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(mysterious music)

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Out in the darkest,

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coldest part of our Solar System,

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a hidden force appears to be disrupting

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the rhythm of space.

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(mysterious music)

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Totally dominates all of that region of space.

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Each thing that's come near the guy,

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flings them into the Sun.

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It's evidence, some say,

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of a massive hidden world orbiting our Sun.

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So we think this thing is as five

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to 10 Earth masses in size.

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It is so, so, so, so dim.

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If you were to compare it with, say,

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like Jupiter, it's probably the difference

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between staring pointblank at a stadium light

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versus looking at a firefly miles away.

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(tense music)

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Now, astronomers across the world

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are racing to be the first to lay their eyes

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on the Sun's lost planet, if it even exists.

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(mysterious music)

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It's never been seen, but there are astronomers

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who are convinced it's there.

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Some call it Planet X.

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Others call it Planet Nine.

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(mysterious music)

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A few years ago, no one could have even imagined

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that there might be another large planet

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silently orbiting the Sun billions of miles past Neptune.

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(mysterious music)

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But now, a new generation of telescopes

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is allowing astronomers to see far beyond

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what they could have ever observed before.

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(mysterious music)

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Just a generation ago,

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even the most powerful telescopes in the world

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struggled to sense anything past the known planets.

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When people first started looking for planets,

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asteroids, anything that moves in the Solar System,

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they didn't buy taking pictures of the sky

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with giant glass photographic plates.

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Photographic plate is a piece of glass

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to paint photographic emotion on it.

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You put on the back of telescope,

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you take the picture of a huge slab of the sky.

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Astronomers worked in complete darkness,

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painstakingly preparing image after image.

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(somber music)

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You put it on the light table,

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and you see it's full of thousands

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and thousands and thousands of stars, galaxies,

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and anything else that happen to be there.

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In this view, I probably see a hundred stars.

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Countless hours were spent comparing images

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taken from the same spot in the sky.

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Night-to-night, galaxies and stars held their positions.

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Planets moved, and other objects circling the Sun,

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however, would appear to shift.

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This is how the great discoveries of the 20th century,

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like Pluto, were made.

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(energetic music)

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Could there be evidence of one more planet

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hiding in a slide like this one?

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The photographic plates, although they have

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this great historic data and historic value,

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it's not sensitive enough

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to have seen Planet Nine, probably.

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The story of the lost planet

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could only begin after more precise digital cameras

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were bolted onto the world's most powerful telescopes

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beginning in the 1990s.

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One of the first major discoveries

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that came out of this upgrade

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was the discovery of the Kuiper Belt.

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With the 1992 discovery of the Kuiper Belt,

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a massive ring of icy debris near Pluto's orbit,

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astronomers finally had proof

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that there was something beyond the planet.

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The moment those first few objects were discovered,

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it was clear that this is just the tip of the iceberg.

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By the end of the 1990s,

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we knew of hundreds of objects

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that were similar to Pluto in terms of their orbits.

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(mellow music)

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Astronomers were now observing

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some three billion miles from Earth,

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but they weren't satisfied.

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And so as soon as the Kuiper Belt was discovered,

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people started saying,

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"Oui, maybe there's something else

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"even farther out there."

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After years of scanning the night sky,

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Mike Brown would answer that suspicion

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with the discovery of Sedna in 2003.

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(mellow music)

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The first hint that something was wrong,

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that the Solar System had some

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additional tricks left up its sleeve,

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came with the discovery of the object, Sedna.

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Sedna, as a physical body, is not too remarkable.

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It's just another icy ball,

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little bit smaller than Pluto.

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But its orbit is completely wild.

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To go one time around the Sun,

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it requires 10,000 years.

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And at its outermost reach of its orbit,

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it goes out to a thousand times

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the distance between the Earth and the Sun.

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No one had ever seen

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anything like Sedna before.

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But what really puzzled scientists

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wasn't just the size of Sedna's orbit.

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It was the shape of Sedna's orbit.

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And the weird part about Sedna's orbit

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is it never comes close to the Sun.

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This was weird, because we thought

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to go on an elongated orbit,

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it need to have gotten close to something,

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and go on on a slingshot and come back around.

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But Sedna never comes close to anything.

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It was clear that there was something else

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in the Solar System that we hadn't seen before.

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Objects in our Solar System

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aren't only impacted by the Sun's gravitational force.

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They're pushed around by each other's

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gravitational force as well.

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(suspenseful music)

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It turns out that is the first clue

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to the existence of Planet Nine,

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even though we tried really hard

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not to jump up and down and say it at the time,

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because we needed a lot more evidence

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before we were convinced

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that there was something out there.

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Everyone realized that Sedna cannot be alone.

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There has to be an additional population of such bodies.

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But for the next 10 years,

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search after search came up empty.

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(mellow music)

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Enters Scott Sheppard,

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at the Carnegie Institution for Science

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in Washington, D.C.

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Using the four-meter Blanco telescope in Chile,

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he and his team began the deepest, widest survey

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of the outer Solar System ever attempted.

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In a matter of days, they make their first discovery.

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And we started observing, and this is what we see.

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We take three images of the sky,

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each image separated by about two hours.

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And we make a movie of those three different images.

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And you see background stars,

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are these circular-type objects,

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and they do not move over a few hours time span.

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We also see very distant galaxies,

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so they are the oval-looking things here and here.

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What we find is this object here is actually moving

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relative to the background stars and galaxies.

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And so, the reason that we found

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this object so interesting

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was because it was so distant.

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And it actually took another year of observations.

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We had to re-observe this object a few months later,

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and then again, a year later,

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to actually get the orbit for the object.

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Sheppard and his partner on the project,

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Chad Trujillo, nicknamed the object Biden,

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and continue to study its orbit.

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What they find only deepens the mystery.

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Like Sedna, Biden orbits the Sun

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but never comes close to the known planets.

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(suspenseful music)

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They then analyze the orbits

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of about a dozen other objects

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found beyond the Kuiper Belt.

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They, too, had unusual elongated orbits.

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Stranger still, when they compare

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all these objects' orbits in three dimensions,

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they realize that they all share a common alignment

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along what astronomers call their argument of perihelion.

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An unknown gravitational force

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appeared to be stretching, clustering,

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and tilting the orbits of these distant objects.

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For Sheppard and Trujillo,

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the data pointed to an uncomfortable conclusion.

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It's kind of almost a crock pot-type thing

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to say that there's a planet in the outer Solar System.

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So, we didn't really want to go that direction,

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but the evidence was there.

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Caltech's Konstantin Batygin

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and Michael Brown were intrigued

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by what Sheppard and Trujillo were suggesting.

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That discovery was the inspiration for me

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and for Mike to go and look at this closer.

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But before they could get on board

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with the idea of a new planet,

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Batygin and Brown needed to better understand

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the impact of the proposed planet's gravitational force

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on the outer Solar System.

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We looked at the data for a long time

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and we realized that our theory of a planet

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made a projection that, in addition to these objects,

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all elongated in one direction,

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there should be another set of objects

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elongated at 90 degrees to these,

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and also twisted by 90 degrees.

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Those are weird orbits that nothing

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that we knew of in the Solar System has.

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So I found these ones, they were twisted

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the way we thought they were twisted.

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They were really distant.

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I didn't know how they were pointed though,

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and so, Konstantin was in my office,

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I'm like, "Konstantin, I found these objects.

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"I'm gonna plot their positions on my screen."

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Press the button, and they were, boom,

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exactly where our theory said they should be.

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I remember this moment.

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This is where I went in my head,

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a switch flip from, this is a kind of cute idea

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that explains the data to,

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"Oh, there is a giant planet out there

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"and we know where it is."

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The race was on to find it.

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(tense music)

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Typically, when you think of proof of something,

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you wanna see a picture of something

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before you believe that it's really there.

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In the case of Planet Nine,

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we don't have a picture of it yet.

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All we have for it for now

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is the gravitational evidence.

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But the gravitational evidence for it

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is really solid.

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(mysterious music)

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Batygin and Brown had found the evidence

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they were looking for, and this wouldn't be the first time

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a planet was predicted before it was ever observed.

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(tense music)

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Because of the shape of Uranus' orbit,

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astronomers understood decades before they saw it

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that another planet, Neptune, must be orbiting nearby.

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A similar phenomenon could be observed near Jupiter.

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Jupiter is a very massive planet

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that is just outside the asteroid belt.

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So we see clustering of asteroids.

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And so even if we never even saw Jupiter,

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we would know Jupiter exists because we see

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clusterings of various main belt asteroid together.

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(tense music)

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Probability that we would see these elongated orbits,

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and they will be clustered the way they are,

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and they will be tilted the way they are,

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the probability is .2%.

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There is a .2% chance, say, two chances of a thousand

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that what we see has just stood a chance.

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(tense music)

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No matter how convinced these teams are

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that another planet is out there,

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until someone observes it,

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it remains only a theory.

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In a way, astronomers are back to where they were

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when telescopes were fitted with glass plates,

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their technology struggling to keep up

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with their biggest ideas.

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(mellow piano music)

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To fully grasp the challenge

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of finding the lost planet,

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you need to understand the scale

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of the known Solar System.

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(mellow piano music)

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So imagine these are the orbits

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of the eight major planets that we know of.

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And here's Neptune, it's the most distant orbit known.

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So if we're gonna plot Planet X's orbit on this graph,

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its minimum distance where we think the planet exists,

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is out this distance.

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It's estimated that the planet's orbit

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takes it some 30 billion miles from the Sun.

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This distant out,

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he Sun would just be another star in the sky.

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Their challenge now

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is to narrow down where on this predicted orbit

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to direct their telescopes.

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It's a daunting task, to say the least.

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(upbeat techno music)

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So what we're gonna do now

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is do a little scaled demonstration

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of looking for Planet Nine

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with a terrestrial telescope.

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And we're gonna go and walk

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about a hundred meters that way

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to get you a sense of scale

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of how big the field of view is.

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(upbeat music)

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The biggest camera we use is called

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Hyper Suprime-Cam on Subaru telescope

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and it covers about 1.5 square degrees of the sky,

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which is about this as viewed from 100 meters away.

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(upbeat techno music)

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So, even though this is a big field of view

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compared to other telescopes on Earth,

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or even in space, this is still

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just a tiny percentage of the full night sky.

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(mellow techno music)

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With each observation,

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astronomers have a near-infinite number

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of targets to choose from,

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each loaded with celestial objects.

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The second challenge is that

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Planet Nine is so far away

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that it appears really, really, really small,

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really tiny on the field of view.

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So, to give you a bit of a sense of scale,

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I have my surgical tweezers here.

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I'm gonna try and create a little model

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of how big Planet Nine would appear on the telescope.

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We have this little white piece of tape.

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This is actually a little big,

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maybe by a factor of a thousand,

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compared to how Planet Nine would actually look,

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but the real issue, the biggest of the challenges,

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is the third one, which is that Planet Nine

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is exceptionally dim, because when you take an object

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and move it really, really, really far away from the Sun,

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it just does not reflect that much light.

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So, a much better analogy

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than this white piece of tape here

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would be a black piece of tape.

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(tense music)

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So, there it is.

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I'm sure it's completely invisible to you,

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but I promise you it's there.

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And that's a pretty good representation

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of what it's like to look for Planet Nine.

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You are looking from 100 meters away

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at a patch of sky this big,

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and you are looking for something

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that is just painfully, just exceptionally dim.

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How Planet Nine got

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so far removed from the other planets

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is one mystery, however, astronomers think they've solved.

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Main theory is probably got it formed

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in the giant planet region,

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probably formed near Jupiter and Saturn.

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And I like the thought

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of a bunch of giant planets out there.

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The early evolution of the Solar System

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is actually a very violent and chaotic place.

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Probably what happened was, Planet Nine came

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a little too close to Jupiter or Saturn

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and got scattered out into this,

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probably this long period orbit.

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Pushed far beyond

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the gravitational reach of its siblings,

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the ejected Planet soon establishes itself

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in its new 20,000-year orbit,

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(tense techno music)

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a relic from a different time and place.

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(tense techno music)

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Most likely, if it formed in the giant planet region,

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it formed a 50% rock and 50% ice.

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(suspenseful music)

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Out here, the gravitational pull

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of this rocky, icy mass would be too much

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for other objects to handle.

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Playing the role Jupiter played among the known planets,

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Planet Nine starts clearing a path

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through the outer regions of the Solar System.

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Any of these smaller objects there,

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if they ever got close to the planet,

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they would basically be ejected

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from the source of gravitation,

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that the planet would swat it like a fly.

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It dominates more of the Solar System

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than all the other planets put together.

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Nothing in the outer part of the Solar System

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is safe for long from Planet Nine.

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Not even Sedna and Biden

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and the other extreme objects,

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which first revealed the possibility

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that this planet could be hiding in our own backyard.

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Those are the only ones that haven't yet

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been destroyed by Planet Nine,

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'cause it just hasn't had time to eat everything yet.

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I think about this sometimes,

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if we existed four billion years from now

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instead of right now, we wouldn't be able to know

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that Planet Nine was there, probably because

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nothing would be left in its region of the sky.

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(suspenseful music)

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Astronomers understand Planet Nine's past.

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They can predict its future.

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But where it is right now remains a mystery,

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(suspenseful music)

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a mystery that the teams from Caltech

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and the Carnegie Institution for Science

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are racing to solve.

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Our survey is trying to cover

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as much sky as possible to very faint depths.

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But we're limited by a technology in our telescopes.

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Sheppard and Trujillo's strategy

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is to keep scanning the sky,

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looking for other objects like Sedna and Biden

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to help point the way to the lost planet.

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We've covered about 30% of sky,

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but we're still not scratching the surface.

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If we wanna find the planet,

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and we're looking for the planet,

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the main goal of our survey

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is to find more of these smaller objects

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that can better narrow down the orbit of the planet.

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Over the past four years, I have spent

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nearly all of my personal research time

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looking for Planet Nine.

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There's nothing else

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that is nearly as important to me

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personally, emotionally, scientifically,

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as finding this planet.

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And for Mike Brown, history shows

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that there may be one more surprise twist in this story.

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Planet Nine may already have been observed.

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Somebody has taken a picture of it already.

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The reason I'm convinced about this, actually,

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is because every major body

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that's been discovered in the Solar System

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for the past 200 years, literally,

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has been seen many, many years before its discovery.

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Uranus was observed for 91 years before.

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People went back and found previous observations

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up to 91 earlier, 21 different times.

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Neptune was predicted by Le Verrier, found by Galle.

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It was observed by Galileo.

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Galileo drew in his notebook a little star

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that turned out to have been Neptune.

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Pluto was seen 16 years before its discovery.

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The trick is not to take the picture of it.

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The trick is to figure out

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which one of those little spots

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that just looks like a star

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is actually not a star, but actually your object.

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No one's life is going to change because

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there's additional planet in the Solar System

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really, really far away.

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But it would completely redefine the scale

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of what we think of as our cosmic home.

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And human beings have always been explorers.

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And understanding our cosmic home

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is something that everyone wants to do.

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It's weirdly wired into our DNA.

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If, or when, the Sun's lost planet

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is finally discovered, it'll be a triumph of science,

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mathematics, and collaboration,

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but for now, it remains invisible, a theoretical planet

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on our lonely epic journey around the Sun,

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waiting, it seems, to be discovered

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and to upend our view of the Solar System forever.

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(mysterious music)