In 1820, French astronomer
Alexis Bouvard noticed something strange.
The gravity of the inner planets
couldn't account for the orbital
movement of the outermost planet, Uranus.
His hypothesis: There must be
a massive object influencing it.
That eventually led mathematicians
to correctly calculate
the location of our eighth planet, Neptune.
And this discovery fueled a mad hunt
to discover more planets using this method.
To give it an approximation,
everybody and their brother
have looked for planets beyond Neptune.
I mean, the moment Neptune was discovered,
everyone wanted to do it because
it was this clear demonstration
that you can use math to discover objects.
Unfortunately, since then,
no major planet in our solar system has been found.
But thanks to modern telescopes,
thousands of smaller objects
have been discovered in the past
few decades beyond Neptune
called trans-Neptunian objects, or TNOs,
like Sedna, one nicknamed The Goblin
and another nicknamed Biden.
And planetary scientists
at the California Institute of Technology
have noticed some anomalies with these objects.
The first is the fact that the orbits
all lie in the same plane.
The second is that they all swing out
into the same direction.
Now, the third is where it gets interesting.
A large fraction of these orbits
that swing out into what look like
they've been arranged are detached
from the orbit of Neptune.
Their hypothesis? A giant theoretical
planet beyond Neptune,
which accounts for the anomalies
of these objects: Planet Nine.
And thanks to a new telescope
equipped with the largest
digital camera ever built,
we may be one sky survey
away from finally finding it.
It will be a game changer because it's big
and it can detect very faint objects.
We will find objects all the way
from near-Earth asteroids out
to trans-Neptunian objects and beyond.
After the discovery of Neptune,
scientists thought there had
to be yet another planet to account
for the remaining orbital anomalies of Uranus.
They called it Planet X.
There were hundreds of attempts
by scientists to find this missing planet
over the next few decades.
But as it turns out,
they were wrong.
Astronomers had incorrectly
estimated Neptune's mass,
later confirmed by Voyager in 1989,
which did account for these remaining anomalies.
The reason Pluto was initially
considered a planet is because
it was an object that was sort of
in the right part of the sky
where Clyde Tombaugh was looking for Planet X,
a predicted object that was supposed
to resolve remaining orbital anomalies of Uranus.
Although Pluto would later
be classified as a dwarf planet,
since it was too small
and too far away to have
any influence on the outer planets.
This was the start of a new era
for discoveries of bodies
in the outer solar system.
Pluto is one of what's thought
to be hundreds of thousands of TNOs.
And the more that are discovered,
the more data seems to point to a ninth planet.
We see this grouping of trajectories,
all these little pieces of debris
that are hanging out in this population
of icy planetesimals beyond Neptune.
All of their orbits tend to be
aligned in a very specific way
that is difficult to ascribe to chance
or to describe in any other way
other than the existence of yet another
planetary mass body hanging out
in the distant solar system.
The aligned orbits hint at
a large body pulling them in the same direction.
They also found each TNO
to be tilted on its axis
in approximately the same way.
And they don't appear to be put there by Neptune.
These orbits should all hug
the orbit of Neptune and many of them don't.
Why not?
Well, the here in Planet Nine
once again comes back into the fold.
You need Planet Nine's gravity,
as it turns out,
to kind of untether these objects
from the orbit of Neptune.
There's other circumstantial evidence as well.
If you compare the sizes of discovered exoplanets,
planets outside our solar system,
the majority fall into categories
known as super-Earths or mini-Neptunes.
On average, each star,
if you look up in the night sky,
has at least one planet.
And most planets that inhabit this galaxy,
if you will, are super-Earths,
these objects that are a few Earth masses.
It's actually quite remarkable
that the solar system,
as efficient as it was at creating planets,
appears to be missing these
short period super-Earth type objects,
which are very common elsewhere.
So if Planet Nine does exist
and is super-Earth size,
it would make our solar system
similar to the majority we've observed.
To make all these calculations,
and determine the potential orbit of Planet Nine,
the team at Caltech put collected data
into computer simulations.
As far as the math allows us to calculate,
it's about five Earth masses.
So it falls in between
an Earth-type planet
and an ice giant, such as Uranus or Neptune.
They also estimate that
it's on a 20,000-year orbit around the sun.
That's 10 times further away
than Neptune is from the sun.
We can calculate the orbit
and the mass of Planet Nine
to a pretty good approximation.
What we cannot do is tell you
where it is on the night sky.
Fundamentally, it's all just gravity.
It's a different type of dynamics
which allows us to infer
the existence of Planet Nine
than was the case with Neptune.
With Neptune, you could say
it's right there and get the orbit kind of wrong.
Here you have information about the orbit,
not so much the information
of where it is on the night sky.
So that's why we're stuck kind
of observing a whole swath of the sky,
which takes a long time
and is indeed difficult to do.
That massive swath of sky
makes finding such a faint,
far away object like finding
a needle in a haystack.
Planet Nine is millions of times
less bright on the night sky than Neptune.
That's really, really dim.
I mean, if you really think about it,
Neptune is not something
you can observe with your naked eye.
You need a telescope to see,
and Planet Nine is millions of times less bright.
Critics say that the team at Caltech
have an observational bias,
having not looked in enough
quadrants of the sky at different
times of year to conclude
that there is in fact orbital clustering.
Essentially, more data is needed.
The good news is we are kind
of entering the next generation
of such surveys now with the commissioning
and the construction of
the Vera Rubin telescope in Chile.
So I think that's going to be
a game changer in this problem.
The Vera Rubin Observatory
is a new telescope that'll be
coming online in the next couple of years
to complete the widest,
most detailed survey of the sky ever conducted.
This Legacy Survey of Space and Time, or LSST,
may finally tell us once and for all,
if Planet Nine is out there.
Rubin Observatory is a next-generation
survey telescope that is set
to observe the entire night sky every few nights,
repeatedly for 10 years.
The idea being we can build up
not only a deep image of the sky,
but also a multicolor movie.
Unlike telescopes
like the Hubble Space Telescope,
the Rubin Observatory's
field of view is extremely wide.
And thanks to its giant 3-gigapixel camera,
which is the size of a small car,
it can also detect incredibly
faint objects lurking in
the outer regions of the solar system,
so then narrow field of view telescopes
like Hubble can take a closer look.
Because it's a survey telescope,
we have a plan to go
and do this survey across the night sky,
but then also process all the images
and reduce those to catalogs
so that astronomers can come along
and then they can look at the catalogs
and they can do searches for unusual objects
or things that match what they're looking for
just using the catalogs that we're producing.
That'll help scientists solve the mysteries
of dark matter, dark energy,
map the Milky Way
and inventory moving objects
throughout the solar system like TNOs.
So, for example, in the solar system,
almost every population of object
that you can think of in the solar system,
we will detect about 10 to 100 times more of.
We will take 2 million images over 10 years,
which is actually 4 million actual
pictures of the night sky.
So each image is 3.2 gigapixels.
And we take about 1,600 images per night.
That's about 20 terabytes of data per night.
To find objects within our solar system,
the telescope will take two images
of every patch of sky,
separated by an hour.
And from the motion of
the points of light over that hour,
they'll be able to distinguish
between faint stars
and objects within our own solar system.
That's a problem for Planet Nine though.
Objects that are far beyond Neptune
will be difficult to detect
from the motion of only an hour.
Another problem is Planet Nine
may be in the Northern Hemisphere sky.
Due to the frequent clear skies
and low light pollution,
Rubin Observatory is strategically
located below the equator in Chile
and will mostly see Southern Hemisphere skies.
But if it is there,
Rubin Observatory is equipped to find it.
I have colleagues who believe in Planet Nine,
and I have colleagues
who don't believe in Planet Nine.
It's really hard to say
definitively that it's out there.
We have to go and look.
So one of the things that LSST
can do is actually contribute
by surveying more of the sky.
So people are already looking
for Planet Nine.
But given the fact that you have
to look at a large amount of sky
and you need to look for very
faint kind of objects,
that kind of can be a slow process
so LSST will be able to do
that much more quickly.
So within the first year of surveying the sky,
LSST could see if there was
a Planet Nine within its survey
footprint down to the magnitudes
that we can see.
The other thing
though that we can do
is provide further constraints
on what the orbit could be,
because a lot of the constraints
so far come from these also relatively rare,
very distant objects.
And we'll find a lot more of those too.
We'll also be able to tell
if it really is just a bias
in the observing data.
Even if the planet isn't found,
it may give astronomers more data
to find it during future surveys.
Or it may show that there are
other explanations for the orbits
and tilts of these TNOs
as some scientists have argued.
Either way, a hunt to find Planet Nine
will undoubtedly lead to the location of new,
undiscovered TNOs
and whatever else is in our neighborhood.
From the perspective of daily life,
no one's going to change
their daily habits,
you know, start brushing
their teeth more or whatever,
you know, because there's a planet
hundreds of times as far away
from the sun as is the Earth.
But at the same time, look,
I mean, it is part of human nature to explore.
It is part of human nature to ask the question
what's out there?
What lies beyond the ocean?
What lies beyond this hill?
What lies beyond this planet?
What lies beyond the edge of the solar system?
There is something deeply satisfying
about asking and answering these questions
that I think is really important
to kind of our collective human experience.