In Our Time - The Planets

Episode Date: May 27, 2004

Melvyn Bragg and guests discuss our knowledge of the planets in both our and other solar systems. Tucked away in the outer Western Spiral arm of the Milky Way is a middle aged star, with nine, or possibly ten orbiting planets of hugely varying sizes. Roughly ninety-two million miles and third in line from that central star is our own planet Earth, in thrall to our Sun, just one of the several thousand million stars that make up the Galaxy.Ever since Galileo and Copernicus gave us a scientific model of our own solar system, we have assumed that somewhere amongst the myriad stars there must be other orbiting planets, but it took until 1995 to find one. ‘51 Pegasus A’ was discovered in the Pegasus constellation and was far bigger and far closer to its sun than any of our existing theories could have predicted. Since then 121 new planets have been found. And now it is thought there may be more planets in the skies than there are stars.What causes a planet to form? How do you track one down? And how likely is there to be another one out there with properties like the Earth’s?With Paul Murdin, Senior Fellow at the Institute of Astronomy in Cambridge; Hugh Jones, planet hunter and Reader in Astrophysics at Liverpool John Moores University; Carolin Crawford, Royal Society Research Fellow at the Institute of Astronomy in Cambridge.

This BBC podcast is supported by ads outside the UK. Thanks for downloading the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash radio 4. I hope you enjoy the program. Hello. Tucked away in the outer western spiral arm of the Milky Way is a middle-aged star with nine or possibly 10 orbiting planets of hugely varying sizes. Roughly 92 million miles and third in line from that central star is our own.

planet Earth, enthralled to our sun, just one of the several thousand million stars that make up the galaxy. Ever since Galileo and Copernicus gave us a scientific model of our own solar system, we've assumed that somewhere amongst the myriad stars there must be other orbiting planets, but it took until 1995 to find one. 51 Pegasus A was discovered in the Pegasus constellation, and was far bigger and far closer to its sun than any of our existing theories could have predicted. Since then, 121 new planets have been found and now it's thought there may be more planets in the skies than there are stars. What causes a planet to form? How'd you track one down? And how likely is there to be another one out there with properties like the Earths?

With me to discuss an area of research that's at the forefront of modern astronomy is Paul Murdin, senior fellow at the Institute of Astronomy in Cambridge. Carolyn Crawford, Royal Society of Research fellow at the Institute of Astronomy in Cambridge, and Hugh Jones, a planet hunter and reader in astrophysics at Liverpool John Moore's University. Paul Morden, let's start with their own solar system. Can you give us a brief outline of its topography, how many planets, how many moons, how many asteroids?

Well, let's start with the main body in the solar system, which is the sun. It's not a planet, it's a star, like all the other stars in the sky. It's by far the most massive object in the solar system. and you could say it's at the centre with everything going round it. I think the broadest definition of a planet would be that everything else in the solar system is a planet of one sort or another. A number of celestial bodies in orbit in a flat plane and a disk,

all orbiting pretty much in the same direction, all marching in a progression around the sun like the traffic flowing round around about. Counting outwards from the sun, the major planets, the ones which have individual names, the ones that are known as the planets, are Mercury, Venus, the Earth and Mars, which are four planets that are pretty much like our own planets, solid surfaces, most of them with atmospheres, and sort of places that you can imagine an astronaut walking on.

Further outwards from there, there's a group of four much bigger planets, Jupiter, Saturn, Uranus and Neptune, much larger, much bulkier, known as the Gaseous planets or the gas giants. I think it's fair to say eight major planets. Beyond Neptune, there's a range of smaller bodies. They're certainly worlds, and I think under a broad definition you could also say they were planets as well, but their origins are probably a little different from the major planets, so they're sometimes classed as a different sort of beast, and they are Pluto, discovered in the 1930s,

the planet Sedna, discovered only in the first months of this year, and other bodies like Quar, and some with just catalog numbers. There's a big gap in the solar system between Mars and Jupiter, and that's full of tens of thousands of smaller bodies, called minor planets or asteroids. Just to give a depiction of the sky, scale of the whole thing, if you imagine the sun as an orange at the center of the solar system, then the earth is a pinhead about 10 meters away, and Jupiter is a P, perhaps 150 meters away,

and Pluto, a grain of sugar, perhaps half a kilometer, three quarters of a kilometer from here. and the edges of the solar system extend on out many times beyond Pluto. What is the thing that distinguishes the sun from everything else? The star, the sun. Because the sun is so large, so much mass in it, so much big force of gravity, so much weight, so much heat in its center, so much pressure, their nuclear reactions take place in the middle of the sun,

as they do in the middle of every star. And these nuclear reactions supply the energy and therefore the light, for example, that we see from a star. The planets are considerably smaller than that. They don't have the same kind of temperatures in their interior. They don't have the same kind of pressures and they don't have the nuclear reactions.

They only shine by reflecting the light from their star. Hugh Jones, how are these planets, let's call them that, let's be possessive about the solar system. How are our planets formed? Well, if we look at other systems out there in the universe, we can see clouds of material from which we can see new stars being born out of. We haven't actually seen planets elsewhere being born definitively

out of these clouds of material, but what we can see is those stars being born out of those clouds of material. And the hypothesis we have is that our sun was was formed out of these similar clouds of material, and then following that formation of our sun, one would be left with a disk of material, which would be rotating around the sun, and out of that disk of material,

there would be smaller cores would be present in that, and onto these cores, we would find material would be gathered, and eventually the disk of material would be probably cleared away by the wind from the sun. Our own sun has a solar wind, wind, and that would clear away these cores, and what one would be left with would be the inner planets. Paul was talking about Mercury, Mars, Venus, Earth, and we would call those

the terrestrial planets, terrestrial being earth-like, and then the outer planets, Jupiter, Saturn, Neptune, Uranus, being the gas giant planets, and around those planets, you would have, in some senses, the rocky cores, like the Earth, like the terrestrial type planets, but in addition to having these rocky cores, they would also have a lot of extra material around them, which would have condensed onto them, the hydrogen and helium, and they're much larger bodies. They would have had a much longer formation process before any material they were forming from would have been cleared away by the star switching on and clearing the material. So let's go over that again.

This cloud of swirling, let's call it dust, just for the sake of a word of it, comes in about how many billion years ago, and it's swirling around all the time, when there's let us We believe our sun was formed around 5 billion years ago. So in it comes, the sun itself or the whole swirl? Well, the sun, we believe the sun would have formed first. Yeah. And then there's stuff going around and knocking into each other, and the heavier stuff is going to the centre.

So gradually objects are being formed out of this cloud. Correct. And then the solar wind is clearing the distances between, so we see what we think of as spaces, but may not be completely clear spaces out there then. and that's that's the way it went. That's our picture.

That's our picture. And so the planets were formed by collision, the dust colliding with each other and accreting. They were formed by some heaviness inside themselves, sort of drawing the gravity to make them into these shapes. To build them, yes, into larger and larger objects. I mean, collisions may well have played a role in the early solar system. It's, although accretion, if you like,

the sort of the snowball of material gathering together in the early dust cloud is one of the processes. It's clear that collisions would probably have played an important role as well. Right, Carolyn Grofford, how was the Earth formed and what happened to give us our moon? Well, the early Earth was very different from the planet we now know because the planets form hot and throughout their life they'll gradually lose this heat. But it means the early Earth was completely molten. How many years ago we talked about?

We're talking, again, probably about four and a half billion years ago. The Earth was completely molten, and this allowed all the elements in it to stratify, so you have the heavy metals sinking down to form the very iron-rich core we have, and the lighter elements rising up through the crust. And during all the like this first 500 million years of the Earth's formation, it's been continually bombarded by the debris

that's still left over from the formation of the planets, slamming into it. And there's one particular impact which is important for the earth. And that's where we think a proto-planet, maybe something the size of Mars, another planet in the making, whacked into Earth. This enormous impact sent off masses of debris out into orbit around the earth and that eventually coalesced to form our moon. And the protoplanet and the Earth merged together to form the Earth that we have now. Now, this, we need an explanation like this for the Moon for several reasons. One is that when you look at the makeup of the Moon,

it's made of much lighter elements than the Earth, which is strange, unless you say it's actually made up from the outer layers of the Earth that were shot into space when this impact happened. Also, the Earth itself has a much heavier core than you'd expect for a planet of its size. Maybe that's from this merging of the two protoplanets to make our Earth. And finally, the Earth-moon system is fairly unusual. It could almost be regarded as a double planet.

And it's a neutral in our solar system to have such a large moon around a single rocky planet. And it's much easier to explain that by saying there was one unusual event within Earth's history that caused that. So this is called the Great Impact, but it's still a theory, isn't it? It means that we have two ages, an astronomical age of four and a half billion years, and a geological age of four billion years. Because when the two predatory planets and the Earth came together, it reformed the geology. Yes, and certainly when you look at,

we can date the Earth and the Moon to be about over 4 billion years ago from radioactive dating of the elements in the crust, and also from meteorites that have come in from the Moon or other planets. New Jones, at the beginning of the program, I mentioned 51 Pegas USA as the extrasolar planet that was found in 1995.

What methods did you have to discover that? and why did it take so very long to find more planets than the Greeks had, as it were, two and a half thousand years ago? Well, I think the key difference, I mean, although planets have been found in the solar system since the Greeks, the key difference with the 51 Pegasus planet is that it's outside of the solar system. So whether we're talking about bodies beyond Pluto, additional planets, I mean Pluto itself has only discovered in the 30s. Whether we're talking about those additional objects in the solar system,

51 Pegasus is really something completely different because it's not a planet going around our own star, the sun. It's going around the star 51 Pegasus. What was special about 51 Pegasus was that we managed to, if you like, hone our techniques for looking for planets to such a level that we're able to see the very small signals, of a planet going around it.

Can you give us some brief notion of what honing the techniques meant? And why did it take so long? Okay. Well, what we've done to find the vast majority of planets going around other suns is to look for the so-called Doppler wobble. And what we're looking at is the motion of the star towards us and away from us. So in very much the same way that Paul was explaining earlier, that Jupiter is the primary planet in the solar system after the sun,

we can imagine that Jupiter and the sun are orbiting around one another. And so the sun, if you like, is wobbling backwards and forward slightly due to the mass of Jupiter and all the other planets as well, but primarily Jupiter. And what we did with 51 Pegasus is to look at the star and to look for that small wobble. And that wobble is at the level of 50 miles an hour or something like that, a motion of 50 miles an hour over a time period of a few days.

And in order to do that, we use a very high resolution spectrometer, which has allowed us to see the individual spectral lines in the Star 51 Pegasus. And we look at those relative to a reference source, and we see those lines changing with, we see the position of those. lines changing with time, which tells us that the star is going first, if you like, away from us and then towards us and then away from us in a regular periodic motion. And that's exactly what we would expect to see if one smaller body is going around a larger body. We're seeing the centre of mass of the large star changing. Carolyn, what is this going to meet? Are we going

to find masses more planets now? Is there a sort of planet burst happening? Yes. Now we're refining the techniques and the telescopes and instrumentation are getting better. There's quite an explosion really since this discovery of planet hunting. The very interesting thing is these solar systems we're finding are actually quite different from what we originally expected. In the case of 51 peg, what was discovered was something of the order of the size of Jupiter orbiting the star every four days, just over every four days, which is incredible because that means that it's nearer to the Sun, then Mercury. Sorry, it's nearer to its host star, the Mercury is from our Sun.

It's about a sixth of the distance between Mercury and the star. And this really challenges our views about solar system formation. From looking at our own solar system and what we understand about its formation, this predicts that any kind of gas giant, any Jupiter-type planet, should form far out in the nebula where conditions are cooler. And to actually see someone, some planet orbiting so close to the star, that's one of these gas giants, is quite a challenge to explain.

Either we've got our theories of the formation of the solar system wrong somehow, maybe not all solar systems form that way, or alternatively, maybe we're right, and this Jupiter-sized planet formed far out in the nebula and has since somehow spiraled in towards the star, which again confronts our notion that once you formed a planet, it stays put.

And again, that's confronting any ideas we have about the stability of solar, solar systems. It also makes you wonder if something that's formed far out in the nebula has sparred in close to the star, what happened to any rocky planets that might have also formed within that solar system? Maybe they've been swallowed up by the star itself. And maybe rocky planets like we have in our own solar system are much rarer than we thought. Paul, would you like to develop that? Could I step back from this a little and make a general overall comment about what's

just been said. I mean, I think this is absolutely revolutionary, and I think it's revolutionary for the following, I think it's a triumph for science, and I think it's, it's, for this reason. For a long time, we thought, human beings thought, that their place in the universe was special, that the earth was a special place, that it was like, completely unlike anything else, and that we were special creatures with a divine specification on our head, the divine, label on us. That was a philosophy of our position in the universe both philosophically and in geometry that was completely transformed by the scientific way of looking at things right from the atomists right through to the development of scientific perspective on

the way of the world. We've developed a completely different idea in which first First of all, our world, the Earth, was a planet like many others, and the inference being that possibly life existed elsewhere on other planets. But then there was speculation about the realization that our sun was like all the other stars, and maybe other stars had planets too. And as we heard from Hugh, that that actually turned out to be the case. but for at least a couple of thousand years, all of the speculation about what other solar systems were like,

or indeed whether there were any other solar systems, was all based on a completely philosophical view of the world like we were typical of things in the universe. And right in 1995, the position changed so much that we actually had actual evidence that this is really the case. I mean, it was a vindication of a kind of scientific perspective on where we are and what we are. And this, I think, is, I'd like to just sort of pick up Paul's sort of the revolutionary aspect of this.

I mean, we went, Copernicus moved the Earth from being the centre of the universe. And if you like, since has changed the definition of planet, you know, once the Earth was no longer at the centre and the Sun was at the centre of our solar system, then we had different types, we developed the notions of different types of planets, terrestrial, gas-downed planets. And here we're now moving into the notion of not just planets around our own sun, but planets around every sun. And we've now got a situation where the planets that we've found this 120 or how many exactly we have today, we're finding them in all different types of environments. And in fact, we're still not sensitive to any of any environment like our own. So the division of planets which is in our solar system is roughly terrestrial and gaseous.

There may be many, many more categorisations. So far, the ones that we've, the extra solar planets that we've found, a few of them fit into the gas giant category, though as Carolyn was alluding to, most of them are actually very different. But we're yet to really look at our own, to constrain the solar systems. The revolution is that we've found all kinds of, of other planetary systems, which are nothing like our own solar system. Given that the Earth is so small and dark, difficult to spot,

how long do you think before you detect a terrestrial planet like our own? That's a very tough one to answer. I mean, in some senses, we have already detected Earth-mass planets around pulsars, around the remnants of failed stars, which is exactly the last place. you'd ever expect to find a planet. So just to go explain what that means, after a star has used up all its nuclear fuel and exploded and got expelled all the outer layers, I'm talking about a very massive star, it would leave behind a very, very small, dense remnant. And what we find

is that we can see some of these small dense remnants to very large distances because they're spinning very fast. They've got very strong magnetic fields. They give out pulses, and we call them pulsars. And we can see that going around pulsars, we have, in fact, got Earth-mass planets. And so those are our first Earth-mass planets. But to call those terrestrial planets, we're not. Now, to find terrestrial planets going around real sun-like stars, I think we're going to have to wait at least 10 years.

We need a new technology for that. But you think you're going to get that. You think the idea of being unique here is, but with the conditions that obtain, on the Earth do seem to be very special and specialised, Paul. Yes. Can you just describe it? Well, Carolyn painted a picture of the evolution of planets that involved really quite chancy and fluky events.

I mean, she described that, I mean, the major one is the collision of this Mars side. Four and a half billion years ago. Mars-sized jaywalking, asteroidy sort of thing with the Proto Earth. I mean, that's a real fluke. It happened, apparently. if you believe the theory, you wouldn't expect it to be the routine thing to happen. So at the same time as there are systematic similarities in the formation of all of the planets, there are also completely chance events.

And some of these chance events have really big, let me pun and use the word impact, on the evolution and the development of the planet. So we could be a fluke one-off? Well, the universe is a very big place and there are a lot of stars in it, there are even more planets, as you said at the beginning. So I'd hesitate to use the word unique for our situation, but I would say that there are features of our situation which are very rare. I mean, our solar system, for example,

is apparently a case of arrested development in that it developed a Jupiter, like all these other planetary systems that have recently been discovered. But our Jupiter drew in from the outer reaches of the solar system to where it is now and then stopped and didn't go on to become a very close Jupiter to our star in the way that these other

planets have planetary systems have developed. So something happened to arrest the development of the solar system. That means that our Earth is in a very stable orbit and remains always just about at the right place in the solar system

and to be the right temperature for water to exist and for us to be able to exist. We've had this collision which created a big moon for us. And the gravitational pull of that big moon also holds the Earth in a particularly stable configuration. And all that means is that there has been a long time in which the Earth, the environment on the Earth,

has been compared with all the other things that might be going on, a rather stable place, and therefore life has had billions of years to evolve. And although life starts pretty briskly and creates bacteria-like things, takes a long time for those bacteria to get together in big colonies and make people. And that process of evolution

can be readily interrupted by all kinds of changes of circumstances. That didn't happen for us. It didn't happen for our world. Our Earth remains stable for billions of years, and as a result, we are here talking. I think it's important to say in this that the extra solar planets that have been found are very much the ones that we could find.

I mean, we're not yet sensitive to our own solar system going around any other sun. So while the Earth's environment is very special and does need a lot of very special circumstances, we've found planets going around something like 10 to 20% of stars in the solar neighbourhood. And around those other 80 or 90% of stars, we haven't yet discovered planets.

But we're thinking that the most likely thing is, in fact, now, that we will find Jupiter-like planets around those stars. So we can't yet pin down the existence of solar system-like architectures. We think the scale and the way that the solar system is made up is important, then we haven't yet constrained at. Of course, we've got to be sensitive to the fact that we've got a paradigm in our minds about how life evolved and what happened that is based upon our own interpretation of what apparently produced us. And just as in the past, people regarded the earth as a special case. And we have to be sensitive to the fact that there are other possibilities.

I was going to ask that, actually. So what other possibilities of life would you, could you speculate? Your jaw drops. Could I come back on that? There seem to be some things which generally people seem to agree and other things they don't. There isn't another element in the periodic table of chemical elements, apart from carbon, that generates chemicals which are of such complexity that you can imagine them producing complicated biological organisms. So probably life everywhere is carbon-based.

Probably you need a liquid environment for all the chemicals to get together and do their biochemical thing. And so water-soluble carbon-based life is what we should be looking for. I think the important thing is to free our minds of the sorts of environments in which we would expect to find life, but to stick with something which is fairly secure, which we should be looking for carbon and water. We said at the beginning, or it's been said, this is a revolutionary, a particularly exciting time.

Could you highlight the, as far as you're concerned, as a planet hunter? What is most exciting for you? Well, we're now entering an era where we're finding all of these whole range of different planets in a whole range of different environments, which allows us to look at the whole planetary formation scenario. and I think that very much links together with the formation of stars as well. So that's one linkage. And then I think the other way, as we've been talking, is in terms of the chemistry and biology of the situation.

I mean, lots of very exciting discoveries in the last few years about life being found in deep volcanic vents, in all kinds of strange environments, even yesterday I saw on the news something about, potential life in Venus's microbes in Venus's atmosphere, the whole range of different possibilities for life. And we're finding that from the astronomy

of where these planets are found, that we're finding this vast range of different environments where life might be found, and we're finding the architectures for all of that. Carlin, what are you most looking forward to discovering? Well, the extrasolar planets are very exciting, but I think one thing that is really coming of age is the search for possibility of life

within our own solar system. He's mentioned about in the clouds of Venus. We're obviously all aware of the recent discoveries with the NASA Rovers on Mars. But there's an upcoming expedition which is going to investigate one of the moons of Saturn, which is where we think we have.

There's a very thick atmosphere and possibly a lot of organic compounds beneath the surface. So I think that's a particularly exciting development that's going to be changing a lot over the next year. Well, thank you all very much. Thanks to Hugh Jones, Paul Merton and Carolyn Crawford. Next week we'll be talking about Babylon. Thanks for listening.

We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programmes about history, science and philosophy at BBC.com.com.com.com.