So the authors here are Batygin and Brown,

and a big part of the reason why we're back down to eight planets

is because Brown is—his other name is—he's better known as @plutokiller

because he's the guy who kinda got Pluto demoted from planet status to just

another dwarf planet by basically discovering lots of other dwarf planets

and pointing out that Pluto really is nothing special, and if we're not careful we're gonna

end up with thousands of planets in the Solar System this way.

So he's killed one, and clearly guilt has gotten the better of him

'cause now he's trying to bring one back.

Basically, they found evidence for a large-ish ninth planet in the outer Solar System,

so they haven't actually seen the planet is the important point to get in first.

They have come across indirect evidence that there probably

is a planet out there somewhere, and because it's indirect evidence,

you can't make very definite statements about it, but it has to be at least 10 times the

mass of the Earth. So you're talking about a serious-sized planet. It's not just, you know,

another little Pluto; it really is a serious planet we're talking about here.

Planets are actually quite hard things to spot, especially ones which are a long way from the Sun,

because the planet doesn't actually give out any light of its own.

All you're seeing is the reflected light from the Sun.

So when you get out to large distances, there isn't a lot of reflected light out there, and so actually

by the time it gets from the Sun to the planet and then reflected back to us, they really are very faint.

Plus, there's a lot of space out there. So the way you typically

find minor bodies like asteroids and things is you take a picture and then you take

another picture and you see what's moved. Now these things are moving relatively

slowly, so it's being quite a tricky exercise to actually track them down.

Having said that, something this big really should be findable. So having now made a

prediction of what it roughly is and roughly where to look for it, I suspect

there will now be a concerted effort and if it's there, it'll probably be found.

So the primary piece of evidence is that there are a whole bunch of these things called

Kuiper belt objects, these sort of low-mass objects in the outer Solar System.

You can see there's a whole family of them which are all pointing in really

pretty much the same direction in space. And if you do a kind of calculations and say,

"Well, what's the chances of all these things being that closely lined up?",

it turns out there's a very low probability that they'll get lined up that closely.

So the Sun is in the middle there, and then all these very elliptical orbits that carry them around the Sun.

­(BRADY) I didn't know Kuiper belt objects had orbits quite so eccentric or came that close to the Sun!

(PROF. MERRIFIELD) There are different families of them, but there are all these ones that cross into

the inner Solar System and then

whiz back out again. Well, it depends on how you define the inner Solar System. We think of them

all being out in this thing called the Oort cloud, but actually quite a lot of them are on these kinds of orbits

that carry them in, erm, within the orbit of Neptune, so they don't actually come into the...

(BRADY) Oh, Neptune, they're not gonna smash into Earth. (PROF. MERRIFIELD) No, they're not gonna smash, no, no, no.

So, yes, don't worry about a Kuiper belt object falling on your head but, um, but, yeah, they do,

they are on these very eccentric orbits. Now, the interesting thing about that: how do they

end up aligned like that? Well, maybe however they got made in the first place

just had some preferential direction in it, and for whatever reason they all ended up

forming on these orbits, because it went -- once you put a planet on an orbit,

it stays on the same orbit, and it will just stay there. But the interesting thing is

because they are coming quite close into the inner parts of the Solar System,

they get relatively close to some of the outer planets, and the net effect of that

over many orbits is that they'll gravitationally interact with them and those

orbits will kinda get scattered, so instead of pointing that way, maybe a bit later

on it will have a close encounter and end up coming out in that direction.

And you can do a calculation to say, "Well, what's the lifetime, and how long will they stay if

they formed in this?", and the answer is at least, on the on the time scale of the life

of the Solar System, it's quite short. If they formed in that pattern, they won't

have stayed in that pattern to the present day. So we know it can't just be the origins of it,

so that means it has to be something holding them together.

Now, there are usually two ways you can kind of hold a pattern of things together

like that: it can either be the gravity of the things themselves holding it

together or it can be kind of something else marshalling it, and so you can do a

calculation to figure out "OK, so how much mass is there out there in these kind

of objects, is it enough for the grav... the sort of mutual gravity of it to hold it together?"

And it turns out it fails by some very large fraction, so, actually, that can't

be the explanation, which means you need something else that's marshalling them,

and that was sort of the first piece of evidence that you need something else

out there in that outer Solar System that actually leads to this arrangement.

People hadn't really thought about what could do that, and one of the nice things

that comes out of this paper is they've done a whole bunch of simulations to say,

"OK, so what could be doing it?" It has to be a reasonably massive body to have this effect.

It has to be about 10 times the mass of the Earth or more, and when you've got a

massive body like that, it turns out that the interaction between something on

this anti-aligned orbit with this collection of things over here is enough

to keep them relatively well marshalled together. And you can see they're not

perfectly aligned, so actually there clearly are things going on but it's enough to

stop them from wandering too far away. And if that were the end of the story,

I would kind of say, "Well, you know, it's one of those things where they found

something, and, yes, it's a low probability but maybe—how did they collect them in

the first place, how did they select the data, which things have they ignored?"

But they found that if you make this Planet Nine on that kind of orbit, it does a

couple of other things as well, which also turn out to be things that we

know about. So, for example, there's another of these minor bodies, Sedna, and it

has a couple of friends as well, which is on a rather strange orbit for these

Kuiper belt objects. Most of these Kuiper belt objects are on orbits which get, at some point in their orbit,

get close to Neptune. Sedna is not on one of those orbits, and so for a long time

people have wondered "Well, how did it end up on that orbit, given that we think that

the way you end up with these kinds of orbits is by close interactions with

with Neptune?" And what they found is that if you got this Planet Nine, once in a while

it kicks something off one of these sort of Neptune encountering orbits onto a

completely different orbit, just like the kind of orbit that Sedna's on.

So now they've got an explanation for where Sedna comes from. And then the final piece

of the jigsaw is they also found, sort of unexpectedly while they were doing this

calculation, is that there's a completely different family of planets as well,

minor planets as well, so these Kuiper belt objects which end up on orbits which are

kind of perpendicular to the plane of the Solar System, so all these things are

more or less in the plane of the solar system.

You end up with another family which is almost perpendicular to it, and they thought,

"Well, that can't be right" and then, but then they went and checked and it turns out

there are a whole bunch of things on these perpendicular orbits as well.

So, by invoking this one rather extreme idea, namely there's a ninth, rather large planet in

the outer Solar System, you can explain three things, and, scientifically,

that starts to look like a win, right? In science, if you can, if you dream up something which only

explains one thing, you kind of think "Well, you know, that could be

anything, could be that, could be something else," but it starts to get more efficient

if you've got one thing you've dreamed up that actually explains three different things.

At that point, you start to say, "Well, that's, maybe that's, you know, the evidence

is building up, maybe that really is the way things work, and maybe there is

that ninth planet lurking out there somewhere." So it'll have to be one of these things that sort

of like an ice giant like Neptune or Uranus, one of those big, gaseous,

very cold bodies. When you do simulations of the formation of the Solar System

or stellar systems with planets around them,

you do, actually, sometimes find that the planetary system starts to form, and then

several of the big planets which are forming have a close encounter with each other,

and one of them kinda gets gravitationally kicked out.

So it is, actually, quite plausible there could be a large body which

probably initially formed rather closer to the Sun but then as the

system formed, that then got kicked out onto one of these orbits. Now, then you have to

do something else to it, because if you just kick it out, it will end up on a very elliptical orbit,

but if it gets gets kicked out and then encounters some of the minor bodies out there or

gas or something that it can gravitationally interact with, then it can get on the

more circular orbit and end up very much like the planet we see.

(BRADY) If you enjoyed this video and want to see even more astronomy, I hope you know

about our other channel, DeepSkyVideos. It's really a lot like this channel,

but astronomy only. We have lots of fun over there, and I hope you might check it out.

(PROF. MERRIFIELD) You can see how much light is lost

coming through that filter 'cause it's not very bright at all.

(BRADY) And if you find the history of science interesting, why not check out my other channel, Objectivity?

Each week we go into the vaults and archives and find all sorts of amazing

objects to do with the history of space, astronomy, physics, chemistry, you name it.

(J. WILLGOOSE, ESQ.) Word. (BRADY) Yeah? (J. WILLGOOSE, ESQ.) Yeah! (BRADY) Liking it?

They're really cool videos and worth a look. I'll put links on the screen and in the video description.