Life and the universe

Professor John D Barrow

Today is our opportunity to learn about life. Mostly we’ll learn, in the first half, about life on Earth, and then we’ll learn about some of the aspects of our environment, our solar system and the wider universe that are rather essential and interesting for life, and about some of the searches for planets elsewhere.

The bad news to start with, with regard to life on Earth, is that to a first approximation life on Earth is extinct. More than 99.9% of all the species that have ever existed on Earth are extinct, and we are a small trace element of this process that happens to have survived, for a rather short period compared with our predecessors like the dinosaurs. So besides being something of a trace element, life on Earth is also really rather rare if you look at it in terms of volume or in terms of mass. I drew up a little accounting of life on Earth by mass, and you have to change your origin of coordinates because of the large numbers. If we measure mass in phytograms, (a phytogram is 10 to the 15 grams), it’s about the mass of a good sized mountain, or of a comet, or of an asteroid. All living things on Earth weigh about 1841 phytograms, but almost all of that, more than 97%, are trees and plants, with a huge number of species, more than 3,500. All the rest, the 2.7% are creepy crawly things primarily animals, and we are just 0.22 of a phytogram. This is the sort of living biomass accountancy on Earth. There’s really not very much that’s alive. The rest is mostly silicone dioxide.

If you look at a picture of the size spectrum of things that walk around and grow on Earth, you get some perspective of the relative size of living things. The largest living thing on the planet, the blue whale, is much bigger than anything that ever went on land. The biggest things tend to be things that live in the sea. If you try to live on the land and get too large, the pressure on your base will eventually become so great that it will break the molecular bonds, so there is a limit to how big you can be without breaking. But if you live in the sea, you have buoyancy to support you and you can be much larger. You’re limited by other considerations of food mechanics. The largest land-going things, the dinosaurs, were not too far away from the limits of size imposed by structural stability, and therefore their legs tended to be rather fat – a good base of support. Size is proportional not to destiny but to density, so what you are going to do as a living thing, what you’re going to be able to do, depends to a very great extent on your size.

To quantify what I indicated just now about strength and weight, your strength is determined by your area, or particularly the cross-sectional area through one of your limbs. If you break your arm, what do you have to do? You have to break a line of molecular bonds through your arm. You have to break a surface. And so, if you were spherical or a cubic shaped living thing, your strength will be proportional to your size squared, but your weight is proportional to your mass, which is proportional to your volume, which is proportional to your sized cubed, if you’re a blob. So you can see how if you just grow in size, if you’re just magnified, all your dimensions get larger and larger, your weight grows faster than your strength, and you will eventually break.

You can test this: if I cubed the strength, it would be proportional to the sixth power of the size, and if I squared the weight, it would be proportional to the sixth power of the size, so the cube of your strength should be directly proportional to the square of your weight.

So let’s test this. I looked up the weight lifting world records. The weight lifted is a measure of the strength of the weightlifter, and we know the weight of the lifter (they’re all in weight classes), it follows this rule beautifully. In all the world weightlifting records you know the weight of the lifter, and you can calculate the square of the weight of the lifter, the cube of their strength and the weight lifted. It follows almost a perfect straight line. So maths really works!

That shows you how if you start to become bigger, there are certain stresses on your system as a living thing. You can’t just get as big as you like and think therefore you’ll be tougher than everybody else. You need more to eat, and therefore you need a much larger range to gather it from. In terms of population density against the size of living things, the smaller you are, the larger your population density. You require less calories to eat, you take up less resources, there’s room for you. But if you become a large ferocious mammal, you need a very large range, and that has problems, because it means you won’t often run into another potential mate if you’re a tyrannosaurus, and you need an awfully large number of calories to sustain you from the environment. So your size affects the number of you and your friends that there are, and also whether it will be possible for you to live on an island, for example. You wouldn’t have hundreds of tyrannosauruses on a small island. There simply wouldn’t be enough room to support them. So the type of planet you live on, whether it’s large continental masses or many small islands, will determine the nature of the living things.

It’s interesting to understand why we’re not overwhelmed by large ferocious predators, and we’re going to ask the question why are ferocious large predators so rare. There are really not that many of them – lions, tigers, jaguars. Each continent has perhaps one or two, but there are really not many of them. Why is that?

Well, think what you have to do if you’re a predator. The main thing is that you have to eat, and what do you eat? You want to eat things that are pretty big, and have lots of calories. If you’re a lion, you don’t spend your time eating grass. You want to catch antelope, things with high protein, calorific content. And what do antelope do? Well, they eat something that’s rather smaller. And there is a food chain in any environment that begins at the bottom, usually with grass and trees, and ends at the top with ferocious carnivores, and at each step, the person below fits into the jaws of the person above. That’s roughly the story, and it’s like a commercial operation, that calories are passing from the flora, trees, plants, up into the bodies of the most ferocious carnivores through a number of steps, and there’s a middleman, as it were, at each step, so that people that eat the grass take a certain percentage of the calories, and they’re then eaten by somebody else, who take another 10%, and the lion at the top finally takes his 10% of the calories at the bottom. So you can see that if you wanted to have a lion that was vastly bigger than existing ones, that lived off the present lions, it’s got an increasingly narrow amount of calories to live off, diminishing returns. The amount of energy available at the base of the food chain determines how far up it you can go, how many ferocious carnivores you can have living off slightly less ferocious carnivores. What’s worrying is that if you are a ferocious carnivore and you want to capture one of the slightly less ferocious carnivores, they tend to be able to run fast and you expend a lot of energy trying to catch them. So a cheetah can’t afford too many unsuccessful pursuits of antelope, otherwise the number of calories that it will get from the antelope when it catches it won’t replenish what’s spent in the chasing game. So this tree you see is rather curious; it shows us why you expect the number of extremely ferocious predators to become really rather small at the top of the food chain, but the answer as to how far you could go up, how many calories there are to go to the top, is determined by how many come in at the bottom.

For the answer to that question, we go to a little bit of astronomy, and it’s to do with photosynthesis, because that’s where the energy comes from at the base of the food chain on Earth. Photosynthesis is a process that involves converting carbon dioxide, oxygen or sulphur, so you can have photosynthesis that works with sulphur dioxide, or hydrogen sulphide, rather than water. Carbon dioxide in the atmosphere is converted to oxygen, and the efficiency of this process on Earth is about 1%. What it’s doing is converting sunlight, solar energy, into a form of energy that’s useable by plants, which can be eaten by us and other animals to power our bodies. Unfortunately, or maybe fortunately if you’re a global warming unenthusiast, only 0.03% of our atmosphere is carbon dioxide, and so that the base number determines the efficiency of photosynthesis and how much energy there is in the food chain to be eaten by animals. So the answer really to the question as to why ferocious predators are so rare is that there’s so little carbon dioxide in the atmosphere.

We know the Earth is a rather beautiful habitable planet with many unusual properties, and I want to pick on a few of those properties first of all to amplify a bit before we look at other planets. If you want to construct or find a habitable planet, there are various constraints you’ve got to worry about. The planet’s got to be big enough to hold on to an atmosphere. The Moon is too small to hold on to an atmosphere; its gravity is not strong enough to retain molecules of oxygen and nitrogen and hydrogen moving at a reasonable ambient temperature around its surface. They just escape; they have speeds greater than the escape velocity for the planet. So you want to be big enough to hold on to your atmosphere. You don’t want to be too big, because once you get too big, the force of gravity on your surface becomes so great that it will break up molecular bonds and crush anything that stands on the surface. There’s a sort of Goldilocks region, an in between world, where things are just right: big enough to hang on to your atmosphere, not too big to crush everything. The giant planets in the solar system like Jupiter and Saturn are really great balls of gas and liquid hydrogen, so if you drop the Earth into Jupiter it would float, for a bit.

The other matter to do with having an atmosphere is that it may be that you need to have a magnetic field. Mars, which has been much in the news, is a biggish planet, it looks as though it might be habitable in some ways, it has a tilt, rotation axis just like the Earth, but it has no magnetic field, and therefore it has no atmosphere. Mars once did have an atmosphere of sorts, but there is a wind of charged particles which are blown out from the surface of the Sun called the solar wind, and those particles, when they impinged upon Mars long, long ago, gradually started to strip off the atmosphere of Mars, and eventually it was blown away, leaving a fairly lifeless, atmosphere-less planet. Earth, on the other hand, unlike Mars, has a magnetic field, and so when those charged particles come from the Sun and impinge on the North Pole area, the magnetic field diverts the charged particles of the solar wind around the Earth, repels them and protects our atmosphere. So if we didn’t have a magnetic field, we would lose our atmosphere too, just like Mars.

It’s always been traditional for astronomers to think that a habitable planet needs to have liquid water on its surface - what astronomers have come to call the habitable zone. So if you think of a star radiating energy, if you sit on a planet at a particular distance, the further away you get, the cooler it will be on your surface. If you get too close, you will be burnt up, like Mercury; get too far away, and you’ll end up completely dead and barren, like Pluto. So again, there’s a sort of in between world where the temperature is okay; it’s high enough to keep water liquid without boiling it.

Recently there have been all sorts of interesting developments in this area within chemistry as well as within astronomy. You can see that if your orbit is circular, and the Earth’s is, then one particular radius keeps you in the right place to have liquid water, but if you have an extremely elliptical orbit, then when you get far away, you’ll freeze, all the water will turn to ice; come close in, it will all evaporate in steam. It’s like being in a rather nasty type of pressure cooker. So being in a very eccentric orbit is quite a difficult situation to maintain liquid water. Recently chemists have discovered that water can behave very differently when it’s present in very thin films. We’re familiar in physics that the behaviour of thin films, When you move in one direction across the surface, it thinks it’s in a real three-dimensional world, but when it tries to move up and down, it thinks it’s living in a sort of a one-dimensional world, and the forces between chemical bonds are quite different in a surface that’s very thin, or a film, than they are in an ordinary solid. It turns out that water, for example, can remain liquid when it’s in a thin film, way below zero centigrade, so the properties change in dramatic ways. Astro-biologists studying the origin of life on Earth are rather interested in the idea that life might develop in inter-facial media, or in thin films, and so one has to take seriously the possibility that the conditions under which liquid water can be maintained might be much wider than we’ve ever imagined before.

I’ve mentioned the fact that we don’t want our planet to get too big otherwise gravity will be too strong on the surface and atomic bonds will break. You try to have mountains that are too high, they’ll liquefy the matter at their base and just sink until they’re at a height where the base can solidify. The last thing is you want a certain amount of stability. We don’t want the temperature of the surface changing in dramatic ways so as to freeze and thaw, and freeze and thaw, conditions.

I have done some numerical simulations of the evolution of our galaxy. In the case of the solar system and other planetary systems, the habitable zone also changes with time, over millions and billions of years, because the star changes its output of energy.

When a galaxy, like the Milky Way, is very young, there’s much explosive star formation activity near the centre, supernovae, and this red region is uninhabitable because it’s bombarded by high energy particles and gamma rays. But what you’re wanting from the galaxy, from this activity, is the production of heavier elements, materials that can be used to make solid planets – dust, lumps and so on – and you need this activity in order to generate that raw material. As time goes on, supernova activity is confined to the inner region, and in this region you’ve got enough raw material to make some planets and planetary system with some probability. You’ve still got some nasty supernova activity and things that will kill you off at the centre. As time goes on, again you start to produce a ring within the galaxy, within which it’s probable and possible for life-supporting planetary systems to develop. In the outer region, there’s not enough raw material; in the inner region, there’s too much nasty activity. So just as in the solar system itself there is a region where it’s most probable for life to evolve and persist, the same is true in galaxies.

If we look in a bit more detail at what life is like within a habitable zone, and we think about planetary life in other worlds, around other stars, it’s interesting to think of essential features which would likely be shared by other inhabitable planetary systems, and if astronomers are sitting on planets around other star systems, they might share thoughts and ideas about the nature of their world with us.

The first is that every star rotates, or every planet also rotates at some rate, and you can see therefore if you have a parent star which you’re orbiting around, your rotation will give you the concept of what we call night and day. There will be a period of darkness, there will be a period of light, and the length will depend on your rotation period. And if you’re in orbit around the star, you will also have the concept of a year. It will be rather different to our year, but nonetheless these recurrent periods will be things that figure both in your science and in your mythology, your general beliefs about the universe.

Because you have a star, you have sunlight, and we believe you would have to have an atmosphere, a gaseous atmosphere of some sort, perhaps containing oxygen like our own planet, in order for there to be activity. One of the interesting things about an atmosphere is that it promotes the evolution of colour vision. When you have an atmosphere, sunlight is scattered by the particles in the atmosphere, and this is why we see that the sky is blue. If we have a sun over there, and we look at the sky over there, not in London perhaps but other times of the year, elsewhere, we’ll see a blue sky. The blue end of the spectrum is scattered the most, and so when you look over there, away from the Sun, you’re seeing light that has been scattered many times through the atmosphere. When you look towards the Sun at sunset, you’re seeing the light that’s been scattered least; it’s at the other end of the spectrum, and therefore it’s red. So the presence of an atmosphere scatters light, it produces colour, and once you have something like colour in the environment – it can also be produced perhaps by photosynthesis, chlorophylls being very active in that process – then natural selection has something to act on, someone can gain an advantage by having a little bit of sensitivity to colour.

Our own colour vision is like a three-dimensional system. It has three axes: one, roughly, is sensitive to light and dark, almost like black and white; one to the yellow/blue axis of colour; and the other to the red/green. You can speculate why that might be. Dark and light, you can tell when there’s light about and when it’s hazardous. Yellow and blue looks rather like the distinction between the Sun and the sky, and red and green, many living things, flora, green, and if you want to pick out things to eat, it’s rather good to be able to pick out red objects against green. So there may be evolutionary reasons why we have the particular colour vision that we do. But the most interesting reason of all, and which we might share with others, is the seasons. It’s always a good test to ask the ordinary person in the street, somebody generally interested in science, why they think there are seasons. It’s a good test question. Most people think it’s something to do with the fact that we’re closer to the Sun or further from the Sun at different times of the year. It’s not, in fact. The reason there are seasons is because the Earth’s rotation axis is tilted with respect to the vertical, where the horizontal is the plane of the orbit in which it’s going around. So as the Earth rotates around the Sun, its rotation axis is tilted at 23 degrees to the vertical, and so you can see that that means that for 6 months of the year the people in one hemisphere are closer to the Sun, the other 6 months, they’re further away. If there was no axial tilt, there would be no seasons. The fact that you’re in an elliptical orbit is actually quite irrelevant, it’s newt and tortoise. Even though you might think in an ellipse you’ll be farther away for part of the time, properties of the ellipse are such that the amount of solar flux that you receive when you’re farther away is exactly equal to what you receive when you’re closer because you move faster when you’re farther away than when you’re close. So if we had a much larger tilt, if it was 60 degrees or even 90 degrees, like Neptune, the seasonal variations would be huge, enough to freeze and melt huge factions of the sea in every annual cycle. So it’s important to have some seasonal variation, but not to have it massively extreme.

That’s a very interesting story about our seasonal variations, and it’s linked very much to the existence of the Moon. A famous picture was taken by Apollo on its first visit to the Moon. These pictures, in the mid-1960s, really aroused people’s environmental awareness for the uniqueness and unusualness of the Earth as an object in space. It was colourful, it was exotic, it was nothing like the Moon, which is dead and cratered, dusty and lifeless. Well, the Moon, despite this rather unappealing propaganda, is actually vitally important. If the Moon didn’t exist, neither would we, and the reason is that the Moon is extremely unusual in the solar system. No other planet has got a moon which is so large compared to itself. So the Moon is almost like a double planet to the Earth, and because of that, it can really be extremely influential. We know that it does exotic things like produce complete eclipses of the Sun due to its coincidental size compared with the disc of the Sun, but it’s a reflection of its huge size. The most interesting thing about the Moon is that it stabilises the long term evolution of the Earth’s climate. How does it do that?

I mentioned that the Earth’s rotation axis points off, its North Pole points off into space in a particular direction, but it doesn’t always point in the same direction. It processes, rather like one of those little tops on a stand, you start it spinning, and it weaves around, processes, in a little circle. The spinning Earth is just the same. It takes about 26,000 years for its North Pole to process around on the sky. At the moment, we’re rather fortunate for quite a while, our rotation axis has been pointing towards a particular star, the one that we call the Pole Star, but for large parts of recorded history, it pointed in the direction where there was no interesting star, and navigators and writers didn’t have a star which they could call the Pole Star. I think I mentioned in one of my previous lectures that Shakespeare has Caesar say “and one is constant as the Northern Star”. It’s a complete anachronism in the sense that in Caesar’s day there wasn’t a Northern Star, but in Shakespeare’s day, there was.

This procession process is very interesting because it’s a rotational movement that can be made to resonate with gravitational perturbations coming from the other planets and their motions in the solar system. The ill-fated Millennium Bridge in London, before it was damped down, if you walked across it, it starts to undergo huge oscillations. There’s a resonance between the frequency of your walking and the natural oscillations of the bridge. The same thing can happen to the Earth’s procession, and when you do the calculations – detailed model of the solar system – the results are really unnerving, because you discover that the planet Mars, for example, has a history in which the tilt angle will have undergone chaotic movement over hundreds of millions of years. Its tilt today is 24 degrees, very similar to the Earth’s, but its history is completely chaotic, the tilt has moved all over the place, and that’s no doubt one of the reasons when you look at photographs of Mars where you see bits of ice all over the place. It’s as though the climate’s been permutated at random every so often, and it has. What happens with the Earth, you do the same calculation, and the same thing should have happened to the tilt axis of the Earth. Every few million years, it should have been moved around, and our whole climate should have dramatically changed, but if you add the Moon, that doesn’t happen.

Without the Moon, the Earth’s tilt angle from a typical starting condition, instead of staying around 23 degrees, would jump all over the place, and we would have huge variations in climate from millennia to millennia, huge changes in melting and freezing of the oceans, and a very unfortunate environment. You would get chaotic, random, huge variations. But the Moon is so big that its gravitational field overwhelms all those perturbations from the other objects in the solar system and acts like a damping force on the resonances, and all you get is a little wobble of the Earth’s rotation axis, plus or minus a fraction of a degree, and that’s very likely a contributing factor, if not the main factor, in the hundred thousand year cycle of the Ice Ages. If you were Mars, you have two moons, they’re rather uninteresting moons, not like our Moon, they’re just asteroids that came a bit too close, they’re tiny, 10 to the 15 grams, one phytogram, just the size of a comet or an asteroid, so they have no effect on the main orbital dynamics of Mars. Similarly, the other outer planets, like Jupiter and Saturn, they’re so far away from the Sun and the other planets, that they don’t have any resonances with their rotation at all. So we’re saved by the Moon, and you begin to see that if you want to set up a habitable scenario somewhere else, you’ve got quite a lot of complicated factors that you might expect to come into play. It’s not enough just to have liquid water. You’ve got to stay in an orbit where there is liquid water. You’ve got to have low seasonal variation, and so on.

Many years ago, I think it was in the early Sixties, Frank Drake began the serious astronomical thinking about life in other worlds, and began what become known as the SETI project, the Search for Extra Terrestrial Intelligence. He produced a famous formula that goes by various names in the subject. Some people call it Drake’s formula. Someone even used to call it the nonsense formula, not wishing to prejudice people’s views in any way. You shouldn’t take the formula very seriously I think, but it just focuses your attention on the sort of factors that you should take into consideration. What Drake was trying to estimate is the number of inhabited or intelligent civilisations that might be able to communicate with us, or we to them, within our galaxy, and what sort of factors would you take into account. The first, R, is just the average rate of the making of stars in the galaxy, so how many stars do you tend to make, 20 to 50 a year or something like that, and then he’s interested in saying, well, what fraction of those are going to be suitable as stars around which planets can orbit, so we don’t want highly unstable stars or ones which produce outbursts or something like that. And then you’ve got to think a little harder and say, well, what fraction of those will have planets? Back in the Sixties and Seventies and until quite recently, nobody had any idea whether planets were a common phenomenon. The fashionable theory of the origin of the solar system when this formula was first conceived imagined that the solar system formed because of a sudden rather random, unusual event, a supernova, an explosion, a catastrophe in our part of the galaxy, and therefore planet formation was not a common generic process as we now know it to be. Then we might want to know how many of those suitable planets are within a habitable zone around their star, and whether they will stay in a habitable zone? Having found those, how many of them allow life to actually originate? This might require the tilt angle of those planets and their rotation to be just right, something about their size to be just right, and so on. And then having given rise to life, certain sorts of life are not very interesting. Okay, we’re interested in intelligent life perhaps, or life that does interesting things. And then how many of those intelligent species wish to communicate? And then finally, L at the end, the most controversial and uncertain factor, what’s the average lifetime of such a civilisation? Do they annihilate themselves, do they poison themselves, does something happen? With high probability in their environment, do they get hit by comets and asteroids? The Moon protects us in a rather good way. So you can see that you would be able to add perhaps more factors, but there really is a long straining process of many factors that have to be satisfied in order to give rise to a communicating collection of intelligent species with any probability. Of course N, the number of sites, might be very strongly affected by the sheer number of stars that there are in the galaxy, but the problem with a formula like this is every single factor in it is enormously uncertain. There’s very few that we know with any precision, and so you can get an answer that varies between zero, one and ten million. So it’s interesting for just sharpening your thoughts about what are the considerations, but not very useful for telling you how many people are likely to be there.

The great mystery of the search for intelligence of course is what’s sometimes known as the great silence. Where is everybody? If the universe is teeming with intelligent forms of life, as Frank Drake and co argued at the beginning, why are they not signalling to us? Some of these things I’ve mentioned in other talks before, there’s all sorts of reasons, and what’s odd about them is that you can think of a reason, and then you can think of the opposite reason usually, which is just as plausible.

So let me give some examples. It could be that we’re too boring. So we’re just another typical form of life and civilisation that’s developed billions of times over our galaxy. We’re entirely predictable in the course of life, and a very advanced civilisation listening to our television programmes and watching what we’re doing – incidentally, that is the first signal that they receive. It’s a rather sombre thought that if you’re a distant listener looking for life on Earth, the things that will be first discovered about life on Earth will be the first television broadcasts that were made, and they were Hitler’s broadcasts during the Second World War. They will be the first pieces of information about planet Earth that will be received. So we may just be like one of those beetles, the hundreds of thousands of species at the beginning, so we’re just too boring.

On the other hand, we might be too interesting. We’ve very particular and unusual and so we’re in a sense in a galactic game reserve, we’re being watched very carefully, studied in a non-interventionist way, scholars are giving courses and conferences and lectures about all the exciting things that are happening at Gresham College and London and the world.

It could be that we’re the first and that’s why we haven’t heard from anyone else, that we’re the first intelligent species in our galaxy. It’s a rather non-Copernican reason, unless there’s some good astrophysical reason that our star is the oldest or something. On the other hand, we might be the last, everybody else has died out, been blown to bits by cometary impacts, so we could be the last.

It could be that life is actually self-regulating, that it extinguishes itself through disease, through exhausting resources on a planet, that you can’t survive very long on a planet, you can’t become an advanced industrial society and persist for very long. If you want to be long lived, you don’t have things like industrial development and so forth, and so you’re not into the business of sending signals and you don’t make yourself known.

Certainly realistic is that we know that planets suffer impacts from comets and asteroids. We’re protected by the Moon and the giant outer planet Jupiter, but we’ve come to realise that this is a dramatic problem for planetary civilisations. In the last century, there were two major impacts that we know of on Earth, one in Argentina, one in Siberia, and then we know in the past there was a catastrophic impact which did away with the dinosaurs. It could be that if you don’t have a protector like the Moon or Jupiter that your civilisation is inevitably destroyed periodically, you’re always being put back to square one in the evolutionary process by impacts. We’ve seen recently the catastrophic consequences of the tsunami in South-East Asia. It wouldn’t have to have been very much bigger if it had been an impact, and you could imagine you could have overwhelmed an entire planet.

An idea which is somewhat unusual, by which I often try to pedal almost in a sort of storybook way, is that it’s always assumed by everybody that somehow consciousness is an inevitable outcome of intelligence, and that if advanced civilisations become intelligent, then they are therefore conscious. It’s quite possible that consciousness is not required for high intelligence, and that there could be extremely intelligent, scientifically-based, living systems in the universe that are not conscious. How could that be, if you try and imagine it? Or at least they’re not conscious about their science; they don’t do as we do. If somebody throws a ball towards a young child, they will probably catch it, but they don’t solve the equations of Newtonian motion, they don’t know anything about mechanics, and F = MA and things like that, but the evolutionary process has engendered in them information about the nature of the world, so unconsciously they know about motion. So when you cross the road, your brain automatically takes into account all sort of aspects of motion, and if the light’s fading, it’s doing calculations involving optics, but you’re not conscious of any of that. So imagine that this innate intuition about the nature of the world had been expanded enormously, so you had that type of intuition about the quantum world, and all sorts of other aspects of nanotechnology. Then you would be able to do things and develop things just on the basis of that innate intuition. You would be highly intelligent but you wouldn’t have any consciousness of that scientific knowledge, in the same way that you don’t when you catch the ball.

Of course perhaps more likely still is that life is totally different, it’s not like anything that we’ve imagined, and I think the best way in which it might be totally different is that it’s fantastically small. The development of modern technology and computing is going to smaller and smaller scale. We talk about nano-engineering, adjusting and manipulating individual atoms and molecules. So if life wants to escape exhausting its resources on a planetary surface and producing huge amounts of pollution, in the very, very long term, what better way to do it than to evolve to become extremely small, undetectably small, imperceptibly small.

As a bit of light relief from that for a moment, let’s say something else about Mars. In pictures of Mars you’ll see bits of the polar caps, the red planet, the face of George Bush (!), and the canals, it’s all there, and that sort of stuff I think had an important impact on our thinking about life in the universe, that Martians became the identikit aliens, so Martian became the sort of by-word for ET throughout the 20 th Century. How did that happen? Why did it happen? There are some interesting people involved.

The first, and the person who started it all off, was Giovanni Schiaparelli, who was a good astronomer at Milan Observatory, who was one of the main19th Century astronomers studying Mars. Mars comes close to the Earth around every nine years, it has a distance of close approach, so that’s why there were all those spacecraft a year ago all lining up to crash on the surface. They were sent to coincide with the close approach. What Schiaparelli did was to publish drawings from observations he’d made at Milan Observatory of the surface of Mars. Incidentally, he’s a famous name in Italy. He’s the uncle of the Italian fashion designer. Those articles he detected on the surface lines and patterns, he called them “canali”, channels in Italian, but of course when Percy Lowell in America and others translated “canali”, they said canals, and so there grew up the idea that there were structures, engineered structures, on the surface of Mars.

Percy Lowell at Flagstaff Observatory in Arizona became really besotted by this idea, and when he repeated the observations, he started giving very exotic classical names to regions on the surface of Mars, making it sound like some lost kingdom of the past. So Lowell wrote many books about the inhabited surface of Mars, and people became convinced that Mars was inhabited, and people started creating messages to transmit there. In the early years of the 20 th Century in particular, this really came to a head.

The person who exploited it in the most interesting and memorable way was HG Wells with his novel The War of the Worlds and the invasion of the Martians. And you can see how the whole thing came to its final dramatic head with Orson Wells’ wonderful radio production of The War of the Worlds which produced a country-wide panic in the United States, so convincing were the radio broadcasts within his radio show, that the Martians were taking over parts of the United States, that literally I think 200,000 people were fleeing Los Angeles to escape from the Martians. So this is really the story as to how Mars became part of our consciousness as representing aliens.

Well, astronomy has changed this story very greatly in just the last few years. New technologies enable astronomers to observe planets now around other stars, and at the last count, and this can change almost on a weekly basis, when I looked a week ago, there were 136 planets now discovered around other stars. The principal method by which this is done, you look at a moving star, and you look at the variations in its light. As it moves in an orbit, the light will be shifted by the Doppler shift, and you look at the little wiggles in the light variation against time, and the speed of the motion against the phase of the orbit, and the deviations from the perfect curve will tell you whether there is another object in the system, whether there is a planet there which is just causing the star to wobble a little bit as it goes round, because the star and the planet will rotate around their common centre of gravity, which will be a little different to if the planet wasn’t there. In some cases, you can then actually look and see a planet passing in transit across the face of a star.

This process has enabled people to look in many, many systems. There’s a selection effect of course. All the planets that you find are very large, much bigger than the Earth. There were probably, until just a few weeks ago, no solid planets, and that’s because you can only see the big ones.

There’s one system in which there are three planets, so there is a solar system amongst them. So the first lesson we have from this is that the formation of planets around stars seems to be a generic process, there’s nothing special about it, and it even seems that having more than one planet is relatively not so special. In the future, we will hope to look for solid smaller planets in the system, and particularly small moons moving around the big planets, just as Titan is a moon of Jupiter.

So planets are common, but there is something about these planets which is very unusual. They’re extremely large because we can only see the large ones, and as time goes on, we will see the small ones. Within 10 or 15 years, we will be able to see Earth-sized planets around other stars and analyse the content of their atmosphere, to see in particular whether there are greenhouse gases, whether there is industrial activity on a planet.

© Professor John D Barrow, Gresham College, 20 January 2005