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The search for our solar system's ninth planet

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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.
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    Just like Bouvard used
    unrecognized observations of Uranus
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    from 91 years before its discovery,
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    I bet that there are unrecognized images
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    that show the location of Planet Nine.
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    It's going to be a massive
    computational undertaking
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    to go through all of the old data,
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    and pick out that one faint moving planet.
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    But we're underway.
  • 13:01 - 13:03
    And I think we're getting close.
  • 13:03 - 13:06
    So I would say, get ready.
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    We are not going to match Le Verrier's
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    "make a prediction,
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    have the planet found in a single night
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    that close to where
    you predicted it" record.
  • 13:15 - 13:19
    But I do bet that within
    the next couple of years
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    some astronomer somewhere
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    will find a faint point of light,
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    slowly moving across the sky
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    and triumphantly announce
    the discovery of a new,
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    and quite possibly not the last,
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    real planet of our Solar system.
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    Thank you.
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    (Applause)
Title:
The search for our solar system's ninth planet
Speaker:
Mike Brown
Description:

more » « less
Video Language:
English
Team:
closed TED
Project:
TEDTalks
Duration:
13:52

English subtitles

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