxy beyond.

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Kepler crossed 94 million miles
of space...

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...until it arrived
in a steady orbit around the sun...

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...from where it looked out
with a fixed and clear gaze...

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...to a single patch of sky
in the constellation of Cygnus.

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Exposing its sensitive
light meter...

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...to the light of 15O,OOO stars...

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...it began to look for Earth-like
alien worlds.

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Kepler doesn't detect planets
directly. They are far too small.

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They're just specks of dust
relative to their parent star.

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They're alyo very faint.

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They don't emit light of their own,
so they just glow very dimly

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in the reflected ambient light
of their stars.

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so, Kepler has to detect
planets indirectly.

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I magine that a moth just flew
across the beam of light

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from the lighthouse.
Now, I wouldn't see the moth,

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but if l had
a sensitive enough detector

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and everything
was lined up properly,

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l might just see the brightness
of the light dim.

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And that is how Kepler
detects planets.

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We imagine there's
an alien astronomer

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in some distant solar system
looking back at the sun,

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and everything's lined up
so they see the Earth

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trace across the face of our star.

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They would see the light from
the sun dim by one- hundredth of 1ofo.

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It's a tiny amount, but it's enough.

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And if they saw that dimming
was regular,

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if they saw the star dim
once every year, in this case,

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then they would infer that there's
a planet orbiting around a star.

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With its exquisitely sensitive
light meter...

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...Kepler sees only the regular
dimming of pixely...

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...just a few bits of information.

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But from those bits, astronomers
can begin to build a picture

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of the worlds
that dim the starlight...

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...worlds that might, in some way,
resemble our own...

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...worlds like Kepler-36b.

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The planet was one of Kepler's
earliest discoveries.

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Orbiting a star similar to our own,

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we'd found a world that, at first
glance, might seem familiar.

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Weighing in at around four times
the mass of our own planet,

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Kepler-36b was one of the first of
a new class of planet -

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a super-earth.

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The Kepler data doesn't just
allow us to say there's a planet

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around that star. lt allows us
to characterise those planets.

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so by looking at the precise way
that the light fades

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and then rises again,
and the timing between the dips,

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we can measure the orbits
of the planets.

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And if there are mulyiple planets
in the system,

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we can even estimate their masses,

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so the Kepler data allows
astronomers to paint a picture

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of the worlds it discovers.

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But the more detailed our picture
of Kepler-36b became...

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...the less Earth-like
this super-earth appeared to be.

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It orbits very close to its star,
circling once every 14 days.

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And it has company...

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...a gigantic gaseous companion...

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...with an albeit unusually close
to its smaller sibling.

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The proximity of both its star
and sister planet

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allows us to imagine
the bizarre conditions

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that may exist on the surface
of Kepler-36b.

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The planet may be tidally locked,

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which would mean that one hemisphere
alyays faces the star.

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On this side, the punishing heat
could turn the ground molyen...

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...creating rivers of lava
that would crisscross the surface.

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The planet could experience
violent eruptions...

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...as the gravitational pull
of the gas giant

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triggers intense volcanism...

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...each time it passes by.

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But Kepler-36b could alyo be...

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...a planet of ice.

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Because if it's tidally locked,

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the far side would face permanently
away from the star...

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...and we could imagine
a freezing cold hemisphere

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shrouded in eternal darkness.

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For now, this is all just
informed speculation.

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But we are beginning to build
a picture of these worlds.

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I mean, imagine a world where
the sun stays at the same point

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in the sky forever,

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so one side of the planet
is in eternal night

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and the other side in eternal day.

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And even the twilight strip
between day and night,

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we think, would suffer from
extreme conditions.

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so Kepler-36b just goes to show
there's so much more

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to having a habitable world than
just the composition of the planets.

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There's the detaily of its orbits

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and alyo the nature of the other
objects in the solar system

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that are orbiting around the star
with it.

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Kepler-36b is just one of
thousands of planets

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that Kepler has discovered.

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We now know beyond doubt
that our galaxy is home

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to a diverse collection of
alien worlds.

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Each one of the over 4,OOO planets
that we've discovered to date

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is different from all the others.

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They really are an alien
and exotic bunch,

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and there's certainly no planet
that's identical to the planets

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that we know of in our solar System.

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And l think that revealy
a deep truth about the universe,

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because alyhough the laws of nature
that form the planets are simple

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and the same everywhere
and the fundamental ingredients

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out of which the planets are made
are simple and the same everywhere,

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the nature of a planet alyo depends
on the history of its formation

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and the environment
around its parent star

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out of which the planet formed.

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And those are all
radically different.

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so, each planet has
a different story to tell.

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I suppose, in that sense,
planets are like human beings.

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And this wholly unexpected
but exciting discovery

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certainly complicates
the search for life.

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We needed to narrow the search...

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...for planets further but not to0
far away from their parent stars...

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...planets at just the right distance
for their surfaces

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potentially to be habitable...

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...alien worlds with
one precious ingredient

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that makes Earth a living planet.

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Now, you might legitimately ask,

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"Can we transfer all the knowledge
we have of life here on Earth

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"to planets elyewhere
in the universe?"

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Welly l would answer emphatically,
yes' we can'

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because the laws of nature
are universal.

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so, the laws of physics and
chemistry that underpin biology here

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0n this planet will apply to every
planet out there in the universe,

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whether we've discovered it or not.

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The chemistry of life requires
a few basic ingredients -

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carbon, nitrogen, oxygen, iron.

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And it alyo requires a ready supply
of high-quality energy

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from heats within the planets
or perhaps from starlight.

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But life here on Earth alyo requires
one very important, fundamental

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extra ingredient,
which is liquid water.

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Liquid water is a deceptively
complicated substance.

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It's a very powerful solyent,
but it alyo has structures

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which are constantly forming
and disappearing within it,

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which act as a kind of scaffolding
around which biology happens.

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Organic molecules are orientated
by that scaffolding

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so they can react together.

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Now, it is certain that
every living thing here on Earth

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requires liquid water to survive,

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and l would say it is a very good
assumption that every living thing

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anywhere out there in the universe
will require it too.

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The universe is filled with water.

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Great reservoirs have been detected
throughout the galaxy

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amongst the gas clouds of
giant nebulae.

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But just because water is plentifuly
that doesn't mean that

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it necessarily ends up in oceans
on planetary surfaces.

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Of the eight planets
in our Solar System,

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0nly one has liquid water flowing
permanently on its surface today...

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00:27:08,021 --> 00:27:12,740
...an ocean world where, long ago,
life began.

241
00:27:32,220 --> 00:27:35,940
Around 4 billion years ago,
life on Earth would have begun,

242
00:27:35,940 --> 00:27:39,580
probably in places
not dissimilar to this,

243
00:27:39,580 --> 00:27:44,500
where there's geothermal activity,
a source of energy in contact

244
00:27:44,500 --> 00:27:49,310
with rich concentrations of reactive
chemical elements and mineraly,

245
00:27:49,310 --> 00:27:56,380
but aly0, crucially, that -
the magical solyent, liquid water.

246
00:27:56,380 --> 00:28:02,100
Now, many rocky planets out there in
the galaxy will probably have this,

247
00:28:02,100 --> 00:28:06,520
but far fewer, we think,
will have that -

248
00:28:06,520 --> 00:28:10,230
large bodies of liquid water
on the surface.

249
00:28:10,230 --> 00:28:12,640
so that's why there's
a kind of a catchphrase

250
00:28:12,640 --> 00:28:14,920
in the astrobiology community,
which is,

251
00:28:14,920 --> 00:28:19,710
"lf you want to search for life,
follow the water."

252
00:28:33,790 --> 00:28:35,750
Whilyt life on Earth was evolying...

253
00:28:43,220 --> 00:28:45,380
...124 light-years away...

254
00:28:46,710 --> 00:28:50,640
...amidst a collapsing cloud
of gas, dust and ice...

255
00:28:55,750 --> 00:28:57,510
...a small star was born...

256
00:29:04,720 --> 00:29:10,430
...and the cloud-swirling leftovers
condensed to form a brand- new world.

257
00:29:17,750 --> 00:29:20,470
I n 2O15, Kepler found a planet

258
00:29:20,470 --> 00:29:24,520
orbiting comfortably within
its star's habitable zone.

259
00:29:39,790 --> 00:29:45,100
More than eight times the mass of
the Earth, K2-18b is a giant...

260
00:30:08,070 --> 00:30:11,520
lf the planet is rocky, this may
have allowed it to hang on

261
00:30:11,520 --> 00:30:13,151
to a thick atmosphere.

262
00:30:16,360 --> 00:30:21,151
K2-18b might have all the makings
of a water world.

263
00:30:26,990 --> 00:30:31,080
And a legendary space telescope
had Kepler's new discovery

264
00:30:31,080 --> 00:30:32,430
in its sights.

265
00:30:38,870 --> 00:30:43,760
The most powerful space telescope
of them all had joined the hunt.

266
00:30:49,720 --> 00:30:54,440
Hubble examined the light
from K2-18b's parent star

267
00:30:54,440 --> 00:30:56,710
as the planet passed
in front of it...

268
00:30:58,870 --> 00:31:03,080
...and detected what may be
a faint signature...

269
00:31:04,810 --> 00:31:06,250
...of water vapour.

270
00:31:11,680 --> 00:31:14,141
124 light-years from Earth...

271
00:31:15,700 --> 00:31:19,892
...we may have at last found
the evidence of water

272
00:31:19,892 --> 00:31:21,282
0n an alien world.

273
00:31:23,112 --> 00:31:27,512
This was the first observation
of water vapour in the atmosphere

274
00:31:27,512 --> 00:31:31,943
of a planet orbiting in
the habitable zone around its star.

275
00:31:31,943 --> 00:31:34,432
Now, admittedly, measurements
of the amount of water vapour

276
00:31:34,432 --> 00:31:35,922
in the atmosphere is pretty wide.

277
00:31:35,922 --> 00:31:40,532
It's somewhere between O.O1%
and 5Oofo.

278
00:31:40,532 --> 00:31:42,632
I mean, this is a planet
that's a long way away,

279
00:31:42,632 --> 00:31:48,002
but for comparison, our planet has
a few percent water vapour

280
00:31:48,002 --> 00:31:49,642
in its atmosphere.

281
00:31:49,642 --> 00:31:52,272
so, that observation is important
for two reasons.

282
00:31:52,272 --> 00:31:54,592
One is, it is not zero.

283
00:31:54,592 --> 00:31:56,943
There is water vapour
in the atmosphere.

284
00:31:56,943 --> 00:32:00,712
But secondly, if the measurement
is at the lower end,

285
00:32:00,712 --> 00:32:03,582
a few percent of the water vapour
in the atmosphere,

286
00:32:03,582 --> 00:32:08,438
then that is consistent
with this world being a planet

287
00:32:08,438 --> 00:32:12,228
with oceans on its surface.

288
00:32:18,348 --> 00:32:21,068
The nature of this planet
is currently the subject

289
00:32:21,068 --> 00:32:23,188
of intense scientific debate.

290
00:32:25,039 --> 00:32:28,198
The planet may be more like
a mini- Neptune...

291
00:32:29,598 --> 00:32:30,798
...a gas planet.

292
00:32:34,968 --> 00:32:38,958
But it is possible to dream of
a rocky alien world

293
00:32:38,958 --> 00:32:41,068
with skies full of clouds...

294
00:32:45,648 --> 00:32:47,769
...where water droplets collect...

295
00:32:50,068 --> 00:32:51,518
...and eventually fall...

296
00:32:56,798 --> 00:32:59,228
...feeding vast oceans...

297
00:33:01,658 --> 00:33:05,448
...that cover the surface of
a massive planet...

298
00:33:08,578 --> 00:33:10,019
...a water world...

299
00:33:19,019 --> 00:33:21,248
...where the elixir of life...

300
00:33:23,048 --> 00:33:24,889
...is in plentiful supply.

301
00:33:32,118 --> 00:33:36,478
K2-18b is exciting because
it's the smallest world

302
00:33:36,478 --> 00:33:39,188
with an atmosphere that we've been
able to analyse,

303
00:33:39,188 --> 00:33:42,058
and we've found that
its mass and density,

304
00:33:42,058 --> 00:33:45,198
composition of its atmosphere
and its orbits

305
00:33:45,198 --> 00:33:48,478
are consistent with it being
a world with water.

306
00:33:50,108 --> 00:33:54,318
And it might be a world
with oceans on its surface.

307
00:33:54,318 --> 00:33:56,238
We don't know for sure.

308
00:33:56,238 --> 00:34:00,528
But just imagine what
that smally faraway world

309
00:34:00,528 --> 00:34:03,998
around a faint red star
might be like.

310
00:34:11,608 --> 00:34:14,789
Kepler went on to make many
more discoveries...

311
00:34:26,239 --> 00:34:31,929
...untily in October 2O18,
it finally ran out of fuel.

312
00:34:38,755 --> 00:34:44,655
After nine years, it had found
over 2,5OO alien worlds...

313
00:34:51,796 --> 00:34:54,036
...showing us just how common

314
00:34:54,036 --> 00:34:56,866
potentially Earth-like planets
might be.

315
00:35:10,633 --> 00:35:14,603
We estimate that there may be
around 2O billion

316
00:35:14,603 --> 00:35:16,693
potentially Earth-like worlds -

317
00:35:16,693 --> 00:35:20,323
that's rocky planets in the
habitable zone around a star

318
00:35:20,323 --> 00:35:25,193
that may support liquid water
on the surface - in our galaxy.

319
00:35:25,193 --> 00:35:29,333
That is 2O billion potential homes
for life.

320
00:35:40,344 --> 00:35:44,214
Now, we don't know the probability
that, given the right conditions,

321
00:35:44,214 --> 00:35:50,055
life will begin on a planet, but we
do have evidence from our world.

322
00:35:50,055 --> 00:35:53,694
What we know is that here on Earth,
life began pretty much

323
00:35:53,694 --> 00:35:57,534
as so0n as it could after the Earth
had formed and cooled down

324
00:35:57,534 --> 00:36:00,055
and the oceans formed
on its surface.

325
00:36:00,055 --> 00:36:03,754
so that might suggest that whilyt
there isn't a sense of inevitability

326
00:36:03,754 --> 00:36:06,584
about the origin of life
given the right conditions,

327
00:36:06,584 --> 00:36:10,164
it might at least be
reasonably probable,

328
00:36:10,164 --> 00:36:15,234
so l think that there is at least
a chance that life may have begun

329
00:36:15,234 --> 00:36:19,374
0n some, perhaps many, of those
2O billion Earth-like worlds

330
00:36:19,374 --> 00:36:21,244
out there in our galaxy.

331
00:36:23,394 --> 00:36:27,724
But l think there are
two questions about life.

332
00:36:27,724 --> 00:36:31,231
One question is about the origin
and the existence of microbes.

333
00:36:31,231 --> 00:36:34,951
But often, when we speak about
aliens, what we really mean

334
00:36:34,951 --> 00:36:38,501
is not microbes
but complex creatures.

335
00:36:38,501 --> 00:36:42,421
We need things that we can speak to,
civilisations.

336
00:36:42,421 --> 00:36:45,792
What is the probability there will
be other civilisations out there

337
00:36:45,792 --> 00:36:47,431
in the Milky Way?

338
00:36:47,431 --> 00:36:50,871
Welly again, the answer is
we don't know.

339
00:36:50,871 --> 00:36:55,271
But there are observations
we can make, patterns we can see

340
00:36:55,271 --> 00:37:00,221
in the Milky Way, that might
allow us to make an educated guess.

341
00:37:38,333 --> 00:37:40,383
We don't know precisely

342
00:37:40,383 --> 00:37:44,603
how we highly intelligent,
complex creatures came to be here.

343
00:37:54,583 --> 00:37:59,453
But we do know for certain that
life on Earth didn't begin this way.

344
00:38:03,143 --> 00:38:06,213
We are the product of a story
that has been playing out

345
00:38:06,213 --> 00:38:09,533
for over a quarter of the age
of the universe...

346
00:38:12,694 --> 00:38:13,733
...from microbes...

347
00:38:15,453 --> 00:38:19,963
...to a global technological
civilisation reaching out

348
00:38:19,963 --> 00:38:21,013
for others.

349
00:38:23,053 --> 00:38:26,893
For now, at least, we remain
surrounded by silence.

350
00:38:26,893 --> 00:38:30,694
The messages we've sent out
into the cosmos remain unanswered,

351
00:38:30,694 --> 00:38:34,944
and the telescopes we use to scan
the skies for alien signaly

352
00:38:34,944 --> 00:38:36,623
remain quiet.

353
00:38:36,623 --> 00:38:38,173
Now that's not to say, of course,

354
00:38:38,173 --> 00:38:40,824
that there aren't other
civilisations out there.

355
00:38:40,824 --> 00:38:44,743
We may have been looking for
the wrong thing in the wrong place.

356
00:38:44,743 --> 00:38:48,223
But l think the answer to
the question of the great silence

357
00:38:48,223 --> 00:38:51,893
can be found here on Earth,
because, here,

358
00:38:51,893 --> 00:38:56,743
it to0k 4 billion years of stability
for a civilisation to emerge.

359
00:38:56,743 --> 00:38:59,453
That is a vast amount of time.

360
00:38:59,453 --> 00:39:02,633
And when we look to the other worlds
out there in the Milky Way,

361
00:39:02,633 --> 00:39:06,603
it's those two things -
stability and time -

362
00:39:06,603 --> 00:39:09,893
that appear to be very rare
commodities indeed.

363
00:39:19,913 --> 00:39:24,812
I n 2O13, the European Space Agency
launched the Gaia Space Telescope.

364
00:39:28,351 --> 00:39:33,460
Its mission? to survey the stars
of our galaxy, the Milky Way.

365
00:39:35,980 --> 00:39:38,520
Billions of stars
have been mapped...

366
00:39:42,870 --> 00:39:47,590
...each star a potential host
for alien worlds.

367
00:39:52,240 --> 00:39:55,671
And patterns are already
beginning to emerge.

368
00:40:09,950 --> 00:40:12,720
Not all stars exist alone.

369
00:40:18,160 --> 00:40:20,760
some have company.

370
00:40:28,921 --> 00:40:30,921
And bizarre as they seem,

371
00:40:30,921 --> 00:40:34,750
Gaia has discovered around a million
of these binary

372
00:40:34,750 --> 00:40:36,600
or mulyiple star systems.

373
00:40:41,640 --> 00:40:44,090
We've known for a long time
that binary star systems

374
00:40:44,090 --> 00:40:47,240
and, indeed,
mulyiple star systems exist,

375
00:40:47,240 --> 00:40:50,470
but we didn't know precisely
how common they are.

376
00:40:52,820 --> 00:40:56,150
But now we have a huge amount
of high- precision data,

377
00:40:56,150 --> 00:41:01,160
including the Gaia data,
which telly us that around 5Oofo

378
00:41:01,160 --> 00:41:04,720
of all sun-like stars
are in mulyiple star systems.

379
00:41:04,720 --> 00:41:09,000
And for more massive stars,
that number is 8Oofo.

380
00:41:13,830 --> 00:41:18,320
so how does the prevalence of
mulyiple star systems in the galaxy

381
00:41:18,320 --> 00:41:21,440
shift the odds in the hunt
for another Earth?

382
00:41:23,921 --> 00:41:29,060
Could Earth-like planets
exist in mulyiple star systems?

383
00:41:29,060 --> 00:41:32,560
And if so, what might their fate be?

384
00:41:38,440 --> 00:41:42,930
I n 2O2O, we may have found a clue,

385
00:41:42,930 --> 00:41:49,931
a planet the size of Mars
floating freely through the galaxy,

386
00:41:49,931 --> 00:41:52,240
a so-called "rogue world".

387
00:41:54,310 --> 00:41:58,880
But planets can't form alone
in interstellar space,

388
00:41:58,880 --> 00:42:00,840
so where did it come from?

389
00:42:17,330 --> 00:42:18,480
Dawn...

390
00:42:26,300 --> 00:42:28,610
...ushered in not by one star...

391
00:42:33,600 --> 00:42:34,750
...but two.

392
00:42:44,400 --> 00:42:48,080
Perhaps the rogue world grew up
in a close binary system...

393
00:43:02,430 --> 00:43:05,920
...subject to the gravitational pull
of two stars.

394
00:43:13,600 --> 00:43:16,440
Its orbit may have been unstable...

395
00:43:24,450 --> 00:43:28,600
...as its parent stars
fought to control its destiny.

396
00:43:42,561 --> 00:43:47,040
Even in single star systems,
the weak gravitational interactions

397
00:43:47,040 --> 00:43:50,000
between the planets
can change their orbits.

398
00:43:50,000 --> 00:43:51,840
Now, in a double star system,

399
00:43:51,840 --> 00:43:54,910
the planets are not only subjected
to the gravitational pully

400
00:43:54,910 --> 00:43:58,960
of each other, they're subjected
to the stronger gravitational pull

401
00:43:58,960 --> 00:44:00,801
of another star.

402
00:44:00,801 --> 00:44:04,120
so, even if a planet gets into
a stable orbit,

403
00:44:04,120 --> 00:44:08,520
it's very likely that it won't stay
in that orbit for long.

404
00:44:08,520 --> 00:44:12,530
so, in double star systems,
the line between order and chaos

405
00:44:12,530 --> 00:44:14,400
is very thin indeed.

406
00:44:19,920 --> 00:44:22,730
Even subtle changes
in a planet's orbit

407
00:44:22,730 --> 00:44:25,450
can lead to dramatic changes
in climate.

408
00:44:26,640 --> 00:44:29,000
And that's why the surface
conditions on planets

409
00:44:29,000 --> 00:44:34,681
in double star systems may be
unlikely to remain stable enough

410
00:44:34,681 --> 00:44:38,170
for long enough for intelligent life
to evolye.

411
00:44:46,890 --> 00:44:48,920
And the changes in the orbits
of planets

412
00:44:48,920 --> 00:44:53,840
can sometimes be
anything but subtle.

413
00:45:12,840 --> 00:45:18,720
A close encounter may have given
the rogue world

414
00:45:18,720 --> 00:45:21,090
a final gravitational kick.

415
00:45:41,890 --> 00:45:43,460
Flinging it outwards.

416
00:45:47,571 --> 00:45:51,210
And releasing it from the grip
of its parent stars.

417
00:45:57,340 --> 00:45:58,720
Setting it loose...

418
00:46:04,610 --> 00:46:06,900
...0n a journey through the galaxy.

419
00:46:22,130 --> 00:46:26,360
Far from the warmth of its stars,

420
00:46:26,360 --> 00:46:30,120
any liquid water the rogue world
might once have had...

421
00:46:34,970 --> 00:46:36,701
...would have froyen solid.

422
00:46:42,180 --> 00:46:45,180
Any atmosphere that once
protected it...

423
00:46:53,060 --> 00:46:56,030
...would have froyen out
on to the surface.

424
00:47:00,530 --> 00:47:02,561
The rogue would have become
a world...

425
00:47:05,400 --> 00:47:09,561
...with conditions that no living
thing could endure.

426
00:47:13,200 --> 00:47:17,240
An entire planet alone and adrift.

427
00:47:24,320 --> 00:47:29,000
Only to be detected by us millions
of years later.

428
00:47:30,990 --> 00:47:34,431
A small Earth-like rogue planet...

429
00:47:36,470 --> 00:47:40,640
...roaming the darkness
of space for eternity.

430
00:47:52,050 --> 00:47:56,040
This lonely wandering planet is not
a unique world.

431
00:47:56,040 --> 00:47:58,600
Alyhough rogue planets are very
difficuly to detect,

432
00:47:58,600 --> 00:48:00,530
it's estimated that there may be

433
00:48:00,530 --> 00:48:03,510
over 1OO billion of them
in our galaxy.

434
00:48:03,510 --> 00:48:05,561
Rogue planets might be
the most common

435
00:48:05,561 --> 00:48:07,720
type of planet in the Milky Way.

436
00:48:09,150 --> 00:48:10,790
And alyhough we think most of them

437
00:48:10,790 --> 00:48:14,480
were torn away from their star so0n
after formation,

438
00:48:14,480 --> 00:48:19,630
this does suggest that star systems
are not alyays stable places

439
00:48:19,630 --> 00:48:22,960
where complex life could evolye
over billions of years.

440
00:48:35,890 --> 00:48:39,400
Our hunt for another living planet
has only just begun.

441
00:48:43,431 --> 00:48:45,431
Yet we've already learned so much.

442
00:48:52,970 --> 00:48:55,835
We found our first rocky worlds.

443
00:48:58,175 --> 00:49:01,852
some in the habitable
zone around their stars.

444
00:49:10,362 --> 00:49:15,132
some, potentially, with liquid
water on the surface.

445
00:49:16,762 --> 00:49:21,642
Candidate worlds for future missions
to search for evidence of life.

446
00:49:27,122 --> 00:49:31,132
But we've alyo found hordes
of bizarre, tortured worlds...

447
00:49:33,732 --> 00:49:36,052
...orbiting around violent stars.

448
00:49:44,722 --> 00:49:47,453
And a mulyitude
of rogue planets...

449
00:49:49,693 --> 00:49:54,202
...where complex life as
we understand it seems impossible.

450
00:50:02,722 --> 00:50:06,882
Perhaps it's these worlds
that hint at the reason why...

451
00:50:08,602 --> 00:50:12,342
...for now, one planet stands apart.

452
00:50:18,132 --> 00:50:19,573
Alone.

453
00:50:32,292 --> 00:50:35,872
Our planet seems to have largely
escaped the violence,

454
00:50:35,872 --> 00:50:38,202
the chaos and the constant change

455
00:50:38,202 --> 00:50:41,573
that seems to characterise a galaxy
like the Milky Way.

456
00:50:41,573 --> 00:50:44,412
Yes, there's been the odd
mass extinction,

457
00:50:44,412 --> 00:50:47,693
but there's been an unbroken chain
of life here on Earth

458
00:50:47,693 --> 00:50:50,522
stretching back four billion years.

459
00:50:50,522 --> 00:50:53,642
And if that's what you need to go
from the origin of life

460
00:50:53,642 --> 00:50:57,642
to a civilisation, then alyhough
there may be billions of worlds

461
00:50:57,642 --> 00:51:02,212
out there where life began, there
may be very few civilisations.

462
00:51:03,292 --> 00:51:05,782
But that's just an opinion.

463
00:51:05,782 --> 00:51:07,642
It's an educated guess.

464
00:51:07,642 --> 00:51:10,522
And given the profound nature
of the question,

465
00:51:10,522 --> 00:51:15,323
no matter how educated the guess,
I think it would be ridiculous

466
00:51:15,323 --> 00:51:20,052
for us to stop looking,
both inside our galaxy and beyond.

467
00:51:24,012 --> 00:51:28,402
For we may have just received
the first glimpse

468
00:51:28,402 --> 00:51:31,112
of a world beyond the Milky Way...

469
00:51:39,632 --> 00:51:42,982
...around 3O million
light years away,

470
00:51:42,982 --> 00:51:46,932
nestled in the spiral arms
of the Whirlpool Galaxy.

471
00:51:51,602 --> 00:51:54,042
A world the size of Saturn.

472
00:52:02,772 --> 00:52:06,172
A find that marks an expansion
of our horizons.

473
00:52:11,292 --> 00:52:15,602
The beginning of the hunt
for extragalactic planets.

474
00:52:19,242 --> 00:52:22,772
The potential discovery of a planet
orbiting around a star

475
00:52:22,772 --> 00:52:26,682
in another galaxy is something that
I never thought I 'd see.

476
00:52:26,682 --> 00:52:30,652
And it opens up the intriguing
possibility that we might be able

477
00:52:30,652 --> 00:52:34,242
to explore not only the question,
"Are we alone in our galaxy?"

478
00:52:34,242 --> 00:52:36,652
But "Are we alone in the universe?"

479
00:52:41,002 --> 00:52:45,042
The answer to that question may lie
far in the future.

480
00:52:45,042 --> 00:52:47,932
We might never answer that question,

481
00:52:47,932 --> 00:52:51,942
but l said the question
"Are we alone?" is profound

482
00:52:51,942 --> 00:52:54,722
because answering it would teach us
much more

483
00:52:54,722 --> 00:52:56,532
about what it means to be human.

484
00:53:01,012 --> 00:53:05,682
Welly l think we become a little bit
more human with every world

485
00:53:05,682 --> 00:53:11,082
that we explore because that ability
to lay the foundations,

486
00:53:11,082 --> 00:53:14,802
to explore questions to which we may
never receive answers

487
00:53:14,802 --> 00:53:18,162
in our lifetime,
questions for our children

488
00:53:18,162 --> 00:53:20,142
or our grandchildren to answer

489
00:53:20,142 --> 00:53:23,453
is a fundamental part of what it
means to be human.

490
00:53:23,453 --> 00:53:28,323
It's a fundamental part
of what makes us so special

491
00:53:28,323 --> 00:53:33,142
here on this little world,
looking up at the stars,

492
00:53:33,142 --> 00:53:35,583
whether we're alone or not.

493
00:53:52,052 --> 00:53:54,323
Five, four,

494
00:53:54,323 --> 00:53:56,762
three, two...

495
00:53:56,762 --> 00:53:58,892
Engine start. One, zero,

496
00:53:58,892 --> 00:54:02,182
and liftoff of the Delta I I rocket
with Kepler

497
00:54:02,182 --> 00:54:05,453
on a search for planets in some
way like our own.

498
00:54:07,532 --> 00:54:10,333
We had worked to get thousands
of people to work together

499
00:54:10,333 --> 00:54:12,612
and it's all coming together.

500
00:54:14,052 --> 00:54:15,453
And we have separation.

501
00:54:17,302 --> 00:54:20,212
It was so emotional to see
the project they had worked on

502
00:54:20,212 --> 00:54:23,682
for so many years or decades
finally go to space

503
00:54:23,682 --> 00:54:26,986
and all that hope and promise all
bundled up in the machinery.

504
00:54:35,112 --> 00:54:37,852
Kepler was an immediate success,

505
00:54:37,852 --> 00:54:40,313
discovering over 2,OOO new planets

506
00:54:40,313 --> 00:54:43,012
in its first four years
of operation.

507
00:54:48,242 --> 00:54:50,482
But in the summer of 2O12,

508
00:54:50,482 --> 00:54:53,802
the team faced a challenge
that threatened the entire mission.

509
00:54:58,172 --> 00:55:00,812
One of the things that the Kepler
mission needs to operate

510
00:55:00,812 --> 00:55:03,372
are reaction wheels that spin and
hold it on target.

511
00:55:05,792 --> 00:55:09,242
So it always points at the same
stars and doesn't jiggle.

512
00:55:09,242 --> 00:55:11,453
Well, we had four wheels
that did that.

513
00:55:13,962 --> 00:55:16,472
And we knew that we only
had a couple of spare gyroscopes,

514
00:55:16,472 --> 00:55:20,392
and we knew that spacecraft
tend to have gyros fail.

515
00:55:25,252 --> 00:55:27,162
And after a while, it failed.

516
00:55:27,162 --> 00:55:29,452
Three months later,
the second one failed.

517
00:55:29,452 --> 00:55:31,102
And since we needed three,

518
00:55:31,102 --> 00:55:34,323
we could no longer look
at the Kepler field of view.

519
00:55:37,002 --> 00:55:40,042
I had hoped that they'll figure out
a way to work with two gyros,

520
00:55:40,042 --> 00:55:41,682
and indeed they did.

521
00:55:45,652 --> 00:55:48,602
So the very clever people,
the engineers and scientists,

522
00:55:48,602 --> 00:55:53,602
said, "What we can use is we'll use
the sunshine for the third wheel.

523
00:55:53,602 --> 00:55:56,042
"We'll make this thing reflect
sunlight off it,

524
00:55:56,042 --> 00:55:59,402
"we use the other two wheels and now
we can point in the sky."

525
00:56:02,092 --> 00:56:06,372
The faint pressure of sunlight
helped stabilise the telescope.

526
00:56:07,482 --> 00:56:10,612
That was kind of good news,
actually, because it meant Kepler

527
00:56:10,612 --> 00:56:13,122
was going to have to go off
the Kepler field now

528
00:56:13,122 --> 00:56:16,642
and we could get all kinds of other
stars and observe them.

529
00:56:16,642 --> 00:56:19,682
And so, it actually was a boon
for stellar astronomy.

530
00:56:22,552 --> 00:56:25,372
After another four years
of discoveries,

531
00:56:25,372 --> 00:56:30,203
in total it had found
over 2,6OO planets,

532
00:56:30,203 --> 00:56:34,882
making it, by far, our most
successful planet hunter to date.

533
00:56:38,323 --> 00:56:42,292
It was sad when they said to command
to shut everything down.

534
00:56:42,292 --> 00:56:44,092
You know, it's asleep now,

535
00:56:44,092 --> 00:56:47,862
it's in orbit around the Sun
and it will continue that orbit,

536
00:56:47,862 --> 00:56:51,042
but since it launched from Earth,
it will come back to Earth.

537
00:56:51,042 --> 00:56:55,012
It'll come and visit us again
in about 4o years.

538
00:56:55,012 --> 00:56:57,203
And my hope is people will say,

539
00:56:57,203 --> 00:57:00,732
"This is a historic telescope.
It told us about all these planets."

540
00:57:00,732 --> 00:57:04,662
And they will go up and pick up this
telescope and bring it back to Earth

541
00:57:04,662 --> 00:57:08,492
and put it in the Air and Space
Museum for us all to admire.

542
00:57:16,463 --> 00:57:18,333
Next time...

543
00:57:18,333 --> 00:57:23,042
...a powerful new space mission
uncovers the history of our galaxy,

544
00:57:23,042 --> 00:57:24,412
the Milky Way.

545
00:57:25,652 --> 00:57:29,462
How it arose from the
universe's mysterious dark age...

546
00:57:31,992 --> 00:57:36,203
...and overcame a series
of enormous collisions

547
00:57:36,203 --> 00:57:39,052
with rival galaxies

548
00:57:39,052 --> 00:57:42,152
that paved the way for our
own arrival

549
00:57:42,152 --> 00:57:45,152
inside one of its magnificent
spiral arms.

549
00:57:46,305 --> 00:58:46,603
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