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♪
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narrator: Earth.
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[ dog barking ]
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the cradle of humanity
throughout our existence.
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♪
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but it won't be forever.
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All it would take would be
one giant meteorite
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to wipe us off
the face of the earth.
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♪
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narrator:
It's not just meteors.
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Our planet will change.
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Our planet could freeze over
or it could heat up.
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Narrator: And our sun will
eventually die.
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We are actually near the end
of habitability of earth.
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To survive in this universe,
we need an insurance policy...
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♪
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...To colonize other worlds.
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Having multiple planets
which are colonized
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is really in our interest
for our own survival.
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Narrator: So can we find
a new home in the galaxy?
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Can we find earth 2.0?
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♪
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-- captions by vitac --
www.Vitac.Com
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captions paid for by
discovery communications
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♪
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for centuries, we only knew
of the handful of planets
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in our own solar system.
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♪
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now astronomers are finding
thousands of new worlds
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around alien stars --
exoplanets.
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We are discovering exoplanets
by the bucketful.
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Plait: There are as many planets
out there as there are stars,
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and there are hundreds of
billions of stars in the galaxy.
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Narrator: But out of billions
of exoplanets,
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are there any that could offer
new opportunities
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for humanity to thrive
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and provide sanctuary
in a dangerous universe?
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Is there an earth 2.0?
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Is earth 2.0 out there?
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That would be truly amazing.
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♪
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narrator: August 2016.
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Astronomers announce
that earth 2.0
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could be closer
than anyone ever expected...
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...A planet orbiting the sun's
nearest stellar neighbor,
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the red dwarf proxima centauri.
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So it turns out that our nearest
star neighbor has an exoplanet.
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It's only about
4 light-years away,
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so that means that it's actually
potentially possible for us
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to get there and to explore it.
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Narrator: Scientists named
the planet after the star,
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proxima centauri b,
or proxima b for short...
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...A world that appears to be
a lot like earth.
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From the way it's tugging
on the star, proxima centauri,
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we know that it has
1.3 times the earth's mass.
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It's roughly the same size
as the earth.
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♪
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narrator: Of the exoplanets
we know about,
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most are uninhabitable
gas giants, like jupiter.
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Proxima b is a rare find,
an earth-sized planet,
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but an earth-sized planet
might not be earthlike.
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A true second earth must also be
the right distance
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from its star.
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The habitable zone,
or some people call it
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the goldilocks region,
is a distance away from the star
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where you're not so close
where you're going to burn up
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and evaporate
all of your liquids,
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and you're not so far away
where you're frigid and cold.
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So it's that special region
where it's just, just right.
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Narrator: Does proxima b
lie in this region?
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Could it have liquid water?
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Proxima centauri b orbits
its star once every 11.2 days,
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so compare that to the earth,
which goes around the sun
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once every 365 days.
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That's because the planet
is much, much closer to the star
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than the earth is to the sun.
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♪
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narrator: Earth orbits
93 million miles from the sun.
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Proxima b orbits
20 times closer,
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under 5 million miles
from its star.
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You might think
that proxima b should be,
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you know, really a fried world,
a burnt-out husk, if you will.
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Narrator: But proxima b's sun is
very different than ours.
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At just over 5,000 degrees
fahrenheit,
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it's half as hot
and roughly 8 times smaller,
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an m-class red dwarf star.
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An m-dwarf that proxima b
is around is much less bright,
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much less hot,
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so you can orbit
much closer to that star
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and be at the same temperature
that we are here on earth.
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♪
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narrator: Proxima b's
tight orbit around the red dwarf
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could make
the planet habitable,
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but it would be
very different from earth.
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The star dominates the sky,
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lighting any oceans
and mountains
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with an alien red glow.
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So proxima b may be
the earth 2.0
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that we've been looking for.
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Narrator: But in 2017,
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the dim red dwarf star
erupts in a way
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that's unlike anything
we've seen before...
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...Blasting the planet
with radiation...
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A megaflare.
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They're like solar flares,
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but they can be
much more powerful.
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In fact, they can outshine
the star itself.
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Narrator: Our sun releases
powerful solar flares
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when its magnetic field
becomes tangled.
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But the megaflare is
10 times stronger
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than our sun's
strongest flares.
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On an m dwarf star,
that magnetic field
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can get a lot more tangled
than on our own sun.
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That means that when a flare
happens,
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it can release
a lot more energy.
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♪
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narrator: Scientists believe
that megaflares like this
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are planet killers.
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♪
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radiation tears
the atmosphere from the planet,
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and these megaflares hit proxima
b roughly once every year.
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Red dwarf stars
are incredibly temperamental.
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They are not good parents
to their planets,
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so if proxima b did have
an atmosphere at one point,
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it would've been stripped away
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by one of these
violent outbursts.
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Narrator: Leaving proxima b
dangerously exposed to space.
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An atmosphere dampens
the temperature gradients
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between light and shadow,
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so in sunlight,
it is just burning hot,
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but right next door in a shadow,
it is freezing cold.
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Narrator:
Without an atmosphere,
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proxima b would
be a barren wasteland,
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blasted by intense radiation
from its star --
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completely uninhabitable.
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Proxima b, our perhaps best shot
at finding earth 2.0 so far,
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is actually
a dried out husk of a world
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that has lost its atmosphere,
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maybe lost any water
that it also harbored,
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simply by being that close
to its parent star.
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♪
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narrator: Proxima b may be
the nearest exoplanet,
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but it's not the only option.
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The future of humanity may lie
in an incredible star system
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just 40 light-years away.
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We've just found
a really exciting system
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where there's not just one
chance to have a new earth,
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but seven.
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♪
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[ explosion ]
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♪
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narrator:
In an unforgiving galaxy,
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finding earth 2.0 could be
the difference
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between extinction and survival.
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It's a pretty wild place
out there.
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Our planet is not going
to be here forever,
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and it would be wonderful
if we could find a place like it
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so that we could live.
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♪
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narrator: The future of humanity
lies on an alien exoplanet.
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The question is, where?
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♪
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2016.
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Astronomers scan the skies
with the new
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transiting planets and
planetesimals small telescope,
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or trappist.
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They look for the flickering
of a star
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caused by the silhouette
of a planet.
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The planet can pass in front
of the disk of its star
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once per orbit,
causing a little mini eclipse,
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a little dimming temporarily
in the light of the star.
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Narrator:
Scientists spot the dimming
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of a nearby red dwarf star
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just 40 light-years from earth,
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the first alien system
detected by the telescope,
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the incredible
trappist-1 system.
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♪
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the trappist-1 discovery
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was a really great
bang for our buck in a sense...
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[ laughs ] because we found
seven exoplanets all at once.
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Narrator: But are
any of these seven planets
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actually habitable?
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With the worlds
of the trappist system,
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there's probably
a range of climates.
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The innermost ones
are probably very hot.
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You might even be looking
at lava worlds.
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Farther away,
they're probably worlds of ice.
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Narrator: But the middle
planets -- d, e, and f --
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are all prime candidates.
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It's exciting to think that
three of the planets
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orbiting trappist-1
are in the "habitable zone,"
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are at the right distance
from that star
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to have liquid water
on their surface.
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♪
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and one planet stands out
as a new earth,
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orbiting just 2.7 million miles
from the star --
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trappist-1e.
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The composition of trappist-1e
suggests that it could have
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a pretty significant iron core,
kind of like the earth does.
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There's a potential there for
a very powerful magnetic field.
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00:12:46,399 --> 00:12:48,399
Like earth, trappist-1e
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00:12:48,401 --> 00:12:51,402
could host
a protective magnetic field,
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deflecting the harsh solar winds
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00:12:54,074 --> 00:12:57,542
and powerful outbursts
that strip away atmospheres.
205
00:12:59,813 --> 00:13:01,479
So magnetic field
is a good thing.
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00:13:01,481 --> 00:13:05,383
It's a kind of a protection
from the evil forces of the star
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00:13:05,385 --> 00:13:07,952
that you're orbiting around.
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Narrator:
And unlike proxima centauri,
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the trappist-1 star
appears unusually quiet.
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Trappist-1 is actually
a very old, much calmer star
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and doesn't undergo a lot
of these huge flares
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like proxima centauri does.
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00:13:23,069 --> 00:13:25,837
And so it's a somewhat perhaps
better system to look
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for an earthlike planet,
an earth 2.0.
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00:13:28,642 --> 00:13:33,077
♪
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00:13:33,079 --> 00:13:36,047
narrator: The data suggests
that trappist-1e
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00:13:36,049 --> 00:13:37,782
could have vast oceans,
218
00:13:37,784 --> 00:13:41,653
a protective atmosphere,
and habitable temperatures.
219
00:13:41,655 --> 00:13:51,596
♪
220
00:13:51,598 --> 00:13:56,935
but living here would be nothing
like living on earth.
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00:13:56,937 --> 00:13:59,070
Oluseyi: The thing to keep in
mind about the trappist system
222
00:13:59,072 --> 00:14:01,306
is that it's very unlike
our own.
223
00:14:01,308 --> 00:14:05,443
The planets are much closer in,
so because they're closer in,
224
00:14:05,445 --> 00:14:08,446
their orbits
are faster and smaller.
225
00:14:08,448 --> 00:14:12,750
Narrator: On trappist-1e,
an entire year
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00:14:12,752 --> 00:14:15,286
takes just 6 earth days.
227
00:14:15,288 --> 00:14:17,021
Can you imagine you're just
basically tearing
228
00:14:17,023 --> 00:14:19,657
your calendar days off day after
day after day really quickly?
229
00:14:19,659 --> 00:14:22,727
Your birthday would be today
and then tomorrow.
230
00:14:22,729 --> 00:14:24,162
Happy birthday, again!
231
00:14:24,164 --> 00:14:26,097
Wedding anniversaries,
you're constantly forgetting
232
00:14:26,099 --> 00:14:30,568
your wedding anniversary,
and it would be hard.
233
00:14:30,570 --> 00:14:33,571
And on this strange and alien
world,
234
00:14:33,573 --> 00:14:39,844
explorers would witness sights
unlike anything seen before.
235
00:14:39,846 --> 00:14:43,147
In a lot of ways, it really is
sort of a science fiction sky,
236
00:14:43,149 --> 00:14:45,917
the kind of things
that are envisioned in movies.
237
00:14:45,919 --> 00:14:48,786
You could look up and see
the other planets in your sky
238
00:14:48,788 --> 00:14:51,723
much like how we can see
our own moon.
239
00:14:51,725 --> 00:14:54,459
You could physically resolve
features on the surface
240
00:14:54,461 --> 00:14:58,596
such as continents
with your own eyes.
241
00:14:58,598 --> 00:15:03,468
But could this planet
be too good to be true?
242
00:15:03,470 --> 00:15:07,705
So we could have a potentially
habitable planet
243
00:15:07,707 --> 00:15:09,641
that's really close to its star,
244
00:15:09,643 --> 00:15:12,410
but other issues arise
when you have a solar system
245
00:15:12,412 --> 00:15:14,078
that's that compressed,
246
00:15:14,080 --> 00:15:17,715
and one of those is
the potential for tidal locking.
247
00:15:20,887 --> 00:15:24,188
Orbiting just a few
million miles from the star,
248
00:15:24,190 --> 00:15:30,795
trappist-1e is likely tidally
locked with one side
249
00:15:30,797 --> 00:15:34,666
facing the star forever.
250
00:15:34,668 --> 00:15:38,236
So you could imagine a situation
where, gosh, it's constant day
251
00:15:38,238 --> 00:15:39,904
and it might just
produce something
252
00:15:39,906 --> 00:15:41,739
that's like a scorched earth,
253
00:15:41,741 --> 00:15:43,808
kind of like what we see
behind me,
254
00:15:43,810 --> 00:15:46,544
but on the other side,
it is constant night,
255
00:15:46,546 --> 00:15:48,246
and so in that case,
it might just be, like,
256
00:15:48,248 --> 00:15:51,849
a frozen wasteland.
257
00:15:51,851 --> 00:15:56,854
And trappist-1e's problems
get even more extreme.
258
00:15:56,856 --> 00:16:00,325
If you have a permanent day side
and a permanent night side,
259
00:16:00,327 --> 00:16:03,261
the night side of the planet
is going to get so cold
260
00:16:03,263 --> 00:16:06,764
that everything just freezes
out, including the atmosphere.
261
00:16:10,737 --> 00:16:13,504
The gases of trappist-1e's
atmosphere
262
00:16:13,506 --> 00:16:15,974
could freeze into solid ice
263
00:16:15,976 --> 00:16:19,877
on the frigid night
side of the planet,
264
00:16:19,879 --> 00:16:24,816
and the gases on the day side
burn away.
265
00:16:24,818 --> 00:16:31,055
The atmosphere thins
and eventually disappears,
266
00:16:31,057 --> 00:16:38,162
and trappist-1e ends up
completely inhospitable.
267
00:16:38,164 --> 00:16:41,899
So even though we found maybe
a perfect planet around a star,
268
00:16:41,901 --> 00:16:43,634
the type of star
and where it's orbiting
269
00:16:43,636 --> 00:16:45,470
could have
a really important effect
270
00:16:45,472 --> 00:16:48,106
as to whether or not
that planet might be habitable.
271
00:16:50,944 --> 00:16:53,444
Narrator:
Despite its apparent potential,
272
00:16:53,446 --> 00:16:58,549
our future is not
in the trappist-1 system.
273
00:16:58,551 --> 00:17:03,621
The search for truly
earthlike planets continues.
274
00:17:03,623 --> 00:17:08,126
In the trappist-1 system,
we find a very earthlike world,
275
00:17:08,128 --> 00:17:11,362
but the star its orbiting
is not very sunlike.
276
00:17:11,364 --> 00:17:14,332
So what we should
be looking for, perhaps,
277
00:17:14,334 --> 00:17:17,468
is a earthlike planet
around a sunlike star.
278
00:17:19,572 --> 00:17:25,810
Narrator: To find earth 2.0,
we need a sun 2.0.
279
00:17:25,812 --> 00:17:29,514
♪
280
00:17:35,822 --> 00:17:40,224
[ explosion ]
281
00:17:40,226 --> 00:17:44,962
♪
282
00:17:44,964 --> 00:17:47,365
narrator: The milky way --
283
00:17:47,367 --> 00:17:50,701
home to hundreds of billions
of stars...
284
00:17:53,006 --> 00:17:57,075
...Ranging from dim,
explosive red dwarves...
285
00:17:59,179 --> 00:18:02,046
...To short-lived
blazing giants.
286
00:18:02,048 --> 00:18:08,086
♪
287
00:18:08,088 --> 00:18:13,091
but in the middle are stars
like our sun.
288
00:18:13,093 --> 00:18:16,260
Strictly speaking,
if we really want earth 2.0,
289
00:18:16,262 --> 00:18:20,498
we need to look for planets
around stars like our sun.
290
00:18:20,500 --> 00:18:22,533
Narrator: Stars like our sun
291
00:18:22,535 --> 00:18:25,470
are calm and stable with
long lives...
292
00:18:27,941 --> 00:18:31,175
...And the habitable zone
lies far enough away
293
00:18:31,177 --> 00:18:35,179
that planets avoid
tidal locking.
294
00:18:35,181 --> 00:18:38,783
We stand a much better chance
of colonizing a planet
295
00:18:38,785 --> 00:18:41,853
around a sunlike star.
296
00:18:41,855 --> 00:18:46,157
We're hunting for planets
in the habitable zone of stars
297
00:18:46,159 --> 00:18:50,828
like our own sun,
and we have found worlds there.
298
00:18:50,830 --> 00:18:55,066
♪
299
00:18:55,068 --> 00:19:00,304
narrator:
Worlds like kepler-452b,
300
00:19:00,306 --> 00:19:04,408
an exoplanet
1,800 light-years away,
301
00:19:04,410 --> 00:19:08,446
orbiting the same type of star
as our sun.
302
00:19:10,717 --> 00:19:13,017
You really couldn't ask
for a more earthlike orbit
303
00:19:13,019 --> 00:19:14,352
around this star.
304
00:19:14,354 --> 00:19:18,089
The year is about 385 days.
We're 365 days.
305
00:19:18,091 --> 00:19:21,792
This really is very much
like the earth.
306
00:19:21,794 --> 00:19:25,429
The planet orbits its star
at roughly the same distance
307
00:19:25,431 --> 00:19:28,966
as the earth orbits the sun,
308
00:19:28,968 --> 00:19:31,769
and could be
very much like home.
309
00:19:33,740 --> 00:19:38,609
Kepler-452b is in
the habitable zone of its star,
310
00:19:38,611 --> 00:19:40,745
so if there is
liquid water there,
311
00:19:40,747 --> 00:19:45,483
there could be oceans and lakes
and rivers and streams
312
00:19:45,485 --> 00:19:48,219
and blue skies and cloudy days.
313
00:19:51,124 --> 00:19:54,692
Narrator: Sounds nice,
but kepler-452b
314
00:19:54,694 --> 00:19:56,727
is a lot larger than earth.
315
00:19:59,866 --> 00:20:04,035
Kepler-452b is
a great earth 2.0 candidate,
316
00:20:04,037 --> 00:20:09,207
but it's sort of like
earth on steroids.
317
00:20:09,209 --> 00:20:14,111
This world is about 5 times
more massive than our own planet
318
00:20:14,113 --> 00:20:17,915
and about 60% wider.
319
00:20:17,917 --> 00:20:22,220
Narrator: Scientists call
large worlds like kepler-452b
320
00:20:22,222 --> 00:20:23,621
super-earths.
321
00:20:23,623 --> 00:20:27,692
♪
322
00:20:27,694 --> 00:20:29,160
durda:
These worlds are maybe
323
00:20:29,162 --> 00:20:31,596
1.5 or 2 times
the size of the earth,
324
00:20:31,598 --> 00:20:35,499
with maybe as much
as 10 times the mass.
325
00:20:35,501 --> 00:20:38,603
Narrator: Could this
super-sized earthlike planet
326
00:20:38,605 --> 00:20:40,972
be our second home?
327
00:20:40,974 --> 00:20:43,140
A planet like this seems to meet
328
00:20:43,142 --> 00:20:45,209
a lot of our standards
for an earth 2.0.
329
00:20:45,211 --> 00:20:47,812
It's around a star like our sun.
330
00:20:47,814 --> 00:20:50,915
It's smack-dab in the middle
of the habitable zone.
331
00:20:50,917 --> 00:20:53,417
The problem is there
are other factors at play.
332
00:20:53,419 --> 00:20:56,087
One of those is simply
the mass of the planet.
333
00:20:56,089 --> 00:21:02,627
♪
334
00:21:02,629 --> 00:21:04,762
narrator: Kepler-452b's size
335
00:21:04,764 --> 00:21:09,033
has an extreme effect
on its gravity.
336
00:21:09,035 --> 00:21:12,970
Because of its incredible mass,
the gravity on the surface
337
00:21:12,972 --> 00:21:16,841
is about twice
what we feel here on the earth.
338
00:21:18,911 --> 00:21:20,711
Narrator: That extra gravity
339
00:21:20,713 --> 00:21:24,382
would make colonizing the planet
difficult.
340
00:21:24,384 --> 00:21:25,850
Just about any chore
you could imagine
341
00:21:25,852 --> 00:21:27,852
doing that you don't
like doing on the earth,
342
00:21:27,854 --> 00:21:30,821
you're going to like it
even less on a planet like that.
343
00:21:30,823 --> 00:21:33,324
When the garbage can weighs
twice as much as it does here
344
00:21:33,326 --> 00:21:35,860
on the earth, that's not going
to be very much fun.
345
00:21:35,862 --> 00:21:38,295
Maybe lebron james and I
will be okay,
346
00:21:38,297 --> 00:21:42,033
but normal humans,
I'm not so sure.
347
00:21:43,436 --> 00:21:47,004
Narrator: And we could be stuck
on the planet's surface.
348
00:21:48,608 --> 00:21:51,242
If you landed on the surface
of one of these super-earths,
349
00:21:51,244 --> 00:21:53,678
it'd be pretty easy
to get down onto the surface,
350
00:21:53,680 --> 00:21:57,248
but it would be very difficult
to get back up.
351
00:21:57,250 --> 00:21:59,383
It's already
incredibly difficult
352
00:21:59,385 --> 00:22:00,685
for us to leave the earth.
353
00:22:00,687 --> 00:22:03,621
Think of our giant engines
and rockets,
354
00:22:03,623 --> 00:22:05,656
these incredible miracles
of engineering
355
00:22:05,658 --> 00:22:07,858
that we need to blast off.
356
00:22:07,860 --> 00:22:12,063
You need twice that
to get off of kepler-452b.
357
00:22:12,065 --> 00:22:15,433
[ thunderclap ]
358
00:22:15,435 --> 00:22:19,970
and to make matters worse,
kepler-452b's atmosphere
359
00:22:19,972 --> 00:22:24,075
is thought to be radically
different from earth's.
360
00:22:24,077 --> 00:22:25,943
In some sense,
how big the planet is,
361
00:22:25,945 --> 00:22:30,081
how massive it is will determine
what its atmosphere is like.
362
00:22:30,083 --> 00:22:32,249
If you have a lot of mass
and a lot of gravity,
363
00:22:32,251 --> 00:22:34,285
you can hold onto a lot of air.
364
00:22:34,287 --> 00:22:36,887
You can have
a much larger atmosphere,
365
00:22:36,889 --> 00:22:40,491
much thicker, much denser, and
higher pressure at the surface.
366
00:22:42,662 --> 00:22:47,531
Narrator: The thick atmosphere
could trap heat from the star.
367
00:22:47,533 --> 00:22:51,035
Surface temperatures become
ferociously hot,
368
00:22:51,037 --> 00:22:52,570
and crushing pressures
369
00:22:52,572 --> 00:22:57,074
make the surface
completely uninhabitable.
370
00:22:57,076 --> 00:22:59,844
So it's possible this planet has
a very thick atmosphere
371
00:22:59,846 --> 00:23:02,546
that's become more of
a runaway greenhouse effect.
372
00:23:02,548 --> 00:23:05,116
The planet has gotten hotter
and hotter over time.
373
00:23:05,118 --> 00:23:07,118
Maybe instead of finding
an earth 2.0,
374
00:23:07,120 --> 00:23:09,687
what we've found is a venus 2.0.
375
00:23:09,689 --> 00:23:14,625
♪
376
00:23:14,627 --> 00:23:18,195
narrator: Super-earths may have
an appealing name,
377
00:23:18,197 --> 00:23:22,400
but their intense gravity would
make them difficult to live on,
378
00:23:22,402 --> 00:23:26,036
and we could not survive
in their thick atmospheres.
379
00:23:26,038 --> 00:23:31,976
♪
380
00:23:31,978 --> 00:23:34,945
so far, all the worlds
we've found have turned out
381
00:23:34,947 --> 00:23:38,616
to be uninhabitable,
382
00:23:38,618 --> 00:23:44,422
but what if our new home
is not a planet?
383
00:23:44,424 --> 00:23:48,192
Earth 2.0 may not be
an exoplanet at all.
384
00:23:48,194 --> 00:23:50,494
It might be an exomoon.
385
00:23:50,496 --> 00:23:55,299
♪
386
00:24:01,707 --> 00:24:06,110
[ explosion ]
387
00:24:06,112 --> 00:24:09,880
♪
388
00:24:09,882 --> 00:24:12,583
narrator: We live in
a cosmic shooting range
389
00:24:14,654 --> 00:24:17,154
where planets die every day,
390
00:24:19,725 --> 00:24:25,996
but backup planets like our own
seem almost impossible to find.
391
00:24:25,998 --> 00:24:30,167
Have we been looking for
the wrong thing?
392
00:24:30,169 --> 00:24:32,002
I think there's
a pretty good chance
393
00:24:32,004 --> 00:24:35,539
that earth 2.0 might not be
a planet per se,
394
00:24:35,541 --> 00:24:39,109
but actually a moon
of a giant planet.
395
00:24:39,111 --> 00:24:40,444
The exciting thing
about an exomoon
396
00:24:40,446 --> 00:24:42,947
is that they could
potentially be habitable.
397
00:24:42,949 --> 00:24:44,148
Thaller: So is it possible
398
00:24:44,150 --> 00:24:46,083
that as we look
at different solar systems,
399
00:24:46,085 --> 00:24:50,321
the real analog for earth 2.0
will turn out to be an exomoon?
400
00:24:50,323 --> 00:24:56,727
♪
401
00:24:56,729 --> 00:24:59,697
narrator: 2017.
402
00:24:59,699 --> 00:25:02,766
The kepler telescope
scanned a sunlike star
403
00:25:02,768 --> 00:25:05,236
8,000 light-years away,
404
00:25:05,238 --> 00:25:07,972
and professor david kipping
and his team
405
00:25:07,974 --> 00:25:12,543
watched the transiting exoplanet
406
00:25:12,545 --> 00:25:17,348
kepler-1625b.
407
00:25:17,350 --> 00:25:22,219
Kepler-1625 was one of
the many thousands of planets
408
00:25:22,221 --> 00:25:24,221
discovered by kepler,
409
00:25:24,223 --> 00:25:26,891
but what made it different
from our perspective
410
00:25:26,893 --> 00:25:29,493
as a moon hunter
was that this is a planet
411
00:25:29,495 --> 00:25:32,897
which was jupiter-sized,
far away from its star,
412
00:25:32,899 --> 00:25:35,065
and apparently
on a near-circular orbit,
413
00:25:35,067 --> 00:25:39,003
so everything that we want
for finding exomoons.
414
00:25:39,005 --> 00:25:43,874
♪
415
00:25:43,876 --> 00:25:47,144
narrator:
The exoplanet kepler-1625b
416
00:25:47,146 --> 00:25:50,681
is an uninhabitable gas giant,
like jupiter.
417
00:25:52,785 --> 00:25:56,020
But it is in the habitable zone,
418
00:25:56,022 --> 00:25:59,890
and that means its moons
would be, too.
419
00:26:02,028 --> 00:26:06,230
Unfortunately, these exomoons
are incredibly hard to see.
420
00:26:09,702 --> 00:26:13,337
The way that kepler
finds exoplanets out there
421
00:26:13,339 --> 00:26:16,240
really does relate
to the size of the planet,
422
00:26:16,242 --> 00:26:17,841
and for moons, it's much,
much more difficult
423
00:26:17,843 --> 00:26:20,978
because it's smaller
so it's harder to detect.
424
00:26:20,980 --> 00:26:22,980
Kipping: The largest moon
in the solar system
425
00:26:22,982 --> 00:26:24,748
is ganymede around jupiter.
426
00:26:24,750 --> 00:26:27,351
It's about 40%
the size of the earth,
427
00:26:27,353 --> 00:26:31,255
and we really very rarely
detect planets that small.
428
00:26:31,257 --> 00:26:33,624
So, of course,
looking for exomoons
429
00:26:33,626 --> 00:26:36,560
is going to be very,
very challenging.
430
00:26:36,562 --> 00:26:41,098
♪
431
00:26:41,100 --> 00:26:43,601
narrator:
In 2018, the team recruited
432
00:26:43,603 --> 00:26:47,004
the powerful
hubble space telescope
433
00:26:47,006 --> 00:26:50,941
and used the data to hunt
for the tiny silhouette
434
00:26:50,943 --> 00:26:54,011
of any moons.
435
00:26:54,013 --> 00:26:56,146
If you have an exomoon
orbiting a planet,
436
00:26:56,148 --> 00:26:58,148
sometimes it's going to
lead the planet
437
00:26:58,150 --> 00:26:59,717
when it transits the star,
438
00:26:59,719 --> 00:27:01,485
and sometimes it's going
to trail behind
439
00:27:01,487 --> 00:27:03,020
as it transits the star,
440
00:27:03,022 --> 00:27:05,856
and you see a little bump
in the transit dip itself
441
00:27:05,858 --> 00:27:07,558
at different places.
442
00:27:09,395 --> 00:27:12,963
Narrator: And the team detected
the signal --
443
00:27:12,965 --> 00:27:18,469
not one, but two objects
orbiting together,
444
00:27:18,471 --> 00:27:23,440
confirmation of the first
exomoon ever discovered.
445
00:27:23,442 --> 00:27:33,050
♪
446
00:27:33,052 --> 00:27:35,619
it was an amazing discovery.
447
00:27:35,621 --> 00:27:37,921
I've been looking for exomoons
my entire career.
448
00:27:37,923 --> 00:27:40,491
For 10 years, we have been
in this quest to try
449
00:27:40,493 --> 00:27:42,893
and find these things.
450
00:27:42,895 --> 00:27:44,228
Caspi: This discovery,
451
00:27:44,230 --> 00:27:47,898
this announcement
was absolutely remarkable.
452
00:27:47,900 --> 00:27:49,933
Not only does it mean
that we might find
453
00:27:49,935 --> 00:27:52,002
earth twins everywhere
in the milky way,
454
00:27:52,004 --> 00:27:53,871
but it gives us
something to strive for,
455
00:27:53,873 --> 00:27:55,472
for human exploration.
456
00:27:55,474 --> 00:28:00,444
♪
457
00:28:00,446 --> 00:28:03,247
narrator:
On this alien exomoon,
458
00:28:03,249 --> 00:28:06,550
the skies would be nothing
like earth's.
459
00:28:06,552 --> 00:28:07,851
Visually, I think it would be
460
00:28:07,853 --> 00:28:09,553
an absolutely stunning place
to be.
461
00:28:09,555 --> 00:28:12,089
You look up in the sky,
and you see this ringed planet
462
00:28:12,091 --> 00:28:14,324
looming huge in the sky.
463
00:28:17,797 --> 00:28:21,265
Narrator: A world that could
be like earth,
464
00:28:21,267 --> 00:28:24,201
only orbiting another planet.
465
00:28:24,203 --> 00:28:29,940
♪
466
00:28:29,942 --> 00:28:34,311
but don't pack your space suit
just yet.
467
00:28:34,313 --> 00:28:35,879
Even though the planet
and the moon
468
00:28:35,881 --> 00:28:37,614
are potentially
the right distance
469
00:28:37,616 --> 00:28:39,383
away from the star
that we might imagine
470
00:28:39,385 --> 00:28:41,785
there being liquid water
on the surface,
471
00:28:41,787 --> 00:28:45,122
both the moon and the planet
are likely gaseous objects
472
00:28:45,124 --> 00:28:47,624
with no solid surface
to speak of.
473
00:28:50,096 --> 00:28:53,030
Narrator: Although the moon
probably isn't habitable,
474
00:28:53,032 --> 00:28:55,466
it is an important step
for finding worlds
475
00:28:55,468 --> 00:28:59,236
like our own in the galaxy.
476
00:28:59,238 --> 00:29:02,106
If we find exomoons
around exoplanets,
477
00:29:02,108 --> 00:29:04,508
that potentially
hugely increases
478
00:29:04,510 --> 00:29:07,644
the number of habitable worlds
that are out there.
479
00:29:07,646 --> 00:29:09,346
We just need more accurate
measurements,
480
00:29:09,348 --> 00:29:10,714
and then all of a sudden,
481
00:29:10,716 --> 00:29:12,983
the universe is going to be
full of exomoons.
482
00:29:12,985 --> 00:29:18,455
♪
483
00:29:18,457 --> 00:29:20,290
narrator: But these worlds
need to be more
484
00:29:20,292 --> 00:29:24,294
than just earth look-alikes.
485
00:29:24,296 --> 00:29:25,696
Everyone gets very excited
486
00:29:25,698 --> 00:29:28,365
when we find earthlike planets
around other stars,
487
00:29:28,367 --> 00:29:31,335
but "earthlike" kind of
just means how big it is
488
00:29:31,337 --> 00:29:33,537
and whether it can support
liquid water
489
00:29:33,539 --> 00:29:35,672
where it is in relation
to its star.
490
00:29:35,674 --> 00:29:39,676
All of that is great,
but it's just not enough.
491
00:29:42,982 --> 00:29:46,183
Narrator: A planet's composition
could be make-or-break
492
00:29:46,185 --> 00:29:47,951
for our new home...
493
00:29:50,089 --> 00:29:53,423
...The difference between
the perfect world
494
00:29:53,425 --> 00:29:55,359
and a ticking time bomb.
495
00:29:55,361 --> 00:30:00,998
♪
496
00:30:07,339 --> 00:30:11,775
[ explosion ]
497
00:30:11,777 --> 00:30:15,612
♪
498
00:30:15,614 --> 00:30:19,950
narrator: The hunt for earth 2.0
is still on.
499
00:30:19,952 --> 00:30:23,687
We've examined intense,
red dwarf systems...
500
00:30:24,957 --> 00:30:27,858
...Massive super-earths,
501
00:30:27,860 --> 00:30:31,495
and alien exomoons
502
00:30:31,497 --> 00:30:36,266
but so far,
there's no place like home.
503
00:30:36,268 --> 00:30:38,702
There are all these criteria
we have to tick off --
504
00:30:38,704 --> 00:30:40,938
a sunlike star,
505
00:30:40,940 --> 00:30:43,941
an orbit that puts it at
about the right temperature,
506
00:30:43,943 --> 00:30:46,944
a solid surface, something
that could retain an atmosphere.
507
00:30:48,981 --> 00:30:52,516
Narrator: But a planet that
appears earthlike on the outside
508
00:30:52,518 --> 00:30:55,586
may not be earthlike
on the inside.
509
00:30:55,588 --> 00:30:59,523
♪
510
00:30:59,525 --> 00:31:01,625
one of the things that makes
our world so unique
511
00:31:01,627 --> 00:31:02,893
is its plate tectonics,
512
00:31:02,895 --> 00:31:04,928
and that actually regulates
our climate.
513
00:31:08,000 --> 00:31:11,368
Narrator: The earth's climate
depends on cycles of materials,
514
00:31:11,370 --> 00:31:14,905
like carbon dioxide and water.
515
00:31:14,907 --> 00:31:18,508
Molecules move between
the earth's molten interior
516
00:31:18,510 --> 00:31:21,912
and the surface through
active plate tectonics
517
00:31:21,914 --> 00:31:23,680
and volcanic eruptions.
518
00:31:25,851 --> 00:31:28,619
These cycles help to regulate
the temperature
519
00:31:28,621 --> 00:31:31,054
and composition
of the earth's atmosphere.
520
00:31:34,560 --> 00:31:37,361
If we were to find another
earthlike planet out there,
521
00:31:37,363 --> 00:31:40,197
and it had geologic activity,
that means that at least
522
00:31:40,199 --> 00:31:43,634
it has the means to sustain
the carbon cycle
523
00:31:43,636 --> 00:31:45,435
and all of these
natural phenomenon
524
00:31:45,437 --> 00:31:50,007
that makes this planet
habitable and sustainable.
525
00:31:50,009 --> 00:31:54,144
♪
526
00:31:54,146 --> 00:31:57,581
narrator: How can we know what's
happening inside a planet?
527
00:31:59,919 --> 00:32:05,389
A clue can be found in
vast ranges across our world --
528
00:32:05,391 --> 00:32:07,090
mountains.
529
00:32:07,092 --> 00:32:13,497
♪
530
00:32:13,499 --> 00:32:17,100
kipping: These topographical
features are an indicator
531
00:32:17,102 --> 00:32:20,170
that the planet is alive
and there is still processes
532
00:32:20,172 --> 00:32:22,072
happening underneath
its surface.
533
00:32:22,074 --> 00:32:25,075
[ cracking, thudding ]
534
00:32:25,077 --> 00:32:26,877
♪
535
00:32:26,879 --> 00:32:28,445
narrator:
Mountain ranges are created
536
00:32:28,447 --> 00:32:31,715
when a planet's
tectonic plates collide,
537
00:32:33,719 --> 00:32:37,120
and even though exoplanets
are light-years away,
538
00:32:37,122 --> 00:32:40,657
astronomers could work out
whether their surfaces
539
00:32:40,659 --> 00:32:43,660
are smooth
or covered in peaks.
540
00:32:47,166 --> 00:32:49,466
Kipping: Those mountain ranges
are poking out,
541
00:32:49,468 --> 00:32:52,869
and depending on which rotation
the planet is in,
542
00:32:52,871 --> 00:32:55,272
the planet will appear
very slightly bigger
543
00:32:55,274 --> 00:32:58,141
or very slightly smaller
depending on the silhouette
544
00:32:58,143 --> 00:32:59,643
which is being cast.
545
00:32:59,645 --> 00:33:03,981
♪
546
00:33:03,983 --> 00:33:07,317
narrator: These tiny changes
in light could be the sign
547
00:33:07,319 --> 00:33:10,454
that an exoplanet
is healthy and active.
548
00:33:13,425 --> 00:33:15,492
But we can only use this method
549
00:33:15,494 --> 00:33:19,162
when a planet
is in front of its star.
550
00:33:19,164 --> 00:33:22,899
What if astronomers
could use starlight itself
551
00:33:22,901 --> 00:33:27,504
to determine
the geology of a planet?
552
00:33:27,506 --> 00:33:30,974
We think that planets form
at roughly the same sort of time
553
00:33:30,976 --> 00:33:32,442
that stars form,
554
00:33:32,444 --> 00:33:36,113
and they all form from this same
giant cloud of material.
555
00:33:38,684 --> 00:33:41,318
And so if you measure
the composition of a star,
556
00:33:41,320 --> 00:33:44,154
then it seems reasonable
to take those values and assume
557
00:33:44,156 --> 00:33:47,090
they're somewhat similar
for the planets as well.
558
00:33:47,092 --> 00:33:49,960
♪
559
00:33:49,962 --> 00:33:52,996
narrator: Astronomers can
work out what chemical elements
560
00:33:52,998 --> 00:33:56,700
are present in the star,
by splitting its light
561
00:33:56,702 --> 00:34:02,072
into different wavelengths,
and any planets around that star
562
00:34:02,074 --> 00:34:05,842
will have a similar
chemical composition.
563
00:34:05,844 --> 00:34:08,812
Composition is actually
a really important part
564
00:34:08,814 --> 00:34:11,548
of whether or not it's actually
going to be habitable.
565
00:34:11,550 --> 00:34:14,918
The composition
really is its geology.
566
00:34:17,656 --> 00:34:20,123
Narrator:
Rocky exoplanets are all made
567
00:34:20,125 --> 00:34:23,260
from the same
basic ingredients --
568
00:34:23,262 --> 00:34:28,331
chemical elements like oxygen,
silicon, and aluminum.
569
00:34:28,333 --> 00:34:30,600
Change the balance of
ingredients,
570
00:34:30,602 --> 00:34:34,771
and you get
very different planets.
571
00:34:34,773 --> 00:34:36,173
If we have some idea
572
00:34:36,175 --> 00:34:38,708
of the composition of
a rocky planet,
573
00:34:38,710 --> 00:34:41,344
we can actually use
that to give us clues
574
00:34:41,346 --> 00:34:45,682
as to whether a world has
or doesn't have plate tectonics.
575
00:34:45,684 --> 00:34:49,352
♪
576
00:34:49,354 --> 00:34:50,987
narrator:
New research indicates
577
00:34:50,989 --> 00:34:53,990
that exoplanets with too much
silicon and sodium
578
00:34:53,992 --> 00:34:58,395
form different types of rock
than those on earth,
579
00:34:58,397 --> 00:35:02,866
creating rigid planets
where plate tectonics stall
580
00:35:02,868 --> 00:35:08,105
and carbon dioxide builds up
with devastating consequences.
581
00:35:10,375 --> 00:35:12,309
Without active geology,
we end up with
582
00:35:12,311 --> 00:35:14,945
maybe a venetian atmosphere.
583
00:35:14,947 --> 00:35:17,414
Thaller: That means there
a runaway greenhouse effect.
584
00:35:17,416 --> 00:35:19,149
It's gotten hotter and hotter.
585
00:35:19,151 --> 00:35:21,051
Gases are baked
out of the rocks.
586
00:35:21,053 --> 00:35:23,186
There's no way to actually
rein them back out,
587
00:35:23,188 --> 00:35:24,955
not a good place for life
at all.
588
00:35:24,957 --> 00:35:29,359
♪
589
00:35:29,361 --> 00:35:33,463
narrator: At worse, the planet
becomes a pressure cooker,
590
00:35:33,465 --> 00:35:36,333
waiting to explode.
591
00:35:36,335 --> 00:35:38,168
If we change the composition
of a planet,
592
00:35:38,170 --> 00:35:39,803
it affects its tectonic system.
593
00:35:39,805 --> 00:35:42,572
That entirely changes
how a planet loses heat,
594
00:35:42,574 --> 00:35:44,407
and the heat builds up
and builds up and builds up,
595
00:35:44,409 --> 00:35:46,943
and then maybe there's
a catastrophic overturn
596
00:35:46,945 --> 00:35:48,211
of the crust.
597
00:35:48,213 --> 00:35:53,383
♪
598
00:35:53,385 --> 00:35:58,555
♪
599
00:35:58,557 --> 00:36:01,591
narrator: The solid crust of
the planet collapses.
600
00:36:04,663 --> 00:36:07,864
Oceans of lava bubble up,
601
00:36:07,866 --> 00:36:13,170
and a greenhouse atmosphere
boils the surface --
602
00:36:13,172 --> 00:36:17,107
a violent end
to a potential new home.
603
00:36:20,112 --> 00:36:21,778
Kipping:
Clearly, you need to know
604
00:36:21,780 --> 00:36:23,046
about the composition
of those planets
605
00:36:23,048 --> 00:36:24,915
before you can start
making statements
606
00:36:24,917 --> 00:36:27,884
about how habitable
those worlds truly are.
607
00:36:30,789 --> 00:36:33,757
Narrator: But there's something
else that a planet needs
608
00:36:33,759 --> 00:36:37,293
to be earthlike,
an invisible shield
609
00:36:37,295 --> 00:36:42,098
that protects it
from the dangers of space,
610
00:36:42,100 --> 00:36:45,602
providing warmth
and life-giving water --
611
00:36:45,604 --> 00:36:47,137
an atmosphere.
612
00:36:47,139 --> 00:36:51,241
♪
613
00:36:57,549 --> 00:37:02,052
[ explosion ]
614
00:37:02,054 --> 00:37:05,222
♪
615
00:37:05,224 --> 00:37:10,493
narrator: The hunt for earth 2.0
has turned up plenty of planets,
616
00:37:10,495 --> 00:37:13,230
but for a planet to be
like earth,
617
00:37:13,232 --> 00:37:17,167
it has to check a lot of boxes.
618
00:37:17,169 --> 00:37:20,170
If you're really looking
for earth 2.0,
619
00:37:20,172 --> 00:37:21,871
then you're gonna
have to find a planet
620
00:37:21,873 --> 00:37:24,641
that's the same mass
and size as earth,
621
00:37:24,643 --> 00:37:26,776
orbiting a sunlike star
622
00:37:26,778 --> 00:37:30,914
at about the same distance
with a similar atmosphere
623
00:37:30,916 --> 00:37:35,085
and a lot of surface water
that's in liquid form.
624
00:37:35,087 --> 00:37:36,319
Good luck.
625
00:37:40,359 --> 00:37:42,292
Narrator: And on the list
of requirements,
626
00:37:42,294 --> 00:37:47,564
an exoplanet's atmosphere
is critical.
627
00:37:47,566 --> 00:37:50,467
It protects the planet
from huge temperature swings.
628
00:37:50,469 --> 00:37:53,236
It protects the planet
from small asteroid impacts.
629
00:37:53,238 --> 00:37:55,572
It protects the planet
from dangerous radiation
630
00:37:55,574 --> 00:37:57,274
from space and from the star.
631
00:37:57,276 --> 00:38:00,010
It is almost literally a shield
around the planet,
632
00:38:00,012 --> 00:38:03,747
protecting us from outer space.
633
00:38:03,749 --> 00:38:07,917
But to also has to be
the right kind of atmosphere.
634
00:38:07,919 --> 00:38:11,921
♪
635
00:38:11,923 --> 00:38:15,258
get it wrong, and the planet
can have crushing,
636
00:38:15,260 --> 00:38:17,694
boiling conditions
on the surface.
637
00:38:20,198 --> 00:38:22,032
Look at our own solar system.
638
00:38:22,034 --> 00:38:23,867
The sun's habitable zone
includes
639
00:38:23,869 --> 00:38:26,903
three different planets,
venus, earth and mars,
640
00:38:26,905 --> 00:38:29,406
but mars has a thin atmosphere
and is too cold.
641
00:38:29,408 --> 00:38:32,242
Venus has too thick
of an atmosphere and is too hot.
642
00:38:32,244 --> 00:38:35,445
We're the only planet
that happens to be just right.
643
00:38:35,447 --> 00:38:40,950
♪
644
00:38:40,952 --> 00:38:43,753
narrator: So far, astronomers
have mostly had to guess
645
00:38:43,755 --> 00:38:48,091
if these exoplanets
have atmospheres,
646
00:38:48,093 --> 00:38:52,762
but now we're looking
for them directly,
647
00:38:52,764 --> 00:38:54,998
searching for
earthlike atmospheres
648
00:38:55,000 --> 00:38:57,400
around earthlike planets.
649
00:38:59,638 --> 00:39:02,372
Straughn:
This is incredibly hard to do,
650
00:39:02,374 --> 00:39:05,675
so in order to look at
the details of these atmospheres
651
00:39:05,677 --> 00:39:07,777
in the glare of the star
652
00:39:07,779 --> 00:39:11,915
requires
incredibly precise technology
653
00:39:11,917 --> 00:39:13,750
and precise measurements.
654
00:39:16,154 --> 00:39:19,122
Narrator: Astronomers detect
atmospheres by watching
655
00:39:19,124 --> 00:39:22,025
a planet pass in front
of the star.
656
00:39:24,363 --> 00:39:28,264
A small fraction of light shines
around the edge of the planet
657
00:39:28,266 --> 00:39:31,301
and through the atmosphere
658
00:39:31,303 --> 00:39:35,271
where molecules like water,
hydrogen, and carbon dioxide
659
00:39:35,273 --> 00:39:41,211
absorb particular wavelengths
of light from the star.
660
00:39:41,213 --> 00:39:43,780
If we can see the light
of the star
661
00:39:43,782 --> 00:39:46,216
shining through
around the planet,
662
00:39:46,218 --> 00:39:48,718
we can maybe deduce
some information about
663
00:39:48,720 --> 00:39:50,320
does it have an atmosphere?
664
00:39:50,322 --> 00:39:51,821
What are the properties
of that atmosphere?
665
00:39:51,823 --> 00:39:54,991
How hot is it?
What's it made out of?
666
00:39:54,993 --> 00:39:56,926
That's how we'll be able
to determine
667
00:39:56,928 --> 00:39:59,796
if things in the atmosphere
might indicate
668
00:39:59,798 --> 00:40:02,399
that the surface
is hospitable to life.
669
00:40:04,803 --> 00:40:07,937
Narrator: So far, we haven't
seen any exoplanets
670
00:40:07,939 --> 00:40:11,174
with atmospheres
that we could live in,
671
00:40:11,176 --> 00:40:13,443
but that's about to change.
672
00:40:13,445 --> 00:40:19,749
♪
673
00:40:19,751 --> 00:40:21,718
scientists around the world
674
00:40:21,720 --> 00:40:24,754
are working on the next
generation of telescopes
675
00:40:24,756 --> 00:40:29,159
to revolutionize
exoplanet astronomy.
676
00:40:29,161 --> 00:40:31,094
We've got some ideas,
and some telescopes
677
00:40:31,096 --> 00:40:34,097
that are gonna be built probably
in the next couple of decades
678
00:40:34,099 --> 00:40:37,367
will be big enough,
will be sophisticated enough
679
00:40:37,369 --> 00:40:39,736
to be able to see
this sort of thing.
680
00:40:41,406 --> 00:40:46,409
Narrator:
Missions like the james webb
space telescope --
681
00:40:46,411 --> 00:40:49,579
seven times more powerful
than hubble,
682
00:40:49,581 --> 00:40:53,583
it should allow us to see
the atmospheres of planets
683
00:40:53,585 --> 00:40:55,685
across the galaxy
684
00:40:55,687 --> 00:41:01,858
and be a tool that
finally finds a second earth.
685
00:41:01,860 --> 00:41:04,427
The key things we'd be looking
for in these atmospheres
686
00:41:04,429 --> 00:41:07,764
are in the infrared part
of the electromagnetic spectrum,
687
00:41:07,766 --> 00:41:11,468
which is where webb
is designed to work.
688
00:41:11,470 --> 00:41:13,136
The james webb
space telescope is,
689
00:41:13,138 --> 00:41:17,540
I believe, going to be the next
really critical mission
690
00:41:17,542 --> 00:41:22,946
to help us in our search for
potentially earthlike planets.
691
00:41:22,948 --> 00:41:26,716
♪
692
00:41:26,718 --> 00:41:31,721
narrator: We're still searching
for that perfect earth twin,
693
00:41:31,723 --> 00:41:35,058
and every day,
694
00:41:35,060 --> 00:41:38,461
we get closed to finding it.
695
00:41:38,463 --> 00:41:42,899
30 years ago,
we had zero exoplanets.
696
00:41:42,901 --> 00:41:45,668
Today, we know of thousands.
697
00:41:45,670 --> 00:41:48,538
With the next generation
of instruments,
698
00:41:48,540 --> 00:41:51,541
we're going to uncover
tens of thousands,
699
00:41:51,543 --> 00:41:56,679
hundreds of thousands,
even millions of exoplanets.
700
00:41:56,681 --> 00:42:00,984
Narrator: All with the ultimate
aim of leaving earth,
701
00:42:00,986 --> 00:42:06,155
a civilization
spread across the stars.
702
00:42:06,157 --> 00:42:08,191
Oluseyi: One of the things
I love about being a human
703
00:42:08,193 --> 00:42:10,493
is the fact that I'm born
with this curiosity.
704
00:42:10,495 --> 00:42:14,230
This curiosity drives us
to explore, explore earth,
705
00:42:14,232 --> 00:42:18,167
explore our solar system
and beyond into the galaxy.
706
00:42:18,169 --> 00:42:21,371
Plait: We'll be learning about
these planets for a long time.
707
00:42:21,373 --> 00:42:23,873
We have just started
this journey.
708
00:42:23,875 --> 00:42:28,478
♪
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