In Our Time - The Kuiper Belt

Episode Date: March 2, 2017

Melvyn Bragg and guests discuss the Kuiper Belt, a vast region of icy objects at the fringes of our Solar System, beyond Neptune, in which we find the dwarf planet Pluto and countless objects left over from the origins of the solar system, some of which we observe as comets. It extends from where Neptune is, which is 30 times further out than the Earth is from the Sun, to about 500 times the Earth-Sun distance. It covers an immense region of space and it is the part of the Solar System that we know the least about, because it is so remote from us and has been barely detectable by Earth-based telescopes until recent decades. Its existence was predicted before it was known, and study of the Kuiper Belt, and how objects move within it, has led to a theory that there may be a 9th planet far beyond Neptune.WithCarolin Crawford Public Astronomer at the Institute of Astronomy and Fellow of Emmanuel College, University of CambridgeMonica Grady Professor of Planetary and Space Sciences at the Open UniversityAndStephen Lowry Reader in Planetary and Space Sciences, University of KentProducer: Simon Tillotson.

This is the BBC. Thanks for downloading this episode of In Our Time. There's a reading list to go with it on our website, and you can get news about our programs if you follow us on Twitter at BBC In Our Time. I hope you enjoyed the programmes. Hello, about four and a half billion years ago, a vast cloud of dust and gas collapsed and gave rise to our solar system. Much of the gas became the sun as much of the dust became the planets.

At the fringes of the solar system, some dust combined with ice to make small, objects and there in what's called the Kuiper Belt they've largely remained unchanged. Occasionally they come much closer as comets. There are larger objects among them too, like Pluto, which we once thought to be a planet. These distant objects orbit the sun in patterns which suggest not only where our known planets used to be, millions of years before, but at the possible existence of a ninth massive planet far beyond Neptune. We need to discuss the Kuiper Belt are Carolyn Crawford, public astronomer at the Institute of astronomy and fellow of Emmanuel

College University of Cambridge. Monica Grady, Professor of Planetary and Space Sciences at the Open University. And Stephen Lari, reader in planetary and space sciences at the University of Kent. Monica Grady, where is the Kuiper Belt?

It's a long, long way away. If we think of Neptune as being 30 times as far away-ish from the sun as the Earth is, the Kuiper Belt is beyond that, up to maybe 50,000 times as far away, to the outermost fringes of the solar system. So if one astronomical unit is 150 million kilometres, you're talking about 150 million times 50,000

kilometres. Well, that's a starter, isn't it? Yes, it's a long way. We can just get our minds around that and then settle back, I think, really. Why do you call it a belt? Well, we tend to think of things in the same way as the Earth goes around the sun. So you've got the sun in the middle, you've got the planets all orbit at the same level, more or less. On the same plane?

On the same plane, yes, which we call the ecliptic. And so we tend to think of the orbits as belts, like we think of the asteroid belt, which is a load of rocks in between Mars and Jupiter. And the Kuiper belt is a similar array or assemblage of rocks. Now, some are above and some are below the plane of the ecliptic, but generally most of them are in that belt. You know them with brilliant scientific precisioners. The KBOs are KBELB object.

That saves a lot of breath. None in my case, I'm afraid. What are these objects? What distinguishes them? They're mainly made of rock and ice. They're irregular shapes. Some of them are nice football shapes like Pluto, which used to be a planet and is now a Kuiper Belt object.

So if you think of lots of Pluto's. obviously we must see them. We know very little about them really, don't we? We know very little about them because... Let's own up to it. This is one of the programmes where we know very little about what we're talking about. Well, we know... We know more than we did 25 years ago.

We know a lot more than we did 25 years ago. In fact, we know a lot more than we did three years ago because of the New Horizons mission that has been to Pluto. But we've seen these objects through very large telescopes. Because they're so far away, that makes them very, very faint. They don't come close. Most of them don't come close to the Earth like comets do, so we don't see them with tails or anything like that.

So they're very dim. They're very dark because they're mainly rock, and any ice that's there is buried. So they don't shine. When I talk about range, so are you able to talk about different sizes? You've talked about Pluto, once a planet now demoted to dwarf planet status and put in as a Kuiper belt object. Can you give us some idea of the range of size

that you think you can see through a telescope who scarcely picked them up? Well, it's... I can't remember offhand how the diameter of Pluto... 250, isn't it? Oh, right, thank you. From the... 250?

2000. Yeah, 2,500 kilometres. Left up and north. I knew it's bigger than series. I knew it's bigger than series. But that sort of size, down to probably individual dust grains. So you're talking a whole range of sizes.

Nothing as big as the Earth that we have seen, nothing as big as Neptune that we have seen yet. Caroline Crawford, how did those objects in the Klafebilt get there? Well, most of them have always been there because they are just fragments of the early solar system that fell to coalesce into planets. If we wind time back, you know, four and a half, billion years ago. You've got the new sun

and it's sitting in a sort of nice fat cocoon of dust and gas and it's that cocoon that goes on to develop the planets. We call that the proto-planetary disk. And within that cloud, and this is what's left of that big interstellar cloud that you described that, you know, collapsed form

the solar system, you've got dust grains and you've got gas particles and the dust grains will stick together and they'll build up slowly through collisions. And you'll eventually get small objects. maybe 10 kilometres across, which have enough gravity that they can pull in material from the surroundings. So if 100,000 years say you build up what we call planetesimals, chunks of rock that are like 100 kilometres across, but at that point, they've kind of pulled, grabbed everything in their

vicinity by the gravity. And the only way you can build them up to bigger objects is if they collide with other planetesimals. Now in the inner solar system, that's easy to do because it's denser. But when you're get to this regime that we're talking out beyond Pluto, all this material in this disc, you've got these planetesimals, but they're so spread out that you don't have these collisions. There isn't enough material to really sort of collide and form a planet. And so these are the vestiges of that period. They've had collisions, so you've got fragments of the planetesimals. You've got these planetesimals.

Some of them have got some way to building up bigger objects, but you've got the dust, grains from the collisions. you've got the chunks, you've got the planetesimals, and it's all part of this very early phase of the solar system formation. And all this comes out of the swirling of dust and gas, and those create objects which then create gravity, which then create mass, which then create what we've got now? More or less, yes. Yes.

I can't think that right, don't you? How long have we known they were there? Well, we've suspected they're there for quite a while. During the last century, there was both Ken Edgeworth in 1943 and also Gerard Koiper, for whom the belt is named in 1951, both put forward this speculation that it's not like the solar system's going to just end when you get to Neptune and Pluto. That disc material is going to carry on.

You're going to have these planetesimals from the sort of partway formation of the planets out there. And both of them speculated that there was this material out there, but the observation technology couldn't detect them at the time. And we didn't actually observe the first kuiperabout object until 1992. So it's a great deal later. And that was by David Dewitt and Jane Liu, who had spent five years searching for these objects. And they discovered the first one in 1992, another one within six months, another two the next year, another four the next year. And then the floodgates opened.

And suddenly we're populating this region out beyond Neptune. And it goes from being this theoretical idea that's left over and should be, to an actual proper part of our solar system. And out there, are they, they're a belt, would be told them. Are they going round the sun in the same way as the planets? Yeah, they're all, well, they're in the same way. Not all of them in the same way, as I think we might hear later, but they are all orbiting the sun on their individual orbit.

So it's just like a big chaotic swarm of lumps of rock and planetesimals all swirling round the sun out beyond Neptune. What was exciting about the discovery of them? What do you think, oh, this is taking it to, and it's obviously taken this to and you place in space. But what was exciting? What were you going to find that you had thought you might never have found before? Well, first of all, if you actually discover something that you have theorised,

is there, that is an enormous vindication of your understanding about the early solar system formation. And it not only reflects that, but it also solves other problems like comets. There are certain comets which reappear really regularly, frequently, like Halley's comet. That comes around every 76 years. Now these aren't expected to live very long. Every time they go near the sun, they'll lose material, and they'll also get perturbed by the gravity of the planets when they go into the inner solar system. So where do these come from? You need a reservoir for these comets to fall into the solar system from.

And this was a puzzle, unless you postulated that there is a reservoir of frozen objects, and the only place to hide it is really out beyond. Neptune. So when you discover the Kuipa Belt and you look at these objects and you see they're made of rock and ice, you suddenly get this source of these comets. Stephen Lowry, what keeps the objects in the cover belt where they are? And I said in the introduction unchanged since the whole Chebang, the cellar system got underway. Can you tell us what keeps them down? Why are they unchanged? Well, the gravity controls everything. So gravity controls the motions of all of the planets, the giant planets, and it controls everything all the way down to the smallest dust range. So gravity is the mutual attraction that any two

objects in the universe exerts upon each other. And so it depends on the amount of stuff on the two objects, and it depends on the distance that those two objects are separated by. So the closer they are, the stronger the gravitational force, the larger they are. then the stronger the gravitational force is again. So that's what holds the solar system together. Now, what keeps the objects in the orbits that they're in is that they have a speed, that they're moving in space. They're moving around the sun.

And so the two things cancel each other out. They're balanced, the speed that they're moving, and the gravitational force that the sun exerts on it. So that keeps it in orbit. Now, the objects are moving very, very slowly in that region of the solar system. compared to asteroids and planets that are much further in. So the further away you get, the slower these objects move.

And the slower they move... Why is that? Why are they moving so slowly? It comes out of the gravitational laws that physicists have been talking about for... Yeah, but what are those? Can you summarise those? Well, there's one called Newton's Laws of Gravitation. That was the very first gravitational law, the first serious one that we started to think about.

And so that's where this distance relationship comes in. that's where it comes from. This is the magic equation that Isaac Newton came up with. What is that equation? It's, okay, you take the mass of each of the two bodies and you multiply them together. The sun on one of these objects. Yes, one of the objects.

And then you divide it by the distance squared. And then there's a constant in there that you have to multiply everything together by it to get it all to work. So it's a very basic, very elegant equation and it just takes into account how big they are and how far they are apart. But the objects have to

moving in order to maintain their orbits. And as I understand if what you've said, there might be tens of thousands, hundreds of thousands of them, but the area is so big that if you I think one of the, get the analogy if you got in a canoe and tried to paddle across the Pacific, it was like the odds

of hitting another canoe also paddling in the Pacific. There's big gaps in between. Right. It's a very, very sparse part of space to the point where the New Horizons mission, which is a NASA mission to Pluto, which is now on its way to the Kuiper belt.

There were no known Kuiper belt objects in the path of the spacecraft when the spacecraft was launched. That's just how sparse the Kuiper belt actually is. So after the spacecraft was launched, they had to search very rigorously using very powerful

telescopes to try and find a Kuiper Belt object that's anywhere near the projected path of the spacecraft. So yes, it is very sparse. So what factors can affect the orbit of these objects? Okay, there's... And we've talked about gravity.

Yes, the main one's gravity, of course. The giant planets exert. A lot of gravitational pull and other bodies in that part of the solar system, particularly Neptune. That's the main thing. Collisions is another thing. So, Kuiper belt objects do collide.

Oh, we've just said there's been a little chance of colliding. It does happen, and it's one of the theories that we think how the so-called short period comments are produced, produced by collisions, between Kuiber Belt objects. So fragments break off, then they get pulled in by the gravitational pull of Neptune in towards the inner solar system.

So those are the two main mechanisms. Now, there are other forces present that mainly affect asteroids closer into the sun, but that far out, it gets gravity that controls things. And also the motions of nearby stars can mix things up too a bit. Monica Grady, what we've been talking about comments, so there's been comments that we mentioned several times. What evidence is there that the Khypipel is the source of comments and what's comets, sorry, and what sort of comets?

Well, we divide comets into two broad categories, short period and long period. So that depends on how frequently they come close to the sun. So, for instance, comet Chorimov Grasimenko, which was the target of the Rosetta mission, and that comes around every six years or so. So it's a very, very short period comet. Now, when we see comets, we can track back their orbit and look to see what the first, this point is they've gone from the sun to see how they then come back round again and in close to the sun. So comets have very long elliptical orbits rather than the circular orbits of the planets.

And so once you've started to trace lots of orbits back of comets, you see they are all coming from, the short period comets are all coming from approximately, you know, a similar place. If you can say, right, actually, and we think that one might have been moved a bit by Jupiter. And so you're looking back in time to where those con. have actually originally come from. And you can see, oh, right, actually, as Carolyn said, there looks to have been this reservoir where they've all come from. Now, that's the short period comets, the longer period comets,

probably come from another reservoir, which we call the Ort Cloud, which is another of these theoretical things that we all believe very firmly is there, but nobody's ever seen it. And that's probably more like a shell. That's what we like. You know it's a shell, even though you haven't seen it. Well, we believe it's encompassing the entire solar system. It's the outer fringe of the solar system.

And this is where when the solar system was forming from this cloud of gas and dust, not all the planets were in the places where they are now. So Jupiter and Saturn were probably much more close into the sun than they are now. And they moved. And as they moved, they skis. a whole host of objects out as far as they could possibly go. And that's what we think makes the Ork Cloud where these comets, long-period comets come from.

Stephen, you want to come in? Oh, that's fine, that's fine. All right. Caroline, we, Pluto was reclassified to the dismay of many people from being a proper planet to being a dwarf planet. Now, how did that happen, and how does it tie up with the KBOs, as we might call them? Well, as the Kuiperaubout began to be populated, you know, more and more of these rocks being discovered, the thing that began to happen sort of around about 2002

was that it became, and the early part of the century, it became quite clear that hidden amongst these Kuiper Belt objects are objects which rival Pluto in terms of their mass and their size. And most of these have been discovered by Michael Brown, his collaborators in California. So like the first one was Kual, about half the size of Pluto, and traveling out 40 times.

the Earth's sun distance. So way out beyond Neptune in a nice circular orbit. And then their whole slew more of these objects. You've got Mackey-Maki. You've got Orcus, which has an orbit similar to Pluto. Other strange ones like Sednao, which follows an 11,000-year orbit around the sun. It sort of goes from, and again, it goes from 80 times the Earth's sun distance out to 900 times that distance. So you're beginning to build up a handful of these objects. at about half the size of Pluto. But the real game changer is in 2005 when the discovery of ERIS is announced.

Now, ERIS is about the same size of Pluto, about 2,400 kilometres across. It's got a little moon. You can measure its mass, by the way, that the moon responds to the gravity of the planet. And you find it's more dense than Pluto. And suddenly you've got a whole host of objects that are similar to Pluto,

and one that is beefier and just as big as Pluto. And at that point, you think, Actually, Pluto, we've got a new context for Pluto. There's a new architecture we understand about the solar system and that maybe we need to re-evaluate that it's more like these other objects that are being discovered than when it was discovered in 1930, it was obvious. The only other things like it were planets.

Well, actually, we've learned an awful lot more since, and that's when you have the re-evaluation. Why did you decide to demote Pluto rather than upping the others and calling those planets as well? Well, first of all, we don't know how many are out there. So you don't want too many planets. Rather the names. Well, it's also, I mean, this is not unprecedented.

So back in the 19th century, asteroids were discovered. The first four asteroids discovered at the first part of the 19th century, they were taken as planets. And it's not until like 50 years later when you begin to realize, actually there's a whole crowd of objects at that distance from the sun, and we discover a new bona fide planet Neptune,

that we re-evaluate them, say, okay, they're not planets, there's something else. They called asteroids. So it's the same kind of thing. But this time, of course, it has to be not just by informal consensus. It has to be ratified by the International Astronomical Union who discover they don't even have rules about, you know, what defines a planet. So it's a good point to define what is a planet.

And they did a mix of both sort of dynamical and geological processes. So first of all, if you're a planet, if you orbit the sun, so that rules out some of the bigger moons of the gas giants. so okay. You've got to be large enough that you've got enough self-gravity that you're pulled

into a spherical shape. So that, depending what you're made of, how much rock, how much ice, it's like 500 kilometres or bigger. But the clincher is the third thing. And this makes the division

between planets and dwarf planets, which is that you have to dominate your orbit. You can't share your orbit around the sun with lots of other objects. And it's this last clause which says that

Eryus and Pluto and Quayer, all of these are dwarf planets, they're no longer classified as planets. Monica. Well, I take issue with what you said about demoting Pluto. I think what you... It's a little light-hearted way. Yes, I know, but I think, I prefer to think... Preparing my defence before you launch your attack.

No, no, no, no. I think, I rather think of promoting Pluto to being a Kuiper Belt object. Because Pluto, you know, ninth planet, boring, far away, we've got no idea what it's like until the New Horizons mission. It's like, oh, it's so far away we don't really care about it. It's not enormous like Jupiter. But now Pluto is one of the biggest of the Kuiper Belt objects. We know loads and loads about it, so it's much more exciting. Now it's a large Kuiper Belt object than when it was just a small planet.

Stephen? Yes, just to address Caroline's earlier point. One of the problems with promoting everything else is that you need to determine the size. That's a very difficult thing to do. One of the main reasons that is that you don't know how well the surface reflects light. In other words, the albedo is a measure of how the fraction of light that reflects from the surface that comes from the sun. That's very difficult to measure. You need to use telescopes that use optical light.

You need telescopes that use infrared light and you need to combine the two things. And it's a very difficult measurement to make. So because these things are so far away, they're unresolved. only look like tiny little point sources of light. And so because of that, you have to use very clever techniques to measure this reflectivity. And if you don't have that, you're really not constraining the size very well. And also the shape, you know, this IAU stipulation that it has to mold itself into a sphere before it gets called a planet.

That's a very difficult thing to do for Khyber Belt objects because we're not seeing anything resolved. So you can, an object can look elongated. simply because it has different colors and color patches on its surface. So you can confuse the matter. So what you need, you need to measure how fast it spins. You need to measure the variation in brightness. And then you can say something about its shape.

But that's very difficult to do if you don't know this reflectivity. So that's a problem. Besides Pluto, we have Plutinos. What are Plutinos? Plotinos, to talk about platinos, I have to talk about orbits. I'll explain just a little bit about orbits. just to put the term platinum in context. So everything goes around the sun

and every planet takes a certain period of time to go around the sun. There's what's called a mean motion resonance between two bodies. In that case, one object will... Take the three to two mean motion resonance, for example. In that case, the body furthest away will go around the sun three times

for every time it takes the other planet to go around twice. So in other words, the orbits line up at periodic times throughout the age of the solar system. The platinos are in one of these three to two mean motion resonances. So in other words, Pluto being the main example of that. So Pluto will go around the sun three times, for the length of time it takes Neptune to go around the sun twice. And so what that means is, so these resonances, these orbital or mean motion resonances can have one of two. two effects, they can stabilize the orbits of the two bodies or they can completely destabilize the orbits of those two bodies. In the case of the Plotinos, it's the former. The orbits of

Pluto become stabilized and they become locked in this configuration. For how long we're not sure. So Pluto was the first one that was discovered in this resonance and there's now been many, many more discovered in this resonance. And so basically it protects them from gravitational encounters with Neptune. So they're able to avoid Neptune, more or less all of the time, sufficiently enough so that they're not scattered towards the inner solar system or out towards beyond the Kuiper Belt. Monica Grady, why is the temperature of objects in the Kaeper Belt are so significant? It's very cold. It's a long way from the sun.

What's very cold? Oh, the rocks and in the Kape Belt. What is the temperature of your very cold? Oh, I see. Oh, I don't know. About minus 220. I was going to say about minus 220 Kelvin. You know, it's not absolute zero. So minus say 400 degrees C or whatever.

Not minus 400. No, minus 220 C. C, sorry, yeah, about 50 Kelvin. Yeah. And so it's so cold that when these objects were forming, the ice could condense. So you've got mixtures of rock and ice there. And as long as they stay out there, that ice stays there as well.

And you've got this very primitive mixture of rock and ice, which probably hasn't changed much in 4.5 billion years. So the fact that you've got preserved there, not just ice and rock, but you've probably got preserved there organic chemicals or carbon-bearing molecules, which were also part of this cloud of gas and dust that formed the solar system. So you've got a reservoir of primordial material. And does that, Carole, are we back to opening these things? Will there be life there?

Is there any chance of life there? Well, is there? You've mentioned organic material. Monica, what do you think, Caroline? Well, there's certainly water in the form of ice, and there's certainly quite complex organic compounds. But here again, the temperature is crucial. At 50 degrees above absolute zero,

you're not going to have the necessary chemical processes that will lead to life. So it is this thing of, again, talking what Monica said, they're pristine samples of the very early solar system, but they've been put out the deep freezer space, so nothing really has had a chance to develop. We think water needs to be liquid, at least. You're going to reach a certain temperature before you can get the necessary chemical reactions,

which may go on and produce life. So these are not going to have life on them. They're far too cold. I agree with that. I mean, for metabolism to work, for energetic reactions, you need a certain temperature, you need a certain energy to be able to react one chemical with another. And if you haven't got that energy in the form of sunshine

and you haven't got that energy in the form of chemical energy which is released in a reaction, then you're very, very unlikely to get bigger molecules built up to make life. Are all these objects like each other? Well, the problem is that we know so little about them in detail. I mean, we've seen a lot of information about Pluto, which has given us information, say, for instance, about the amount of nitrogen there,

the nitrogen bearing ice that's on the surface. We've sort of assumed that they're more or less the same sort of stuff, but we haven't got enough detailed information about enough of them, as Stephen was saying, the parameters in terms of the light curves to show how different they are, we just don't have enough information at the moment. Stephen, I'm going to throw another heavy one out of you.

What is the scattered disc? Where does that fit into the scheme of our things this morning? Okay, so we've been talking about the Kuiper Belt. Kuiper belt, all of the... The Kuiper Belt is really just one component of what we call transneptune object. It's just one subpopulation of that entire group. So no discussion is complete of the Kuiper Belt

without mentioning the scattered disk. So the scattered disk are transneptudean bodies that tend to have very stretched orbits. So most of the planets orbit around the sun in nearly circular orbits. Smaller bodies tend to go around the sun in elliptical orbits. So just take the circle and stretch it a bit. That's what the orbit of a typical Kuiber Belt object looks like.

so so the I'm going to me that again I was asking about the scattered disk yeah the scattered discs so the so the scattered desks do have very elliptical orbits they got those orbits by

gravitational scattering by the giant planets when the solar system was forming so there was a lot of planetary movement in the earliest stages in the first one billion years of the formation of the solar system the giant planets moved in their orbits we call that giant planet's giant planet migration.

So the very strong gravitational influence of those planets sent some of those objects into very stretched orbits. They also have very inclined orbits. So what I mean by that is we take the plane of the solar system. This is the plane that all of the planets orbit in. We call that the ecliptic plane. The orbits of the scattered disk tend to be very inclined relative to that reference plane.

So they have very high inclinations. They have very high eccentricities, which is the degree of elasticity of the orbits. And so they are the most likely source of the short period comets. It's not actually the traditional Kuiper belt objects that are in this torus or belt that orbits around the sun. They most likely come from the scattered disk. So just to return to your question about how a light the Kuiper belt objects are, because as we've heard, they're really faint, they're very difficult to distinguish. anything about their size. These orbits become far more important because there's so little

you can learn from an observation, unless you fly past something like Pluto. The best you can tell is it's orbital. It's orbit around the sun. So we group the kuiper belts and shared categories in terms of their orbits. We've already heard kind of three classifications. You've got the plutinos which are locked into that. It's a repeating pattern with Neptune. You've got the main coipe about and Stevens just told us about the scattered disc, the sort of much more far ranging objects. So we group them according to their orbits.

And the other sort of fundamental thing you can get is their colour. Just as a broad brush look at the colour of the object tells you perhaps how similar it might be to Pluto, you know, how many, how much of this red hydrocarbon sludge it might have on its surface or

how much fresh ice has been exposed, perhaps through impacts. So we were really dealing with very bare essentials and just extrapolate by these shared characteristics. And I just wanted to say that that's another of the things that sets Pluto apart from the main planets, is that its orbit is slightly inclined. Its orbit is not completely within the ecliptic. Further out from the KBOs, we've got this wonderful name Oort Cloud,

which may or not exist, but you're assuming it exists. So what's happening there? Well, you know, let's be controversial. Beyond the donut, let's be controversial here. People believe in God. Astronomers believe in the Ork Cloud. You know, it's like it's a hypothetical construct which helps us understand the extent of the sun's gravitational pull.

We have to have, you know, we've got lots of stars. Where does the influence of one star end and the next star begin? You tell me. Yeah. Well, at the Ork Cloud, at this. that shell of icy bodies that we believe infest this region, 50,000 or so times as far away from the sun as the Earth is. You think of it as a cloud.

You think of it as a cloud of unknowing, really, circling beyond the donut of the KBOs, or not circling? I think of it as a shell, all right? So if you think of a football with the sun in the middle, then the Oort cloud are on the outer. outer skin of the football, the surface. And yes, they're all going around the sun, you know, sort of, you know,

but we did say that we gave people to understand earlier on that the KBOs are at the outer edge, where the outer donor ringing around the solar system. Oh, no, no, they're not the outer, they're not the outer edge. I mean, the, the, the, the, the, the, KBO starts at about 30 A.U., 30 sun-earth distances.

And they, and they're sort of going, we tend to think of them as, as a belt, rather than a shell. And you've got this big distance. between 30 and 50,000 and we don't know whether it's 50,000 it might be 100,000, we don't really know. That isn't empty space

as far as we are aware. We sort of assume that there will be odd bits and pieces there, but our telescopes just can't see as far. I'm beginning to sound like a real astronomer here, I need a bigger telescope, you know. Our telescopes just can't see beyond a further

distance. One second, see if you're just to push it to the next one and then ask you, beyond that, there's a hint of something else in that there may be a massive unknown ice planet waiting for us. Carolyn, you've got your finger up there. Well, it's not beyond. It's not beyond the old cloud. It's not beyond the old cloud.

It's within the Kuiper belt. Well, perhaps right at the end of the Kuiper belt. In the end of the Kau. Anyways, out there in the old cloud or beyond the old cloud. No, it's within the old cloud. It's part of the orchard. Considering you don't know if it exists at all, you're having a good old argument about it. Well, I mean, it is incredibly speculative.

I mean, this is just the idea of a planet nine and it's been revived over the last year. If it's there, it's revealed by the orbits as some of these extreme co-po about objects in the scattered disc that Stephen was talking about. And they should all be fairly sort of chaotic in terms of their orbits. And as he described, they're very stretched, elongated orbits.

And when you look at six of these biggest scattered disk objects, all their orbits line up, they all kind of point in the same direction. They're all tilted out of the plane in the same way. They should be random. The fact they're all aligned, one of the ideas from, you know, computer simulations and dynamical modelling is that they're being pulled around by a massive object. Now, if it's there, it's going to have, it's going to be about 10 times the mass of Earth, and it's going to be 20 times further out than Neptune. So this is a kind of planet. We don't have any in a solar system, if it's there.

It's like a super Earth or a mini-Neptune that we see around other planets. So this is, Still quite speculative. You're extrapolating from the orbits of six objects in the scattered disk and saying that they indicate there's this massive object out there. People are looking for it with some of the biggest telescopes. I'm using the prediction. So if it's there, one hopes they'll find it. But I will stress it is speculative.

But if they found it, it would be so exciting. Because first of all, you're finding another bona fide planet in the solar system. And then he was got the question, well, how did it get there? Because if you've got this massive planet out there, it can't have been formed out there. It must have been thrown out there by something going on in the early solar system when the planets are colliding and forming. Or, you know, in the extreme case,

we might have stolen it from another star. Who knows? Stephen, you know what to come in? Yeah, just a small point. My main point was on the Orc Club, but we moved away from it. I agree with you, it's very low number statistics. It's like six objects. So the orbits of about six KBOs have been analysed.

And they all have very similar orbits. and so this theoretical planet is out of sync with this and so that tends to indicate the presence of this planet but it is very uncertain. We've got a rough idea of where it might be. Can I ask you to tell us how, or do discoveries in the Khyberberg, change our understanding of the origins of the solar system?

Yes, well, that's a big question. For me, that's the most exciting thing. The most exciting thing about this is the orbits. these resonances that we talked about earlier. So when the first one was discovered in 1992, it was more of the classical type. But then more and more platinos are being discovered. Now, we knew about the platino resonance,

but before the first one was discovered. So it turns out the dynamical structure of the Kuiper Belt is far more complex than we thought it would be. And one of the unexpected outcomes of that is evidence of planet migration. We talked about it earlier, but the structure of the Kuiper Belt tells us that the giant planets were moving very considerably in the early stages of the solar system in the first one billion years

to the point where even Neptune and Uranus actually switched places completely due to this mutual gravitational interaction between the small Kuiper Belt objects and the giant planets. Believe it or not there's a significant exchange of energy between the two kinds of body that you can move both quite significantly. Collins give us some idea of the excitement of the arrival of a new planet but can you tell us Monica that what you hope to discover that's new about the solar system which to a certain extent has been very well examined as

over the last, to the century, certainly over the last 50 hundred years. What new is it that you're expecting to find realistically? Well, the new sorts of things that have already come to light from the new horizons mission to Pluto is something which I believe caroled, referred to as a hydrocarbon sludge. I mean, and that was new and unexpected.

The fact... So what does that add to what we know? How does it make a difference to the way we think of the solar system? Well, what we're seeing is that there are materials there in greater quantities than we'd actually thought. And so what does that lead to? And what that leads to is an idea of the distribution of materials in the solar system putting more strength on the theoretical models of planet movement that Stevens just described

and the exchange of material between planets and the idea that actually, you know, there's perhaps more ice, a lot more ice, closer into the sun than we have traditionally believed. So, sorry to go on, but so what would that mean if there's more ice, Karen, what would that mean? So perhaps your sludge leads to more ice and more ice leads to a different view of the solar system. So what about that? Well, sludge really gets in the way of ice. I mean, so that might account for the red colours,

some of these objects being red. But here the key thing is ice. And it is all to do with the habitability of planets. Because when the Earth formed, it would be in hot, barren, hostile, dry. It's not going to have water on. All the water would have boiled away if it were there, which it wasn't as it formed. So you've got to deliver that ice from somewhere.

and we think there's this period 3.8, 3.9 billion years ago where you have this bombardment of debris. When the planet's migrating through the disk, when there's all this collision going on, this cometry ice hits the surface of the earth, delivers the water we need for our oceans. Now, of course, there are problems with this theory, but that ice, that water has to come from somewhere

and is going to have to come from the outer parts of the solar system. Along with the hydrocarbon sludge as well and all the other stuff. And we can see from Pluto that actually Pluto is more active, than people had thought. It's got more moons than people had thought. So the whole structure of how these bodies interact with each other, we're learning all the time. You know, a planet that we don't even know about may or may not be out there. It's the sort of thing that, well, it's really mysterious. This orc cloud which defines the gravitational limit of the sun, you know, have things been scattered onwards from other stars, which might be different from our own material. Stephen. Just a quick point on Monag's discussion there. One of the big findings from the Pluto fly-by mission, the New Horizons mission, was that it may have an internal ocean to it. And that was completely unexpected. We expect internal oceans beneath the surface of some of the icy moons of Jupiter and Saturn, for example, because there's gravitational directions that can heat the inside to produce these oceans. But to see this in something like Pluto that's too far away for this tidal direction to happen is a major finding. So we've learned something major there. But one of the quick point, one of the reasons why we study the compositions, what they're made of and what their dynamical properties are,

it tells us what the conditions were like in the earliest stages of the formation of the solar system. We want to know what it was like, how the planets formed, by studying something like the Kuiper Belt, which are among the least altered remnants from that era. That gives us the best chance of doing that. Monica. And the Rosetta Mission to a comet was so important because it showed us some of these things on an object that's coming in,

from in effect the Kuiper Belt or it's been out there at some stage of its life. But we can only have a mission to a comet infrequently because they're expensive. But using telescopes and dynamic modelling, we can get a lot more information on a lot more objects. And that's really important. And we are going to be flying past another Kuiper Belt object beyond Pluto. The New Horizons mission is carrying on and on the 1st of January in 2019. It's going to fly by one of these tiny Kuiper Belt. about objects. Only 50 kilometres across,

very red, but we're going to learn a lot more from that. Well, thanks very much. Carolyn Crawford, Monica Grady and Stephen Lower. Next week we'll be talking about Elizabeth Gaskell's novel, North and South, set around the cotton mills of Victoria in Manchester or Milton, as she called it. Thank you very much for listening. And the In Our Time podcast gets some extra time now with a few minutes of bonus material from Melvin and his guests.

Well, there's a lot to miss out in, four to three minutes, isn't there? But you managed to get an awful lot in. Yes. There was one big thing we can talk about if you like, and that is the existence of a possible fifth giant planet. What, another one? Yes, yes. So this is different.

You can never have too many hidden giant planets. I've waited for an opportunity and I didn't get it. Oh, you should have left in. Oh, well, okay, so there are four giant planets. Okay, so, and we know that they moved quite a bit because of the dynamical structure of the Khyper Pelt that we see. So this is Jupiter, Saturn, Uranus and Neptune you're talking about? These are the four main ones.

Now, there's been a new model that's been produced that indicates that there may have been a fifth ice giant like Uranus or Neptune. But this is the one that Carolyn was talking about. No, it's not the same one. It's a different one. So how is it different one? Because, well, they say that this fifth planet that was forming alongside the other giant planets

was probably flung out, but the orbit that would likely to have entered into is very different to this planet line. They say it's unlikely to be the same thing. Is it not in the ocean? the Earth cloud then? Oh, the out clouds too far away? Well, well, that's if it's still bound to the solar system,

it may not be. So it may be, it may have completely left our own planetary system complete. You gave me a hard time about the Earth cloud, that the planet was beyond the Earth cloud, fair enough. That's how I read the note, that there was the Earth cloud, and then beyond it was this there's this, this, this, this,

this, this, this, this, large, ice, massive ice planet awaiting to be discovered or uncovered. Right, no, that's... It's inside the Earth cloud. Yeah, because the The orc cloud is the furthest distance that things can be gravitationally balance. So what after that? We fall out of the solar system. Well, when you're into another stellar system.

It sort of melts into another one. Yeah, yeah. Yeah. And so the idea is if you've got the sun and it's aught cloud and you've got another star and it's aught cloud, they sort of butt up against each other and you've got another one here and another one here. And they interchange presumably materials. And because the sun isn't static, it's moving within the galaxy.

And this star here is moving within the galaxy, you know, they're moving at different rates. And you get exchange of materials, and you get materials periodically, periodically, episodically, scattered inwards, which you get, which will then refresh the Kuiper belt. But I wanted to ask, because I don't know very much about this planet nine, how inclined is its orbit to the ecliptic?

It's fairly inclined, as far as I remember, because it sort of counted. The tens of degrees. Right. The six trans-Naptunia objects are quite inclined and sort of counterbalances them on the other side. So it is again well out of the eclipse. So it's not a planet then if it's not in the eclipsic.

Thinking back to the rules, if it's got its own orbit and it's going around the sun, I don't think it's ruled out. Doesn't that have to line, has it? The information doesn't play a part in the definition. But we don't know whether it's cleared its orbit, though, do we? Well, should we find it first? Yeah, well,

I noticed an abstract put forward by some of the members of the New Horizons team where they're pushing very hard for changing this definition of a planet and doing away with the requirement to clear your orbit in order to be a planet.

But another problem is the idea that it has to be spherical. I have a problem with that slightly because to what degree where's the dividing line? Where's the cutoff point in how stretched your object is? So obviously nothing

perfectly spherical. Everything's in ellipsoid of some sort. So where is your cut off point? But would you be happy having series as a planet? As a dwarf planet? No, probably not. It's not quite spherical enough for me, I don't think. It's big, it's big, but it's... It's a thousand kilometres across,

ish. Yeah. Well, no. So it's borderline for me, sorry. I was you going to say another thing that was I would have liked to touch on is the fact that we've just been talking about the quote about around our sun. And Monica's just touched on the fact that we see huge belts of material around other stars. So you get very young, nearby stars where you can actually resolve the fact they've got some structure around them.

So you get really young stars where you see huge dusty disks out way beyond the distance of Pluto, where Pluto would be in their system, which is just made up of all these fragments. So you're at an early stage where those planetesimals are colliding. There's lots of dust being created that gets blown out by the system. sun. So we do see evidence for all these phases around the stars. And also there are some sun-like stars with planetary systems around them where they discover huge amounts of carbon monoxide. And if you've got carbon monoxide, you must have water ice usually. So we find evidence for coipers, containing icy, dusty material at about the same distance out from

their star as ours is from the sun. So this is not something just about our solar system. When we look at other stars, we can see the difference between their co-oper-bouts and ours, and sometimes it's very interesting, they're a lot more massive. There are a couple of systems where they have much more ice, much more material in their cope-a-belt, and they don't have any giant planets inside. So you then start in the inner part of the solar system. You then have to say, well, how important is having one of these gas giants into pulling in comets onto your planet? to make them habitable. Do you need something like Jupiter

to bring the water into a planet to make it habitable? So it kind of adds a dimension to looking for Earth-like twins. Maybe it's not enough that it's about the same size, same mass, about the same temperature. You also need to have the rest of the solar system

to match to deliver the water. Absolutely. I couldn't agree more with that, particularly Jupiter. If Jupiter didn't exist, neither would we? So, I mean, what if Jupiter had stayed where it was when the solar solar? I don't know. If Jupiter didn't exist now that it would become a T-shirt.

I'd buy one. The producer is coming in with a big announcement. If you say tea, Melvin, you might have to your coffee. I'll take a couple of things. There are many more science and discussion programs from Radio 4 to download for free. Find these on the website at BBC.co.com. UK slash Radio 4.