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Our solar system is a strange thing.

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It's radically different from
all the other planetary systems

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we see across the galaxy.

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We are an exception
rather than the rule.

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To understand why,
scientists peer deep

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into our solar system's
secret history

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and find a dark and violent past
of planetary homicide

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on a mass scale.

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The solar system is a ghost
of what it used to be.

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You end up with
the last survivors

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being a bunch of freaks.

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Our home could be
one of those freaks.

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The rock beneath
our feet could have been

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from long-dead planets.

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And those long-dead
planets could help explain

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why there's life on Earth.

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In a spiral arm of a huge galaxy
called the milky way

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spins an extraordinary
planetary system,

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our solar system.

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For millennia, it was
the only one we knew,

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but all that has changed.

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Astronomers have discovered more
than 2,600 planetary systems

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to date, but none of them
are quite like our own.

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The incredible thing
about astronomy

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is when you look out
into the universe,

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and you realize you have
completely misinterpreted

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your own home.

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So one important thing
we've learned

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in discovering planets
around other stars

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is that our system
isn't the normal system.

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It's not what we see everywhere.

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In fact, as we discover
more planets

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orbiting other stars,

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we see that ours is an oddball.

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Most other solar systems look
completely different than ours.

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February 2017, NASA makes
a huge announcement

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about a system
in the Aquarius constellation,

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Trappist-1.

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Trappist-1 is a little unusual.

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It's a little bit smaller
and cooler than the Sun,

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but it has seven planets
orbiting it.

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And you think, "well, that's not
that peculiar. We have eight."

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But these are seven
roughly Earth-sized planets.

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There's not a lot
of variety there,

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and they also orbit
the star very close in.

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All seven planets
somehow orbit closer

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to their star than Mercury,

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our innermost planet,
does to our sun.

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Perhaps one of
the greatest puzzles

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that have come out of finding
planets around other stars

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is that they typically
have orbits

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well inside
the orbit of Mercury.

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It's really odd, in my view,

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that the solar system
is hollowed out.

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There's nothing inside
of Mercury's orbit.

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Why is that?

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The mystery
of the missing inner planets

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is like a cosmic whodunit,

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turning scientists
into detectives asking,

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"do we really know how the story
of our solar system unfolds?"

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The whole process is,
like, cosmic CSI.

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You're trying to put together
the clues to find out something

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that happened when nobody else

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was there to watch it happen.

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Like detectives,
scientists start

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with the simplest explanation.

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So in some ways,
the early solar system

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is like a pool game.

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All the billiard balls represent
the pieces, the building blocks,

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the planetesimals
or the planetary embryos

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that are going to come together

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eventually to build
the final system of planets.

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In planetary formation,

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the simplest theory is that
the planets all formed

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where we find them now.

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It's called the classical model.

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How does this play out?

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We start with
a few planetesimals

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that collide with each other,

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and they grow
a little bit larger

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in this region
of the solar system.

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This process continues, and you
grow all the way up to planets,

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with each planet in each of
the zone of the solar system

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creating material just
from its neighborhood

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and no one's really
moving around very far.

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But this classical model
can't explain why

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our inner solar system

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is missing all kinds
of material.

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The classical model has
no natural explanation

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for why Mercury is the last
thing that we know of,

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inward towards the Sun,

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that there are no planets,
no asteroids,

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nothing inside Mercury,
is still a mystery

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that the classical model
can't easily explain.

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The area close to our
sun isn't just missing asteroids

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and small planets.

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It's also missing
really big ones.

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The very first exoplanets,

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the very first alien
worlds we discovered,

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were Jupiter-mass
or bigger planets

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orbiting their starts
very closely,

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even closer than
Mercury orbits the Sun.

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Astronomers
have so far discovered

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around 300 gas giants

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scorchingly close to their suns.

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They call them hot Jupiters...

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But how they form is a mystery.

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Gas giants like Jupiter
should be born out of the cold,

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far from their suns.

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It's very hard to imagine
hot Jupiters forming

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where we see them today.

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The temperatures,
the distances from the star,

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where we find hot Jupiters
are so hot,

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it's hard to imagine
any material

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condensing out
of the solar nebula.

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This kick-started the idea

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that maybe these hot Jupiters,
as they were called,

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may have actually formed
farther out,

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like near where our
Jupiter is now,

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and in the early solar system

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they started migrating inward
toward their star.

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So what happens when
a planet the size of Jupiter

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moves inward?

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Can this help explain the inner
solar system's missing mass

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and answer why
we don't have a hot Jupiter?

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To find out, Kevin Walsh
and colleagues simulate

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the first 10 million years
of the solar system.

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They call this model
the grand tack.

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The grand tack model
is a scenario

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designed to help understand

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how the terrestrial planets
could have formed,

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thinking about
what the giant planets

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might have been doing
in the early solar system.

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The planets form within
a thick disk of gas and debris

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that surrounds
the newly-formed sun.

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The grand tack model simulates
what happens if Jupiter moves

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in towards the Sun
through this disk.

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It's pushing all of
the asteroids in its path

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into the inner solar system.

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All of that material is what is
going to come together

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to form the rocky planets.

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Jupiter's immense
gravity pulls in more

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and more material,

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forming a dense wave of debris
bulging out behind it.

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The pressure of this bulge
pushes Jupiter further inwards.

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Like a wrecking ball,
Jupiter should clear out

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all the planet-building material

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from the entire
inner solar system

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and become a sun-hugging
hot Jupiter,

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but something checks
Jupiter's path of destruction.

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If Jupiter had hung around
much longer

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in the inner solar system,
we wouldn't be here,

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so something must have
drawn it out very rapidly.

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And what could possibly move
a big, massive planet rapidly?

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And the answer is another big,
massive planet.

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Saturn...

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It forms just after Jupiter,

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and is hot on Jupiter's heels,

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as it, too,
migrates towards the Sun.

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Saturn is pretty big itself.

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The combined effect of
the two giant planets

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migrating is that once

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Saturn is large enough,
it can actually change the way

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that the gas disk is interacting
with both the planets.

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And it can stop Jupiter's inward
migration and help to turn

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Jupiter around

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and almost pulls it back
to the outer solar system.

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Like a sailboat
switching direction,

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Jupiter tacks away from the Sun.

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The behavior of the giant
outer planets

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leaves our solar system
with no hot Jupiter

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and has dramatic effect
on the small inner planets, too.

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As it's coming back outwards,

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what has Jupiter done to
the inner solar system?

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It has removed all of the
material in its path,

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all the way down to where
we find the Earth today.

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And all of that material pushed
into essentially a narrow band

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in the inner solar system is
what is going to come together

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to form the rocky planets
as we find them today.

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According to
the grand tack model,

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without Jupiter,
the rocky terrestrial planets

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of the inner solar system
might have never formed.

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One of those planets is Earth.

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So as much as we owe
our existence to Jupiter,

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we also owe it to Saturn

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because if Jupiter had kept
moving in closer to the Sun,

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we almost certainly
wouldn't be here now.

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The grand tack
may provide a vital chapter

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in the story
of Earth's formation.

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That answers why
we're missing a hot Jupiter

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but doesn't explain
why there's nothing

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between Mercury and the Sun,

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nor why we're missing one of
the most common types of planet

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in the whole galaxy,

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a giant rocky world
up to 10 times

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the size of Earth...
A super-Earth.

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The Kepler space telescope

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leads the charge in the hunt

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for exoplanets
around other stars.

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It's confirmed
over 2,000 new worlds.

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The single-largest
finding to date,

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over one-third of those planets
are super-Earths.

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A super-Earth is a type
of rocky planet

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that has a mass a few times
the mass of the Earth,

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and as we look around the
galaxy, we find them all over.

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Our solar system
doesn't have one,

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and you have to ask
the question... why not?

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What's different about us?

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In our solar system,
planets range in mass,

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with Jupiter being the largest

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and Mercury the smallest.

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But weirdly, we have nothing
in the super-Earth-size range,

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which is between
Earth and Uranus.

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Why is there such
a big gap in masses

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between the Earth and Uranus,

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which is roughly a dozen
times the Earth's mass?

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It's a big jump
from one to a dozen.

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Why?

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In 2015,

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Konstantin Batygin
tries to find the answer

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to why we have no super-Earths,

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and why there is nothing
within the orbit of Mercury.

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He reconstructs the grand tack
model with one key difference...

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The simulations start
with six super-Earths

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in our solar system center

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in a tight orbit around the Sun,

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typical of other systems
we've observed.

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He calls this new model
the grand attack.

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One of the realizations
that has come out

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of studying
the grand tack scenario

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is that Jupiter's migration
would have really unleashed

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a veritable grand attack

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upon the inner solar system.

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The grand attack model
ramps up Jupiter's action

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so it sends swarms
of giant asteroids and planetary

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embryos into the inner
solar system on tight-knit,

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overlapping orbits.

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The result?

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Carnage.

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Each big body
will experience a collision

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with another big body
once every 20 to 200 orbits.

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This is exceptionally fast
on cosmic-time scales.

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What this means is that
you take the entirety

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of that overlapped
population of bodies,

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and you smash them up
into smaller debris.

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Jupiter is like a little kid
with a hammer, you know?

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It just comes in and is
whacking around at everything,

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and it's making a mess
of the inner solar system.

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It's a game
of cosmic pool on steroids.

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Let's start with our Jupiter,

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for our model here
of the solar system.

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It's causing a bunch of
very violent collisions

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between all of this debris
that it's sweeping up.

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These huge collisions are
making an enormous amount

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of really small material,

256
00:14:03,950 --> 00:14:06,850
which can drift really fast
inward in the solar system

257
00:14:06,860 --> 00:14:11,330
due to the drag
from the gas around the Sun.

258
00:14:11,330 --> 00:14:14,730
These planetesimals
collide over and over,

259
00:14:14,730 --> 00:14:17,770
pulverized to
the size of gravel.

260
00:14:17,770 --> 00:14:20,900
For the smaller debris,
hitting this dense gas cloud

261
00:14:20,900 --> 00:14:25,710
around the Sun is like
plowing into a headwind.

262
00:14:25,710 --> 00:14:29,040
The swarm of rubble loses
the momentum that keeps it

263
00:14:29,050 --> 00:14:33,180
in orbit around the Sun
and starts spiraling in,

264
00:14:33,180 --> 00:14:36,820
but it hits roadblocks.

265
00:14:36,820 --> 00:14:40,050
So as all this debris rushes
inward in a big wave,

266
00:14:40,060 --> 00:14:43,560
it gets dragged in until it gets
stuck behind the super-Earths.

267
00:14:47,060 --> 00:14:51,230
Debris builds up until the
super-Earths finally give way.

268
00:14:55,140 --> 00:14:56,540
The super-Earths are like a dam

269
00:14:56,540 --> 00:14:59,270
that can't quite resist
the flow of water

270
00:14:59,280 --> 00:15:02,840
and begins to recede
and eventually gets

271
00:15:02,850 --> 00:15:05,010
kind of pushed onto
the surface of the Sun,

272
00:15:05,010 --> 00:15:09,120
together with the flux
of collisional debris.

273
00:15:09,120 --> 00:15:11,790
It's a remarkably swift process.

274
00:15:11,790 --> 00:15:17,490
In just 20,000 years, all the
super-Earths crash into the Sun.

275
00:15:22,530 --> 00:15:24,270
After the dramatic evolution

276
00:15:24,270 --> 00:15:26,600
of the inner solar system,
there's only a fraction

277
00:15:26,600 --> 00:15:28,200
of the original mass left.

278
00:15:28,200 --> 00:15:31,410
The solar system is a ghost
of what it used to be.

279
00:15:34,380 --> 00:15:37,240
There's nothing
in the first 39 million miles

280
00:15:37,250 --> 00:15:38,250
from the Sun.

281
00:15:40,480 --> 00:15:42,950
But slightly farther out,

282
00:15:42,950 --> 00:15:45,990
there's a narrow ring
of rocky debris,

283
00:15:45,990 --> 00:15:51,290
about 10% of the original
material swept in by Jupiter,

284
00:15:51,290 --> 00:15:54,730
just enough to rebuild
the inner solar system.

285
00:15:57,100 --> 00:16:02,340
A few survivors, small
planetesimals start to regroup.

286
00:16:02,340 --> 00:16:04,070
Over millions of years,

287
00:16:04,070 --> 00:16:08,440
four small, rocky planets form.

288
00:16:08,440 --> 00:16:10,140
Mercury, Venus, Earth, and Mars

289
00:16:10,150 --> 00:16:12,580
form from this leftover debris.

290
00:16:12,580 --> 00:16:15,780
The planet that we're
standing on may not be

291
00:16:15,790 --> 00:16:20,590
an original generation
solar system planet.

292
00:16:20,590 --> 00:16:23,520
It's kind of like building
a house with cinder blocks

293
00:16:23,530 --> 00:16:27,360
from a house that sat on
that spot but was demolished.

294
00:16:27,360 --> 00:16:30,900
Not only are we breathing the
atmospheres of long-dead stars,

295
00:16:30,900 --> 00:16:32,300
the rock beneath our feet

296
00:16:32,300 --> 00:16:36,440
could have been
from long-dead planets.

297
00:16:36,440 --> 00:16:39,210
So Earth could be
second-generation planet

298
00:16:39,210 --> 00:16:41,640
formed from the wreckage
of the grand attack.,

299
00:16:44,150 --> 00:16:46,350
But there was one thing
the super-Earths

300
00:16:46,350 --> 00:16:49,250
took with them
to their fiery grave...

301
00:16:49,250 --> 00:16:51,790
The supply of hydrogen
and helium

302
00:16:51,790 --> 00:16:55,190
in the inner solar system.

303
00:16:55,190 --> 00:16:56,590
When you look at
the Earth's atmosphere now,

304
00:16:56,590 --> 00:16:58,930
we don't have any hydrogen
or helium in it.

305
00:16:58,930 --> 00:17:01,160
There was hydrogen
and helium in the disk

306
00:17:01,160 --> 00:17:03,300
where the inner planets formed,

307
00:17:03,300 --> 00:17:05,230
but that became part
of the super-Earths,

308
00:17:05,230 --> 00:17:07,130
the first generation planets.

309
00:17:07,140 --> 00:17:09,870
When Jupiter came in
and dropped them into the Sun,

310
00:17:09,870 --> 00:17:12,140
they took their hydrogen
and helium with them.

311
00:17:12,140 --> 00:17:14,840
So the composition of
the the air around you right now

312
00:17:14,840 --> 00:17:16,010
may be due to the fact

313
00:17:16,010 --> 00:17:17,980
that we're
a second-generation planet.

314
00:17:21,180 --> 00:17:23,680
Coming in second
might not sound great,

315
00:17:23,690 --> 00:17:28,660
but maybe second place
is the reason we're here.

316
00:17:28,660 --> 00:17:32,230
So, the Earth we now see
is Earth 2.0.

317
00:17:32,230 --> 00:17:36,100
Would Earth 1.0 have been
conducive to life?

318
00:17:36,100 --> 00:17:38,930
That's an interesting question.

319
00:17:38,940 --> 00:17:42,500
Earth's atmosphere is
a fertile blend of gases

320
00:17:42,510 --> 00:17:46,640
that allows life as
we know it to flourish,

321
00:17:46,640 --> 00:17:50,140
an atmosphere that might have
been completely different

322
00:17:50,150 --> 00:17:53,950
if Earth was
a first-generation planet.

323
00:17:53,950 --> 00:17:58,090
It's entirely possible
that life, like us,

324
00:17:58,090 --> 00:18:01,090
needs to have
a second-generation planet

325
00:18:01,090 --> 00:18:04,790
to arise in the first place.

326
00:18:04,790 --> 00:18:07,160
Could our planet be more unusual

327
00:18:07,160 --> 00:18:08,500
than we'd ever thought?

328
00:18:11,000 --> 00:18:14,970
How special is the
Earth in a cosmic setting?

329
00:18:14,970 --> 00:18:17,640
We don't really know the final
answer to this question,

330
00:18:17,640 --> 00:18:19,840
but evidence is beginning
to point to the fact

331
00:18:19,840 --> 00:18:22,510
that the Earth is actually
kind of rare,

332
00:18:22,510 --> 00:18:25,180
and we should really
appreciate our planet.

333
00:18:27,350 --> 00:18:29,980
Finding out what
happened to our solar system

334
00:18:29,990 --> 00:18:34,620
is like studying
a cosmic crime scene.

335
00:18:34,620 --> 00:18:37,290
To reveal the solar system's
secret history,

336
00:18:37,290 --> 00:18:41,200
we need to look
in unusual places,

337
00:18:41,200 --> 00:18:44,100
the last surviving pieces
of the violence

338
00:18:44,100 --> 00:18:45,930
from which our home was born.

339
00:19:04,620 --> 00:19:08,080
Our solar system is
a celestial cold case,

340
00:19:08,090 --> 00:19:11,850
and it's hiding the traces
of its violent past.

341
00:19:11,860 --> 00:19:16,790
What you have now is
a crime scene that has dried up,

342
00:19:16,790 --> 00:19:19,130
and you're trying
to find little clues as to

343
00:19:19,130 --> 00:19:21,930
what happened
4 1/2 billion years ago.

344
00:19:21,930 --> 00:19:26,840
It's a really, really
difficult problem to solve.

345
00:19:26,840 --> 00:19:29,670
Solid evidence
could be hard to find,

346
00:19:29,670 --> 00:19:32,910
but sometimes we get lucky.

347
00:19:32,910 --> 00:19:34,940
We don't have a time machine,
so it's hard to go back

348
00:19:34,950 --> 00:19:36,350
in time 4 1/2 billion years

349
00:19:36,350 --> 00:19:40,620
and look at the solar system and
see what it was doing back then.

350
00:19:40,620 --> 00:19:43,350
However, sometimes
nature provides,

351
00:19:43,350 --> 00:19:45,020
and if you don't have
a time machine,

352
00:19:45,020 --> 00:19:48,420
sometimes a time capsule
will do just as well.

353
00:19:48,430 --> 00:19:50,360
And in fact,
we have time capsules

354
00:19:50,360 --> 00:19:51,760
of the early solar system,

355
00:19:51,760 --> 00:19:53,700
and we call them meteorites.

356
00:19:56,970 --> 00:19:59,430
Most meteorites
are chunks of asteroids

357
00:19:59,440 --> 00:20:00,840
that fall to Earth.

358
00:20:00,840 --> 00:20:02,400
Depending on their origin,

359
00:20:02,410 --> 00:20:05,810
they come in different
shapes and sizes.

360
00:20:05,810 --> 00:20:08,110
Asteroids really are
like space fossils

361
00:20:08,110 --> 00:20:12,610
because they were formed
4 1/2 billion years ago,

362
00:20:12,620 --> 00:20:15,780
but they've basically
remained dead.

363
00:20:15,790 --> 00:20:18,550
They are the leftovers, the
remnants of planet formation.

364
00:20:18,560 --> 00:20:22,320
They're the last little bits
that haven't become planets yet.

365
00:20:22,330 --> 00:20:26,760
To understand why meteorites
are such useful clues,

366
00:20:26,760 --> 00:20:30,400
we first need to know
how planets form.

367
00:20:30,400 --> 00:20:33,740
It's a process called accretion.

368
00:20:33,740 --> 00:20:37,040
The cloud of hot gas swirling
around the Sun condenses

369
00:20:37,040 --> 00:20:40,040
and clumps into larger
and larger bodies.

370
00:20:40,040 --> 00:20:44,110
We find traces of this process
inside meteorites,

371
00:20:44,110 --> 00:20:49,550
in tiny mineral beads
called chondrules.

372
00:20:49,550 --> 00:20:51,620
Chondrules are literally
the seeds

373
00:20:51,620 --> 00:20:54,920
of all of the structure
in our solar system.

374
00:20:54,930 --> 00:20:56,630
Most chondrules condense

375
00:20:56,630 --> 00:20:58,430
out of the cloud of hot gas

376
00:20:58,430 --> 00:21:02,760
around the Sun
as the solar system forms.

377
00:21:02,770 --> 00:21:05,570
Chondrules have been
described poetically

378
00:21:05,570 --> 00:21:09,500
as droplets of fiery
rain that have solidified.

379
00:21:09,510 --> 00:21:14,280
They are little globules
of silicate melt

380
00:21:14,280 --> 00:21:17,310
that were produced in
the very earliest history

381
00:21:17,310 --> 00:21:18,780
of our solar system.

382
00:21:18,780 --> 00:21:21,420
These globules of melt
solidified to form

383
00:21:21,420 --> 00:21:23,120
these little spheres.

384
00:21:23,120 --> 00:21:25,690
It really tells us about the
process of agglomeration

385
00:21:25,690 --> 00:21:30,790
of smaller objects
to form larger bodies.

386
00:21:30,790 --> 00:21:32,630
But some chondrules tell us

387
00:21:32,630 --> 00:21:34,960
not only about a planet's birth,

388
00:21:34,970 --> 00:21:36,770
but also its death.

389
00:21:47,210 --> 00:21:49,040
A meteorite called gujba

390
00:21:49,050 --> 00:21:53,480
contains two very different
kinds of globules.

391
00:21:53,480 --> 00:21:57,250
So a gujba is a type
of meteorite

392
00:21:57,250 --> 00:22:00,760
that's made up
of little spherials

393
00:22:00,760 --> 00:22:02,620
of silicate material

394
00:22:02,630 --> 00:22:05,590
as well as spherials
of iron-nickel metal,

395
00:22:05,600 --> 00:22:08,360
and it's very unusual
in that regard.

396
00:22:08,370 --> 00:22:12,530
These metal spherials that
we find in gujba are formed,

397
00:22:12,540 --> 00:22:15,470
we think,
around 5 or 6 million years

398
00:22:15,470 --> 00:22:19,310
after the solar system forms.

399
00:22:19,310 --> 00:22:21,710
At that point,
there was not enough hot gas

400
00:22:21,710 --> 00:22:23,110
lingering in the disk

401
00:22:23,110 --> 00:22:27,120
to form the chondrules
we see in the gujba.

402
00:22:27,120 --> 00:22:30,750
So how did this globules form?

403
00:22:30,750 --> 00:22:33,060
The only way to really produce
these globules

404
00:22:33,060 --> 00:22:35,620
is another process,

405
00:22:35,630 --> 00:22:38,260
and we think in this case
it was some kind of process

406
00:22:38,260 --> 00:22:41,830
like collisions.

407
00:22:41,830 --> 00:22:44,430
Collisions so violent
they vaporized

408
00:22:44,440 --> 00:22:46,970
the silicates and metals.

409
00:22:46,970 --> 00:22:52,770
Solid turns to gas, and
then gas to liquid.

410
00:22:52,780 --> 00:22:55,810
But what planetary body
contains enough metal to be able

411
00:22:55,810 --> 00:23:00,780
to produce the droplets
we see in gujba?

412
00:23:00,780 --> 00:23:04,650
Only something big enough
to have an iron core.

413
00:23:06,520 --> 00:23:08,060
When an object grows
large enough,

414
00:23:08,060 --> 00:23:10,890
its gravity become strong enough
that it differentiates,

415
00:23:10,890 --> 00:23:12,690
and what we mean by that is,

416
00:23:12,700 --> 00:23:15,160
heavy stuff is pulled down
and sinks into the center,

417
00:23:15,170 --> 00:23:16,900
and lighter stuff floats
to the top,

418
00:23:16,900 --> 00:23:22,840
so you have a differentiation
of material.

419
00:23:22,840 --> 00:23:23,656
Earth is a classic example of
a body that is differentiated.

420
00:23:29,410 --> 00:23:33,550
while heavy metals remain in the crust

421
00:23:33,550 --> 00:23:36,120
The crust and the core
are separated

422
00:23:36,120 --> 00:23:41,560
by a molten silicate layer
known as the mantle.

423
00:23:41,560 --> 00:23:45,160
Gujba is a perfect example
of the fact

424
00:23:45,160 --> 00:23:48,360
that you had large
planetary bodies

425
00:23:48,370 --> 00:23:52,000
that were differentiated
into irons and silicates,

426
00:23:52,000 --> 00:23:54,000
and they were colliding
at velocities

427
00:23:54,000 --> 00:23:56,140
great enough
to scatter their pieces

428
00:23:56,140 --> 00:23:57,740
out into the nebula again.

429
00:23:57,740 --> 00:23:59,940
That is just amazing to me,

430
00:23:59,940 --> 00:24:03,510
that this really,
really violent process

431
00:24:03,510 --> 00:24:07,780
in history that's captured
in these tiny little fragments.

432
00:24:10,950 --> 00:24:13,420
Gujba reveals
the differentiated planets

433
00:24:13,420 --> 00:24:16,590
were commonplace
as early as 5 million years

434
00:24:16,590 --> 00:24:19,160
after the formation
of the solar system.

435
00:24:21,600 --> 00:24:23,330
There were so many
of these planets,

436
00:24:23,330 --> 00:24:25,870
they often smashed together.

437
00:24:28,340 --> 00:24:31,370
If you go down this
path of planet formation

438
00:24:31,380 --> 00:24:32,870
by giant impacts,

439
00:24:32,880 --> 00:24:34,680
you end up with
the last survivors

440
00:24:34,680 --> 00:24:38,380
being a bunch of freaks.

441
00:24:38,380 --> 00:24:40,380
Freaks built
from the dead embers

442
00:24:40,380 --> 00:24:41,820
of past generations.

443
00:24:44,620 --> 00:24:48,390
And this violent scenario
raises the question,

444
00:24:48,390 --> 00:24:50,390
just how many planets
did it take

445
00:24:50,390 --> 00:24:52,530
to build the inner solar system?

446
00:24:54,670 --> 00:24:58,600
Simulations say at least 30,

447
00:24:58,600 --> 00:25:00,770
but just four survived.

448
00:25:19,180 --> 00:25:21,280
The secret history
of the solar system

449
00:25:21,290 --> 00:25:24,250
is hard to interpret,

450
00:25:24,260 --> 00:25:27,260
but astronomers devise
radical new solutions

451
00:25:27,260 --> 00:25:28,760
to connect the dots.

452
00:25:31,260 --> 00:25:33,530
The grand tack model helps
explain why we see

453
00:25:33,530 --> 00:25:35,270
no hot Jupiter planet.

454
00:25:37,540 --> 00:25:40,300
The grand attack model
provides an answer

455
00:25:40,310 --> 00:25:43,040
to the lack of material
within Mercury's orbit

456
00:25:43,040 --> 00:25:46,110
and why our solar system
has no super-Earths.

457
00:25:48,510 --> 00:25:53,220
These are theories designed
to crack the cosmic cold case,

458
00:25:53,220 --> 00:25:55,990
but to understand
our red neighbor, Mars,

459
00:25:55,990 --> 00:26:00,390
scientists need another
simulation, a 30-planet pileup.

460
00:26:03,800 --> 00:26:05,730
When you look at the action
in a solar system,

461
00:26:05,730 --> 00:26:08,100
you essentially have our
smallest planet on the inside

462
00:26:08,100 --> 00:26:09,400
and then it gets
larger and larger

463
00:26:09,400 --> 00:26:11,230
as you go from Venus
to the Earth,

464
00:26:11,240 --> 00:26:14,970
so you would naturally expect
Mars to be larger than it is.

465
00:26:14,970 --> 00:26:18,110
It should be 10 times bigger
than it is, but it's not.

466
00:26:21,250 --> 00:26:24,510
Mars' size isn't
its only mystery.

467
00:26:24,520 --> 00:26:28,020
It's also much older
than we expected.

468
00:26:28,020 --> 00:26:31,350
Scientists have refined
the age of Mars' mantle

469
00:26:31,360 --> 00:26:32,990
based on the chemical
composition

470
00:26:32,990 --> 00:26:37,090
of a piece of martian meteorite.

471
00:26:37,100 --> 00:26:40,500
The sample blew off from the
planet during a violent impact

472
00:26:40,500 --> 00:26:44,800
and made its way to Earth.

473
00:26:44,800 --> 00:26:47,270
It revealed that Mars
formed rapidly,

474
00:26:47,270 --> 00:26:51,980
within the first 2 million years
of the solar system's birth,

475
00:26:51,980 --> 00:26:54,980
well before the Earth.

476
00:26:54,980 --> 00:26:57,580
Mars is small, and Mars
formed really,

477
00:26:57,580 --> 00:27:01,180
really fast compared
to what it should have.

478
00:27:01,190 --> 00:27:02,820
The Earth is 10 times
more massive.

479
00:27:02,820 --> 00:27:04,920
It formed in 100 million years.

480
00:27:04,920 --> 00:27:06,620
Mars formed in 2 million years.

481
00:27:06,630 --> 00:27:08,060
This doesn't make sense.
It's our neighbor.

482
00:27:08,060 --> 00:27:10,130
It should look just like us.

483
00:27:10,130 --> 00:27:14,430
Everything about Mars
feels wrong.

484
00:27:14,430 --> 00:27:17,300
These two mysteries
might help explain one another.

485
00:27:17,300 --> 00:27:18,940
Scientists think it's possible

486
00:27:18,940 --> 00:27:21,600
that around 30 other
similar planets

487
00:27:21,610 --> 00:27:23,710
formed alongside Mars

488
00:27:23,710 --> 00:27:26,410
within the first 2 million
years of the solar system.

489
00:27:29,750 --> 00:27:32,520
So what happened
to this 30-planet pileup?

490
00:27:32,520 --> 00:27:34,820
Time for another game
of cosmic pool.

491
00:27:40,130 --> 00:27:43,230
So in this model,
we very quickly form

492
00:27:43,230 --> 00:27:45,900
20 or 30 Mars-sized planets.

493
00:27:45,900 --> 00:27:47,530
This is a pretty
jam-packed system.

494
00:27:47,530 --> 00:27:49,200
The planets are pretty
close to each other,

495
00:27:49,200 --> 00:27:54,070
and it's just on
the hairy edge of stability.

496
00:27:54,070 --> 00:27:57,340
This colony of Mars-sized
planets builds rapidly.

497
00:27:59,080 --> 00:28:00,940
In the early days
of the solar system,

498
00:28:00,950 --> 00:28:03,180
there's enough gas around
to keep their orbits

499
00:28:03,180 --> 00:28:07,380
from crossing each other,

500
00:28:07,390 --> 00:28:10,790
but after 20 million years,
the gas has gone,

501
00:28:10,790 --> 00:28:13,460
and their orbits
start to intersect.

502
00:28:15,530 --> 00:28:17,360
When it goes unstable,

503
00:28:17,360 --> 00:28:19,760
it's then a pretty loud
and chaotic place.

504
00:28:19,760 --> 00:28:21,930
As Mars-sized bodies collide
with each other

505
00:28:21,930 --> 00:28:23,900
to build the Earth and Venus,

506
00:28:23,900 --> 00:28:26,570
we get a series of huge,
violent collisions.

507
00:28:29,740 --> 00:28:31,740
Over the next 100 million years,

508
00:28:31,740 --> 00:28:35,210
the Mars-sized protoplanets
annihilate each other

509
00:28:35,210 --> 00:28:38,820
to eventually form
second-generation planets,

510
00:28:38,820 --> 00:28:42,320
Venus and Earth.

511
00:28:42,320 --> 00:28:45,920
Yet one planet stood back
and watched from the sidelines.

512
00:28:48,160 --> 00:28:53,460
That planet was Mars, and that
is the secret to its old age,

513
00:28:53,470 --> 00:28:56,600
compared to Earth.

514
00:28:56,600 --> 00:28:59,340
If Mars is indeed older
than the Earth,

515
00:28:59,340 --> 00:29:03,510
that would imply that it's one
of the original planetary

516
00:29:03,510 --> 00:29:05,240
embryos of the solar system.

517
00:29:05,240 --> 00:29:07,340
Mars is essentially done early.

518
00:29:07,350 --> 00:29:09,510
It is on the outside
of this whole process,

519
00:29:09,510 --> 00:29:11,980
sitting out not accreting
any more mass

520
00:29:11,980 --> 00:29:13,780
and watching while
the Earth and Venus

521
00:29:13,790 --> 00:29:16,350
form out of the other
big bodies that have been built.

522
00:29:18,690 --> 00:29:20,890
So what prevented Mars
from colliding

523
00:29:20,890 --> 00:29:23,330
with the rest
of the planetary embryos?

524
00:29:25,760 --> 00:29:30,170
The answer... Jupiter.

525
00:29:30,170 --> 00:29:33,670
During the grand tack, Jupiter
moved to the same distance

526
00:29:33,670 --> 00:29:36,740
from the Sun
that we find Mars today,

527
00:29:36,740 --> 00:29:41,040
and in the process ate
the red planet's lunch.

528
00:29:41,050 --> 00:29:42,850
Jupiter
removes all of the material

529
00:29:42,850 --> 00:29:45,550
that Mars otherwise would
have been building on

530
00:29:45,550 --> 00:29:47,680
for the next tens
of millions of years,

531
00:29:47,690 --> 00:29:50,020
essentially clears out a big
chunk of the solar system

532
00:29:50,020 --> 00:29:51,850
and starves Mars.

533
00:29:51,860 --> 00:29:53,420
If Jupiter were not there,

534
00:29:53,430 --> 00:29:54,760
than we would have expected Mars

535
00:29:54,760 --> 00:29:57,590
to have formed a fully-fledged
super-Earth planet.

536
00:29:59,730 --> 00:30:03,000
Earth formed from
the wreckage of this pileup,

537
00:30:03,000 --> 00:30:06,170
but a reminder
of this population lives on,

538
00:30:06,170 --> 00:30:11,640
every time we look to the night sky...
The Moon.

539
00:30:11,640 --> 00:30:14,780
We were convinced we knew
how the Moon formed.

540
00:30:14,780 --> 00:30:18,550
Turns out, we were
completely wrong.

541
00:30:39,730 --> 00:30:42,870
A distance observer
studying our solar system

542
00:30:42,870 --> 00:30:47,070
would notice something
strange right away...

543
00:30:47,070 --> 00:30:50,710
The size of Earth's moon.

544
00:30:50,710 --> 00:30:54,880
Most planet's moons
are tiny by comparison.

545
00:30:54,880 --> 00:30:57,010
How did we get a moon so big?

546
00:30:59,220 --> 00:31:01,220
For a while we've realized
it couldn't have formed

547
00:31:01,220 --> 00:31:02,550
at the same time as the Earth.

548
00:31:02,560 --> 00:31:04,720
It just doesn't make sense.

549
00:31:04,720 --> 00:31:06,860
The standard idea of
the Moon's formation

550
00:31:06,860 --> 00:31:08,990
is that an object
about the size of Mars

551
00:31:09,000 --> 00:31:10,760
collided with the early Earth.

552
00:31:10,760 --> 00:31:13,500
A lot of the debris was thrown
into orbit around the Earth,

553
00:31:13,500 --> 00:31:16,630
and it coalesced
to form the Moon.

554
00:31:16,640 --> 00:31:20,910
We call
this Mars-sized object Thea,

555
00:31:20,910 --> 00:31:24,440
but exactly when
and how the Moon formed

556
00:31:24,440 --> 00:31:27,650
remains a mystery.

557
00:31:27,650 --> 00:31:29,710
Ever since the Apollo missions,

558
00:31:29,720 --> 00:31:32,280
we've been searching
for a piece of lunar rock

559
00:31:32,290 --> 00:31:33,920
that can unlock this secret.

560
00:31:36,360 --> 00:31:40,290
Melanie Barboni's team at UCLA
is one of the few groups

561
00:31:40,290 --> 00:31:44,630
authorized to analyze these
precious lunar samples.

562
00:31:47,500 --> 00:31:48,930
But there's a problem...

563
00:31:48,940 --> 00:31:51,100
Most moon rock is contaminated

564
00:31:51,100 --> 00:31:56,570
and damaged by violent events
in the more recent past.

565
00:31:56,580 --> 00:31:58,340
Asteroids hit the Moon,
and there are geological

566
00:31:58,340 --> 00:31:59,810
processes
that do a lot of mixing,

567
00:31:59,810 --> 00:32:02,910
and it's very difficult
to find a pristine sample

568
00:32:02,920 --> 00:32:07,080
from the Moon's very formation.

569
00:32:07,090 --> 00:32:09,520
Melanie and her team
have come up with a novel

570
00:32:09,520 --> 00:32:11,320
answer to that problem.

571
00:32:11,320 --> 00:32:13,660
Rather than date
the entire rock,

572
00:32:13,660 --> 00:32:18,200
they isolate a tiny, pristine
crystal within a lunar sample,

573
00:32:18,200 --> 00:32:20,730
known as a zircon.

574
00:32:20,730 --> 00:32:22,370
Now we don't want
the whole rock,

575
00:32:22,370 --> 00:32:26,700
we want only tiny zircon
that are inside those rocks.

576
00:32:28,610 --> 00:32:29,940
These zircons formed

577
00:32:29,940 --> 00:32:32,840
just after Thea's collision
with Earth.

578
00:32:32,850 --> 00:32:34,850
Once the molten crust
of the Moon,

579
00:32:34,850 --> 00:32:37,720
it cooled and solidified.

580
00:32:37,720 --> 00:32:40,320
This is much smaller
than the grain of sand

581
00:32:40,320 --> 00:32:42,320
you find on the beach.

582
00:32:42,320 --> 00:32:44,890
Zircon is the most perfect clock

583
00:32:44,890 --> 00:32:46,890
that nature gave us
to date the Moon

584
00:32:46,890 --> 00:32:49,230
because it's very resistant.

585
00:32:49,230 --> 00:32:52,030
Here you can see
its surface is very smooth.

586
00:32:52,030 --> 00:32:55,130
There is no fractures on it.

587
00:32:55,130 --> 00:32:58,140
Zircons tick off time
like clocks.

588
00:32:58,140 --> 00:33:00,710
They contain large
radioactive elements

589
00:33:00,710 --> 00:33:03,240
that decay into smaller ones.

590
00:33:03,240 --> 00:33:05,210
Scientists can tell
how old the crystal

591
00:33:05,210 --> 00:33:09,010
is by measuring
the radioactive decay.

592
00:33:09,020 --> 00:33:14,890
The zircons Melanie found
rewrite the history of the Moon.

593
00:33:14,890 --> 00:33:20,120
The Moon is around
140 million years older

594
00:33:20,130 --> 00:33:21,760
than what we thought.

595
00:33:21,760 --> 00:33:23,690
This means the Moon formed

596
00:33:23,700 --> 00:33:26,300
no later than 60 million years

597
00:33:26,300 --> 00:33:28,670
after the birth of the Sun.

598
00:33:28,670 --> 00:33:32,070
This places the formation
of the Moon right in the middle

599
00:33:32,070 --> 00:33:35,810
of the destruction of
the 30-planet pileup.

600
00:33:35,810 --> 00:33:39,540
It's entirely possible
that Thea was once a member

601
00:33:39,550 --> 00:33:43,610
of this colony
of Mars-sized objects.

602
00:33:43,620 --> 00:33:45,580
It wasn't just some Mars object

603
00:33:45,590 --> 00:33:47,020
that was out in
the outer solar system

604
00:33:47,020 --> 00:33:49,150
and came careening in
and smashed into us.

605
00:33:49,160 --> 00:33:53,020
It was one of these
no-longer-existing planetesimals

606
00:33:53,030 --> 00:33:56,760
that slammed into us
and formed the Moon.

607
00:33:56,760 --> 00:33:59,430
But scientists looking
for traces of Thea

608
00:33:59,430 --> 00:34:03,370
on the Moon draw blanks.

609
00:34:03,370 --> 00:34:04,970
One of the intriguing things
about moon rocks

610
00:34:04,970 --> 00:34:07,810
is how similar they are,
chemically, to rocks on Earth.

611
00:34:07,810 --> 00:34:09,940
It has the same
geochemical fingerprints,

612
00:34:09,940 --> 00:34:12,140
the oxygen isotopes,
of the Earth,

613
00:34:12,150 --> 00:34:14,250
and all the other chemical
isotopes of the Earth.

614
00:34:14,250 --> 00:34:16,650
It looks just like Earth rock.

615
00:34:16,650 --> 00:34:19,480
If the Moon really is the
product of one giant collision,

616
00:34:19,490 --> 00:34:21,320
well, whatever hit the Earth,

617
00:34:21,320 --> 00:34:23,620
there should be different
proportions of that on the Earth

618
00:34:23,620 --> 00:34:26,590
as opposed to the Moon,
but we don't find that.

619
00:34:28,460 --> 00:34:31,730
The Moon is identical
to material from the Earth,

620
00:34:31,730 --> 00:34:35,870
except it's missing heavy
elements, iron and nickel,

621
00:34:35,870 --> 00:34:37,740
found in the Earth's core.

622
00:34:37,740 --> 00:34:41,840
Instead, it mainly contains
lighter rocky elements

623
00:34:41,840 --> 00:34:44,640
found in the Earth's
crust and mantle.

624
00:34:44,640 --> 00:34:46,910
Why?

625
00:34:46,910 --> 00:34:48,210
It wasn't a head-on collision.

626
00:34:48,210 --> 00:34:49,980
It was a grazing collision.

627
00:34:49,980 --> 00:34:51,780
Now, that's important because
the heavy material

628
00:34:51,780 --> 00:34:54,220
was starting to sink
into the center of the Earth,

629
00:34:54,220 --> 00:34:55,951
and the lighter stuff
was floating to the top.

630
00:34:55,960 --> 00:34:58,090
And if this were
a grazing collision,

631
00:34:58,090 --> 00:35:00,620
then that lighter material
would have been splashed out,

632
00:35:00,630 --> 00:35:02,660
and that's what would
have formed the Moon.

633
00:35:02,660 --> 00:35:05,030
And the Moon is, in fact,
less dense than the Earth,

634
00:35:05,030 --> 00:35:07,830
which makes sense if it formed
from this lighter material

635
00:35:07,830 --> 00:35:09,330
that was near the top.

636
00:35:09,340 --> 00:35:11,800
It looks like you took a blob
of the Earth's mantle

637
00:35:11,800 --> 00:35:14,070
and just put it into space
around the Earth.

638
00:35:14,070 --> 00:35:17,010
A single head-on
collision would leave traces

639
00:35:17,010 --> 00:35:21,550
of both Thea and Earth's core
on the Moon,

640
00:35:21,550 --> 00:35:25,250
but a glancing blow wouldn't
knock off enough material

641
00:35:25,250 --> 00:35:28,790
to form a moon as big as ours.

642
00:35:28,790 --> 00:35:30,920
One way we can end up
with the Earth-moon system

643
00:35:30,920 --> 00:35:33,490
that we see today
and solve all these problems,

644
00:35:33,490 --> 00:35:36,190
is that instead of having
one big collision,

645
00:35:36,200 --> 00:35:39,400
there were a series of
several smaller collisions.

646
00:35:43,240 --> 00:35:47,070
Each impact grazes off
a section of Earth's crust

647
00:35:47,070 --> 00:35:49,770
that forms a ring
around our planet.

648
00:35:49,780 --> 00:35:51,380
With each small collision,

649
00:35:51,380 --> 00:35:54,710
material would have been thrown
into orbit around the Earth.

650
00:35:54,710 --> 00:35:57,280
Eventually this
collisional debris merges

651
00:35:57,280 --> 00:36:01,290
to form a small,
new moon, a moonlet.

652
00:36:01,290 --> 00:36:03,990
Now after several of
these collisions,

653
00:36:03,990 --> 00:36:07,120
you'll have debris from each
collision circling the Earth.

654
00:36:07,130 --> 00:36:09,090
Some of it is still in
the form of debris,

655
00:36:09,100 --> 00:36:11,300
some of it is in
the form of moonlets.

656
00:36:11,300 --> 00:36:16,130
Eventually they coalesce
to form our current moon.

657
00:36:16,140 --> 00:36:18,270
It seems our moon may well be

658
00:36:18,270 --> 00:36:19,940
the product of a series

659
00:36:19,940 --> 00:36:24,110
of cosmic collisions
in the early solar system.

660
00:36:24,110 --> 00:36:28,610
What we see when we look up in
the sky now isn't the moon,

661
00:36:28,610 --> 00:36:31,920
but it's basically
the last moon that survived.

662
00:36:31,920 --> 00:36:34,450
It was just the one
that happened to be there

663
00:36:34,450 --> 00:36:36,620
when all of these
impacts stopped.

664
00:36:40,330 --> 00:36:43,560
The closer we come to
understanding our violent past,

665
00:36:43,560 --> 00:36:47,670
the more we appreciate
the calm of the present,

666
00:36:47,670 --> 00:36:50,330
but as we try
to predict our future,

667
00:36:50,340 --> 00:36:54,140
it seems we are destined
for chaos once again,

668
00:36:54,140 --> 00:36:58,510
as a distant mystery planet
in the outer solar system

669
00:36:58,510 --> 00:37:00,880
moves in from the cold.

670
00:37:19,730 --> 00:37:22,230
The birth of our solar system...

671
00:37:22,230 --> 00:37:26,670
Violent, chaotic, catastrophic.

672
00:37:26,670 --> 00:37:29,570
When we look at the solar
system when it was very young,

673
00:37:29,570 --> 00:37:31,740
all of our models pretty much
say the same thing.

674
00:37:31,740 --> 00:37:33,710
It was not nice and orderly.

675
00:37:33,710 --> 00:37:35,240
It was a disaster.

676
00:37:35,240 --> 00:37:37,280
And then things settled down.

677
00:37:37,280 --> 00:37:40,710
Life had a chance to take hold
and evolve under very stable,

678
00:37:40,720 --> 00:37:42,480
very friendly conditions.

679
00:37:42,480 --> 00:37:44,050
So when you look around
right now,

680
00:37:44,050 --> 00:37:47,550
you're seeing the story of
an ancient, violent past

681
00:37:47,560 --> 00:37:50,220
that has smoothed out
into the wonderful environment

682
00:37:50,220 --> 00:37:51,990
we know today.

683
00:37:51,990 --> 00:37:54,290
Our solar system
might seem stable,

684
00:37:54,300 --> 00:37:57,560
but there is still something
very strange about it.

685
00:37:57,570 --> 00:38:00,030
There is still one
enduring mystery,

686
00:38:00,030 --> 00:38:03,940
and that is why
the solar system tilted?

687
00:38:03,940 --> 00:38:05,300
The eight planets orbit

688
00:38:05,310 --> 00:38:08,540
in roughly the same flat plane,

689
00:38:08,540 --> 00:38:11,610
but compared to the spin axis
of the Sun,

690
00:38:11,610 --> 00:38:17,750
that plane is tilted, making
the Sun look lopsided.

691
00:38:17,750 --> 00:38:19,550
And it turns out, when
you look at the Sun's tilt,

692
00:38:19,550 --> 00:38:21,950
it's actually
tipped by 6 degrees,

693
00:38:21,960 --> 00:38:23,690
the plane of the solar system.

694
00:38:23,690 --> 00:38:25,190
And that may not sound
like a lot,

695
00:38:25,190 --> 00:38:26,730
but it's actually quite a bit

696
00:38:26,730 --> 00:38:29,630
compared to the tilts of all
the planets of the solar system,

697
00:38:29,630 --> 00:38:31,060
and this is an outlier.

698
00:38:31,070 --> 00:38:32,430
It's strange.

699
00:38:32,430 --> 00:38:34,630
What could have done that?

700
00:38:34,640 --> 00:38:37,000
The tilt contradicts
what we know about

701
00:38:37,000 --> 00:38:39,000
how the solar system formed,

702
00:38:39,010 --> 00:38:43,740
a spinning cloud
collapses into a disk.

703
00:38:43,740 --> 00:38:48,350
The spinning disk then becomes
the Sun and all the planets.

704
00:38:48,350 --> 00:38:52,550
It should all be spinning
on the same axis.

705
00:38:52,550 --> 00:38:55,990
So one possible way that
you could change the orientation

706
00:38:55,990 --> 00:38:58,520
of the pull of the Sun relative
to the plane the planets are in,

707
00:38:58,530 --> 00:39:00,290
is if there was
something out there

708
00:39:00,290 --> 00:39:03,660
tugging on the planets
for a very long time.

709
00:39:03,660 --> 00:39:06,530
2016, Cal tech astronomers

710
00:39:06,530 --> 00:39:09,540
Konstantin Batygin
and Mike Brown

711
00:39:09,540 --> 00:39:13,940
claim they've found
the missing something...

712
00:39:13,940 --> 00:39:20,650
Planet 9, a theoretical
giant orbiting off-kilter

713
00:39:20,650 --> 00:39:23,720
in the far reaches
of the solar system.

714
00:39:23,720 --> 00:39:28,190
Planet 9 resides on a long
and substantial orbit,

715
00:39:28,190 --> 00:39:33,430
and it itself is pretty massive,
about 10 Earth masses or so.

716
00:39:33,430 --> 00:39:36,600
If it's orbiting the Sun on a
highly elliptical tilted orbit,

717
00:39:36,600 --> 00:39:38,200
every time
it gets close to the Sun,

718
00:39:38,200 --> 00:39:40,930
it's going to tug on the planets
just a little bit.

719
00:39:40,940 --> 00:39:44,400
But over hundreds and thousands
of orbits, it can actually tip

720
00:39:44,410 --> 00:39:46,940
the orbits of all the planets
in the solar system,

721
00:39:46,940 --> 00:39:49,340
but it won't tip the Sun.

722
00:39:49,340 --> 00:39:52,340
Over billions of
years, the planetary system

723
00:39:52,350 --> 00:39:57,150
slowly twists out of alignment
with its original plane.

724
00:39:57,150 --> 00:39:59,220
Planet 9's distant reach

725
00:39:59,220 --> 00:40:03,260
may solve the mystery
of the solar system's tilt,

726
00:40:03,260 --> 00:40:06,990
but it might also have
a disastrous effect

727
00:40:06,990 --> 00:40:10,800
on the outer planets as
the Sun starts to die.

728
00:40:19,010 --> 00:40:21,240
the Sun, just like every other
star in the universe,

729
00:40:21,240 --> 00:40:22,670
has a life cycle.

730
00:40:22,680 --> 00:40:23,810
It was born.

731
00:40:23,810 --> 00:40:26,280
It is currently living its life,
and it will die.

732
00:40:26,280 --> 00:40:29,820
As it dies, it will bloat up
into a red giant star,

733
00:40:29,820 --> 00:40:32,780
and then the outer layers
will begin to drift away.

734
00:40:32,790 --> 00:40:34,090
Now what that means is that

735
00:40:34,090 --> 00:40:36,820
the Sun will be losing
mass very quickly.

736
00:40:36,820 --> 00:40:39,690
The thing that holds us
in orbit around the Sun

737
00:40:39,690 --> 00:40:42,260
is the gravitational
pull of the Sun.

738
00:40:42,260 --> 00:40:45,400
Jupiter, Saturn,
Uranus, and Neptune

739
00:40:45,400 --> 00:40:46,800
will move away from the Sun

740
00:40:46,800 --> 00:40:49,430
as its outer layers expand,

741
00:40:49,440 --> 00:40:52,740
but that's not the case
for distant Planet 9.

742
00:40:52,740 --> 00:40:55,170
Scientists think
that Planet 9 orbits

743
00:40:55,180 --> 00:40:58,040
so far out that as the Sun dies,

744
00:40:58,050 --> 00:41:01,550
it will loosen its gravitational
grip on the planet.

745
00:41:01,550 --> 00:41:05,480
Planet 9 starts to feel
the influence of other objects

746
00:41:05,490 --> 00:41:07,520
more than the Sun.

747
00:41:07,520 --> 00:41:10,260
It turns out a passing
star, for example,

748
00:41:10,260 --> 00:41:11,490
could affect its orbit,

749
00:41:11,490 --> 00:41:14,760
or even tides
from the galaxy itself,

750
00:41:14,760 --> 00:41:18,000
our galaxy's gravitational field
can affect this planet,

751
00:41:18,000 --> 00:41:20,370
and drop it into
the inner solar system.

752
00:41:20,370 --> 00:41:22,700
This change in Planet 9's orbits

753
00:41:22,700 --> 00:41:25,370
could be disastrous
for the solar system.

754
00:41:25,370 --> 00:41:26,710
And if that happens,

755
00:41:26,710 --> 00:41:29,680
it could actually
wreak havoc on the gas giants

756
00:41:29,680 --> 00:41:31,610
Jupiter, Saturn, Uranus,
and Neptune

757
00:41:31,610 --> 00:41:33,210
and distort their orbits,

758
00:41:33,210 --> 00:41:35,080
maybe dropping them into the Sun

759
00:41:35,080 --> 00:41:37,480
or flinging them out of
the solar system, as well.

760
00:41:40,620 --> 00:41:43,290
Planet 9 could make
the death of the solar system

761
00:41:43,290 --> 00:41:46,430
just as violent as its birth.

762
00:41:46,430 --> 00:41:48,960
Right now we're in this
wonderful sweet spot

763
00:41:48,960 --> 00:41:51,460
where life can evolve
and take hold between

764
00:41:51,470 --> 00:41:54,970
two eras of almost
unimaginable violence.

765
00:41:54,970 --> 00:41:58,400
If there is a Planet 9,
then it's kind of a rehash

766
00:41:58,410 --> 00:42:00,170
of what happened
in the early solar system,

767
00:42:00,170 --> 00:42:01,840
when everything
was really chaotic

768
00:42:01,840 --> 00:42:04,110
because of a giant planet
moving inward.

769
00:42:04,110 --> 00:42:07,710
The same thing
could happen again.

770
00:42:07,720 --> 00:42:09,210
Born in chaos.

771
00:42:09,220 --> 00:42:11,850
Perhaps ending in worse.

772
00:42:11,850 --> 00:42:13,420
One thing is clear...

773
00:42:13,420 --> 00:42:16,460
What we thought we knew
of our cosmic home

774
00:42:16,460 --> 00:42:18,120
grows more intriguing

775
00:42:18,130 --> 00:42:22,490
with each new clue
to this once cold case,

776
00:42:22,500 --> 00:42:25,400
now a very hot one.