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  • It's evidence for a distant giant planet in the Solar System.

  • 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 ishis other name ishe'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 maybehow 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.

It's evidence for a distant giant planet in the Solar System.

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新九大行星--60個符號 (The New Ninth Planet - Sixty Symbols)

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    林宜悉 發佈於 2021 年 01 月 14 日
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