WEBVTT

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I'm going to tell you a story
from 200 years ago.

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In 1820, French astronomer Alexis Bouvard

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almost became the second person
in human history to discover a planet.

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He'd been tracking the position
of Uranus across the night sky

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using old star catalogs,

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and it didn't quite go around the Sun

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the way that his predictions
said it should.

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Sometimes it was a little too fast,

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sometimes it was a little too slow.

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Bouvard knew that
his predictions were perfect.

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So it had to be that those
old star catalogs were bad.

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He told astronomers of the day,

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"Do better measurements."

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So they did.

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Astronomers spent the next two decades

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meticulously tracking the position
of Uranus across the sky,

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but it still didn't fit
Bouvard's predictions.

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By 1840, it had become obvious.

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The problem was not
with those old star catalogs,

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the problem was with the predictions.

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And astronomers knew why.

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They realized that there must be
a distant, giant planet

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just beyond the orbit of Uranus

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that was tugging along at that orbit,

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sometimes pulling it along a bit too fast,

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sometimes holding it back.

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Must have been frustrating back in 1840

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to see these gravitational effects
of this distant, giant planet

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but not yet know how to actually find it.

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Trust me, it's really frustrating.

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(Laughter)

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But in 1846, another French astronomer,

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Urbain Le Verrier,

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worked through the math

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and figured out how to predict
the location of the planet.

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He sent his prediction
to the Berlin observatory,

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they opened up their telescope

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and in the very first night
they found this faint point of light

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slowly moving across the sky
and discovered Neptune.

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It was this close on the sky
to Le Verrier's predicted location.

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The story of prediction
and discrepancy and new theory

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and triumphant discoveries is so classic

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and Le Verrier became so famous from it,

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that people tried to get in
on the act right away.

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In the last 163 years,

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dozens of astronomers have used
some sort of alleged orbital discrepancy

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to predict the existence
of some new planet in the Solar system.

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They have always been wrong.

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The most famous of these
erroneous predictions

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came from Percival Lowell,

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who was convinced that there must be
a planet just beyond Uranus and Neptune,

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messing with those orbits.

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And so when Pluto was discovered in 1930,

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at the Lowell Observatory,

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everybody assumed that it must be
the planet that Lowell had predicted.

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They were wrong.

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It turns out, Uranus and Neptune
are exactly where they're supposed to be.

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It took 100 years,

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but Bouvard was eventually right.

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Astronomers needed to do
better measurements.

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And when they did,

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those better measurements
had turned out that

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there is no planet just beyond
the orbit of Uranus and Neptune

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and Pluto is thousands of times too small

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to have any effect on those orbits at all.

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So even though Pluto
turned out not to be the planet

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it was originally thought to be,

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it was the first discovery
of what is now known to be

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thousands of tiny icy objects
in orbit beyond the planets.

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Here you can see the orbits of Jupiter,

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Saturn, Uranus and Neptune,

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and in that little circle
in the very center is the Earth

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and the Sun and almost everything
that you know and love.

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And those yellow circles at the edge

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are these icy bodies
out beyond the planets.

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These icy bodies are pushed and pulled

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by the gravitational fields of the planets

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in entirely predictable ways.

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Everything goes around the Sun
exactly the way it is supposed to.

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Almost.

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So in 2003,

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I discovered, what was at the time,

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the most distant known object
in the entire Solar system.

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It's hard not to look
at that lonely body out there

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and say, oh yeah sure,
so Lowell was wrong,

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there was no planet just beyond Neptune,

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but this, this could be a new planet.

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The real question we had was,

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what kind of orbit
does it have around the Sun?

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Does it go in a circle around the Sun

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like a planet should?

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Or is it just a typical member
of this icy belt of bodies

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that got a little bit tossed outward
and it's now on its way back in?

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This is precisely the question

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the astronomers were trying
to answer about Uranus 200 years ago.

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They did it by using
overlooked observations of Uranus

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from 91 years before its discovery
to figure out its entire orbit.

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We couldn't go quite that far back,

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but we did find observations
of our object from 13 years earlier,

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that allowed us to figure out
how it went around the Sun.

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So the question is,

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is it in a circular orbit
around the Sun, like a planet,

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or is it on its way back in,

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like one of these typical icy bodies?

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And the answer is

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no.

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It has a massively elongated orbit

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that takes 10,000 years
to go around the Sun.

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We named this object Sedna,

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after the Inuit goddess of the sea

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in honor of the cold, icy places
where it spends all of its time.

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We now know that Sedna,

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it's about a third the size of Pluto

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and it's a relatively typical member

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of those icy bodies out beyond Neptune.

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Relatively typical,
except for this bizarre orbit.

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You might look at this orbit and say,

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"Yeah, that's bizarre,
10,000 years to go around the Sun,"

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but that's not really the bizarre part.

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The bizarre part is
that in those 10,000 years

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Sedna never comes close
to anything else in the Solar system.

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Even at its closest approach to the Sun,

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Sedna is further from Neptune

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than Neptune is from the Earth.

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If Sedna had had an orbit like this,

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that kisses the orbit of Neptune
once around the Sun,

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that would have actually been
really easy to explain.

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That would have just been an object

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that had been in a circular
orbit around the Sun

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in that region of icy bodies,

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had gotten a little bit
too close to Neptune one time,

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and then got slingshot out
and is now on its way back in.

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But Sedna never comes close
to anything known in the Solar system

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that could have given it that slingshot.

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Neptune can't be responsible,

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but something had to be responsible.

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This was the first time since 1845

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that we saw the gravitational effects
of something in the outer Solar system,

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and didn't know what it was.

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I actually thought I knew
what the answer was.

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Sure, it could have been some
distant, giant planet

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in the outer Solar system,

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but by this time, that idea
was so ridiculous

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and had been so thoroughly discredited

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that I didn't take it very seriously.

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But 4.5 billion years ago,

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when the Sun formed on a cocoon
of hundreds of other stars,

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any one of those stars

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could have gotten
just a little bit too close to Sedna

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and perturbed it onto the orbit
that it has today.

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When that cluster of stars
dissipated into the galaxy,

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the orbit of Sedna would have been
left as a fossil record

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of this earliest history of the Sun.

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I was so excited by this idea,

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by the idea that we could look

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at the fossil history
of the birth of the Sun,

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that I spent the next decade

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looking for more objects
with orbits like Sedna.

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In that ten-year period, I found zero.

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(Laughter)

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But my colleagues Chad Trujillo
and Scott Sheppard, did a better job,

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and they have now found several objects
with orbits like Sedna,

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which is super exciting.

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But what's even more interesting

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is that they found that all these objects

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are not only on these distant,
elongated orbits,

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they also share a common value
of this obscure orbital parameter

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that in celestial mechanics we call
argument of perihelion.

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When they realized it was clustered
in argument of perihelion,

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they immediately jumped up and down,

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saying it must be caused
by a distant, giant planet out there,

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which is really exciting,
except it makes no sense at all.

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Let me try to explain it
to you why, with an analogy.

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Imagine a person walking down a plaza

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and looking 45 degrees to his right side.

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There's a lot of reasons
that might happen,

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it's super easy to explain, no big deal.

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Imagine now many different people,

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all walking in different
directions across the plaza,

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but all looking 45 degrees
to the direction that they're moving.

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Everybody's moving
in different directions,

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everybody's looking
in different directions,

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but they're all looking 45 degrees
to the direction of motion.

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What could cause something like that?

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I have no idea.

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It's very difficult to think of any reason
that that would happen.

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(Laughter)

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And this is essentially
what that clustering

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in argument of perihelion was telling us.

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Scientists were generally baffled
and they assumed it must just be a fluke

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and some bad observations.

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They told the astronomers,

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"Do better measurements."

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I actually took a very careful look
at those measurements, though,

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and they were right.

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These objects really did all share

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a common value of argument of perihelion,

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and they shouldn't.

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Something had to be causing that.

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The final piece of the puzzle
came into place in 2016,

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when my colleague, Konstantin Batygin,

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who works three doors down from me, and I

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realized that the reason
that everybody was baffled

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was because argument of perihelion
was only part of the story.

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If you look at these
objects the right way,

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they are all actually lined up
in space in the same direction,

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and they're all tilted in space
in the same direction.

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It's as if all those people on the plaza
are all walking in the same direction

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and they're all looking
45 degrees to the right side.

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That's easy to explain.

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They're all looking at something.

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These objects in the outer Solar system
are all reacting to something.

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But what?

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Konstantin and I spent a year

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trying to come up with any explanation
other than a distant, giant planet

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in the outer Solar system.

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We did not want to be the 33rd and 34th
people in history to propose this planet

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to yet again be told we were wrong.

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But after a year,

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there was really no choice.

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We could come up with no other explanation

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other than that that there is a distant,

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massive planet on an elongated orbit,

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inclined to the rest of the Solar system,

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that is forcing these patterns
for these objects

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in the outer Solar system.

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Guess what else a planet like this does.

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Remember that strange orbit of Sedna,

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how it was kind of pulled away
from the Sun in one direction?

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A planet like this would make
orbits like that all day long.

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We knew we were onto something.

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So this brings us to today.

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We are basically 1845, Paris.

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(Laughter)

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We see the gravitational effects
of a distant, giant planet,

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and we are trying to work the calculations

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to tell us where to look,
to point our telescopes,

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to find this planet.

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We've done massive suits
of computer simulations,

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massive months of analytic calculations,

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and here's what I cal tell you so far.

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First, this planet,
which we call Planet Nine,

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because that's what it is,

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Planet Nine is six times
the mass of the Earth.

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This is no slightly-smaller-than-Pluto,

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let's-all-argue-about
whether-it's-a-planet-or-not thing.

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This is the fifth largest planet
in our entire Solar system.

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For context, let me show you
the sizes of the planets.

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In the back there,
you can the massive Jupiter and Saturn.

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Next to them, a little bit smaller,
Uranus and Neptune.

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Up in the corner, the terrestrial planets,
Mercury, Venus, Earth and Mars.

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You can even see that belt

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of icy bodies beyond Neptune,
of which Pluto is a member,

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good luck figuring out which one it is.

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And here is Planet Nine.

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Planet Nine is big.

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Planet Nine is so big,

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you should probably wonder
why haven't we found it yet.

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Well, Planet Nine is big,

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but it's also really, really far away.

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It's something like
15 times further away than Neptune.

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And that makes it about 50,000 times
fainter than Neptune.

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And also, the sky is a really big place.

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We've narrowed down where we think it is

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to a relatively small area of the sky,

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but it would still take us years

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to systematically cover
the area of the sky

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with the large telescopes that we need

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to see something that's
this far away and this faint.

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Luckily, we might not have to.

NOTE Paragraph

00:12:34.729 --> 00:12:39.618
Just like Bouvard used
unrecognized observations of Uranus

00:12:39.642 --> 00:12:42.387
from 91 years before its discovery,

00:12:42.411 --> 00:12:46.192
I bet that there are unrecognized images

00:12:46.216 --> 00:12:49.078
that show the location of Planet Nine.

00:12:49.984 --> 00:12:53.061
It's going to be a massive
computational undertaking

00:12:53.085 --> 00:12:55.389
to go through all of the old data,

00:12:55.413 --> 00:12:58.331
and pick out that one faint moving planet.

00:12:59.276 --> 00:13:00.641
But we're underway.

00:13:00.665 --> 00:13:02.990
And I think we're getting close.

NOTE Paragraph

00:13:03.014 --> 00:13:05.514
So I would say, get ready.

00:13:05.538 --> 00:13:09.513
We are not going to match Le Verrier's

00:13:09.537 --> 00:13:10.705
"make a prediction,

00:13:10.729 --> 00:13:12.633
have the planet found in a single night

00:13:12.657 --> 00:13:14.919
that close to where
you predicted it" record.

00:13:14.943 --> 00:13:18.882
But I do bet that within
the next couple of years

00:13:18.906 --> 00:13:21.319
some astronomer somewhere

00:13:21.343 --> 00:13:23.573
will find a faint point of light,

00:13:23.597 --> 00:13:25.866
slowly moving across the sky

00:13:25.890 --> 00:13:29.174
and triumphantly announce
the discovery of a new,

00:13:29.198 --> 00:13:31.635
and quite possibly not the last,

00:13:31.659 --> 00:13:33.792
real planet of our Solar system.

NOTE Paragraph

00:13:34.183 --> 00:13:35.334
Thank you.

NOTE Paragraph

00:13:35.358 --> 00:13:38.877
(Applause)