The search for our solar system's ninth planet

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

In 1820, French astronomer Alexis Bouvard

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

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

using old star catalogs,

and it didn't quite go around the Sun

the way that his predictions
said it should.

Sometimes it was a little too fast,

sometimes it was a little too slow.

Bouvard knew that
his predictions were perfect.

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

He told astronomers of the day,

"Do better measurements."

So they did.

Astronomers spent the next two decades

meticulously tracking the position
of Uranus across the sky,

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

By 1840, it had become obvious.

The problem was not
with those old star catalogs,

the problem was with the predictions.

And astronomers knew why.

They realized that there must be
a distant, giant planet

just beyond the orbit of Uranus

that was tugging along at that orbit,

sometimes pulling it along a bit too fast,

sometimes holding it back.

Must have been frustrating back in 1840

to see these gravitational effects
of this distant, giant planet

but not yet know how to actually find it.

Trust me, it's really frustrating.

(Laughter)

But in 1846, another French astronomer,

Urbain Le Verrier,

worked through the math

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

He sent his prediction
to the Berlin observatory,

they opened up their telescope

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

slowly moving across the sky
and discovered Neptune.

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

The story of prediction
and discrepancy and new theory

and triumphant discoveries is so classic

and Le Verrier became so famous from it,

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

In the last 163 years,

dozens of astronomers have used
some sort of alleged orbital discrepancy

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

They have always been wrong.

The most famous of these
erroneous predictions

came from Percival Lowell,

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

messing with those orbits.

And so when Pluto was discovered in 1930,

at the Lowell Observatory,

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

They were wrong.

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

It took 100 years,

but Bouvard was eventually right.

Astronomers needed to do
better measurements.

And when they did,

those better measurements
had turned out that

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

and Pluto is thousands of times too small

to have any effect on those orbits at all.

So even though Pluto
turned out not to be the planet

it was originally thought to be,

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

thousands of tiny icy objects
in orbit beyond the planets.

Here you can see the orbits of Jupiter,

Saturn, Uranus and Neptune,

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

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

And those yellow circles at the edge

are these icy bodies
out beyond the planets.

These icy bodies are pushed and pulled

by the gravitational fields of the planets

in entirely predictable ways.

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

Almost.

So in 2003,

I discovered, what was at the time,

the most distant known object
in the entire Solar system.

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

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

there was no planet just beyond Neptune,

but this, this could be a new planet.

The real question we had was,

what kind of orbit
does it have around the Sun?

Does it go in a circle around the Sun

like a planet should?

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

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

This is precisely the question

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

They did it by using
overlooked observations of Uranus

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

We couldn't go quite that far back,

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

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

So the question is,

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

or is it on its way back in,

like one of these typical icy bodies?

And the answer is

no.

It has a massively elongated orbit

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

We named this object Sedna,

after the Inuit goddess of the sea

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

We now know that Sedna,

it's about a third the size of Pluto

and it's a relatively typical member

of those icy bodies out beyond Neptune.

Relatively typical,
except for this bizarre orbit.

You might look at this orbit and say,

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

but that's not really the bizarre part.

The bizarre part is
that in those 10,000 years

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

Even at its closest approach to the Sun,

Sedna is further from Neptune

than Neptune is from the Earth.

If Sedna had had an orbit like this,

that kisses the orbit of Neptune
once around the Sun,

that would have actually been
really easy to explain.

That would have just been an object

that had been in a circular
orbit around the Sun

in that region of icy bodies,

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

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

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

that could have given it that slingshot.

Neptune can't be responsible,

but something had to be responsible.

This was the first time since 1845

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

and didn't know what it was.

I actually thought I knew
what the answer was.

Sure, it could have been some
distant, giant planet

in the outer Solar system,

but by this time, that idea
was so ridiculous

and had been so thoroughly discredited

that I didn't take it very seriously.

But 4.5 billion years ago,

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

any one of those stars

could have gotten
just a little bit too close to Sedna

and perturbed it onto the orbit
that it has today.

When that cluster of stars
dissipated into the galaxy,

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

of this earliest history of the Sun.

I was so excited by this idea,

by the idea that we could look

at the fossil history
of the birth of the Sun,

that I spent the next decade

looking for more objects
with orbits like Sedna.

In that ten-year period, I found zero.

(Laughter)

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

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

which is super exciting.

But what's even more interesting

is that they found that all these objects

are not only on these distant,
elongated orbits,

they also share a common value
of this obscure orbital parameter

that in celestial mechanics we call
argument of perihelion.

When they realized it was clustered
in argument of perihelion,

they immediately jumped up and down,

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

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

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

Imagine a person walking down a plaza

and looking 45 degrees to his right side.

There's a lot of reasons
that might happen,

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

Imagine now many different people,

all walking in different
directions across the plaza,

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

Everybody's moving
in different directions,

everybody's looking
in different directions,

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

What could cause something like that?

I have no idea.

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

(Laughter)

And this is essentially
what that clustering

in argument of perihelion was telling us.

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

and some bad observations.

They told the astronomers,

"Do better measurements."

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

and they were right.

These objects really did all share

a common value of argument of perihelion,

and they shouldn't.

Something had to be causing that.

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

when my colleague, Konstantin Batygin,

who works three doors down from me, and I

realized that the reason
that everybody was baffled

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

If you look at these
objects the right way,

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

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

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

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

That's easy to explain.

They're all looking at something.

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

But what?

Konstantin and I spent a year

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

in the outer Solar system.

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

to yet again be told we were wrong.

But after a year,

there was really no choice.

We could come up with no other explanation

other than that that there is a distant,

massive planet on an elongated orbit,

inclined to the rest of the Solar system,

that is forcing these patterns
for these objects

in the outer Solar system.

Guess what else a planet like this does.

Remember that strange orbit of Sedna,

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

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

We knew we were onto something.

So this brings us to today.

We are basically 1845, Paris.

(Laughter)

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

and we are trying to work the calculations

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

to find this planet.

We've done massive suits
of computer simulations,

massive months of analytic calculations,

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

First, this planet,
which we call Planet Nine,

because that's what it is,

Planet Nine is six times
the mass of the Earth.

This is no slightly-smaller-than-Pluto,

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

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

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

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

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

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

You can even see that belt

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

good luck figuring out which one it is.

And here is Planet Nine.

Planet Nine is big.

Planet Nine is so big,

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

Well, Planet Nine is big,

but it's also really, really far away.

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

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

And also, the sky is a really big place.

We've narrowed down where we think it is

to a relatively small area of the sky,

but it would still take us years

to systematically cover
the area of the sky

with the large telescopes that we need

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

Luckily, we might not have to.

Just like Bouvard used
unrecognized observations of Uranus

from 91 years before its discovery,

I bet that there are unrecognized images

that show the location of Planet Nine.

It's going to be a massive
computational undertaking

to go through all of the old data,

and pick out that one faint moving planet.

But we're underway.

And I think we're getting close.

So I would say, get ready.

We are not going to match Le Verrier's

"make a prediction,

have the planet found in a single night

that close to where
you predicted it" record.

But I do bet that within
the next couple of years

some astronomer somewhere

will find a faint point of light,

slowly moving across the sky

and triumphantly announce
the discovery of a new,

and quite possibly not the last,

real planet of our Solar system.

Thank you.

(Applause)