Is Anybody Out There - Mathematics of Life

Mathematics of Life (2011)

Chapter 18. Is Anybody Out There?

Armed with an understanding of the different meanings that might be assigned to the term ‘life’, we can now return to the question raised at the beginning of the previous chapter: Does life exist outside our own planet?

Scientists have never observed an alien life form, except perhaps some tiny fossils in a meteorite, found in Antarctica and designated ALH 84001, which some scientists think are evidence of past life on Mars (Figure 74, see over). In 1996, NASA scientists announced that the meteorite, thought to come from Mars, contained tiny fossil bacteria. That claim remains controversial, and seemed to have been comprehensively demolished until a recent reappraisal left a tiny bit of room for hope, by answering some of the original objections to a biological origin.1 It’s pretty clear that the meteorite did come from Mars. Trapped gases in tiny bubbles in the rock match the profile of Mars’s atmosphere extremely well, and calculations indicate that rock could have been blasted out of the Martian surface by an impact with a small asteroid. And if that happened, some of the debris from the blast could have ended up impacting the Earth and landing on the ground in the Antarctic, where this particular meteorite was found. The rock does contain strange, tiny shapes, but whether these shapes were once alive is the key issue – many think that the ‘fossils’ might be the result of non-biological processes. It’s difficult to give a definitive answer. Since extraordinary claims require extraordinary evidence, the burden of proof is on those who assert that the shapes are fossils of once-living organisms.

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Fig 74 Suspected fossil bacterium in a meteorite from Mars. It is less than onethousandth of a millimetre long.

Can we say anything genuinely scientific about life on other planets?

I think we can, thanks yet again to the role of scientific inference.

Our current understanding of the origins of Earthly life suggests that there is nothing particularly special about our planet, so we should expect to find life elsewhere. Even intelligent life, aliens. Some scientists disagree, and argue that the Earth is special, so that life elsewhere may be very unusual, and complex life very unusual indeed. In their book Rare Earth, Peter Ward and Donald Brownlee make a persuasive case for this assertion, listing numerous features of the Earth and the Solar System that make it particularly suitable for life.2 They accept that life elsewhere is entirely possible, but expect it mostly to be at the level of bacteria. Intelligence, they argue, will be very rare indeed. Others go even further, and assert that our planet is unique: the only place in the entire, vast universe where life exists.

Scientific opinions on the prospects for alien life take one of three broad positions:

• Alien life does not exist (not by definition, but by sensible scientific principles).

• Alien life does exist but must be very like terrestrial life.

• Alien life does exist and much of it is totally unlike terrestrial life.

The first position is rather negative, but since there is currently no convincing evidence of alien life, it is safe from attack, at least for now. However, current scientific understanding of the origins of life by natural physical and chemical processes does not limit life to one planet among 200 billion galaxies, each having on average up to 400 billion stars, a significant proportion of which (maybe one in four) probably has several planets. So it would be a big surprise if the Earth really is the only place in the universe where life exists, even if it has to be exactly like ours.

The second position is the most respectable scientifically, though not the most imaginative. We know that life forms like ours are definitely possible, and we don’t know for sure that anything different can occur naturally. (Unnaturally is another matter.) The emerging science of astrobiology, or exobiology, combines Earthly biology with astronomy. Until recently it was almost solely focused on the prospects for Earth-like life, requiring an Earth-like planet. The more we understand about the origins of life on Earth, the more stringent these requirements become, and our best estimate of the chance of finding such kinds of life is correspondingly reduced.

However, the third position is slowly gaining ground. There are many valid scientific reasons to think that alien life need not be exactly like ours. One of the most important characteristics of life is that it is adapted to its environment – the basic feature of evolution. There seems to be no good reason why organisms could not evolve in, and become just as adapted to, an environment that differs from anything found on Earth. Insisting on an Earth-like environment as a prerequisite for life seems too narrow. It would be like Victorian explorers expecting all human beings to resemble Victorians in dress, manners and social structure, and ruling out the African forests as a human habitat on the ground that there aren’t any milliners’ shops there. Many astrobiologists are coming round to the view that while other Earths (just like ours) may indeed be rare, alien life might exist on worlds that differ from ours. A better term for this view is xenobiology, the biology of strange life; ‘xenoscience’ might be even better,3 to make the point that alien ‘biology’ might be radically different from anything in our biology textbooks.

How likely is it that the Earth is the only planet, anywhere in the universe, with intelligent life?

To keep the numbers simple, assume there are 1022 planets. By the law of large numbers, in order for there to be one planet on average with intelligent life, the probability of a planet harbouring intelligence must be 10-22, one chance in ten sextillion. If the probability is 100 times bigger, then we expect 100 such planets; if it’s 100 times smaller, we expect 1/100 such planets – which I’m inclined to interpret as ‘none’. It therefore requires some very precise cosmological fine-tuning to hit the magic number 1 that makes our own world unique. It seems unlikely that any plausible physical mechanism could translate the probability of intelligent life into a specific number of planets. So either Earth won the jackpot in a cosmic lottery, or we are not alone.

Standard calculations indicate that in the critical case when the probability is exactly 10-22, the probability of intelligent life being unique is 37%. The probability of no planets with intelligent life is also 37%, and the probability of two or more is 26%.4 Those aren’t bad odds, but it is a sobering thought that even when the universe is exquisitely fine-tuned for humans to be unique, we should expect no worlds with intelligent beings just as often as a unique one. And more than one is almost as likely.

If some day we discover that we really are alone, then either we’ll need a better mathematical model, or we’ll be forced to conclude that some kind of cosmic destiny has arranged for us to be unique. Right now, the best guess is that we are not alone. Planets with intelligent life are probably rare, but the universe is so vast that if there were about a quadrillion such worlds in the universe, all currently harbouring intelligent life,5 then there would be less than ten thousand in our own galaxy. On average, the nearest one would be about a thousand light years away. So the universe could be teeming with life, and we’d never encounter it.

A quick look on the Internet reveals that many non-scientists are convinced not only that aliens exist, but that they have already paid us a visit. However, the US Government has instituted a coverup, so we don’t see aliens walking around our neighbourhood. I’m willing to believe that the US Government – or any government – could decide to cover up alien visitations; however, I doubt it could succeed for long, and I don’t think it has – because I don’t think there’s anything to cover up.

The main reason for disbelieving these tales of alien visitations is not political opinion, or the mindset of the believers, but the science of alien life – which is thriving, and entirely scientific, despite a complete absence of observations of aliens.

It is possible to do good science, even when the subject under discussion has never been observed. Physicists have done a huge amount of work on the Higgs boson, a particle predicted by the ‘standard model’ of subatomic particles, but no one has yet observed one. In fact, that’s what the Large Hadron Collider, which famously broke down days after it was first switched on, is looking for. Assorted nations have committed $9 billion to get it built and keep it running. It is now back in action, but unlikely to find the Higgs boson before this book is published.

No one has ever observed a superstring, but string theorists have devoted a lot of effort to these hypothetical objects because they have the potential to unify quantum theory and relativity. No one has observed a universe coming into existence, but on the whole, cosmologists don’t resign and get jobs as hedge fund managers because of that. No one has observed the interior of a black hole, the birth of a Neanderthal, the emergence of life on land, a herd of wandering sauropods or the gravitational field of the Andromeda Galaxy. Not directly, and in many cases not at all. But these areas are all solid parts of the scientific enterprise.

In fact, you could do a lot of excellent science in a quest to prove that aliens don’t exist. Rare Earth is a case in point.

Science is not simply a matter of direct observation. It is an intricate interplay between theory and experiment, and the experiments are often indirect. The strength of science is inference. No one alive in the past million years observed the evolutionary divergence of humans from chimpanzees, but scientists are in no doubt that this event happened, because many independent lines of investigation all point inevitably to that conclusion. The fossil evidence, the observed ages of the rocks that contain them, the immensely detailed DNA evidence, and the biochemistry of chimp and human bodies all make this contention at least as certain as the Earth going round the Sun.

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Fig 75 Picture alleged to be of a ‘grey’ alien.

Martian microfossils aside, the main claims of alien observations come from ufologists, New Agers and believers in the paranormal. Abduction is a common scenario. The typical ufological alien, often called a ‘grey’, is humanoid, shorter than us, with a greyish skin, a large head, huge oval eyes and tiny nostrils (see Figure 75). Its bodily proportions are different from the human, and its limbs have different joints. Greys dominate reports of encounters with aliens to an extent that is quite remarkable: 90% in Canada, 65% in Brazil and 40% in the USA. In Europe the figure drops to around 20%, and Britons, eccentric to the last, reach only 12%.6

Ironically, the main reason for disbelieving such reports of alien visitors is not that greys are too strange to be credible. It is the exact opposite: they are not strange enough. Greys are the wrong kind of alien. They are far too similar to us. And the science that justifies that claim is good, solid, terrestrial biology.

From the 1960s, my biologist friend Jack Cohen gave over 300 lectures to schools about life on other planets. One of the key scientific principles he explained to them was how to decide which features of life on our planet are likely to be found in alien life forms, if they exist, and which features are merely accidents of evolution on this world and would not be expected elsewhere. I say ‘likely to’ because the discussion has to be theoretical at this stage in our exploration of the universe.

Cohen distinguished these two types of feature, calling them universals and parochials. The words can be used both as adjectives (‘this is a parochial feature’) and as nouns (‘this feature is a parochial’). Five digits on a hand is a parochial, but appendages that can manipulate objects are universal. Wings covered in feathers are parochial, but the ability to fly in an atmosphere is a universal. Daisies are parochial, but photosynthesis – obtaining energy from light – is universal. The term ‘universal’ does not mean that such creatures will exist everywhere, not even on every suitable planet. Flight needs an atmosphere, for instance, but we don’t expect every planet with an atmosphere to have flying creatures. Universals are features that are very likely to evolve on other suitable worlds. Parochials, on the other hand, are local accidents, and we would not expect to see them elsewhere.

The creatures of our planet, at any level of detail, are mostly parochial instances of universals. Each particular type of eye – and there are hundreds of clearly different structures – is a parochial, but vision is a universal. Legs differ from one creature to the next, but locomotion is a universal. Such examples motivate a test that can distinguish the two types of feature. Did the feature evolve just once, or many times independently? ‘Just once’ allows for subsequent modification in many different descendant species. ‘Independently’ means that the different instances have no such evolutionary connection.

In humans, the airway and foodway cross, resulting in many deaths each year from choking. We share this structure with most mammals. It’s a very poor ‘design’, and goes back to an evolutionary accident. About 350 million years ago there were no large land creatures, but in the seas and oceans were many fish. Some had lungs on the top of their bodies, some had lungs underneath. (Taking in some of the planet’s atmosphere to power chemical reactions is universal: where the associated organs go is parochial.) The lobe-finned fishes that evolved into land animals, mentioned as transitional forms in Chapter 5, happened to have their lungs underneath. Our own awkward arrangement is a consequence. It is reasonable to think that some other fish might have made the evolutionary transition to land instead, even on this planet; certainly there seems to be little to stop an analogous event happening elsewhere. So foodway-crossing-airway is a parochial.

Fossil evidence supports the view that the transition from sea to land was gradual. Fish did not suddenly come out on land, contrary to the Gary Larsen ‘Far Side’ cartoon in which three young baseballplaying fish are gazing wistfully at the ball, which has popped out onto the land. The transition from fins to limbs probably happened while the fish were still in the sea, scuttling around in the shallows and increasingly using their fins to push against the mud and sand. Their ability to ‘walk’ along the seabed evolved in concert with the structure of their limbs. By the time they were inhabiting the land permanently, their fins had changed to legs. Pentadactyl limbs, ending in five digits, evolved during this period, and were transmitted from the fishes that scuttled in the shallows to amphibians, then to reptiles, then to mammals and us. The entire structure of our limb joints is parochial. However, possessing jointed limbs is universal. It evolved independently in insects, for example.

The ‘evolved here many times’ test for a universal is closely linked to Earth’s particular evolutionary history. Intelligence is a universal by that definition: it has evolved in the octopus and the mantis shrimp, as well as mammals. But human-level intelligence – what Cohen and I call ‘extelligence’, the ability to store cultural capital and know-how outside ourselves in a form that can be widely accessed – seems to have evolved only once on Earth. Dolphins are smart but they don’t have libraries. So extelligence fails the evolutionary test for being a universal. However, it’s reasonable to argue that it ought to be a universal. Our specific brain structure is parochial, and even brains as such may be, but extelligence is a generic trick, offering clear evolutionary advantages, rather than a specific accident of heredity. Perhaps we haven’t waited long enough for it to arise again.

This suggests that we should broaden our definition, replacing it by a more theoretical version. A parochial is a specific feature that appears to have arisen by accident, and would be unlikely to occur in the same form in a rerun of Earthly evolution. A universal is a general feature that could be realised in many different ways and appears to offer a clear evolutionary advantage, and would probably emerge again if evolution were to be reset and run a second time. Agreed, this definition leaves room for debate, but the whole idea is intended as a guideline, not as a hard and fast rule. It is a guideline for how to think about the possibility of life on other planets.

How good a guideline it is will depend on whether we find such life, and what it looks like if we do.

Most of us absorb our images of aliens from television and movies. Mine came initially from the Eagle comic, and the exploits of Dan Dare, Pilot of the Future, tangling with green-faced Venusian Treens and the evil Mekon, who had an enormous head and a small body, and rode on some sort of antigravity cushion. Comics had more room for imagination than movies or TV. In those media, before computer graphics reached its current levels of realism, aliens had to be thinly disguised human actors, insects magnified to gigantic proportions, or invisible presences that glowed in the dark, emitted sparks or disturbed the air and moved the curtains. Now they can be impressively detailed creatures that inspire terror, like the mother alien in Alien, or they can be cute and cuddly like the Ewoks in Return of the Jedi. And that is what their designers intend, and it is why the aliens in the media mislead us about what real aliens might look like.

Media aliens are invented in order to stimulate specific human emotions. This makes them hopelessly parochial, and many of their features make no scientific sense at all. The aliens in Alien grew inside a human body until they reached the size of a cat, and then burst out gorily through the chest wall. Leaving aside the question of why the victims often didn’t know they had a cat-sized lump inside them, how did a creature on another planet evolve to exploit the biochemistry of a human body? They might just possibly be generalist parasites that can make use of a variety of other creatures as hosts, but it is almost impossible for that kind of generalism to evolve. Parasites co-evolve with their hosts, and are usually very specialised. Dog fleas can’t survive for long without a dog, even though they may temporarily infest a human.

The great universal is evolution. That is how life can diversify, and some of it becomes more complex, while growing ever more suited to the conditions on its home planet. A real alien would have evolved in some environment elsewhere in the universe, and it would be adapted to that environment. So, in order to invent scientifically credible movie aliens, you would have to invent a plausible environment and evolutionary history as well. Mother alien and her parasitic children don’t work. But few movie or TV producers go to those lengths. ‘It’s science fiction, it doesn’t have to make sense,’ they seem to think. But this is a recipe for unconvincing entertainment, bad science fiction, and even worse science.

Another source of images for aliens is human culture. Previous generations saw – well, some thought they saw, and many believed in – ghosts and pixies and other supernatural creatures. Claims of alien visitations and abductions are part of a long tradition of fearsome supernatural creatures, presented in terms that made sense to the culture concerned. They all probably have the same origins: sleep paralysis, in which we can become awake while still in the dreaming state, where our limbs refuse to move and our critical faculties are suppressed. The dreamed abduction seems real because the part of the brain that distinguishes dreams from reality is not functioning, and we experience a feeling of terror because we can’t move.

Specific images and aspects of these supernatural visitors spread through the culture. Even people who don’t believe in UFOs ‘know’ that aliens have huge, dark eyes and big heads. That’s how you tell they’re aliens. Actually, it’s how you tell they are fictions. Greys are too humanoid: they are built from minor variations on human parochials. They are the lazy way to invent an alien: make it like us but change a few features for dramatic effect. It’s not just the shape – bipedal, head at the top, human-shaped skull (just exaggerated). Greys breathe our air – but few creatures that had evolved on another world would be able to do that, unless that world’s atmosphere were very similar to ours. Even humans have trouble on this planet if they merely move to high altitude, where the air has a similar composition but is thinner. Only people who grew up at such heights are comfortable there. Go to Peru, and you’ll see what I mean.

That said, there is room for disagreement. There is also a long tradition of scientific theorising about aliens, arguing that they must be very much like us – maybe not in form, but in their biochemistry and the kind of environment they could live in. The fledgling science of astrobiology mostly takes what we know of Earthly biology and projects it onto the background of alien worlds which we know about through astronomy. The search for alien life then becomes the search for habitable worlds, where ‘habitable’ means that we could live there. Or something very similar to us, adapted to local conditions as Peruvians are to altitude.

If you think aliens should resemble us, you can assess the prospects for aliens by starting with life on Earth. How does Earthly life (identified simply with ‘life’) work? Despite its diversity, everything relies on DNA, RNA and a very standard system of molecular machinery. There are some variations, but they are slight. What do Earth’s creatures need to survive? Water, oxygen, land to live on, comfortable temperatures, low levels of radiation. Energy from the Sun. A stable environment – well, fairly stable: not too many earthquakes, volcanic eruptions, tsunamis, forest fires, or inbound comets and asteroids.

Does this mean that all alien life forms would need the same? That they, too, would have the same kind of DNA? The argument for this view boils down to one simple fact: the only life that we know anything about exists on this planet. All else is hypothetical. From this point of view, the only sensible scientific position is that our kind of life is the only kind of life. Don’t agree? Then show me.

If you insist. There are good reasons to suppose that DNA may not be the only game in town. In the previous chapter we saw that virtually every major player in the Earth’s biochemistry can be modified, and still work. You can change the molecular structure of DNA. You can use a different genetic code to turn sequences of DNA bases into amino acids, the basic building blocks of proteins. You can even change the number of bases that encodes an amino acid, from the usual three to four. You can change the list of amino acids. You can use different proteins for specific functions. Life can exist without oxygen, without sunlight, and – according to a conference held at the Royal Society a few years ago – without water.

Forms of life that don’t use carbon – oxygen chemistry as a source of energy have to do something else, and one possibility uses sulphur and iron instead. Günter Wächtershäuser, a chemist and patent lawyer, suggested that life on Earth first arose in a hydrothermal vent on the ocean floor, exploiting the chemicals that are common in such places, notably iron sulphide.7 With the aid of small quantities of catalysts such as nickel and cobalt, hot water flowing over iron sulphide can form reasonably complex organic molecules, known as metallo-peptides. Some experimental support exists for these ideas, but it is not clear just how complex this type of chemistry can become. It is, however, a plausible alternative to carbon chemistry, and might have given rise to primitive forms of life.

Less hypothetically, there is a lake at the bottom of the Mediterranean Sea, 3.5 kilometres down, west of Crete. I say ‘lake’ because a dense layer of unusually salty water has collected there, pooling on the seabed. It contains hardly any dissolved oxygen, but a lot of hydrogen sulphide, which oozes out from a thick layer of mud. The only life that ought to exist in the lake is anaerobic bacteria, which don’t require oxygen. But in fact there are small, complex animals, with a hydrogen – sulphur metabolism. Bill Martin, an evolutionary biologist, believes that these animals change our view of the origin of eukaryotes.8

The orthodox theory is that eukaryotes evolved because of a massive build-up in the ocean and the atmosphere of oxygen, the waste product of photosynthetic bacteria and algae. Oxygen is a potential source of energy, so organisms could evolve to exploit it. Mitochondria, vital to eukaryotes, do just that; they also protect the cell against the toxic effects of oxygen (things burn in it). Martin argues that oxygen is reactive only when it is in the form of free radicals, but mitochondria create free radicals, so they make the problem worse. And extracting energy from oxygen is so complex that it must have taken a long time to evolve. So for billions of years the oceans would have been jam-packed with hydrogen sulphide, and the anaerobes would not have been poisoned by oxygen. So maybe it wasn’t oxygen that led to the eukaryotes, but hydrogen and sulphur. The animals in the undersea lake may be relics of that process, though they will surely have been changed by more than a billion years of evolution. If Martin is right, Earthly life began as unearthly life. Since oxygen wasn’t needed here, it’s silly to think that it must be necessary for aliens.

There is another argument against the uniqueness of DNA and the associated chemistry. Agreed, there are millions of different species on planet Earth, all using the same biochemistry, but that doesn’t imply that nothing else is possible, because all evolved from the same primitive ancestral forms. Life reproduces: as soon as anything works, you find it everywhere. In a sense, those millions of species provide no more compelling evidence for the necessity of a particular biochemical scheme that any single one of them would.

Not only that: even on this planet, life exists in wildly different habitats, so different that until recently biologists denied that life could ever exist in some of them. These life forms are primitive, on the level of bacteria. They are collectively known as extremophiles – organisms that can survive in extremes. Some live happily in boiling water, others in water that has been supercooled, below its normal freezing point. Some have been found three kilometres underground, some in the stratosphere, and some can survive radiation levels that would be fatal to all other life forms. Late in 2010, a team led by Stephen Giovannoni of Oregon State University drilled nearly 1,400 metres into the bed of the Atlantic Ocean, where they found bacteria thriving at a temperature of 102°C.9 NASA scientists have reported that some bacteria in a Californian lake use arsenic – poisonous to most organisms – in place of the usual phosphorus, though this finding is controversial.

The term ‘extremophile’ reflects unconscious human bias. To a creature that lives in boiling water, it is we who are occupying an extreme environment. Somehow we survive the appalling cold of a British summer. The word ‘survive’ suggests difficulty, but an extremophile is not clinging desperately to the edge of survival in its boiling hot pool: it is comfortable there, and would die if it moved into water that was merely scaldingly hot. The same goes for the other class of extremophile, able to live in freezing cold. Our environment would normally be far too warm for it.

I find it strange that both boiling hot and freezing cold are somehow lumped together into the single category ‘extreme’, but the stuff in between is different. It smacks of parochialism, and reminds me all too closely of Goldilocks and the Three Bears.

I’ll come back to that.

Not so long ago, many people, including scientists, had what they thought was a very good reason to believe that the Earth is unique, the only planet that can support life. Their reasoning went much further: they ‘knew’ that the Earth was the only planet in the Solar System that could support life, because the Solar System was the only place in the universe that had planets. In support of this view was a clear statement of fact: planets round other stars had never been observed, so their very existence was hypothetical. Hypotheticals have no place in solid science: the only reason to think that other planets might exist was pure speculation, based on our limited knowledge about the formation of the Solar System.

To be sure, this suggested the exact opposite: that there is nothing very special about the Sun, so similar processes have probably occurred elsewhere. That meant planets. It was plausible, but there was no proof, so it wasn’t science.

This particular line of thinking has gone the way of the dodo. As I write, we know of 518 planets circling other stars10: the technical term is ‘exoplanets’. More are being discovered every week. It is becoming obvious that a significant proportion of the stars in the universe have planets; possibly most of them. We may never be able to observe the bulk of these worlds directly, but a random sample usually represents a wider truth. Planets are no longer the issue. The only reason we didn’t observe them earlier was that we lacked the technology to detect them. So the frontier of the debate has retreated to the existence of Earth-like planets. Almost all known exoplanets are huge, bigger than anything in the Solar System, dwarfing even Jupiter. The diehards retreated to a previously prepared position, now insisting that the evidence merely proves the existence of gigantic planets that could scarcely be more different from Earth – which to them means no possibility of life on such worlds. Again, there is actually a good reason why most of the known exoplanets do not resemble Earth: the methods used to detect exoplanets work best when the planet is very large.

Improved observation techniques have pushed that frontier back too: we now know of much smaller exoplanets, and can already detect the main gases in their atmospheres. In 2008 Mark Swain’s team at the Jet Propulsion Laboratory in California made the first detection of an organic molecule, methane, on an exoplanet.11 They found it on HD 189733b, a ‘hot Jupiter’ about 63 light years from Earth. Water vapour has been found on GJ 1214b,12 and the same methods should be able to find oxygen.

This suggests that it is unwise to dismiss reasonable possibilities on the ground that there are no observations to support them. You need independent evidence that those possibilities are intrinsically unlikely. A lack of observations can change at any moment with the invention of a new technique. So the current absence of evidence of alien life forms that differ from those on Earth may simply be due to the absence of evidence of alien life forms. Just as the absence of evidence of Earth-like planets was, until recently, due to the absence of evidence of planets – which was not evidence for the absence of planets, Earth-like or not.

The case that complex life elsewhere in the Galaxy, or the universe, is very uncommon has been made, eloquently and comprehensively, in Rare Earth. Ward and Brownlee list a large number of special features of our planet, all supposedly necessary for life to exist, and then work out how likely such a combination of features is. Their result is: very unlikely indeed. They don’t rule out simple life forms, such as bacteria, but they argue persuasively that anything even as complex as, say, a goldfish, must be a very rare thing in our universe. They don’t claim that Earth is unique in having such creatures, but other Earths, if they exist at all, will be very thinly spread.

I’ll list some examples of these features – they are based on solid science, much of it surprising and recent, and of interest in its own right. I’ll restrict attention to three astronomical ones. The first two are relatively new; the third is much older.

1. Jupiter protects the inner planets, Earth among them, from being bombarded by comets. A dramatic instance of this process was the break-up of comet Shoemaker – Levy 9 in 1994. Earlier, the comet had swung close to Jupiter, and was diverted in its orbit so that it would return after a few years. As it approached the giant planet, it broke into twenty pieces, which slammed into Jupiter releasing the energy of six million megatons of TNT – roughly six hundred times the world’s total store of nuclear weapons. If any one of those fragments had hit the Earth, nothing higher than bacteria would have survived, and probably not even them. Without Jupiter, comets would be hitting the Earth every twenty years or so.

2. The Earth’s Moon keeps our planet’s axis of rotation stable. Mathematical calculations show that a world lacking a moon that is large compared with itself will suffer erratic changes in the direction of its axis over periods of tens of millions of years.13 Such large moons are uncommon; it is thought that ours originated from a massive collision between Earth and a body the size of Mars during the early stages of the formation of the Solar System. Such collisions are rare.

3. The Earth is situated within the Sun’s habitable zone: a hollow shell of space inside which liquid water can exist on a planet’s surface. Get too close to the Sun, and water will turn to steam and may boil away entirely; too distant, and it will freeze. The habitable zone is limited: Mercury and Venus, close to the Sun, are on the inside of it; Mars, Jupiter, Saturn, Uranus and Neptune are outside it. We got lucky.

Rare Earth lists several dozen such features, and they are regularly trotted out in television science programmes as proof that Earth is very close to unique. However, the case for Earth’s rarity, like premature news of Mark Twain’s death, has been greatly exaggerated.

Indeed, the importance of each item in the list has been exaggerated. Worse, the significance of any such list has been exaggerated. I’ll go through the three items above in turn, and then turn to my more general objection.

Jupiter. As Shoemaker – Levy 9 shows, there are occasions when Jupiter does indeed protect the Earth from comets. But that does not imply that it always has a beneficial effect. It can also divert incoming comets, causing one that might have missed the Earth to hit.

World governments and NASA are starting to worry about NEOs: near-Earth objects. These are lumps of cosmic rock whose orbit round the Sun can bring them close to Earth. Many are asteroids, bodies ranging in size from a tennis ball to one-third of the diameter of the Moon, although the largest bodies in near-Earth orbits today are much smaller than that. Asteroids are found by their thousands in the asteroid belt between Mars and Jupiter.

Indeed, most Earth-crossing asteroids probably originated in the asteroid belt. If they had stayed there, they would pose no danger to our world. What made their orbits change, to become Earth-crossing?

Jupiter.

Mathematical calculations show that Jupiter has a big effect on asteroid orbits. In fact, as the most massive planet in the Solar System, Jupiter has a big effect on the orbits of small bodies. One of the things that Jupiter can do is disturb suitable asteroids, causing their orbits to elongate until they cross the orbit of Mars. This makes them Mars-crossing, not Earth-crossing. But now, if they come close to Mars, they can be diverted again, and now their orbits can cross the orbit of our own Blue World. So Jupiter centres the ball, and Mars scores.

Jupiter has two faces. One is that of protector. The other hurls rocks at us.

Ward and Brownlee discuss this, and argue that the occasional asteroid impact may be good for evolution, shaking up the biosphere. And so it might, but I wonder why asteroids are beneficial in this respect, while comets are not. It seems like special pleading. In fact, the presence of Jupiter may do more harm than good.

The Moon. Unlike most astronomers, I’m not convinced that the current theory of the origin of the Moon is correct,14 but whatever the mechanism was, it does seem likely that satellites whose size is comparable to that of their primary planet are quite rare. So I’ll concede that. And I agree that the presence of such a body does stabilise the axial tilt. However, it is not at all clear that if a planet’s axis changes its direction over a period of tens of millions of years, which is what the mathematics says, then this poses an insuperable problem for evolution. Earth’s creatures have coped with ice ages that come and go every ten or twenty thousand years, which is far more rapid than a change in axial tilt. Land creatures can move as the climate shifts – we’re talking a few hundred metres per year, and they’re moving faster than that today in response to climate change – unless they run out of land, which can happen. (Our elderly cat moves faster than that when chasing a mouse, but I’m referring to changes in average geographical location.) Birds can fly across open sea. And ocean creatures wouldn’t notice any difference. Since it is generally agreed that life began in the Earth’s oceans, and got very complex there, then the tilt of the Earth’s axis doesn’t matter a hoot.

Habitable zone. The habitable zone of a star is often referred to as the Goldilocks zone because it’s ‘just right’. The problem with habitable zone ideas is not that the concept is completely silly, but that it is too simplistic. It is, for example, not at all clear whether the Earth is within the Sun’s habitable zone. An airless Earth might have boiling hot surface temperatures, like the Moon when the Sun is overhead; on the other hand, one with too little carbon dioxide or a white reflecting surface might be covered in ice, as some of the planet is now, and most of it was during the period of Snowball Earth about 700 million years ago. Both Mars and Venus might support liquid water in suitable circumstances, but in those that currently prevail, Mars can get down to 15 degrees below zero and Venus is hot enough to melt lead. The highest surface temperature recorded on Mars to date is 27°C, but only on rare summer days.

It’s worth taking a closer look at the mathematics of habitable zones, to see where the difficulties arise. The calculations start from a central idea in the physics of heat, called a black body. A green object looks green, to the human visual system, because the object reflects sunlight in a range of wavelengths that our brains interpret as ‘green’. A black object does not reflect any wavelengths in the visible range; black is the brain’s default for such objects. A physicist’s black body is an idealised and extreme version of this: it reflects no electromagnetic radiation whatsoever.

However, reflection is not the only way for an object to emit radiation. A black body at a temperature of zero degrees kelvin – ‘absolute zero’, the lowest possible temperature – would emit no radiation of any kind. But at any other temperature, a black body does emit radiation; it just doesn’t do this by reflection. Instead, it glows incandescently, like a red-hot iron bar. The intensity of radiation emitted depends on the temperature and the wavelength of the radiation. Classical physics predicts that a black body should emit an infinite amount of energy, but that makes no sense. In 1901 Max Planck derived a new formula that agreed with observations, and this was later interpreted as evidence for a quantum world.

Planck’s law can be used to derive a formula for the temperature of a planet orbiting a star, and that lets us calculate where the inner and outer edges of the habitable zone are. There are two versions of the formula. The simplest one models the planet as a black body. However, a real planet will reflect some of the radiation that hits it, and the second model takes this into account by incorporating an extra quantity into the formula. It is called the albedo of the planet, which is the fraction of incoming radiation that is reflected away.

Only the first version can yield a habitable zone that depends only on features of the star. As soon as albedo comes into play, the habitable zone also depends on features of the planet – real or hypothetical – that is under consideration. The second version is more general than the first: if we set the albedo to zero, the value for a black body, we recover the first model. The formula relates the planet’s temperature to the star’s size, its surface temperature, the distance from the star to the planet, and the planet’s albedo.15

First, let’s calculate what the Earth’s temperature would be if its albedo were zero, the value for a black body. The answer is 279 K, or 6°C, placing us just inside the habitable zone. However, if we use the observed albedo, which is 0.3, the temperature becomes 254 K, which is -19°C – well below the freezing point of water. So assuming the correct albedo leads to the paradoxical result that the only known habitable planet in the universe does not lie inside its star’s habitable zone.

To locate the outer edge of the habitable zone, we consider a hypothetical planet whose surface temperature is the freezing point of water, 273 K. Then we solve the equation to derive the distance. For the inner edge, we do the same thing, but using the temperature of boiling water, 373 K. Again, there are two versions. For an albedo of zero, the value for a black body, the Sun’s habitable zone extends from 83 million to 156 million kilometres. If we set the albedo to 0.3, the measured value for the Earth, then the habitable zone stretches from 69 million to 130 million kilometres.

The average distances of the four inner planets from the Sun, in kilometres, are 58 million for Mercury, 108 million for Venus, 150 million for Earth, and 228 million for Mars. So for albedo 0 the Earth just scrapes inside the Sun’s habitable zone ... but so does Venus. For albedo 0.3, only Venus lies inside the habitable zone. The Earth and Mars are too cold, Mercury too hot.

Why, then, is the Earth habitable? Because its atmosphere contains greenhouse gases, mainly carbon dioxide and water vapour, which trap incoming radiation and make it warmer than it would be if no atmosphere were present. But the usual concept of a habitable zone does not take the planet’s atmosphere into account. The idea that a star has a habitable zone, independent of properties of the relevant planet, is an oversimplification. Of course, in a broad qualitative sense it is true that if a planet is too near its star then any water present on its surface will boil, and if it’s too far away the water will freeze. But ‘habitable zone’ lends a misleading air of precision.

Greenhouse warming is just one of a huge variety of effects that between them pretty much demolish ‘habitable zone’ as a useful concept. The surface temperature of a planet depends on many factors, only one of which is how far it is from its star, for a given heat output. For example, clouds and ice can increase the albedo, cooling the planet; so can sulphur dioxide. Carbon dioxide, methane and water vapour can warm it. Feedback loops between different factors further complicate the possibilities: warming seas can create clouds that reflect heat and light back, decreasing ice cover can allow more heat and light in.

Even taking all this into account, it is not true that the only place where liquid water can exist is on the surface of a planet in the habitable zone. For example, it used to be thought that Mercury was locked in a spin – orbit resonance, rotating once during the same time it took to revolve once round the Sun. If so, the same side would always face the Sun, just as the same side of the Moon always faces the Earth (give or take a bit of wobbling, known as libration). In fact Mercury does not do this, but there’s every reason to expect that some worlds somewhere in the Galaxy might be very close to their star – much closer than the habitable zone defined above – and locked in such a resonance. In fact, this is the case for at least one exoplanet.16 If so, one side of the planet would be very hot, the other side very cold ... and in between there would be a belt with more moderate temperatures, suitable for liquid water to exist. ‘Just right’, in fact.

Astronomers are almost certain that liquid water exists on a number of bodies in our Solar System that are well outside the Sun’s habitable zone. Paramount among these is Europa, a satellite of Jupiter. There is convincing evidence that Europa, one-quarter the diameter of the Earth, has an ocean that contains as much water as all of Earth’s oceans put together. Yet Europa’s surface is solid ice. So where is the ocean?

Under the ice.

Measurements of Europa’s magnetic field reveal changes that currently seem to be consistent with only one thing: a worldgirdling ocean upon whose surface the ice floats. The water is kept warm by heat generated in Europa’s core, probably caused by repeated squeezing by Jupiter’s huge and powerful gravitational field. Jupiter’s inner three major satellites, Io, Europa and Ganymede, are trapped in an orbital resonance: while Io goes round four times, Europa goes round twice and Ganymede once. This creates unavoidable tidal forces, and squeezing causes friction, which heats the core. This is not such an outlandish scenario: the Earth’s continents and seabed of solid rock float on a vast underground ocean of magma.17

Europa is not alone in having such an ocean. Ganymede and Callisto may have one too, Io probably has one but it’s sulphur, not water, and Saturn’s moon Titan may have a subsurface ocean of slushy liquid methane.

Finally, there is the obvious point that the Earth’s own extremophiles live in conditions that are outside the habitable zone: water at temperatures above its normal boiling point, and below its normal freezing point. Not far outside, but outside all the same. Could they have evolved in such extreme conditions? That’s less clear, but we’ve already seen that a plausible theory of the origin of life has it evolving first as ... extremophiles.

Protective gas giants, stabilising moons, and Goldilocks orbits ... Rare Earth lists dozens of such factors, and like those three, most of them are open to serious challenge. But there is a more general issue, a mathematical point: logic.

It is all very well to list dozens of special features of the Earth, all of which definitely played a significant role in the evolution of life. But it is wrong to conclude from this (alone) that those features are necessary for life. The correct conclusion is that they were sufficient. ‘Sufficient’ means that with them, life arose. ‘Necessary’ means that without them, it would not have arisen. The two are different, and it is the first that the list of features supports. For you to get wet, it is sufficient to stand outside in the rain without protection. But that’s not necessary. You can fall in a lake or take a bath instead.

Evolution is a universal, and its main feature is that creatures evolve to suit their habitat. If some form of life can exist in some habitat, even if it seems hostile to us, then life can evolve to do so. It doesn’t care about our opinions, because we’re not going to be living there. If we approach questions about alien life with the tacit assumption that the only sensible form of life is us, we will ignore all the other possibilities. The word ‘extremophile’ is humancentred: it starts from where we are, and declares that to be what’s sensible and reasonable. The further away we go from our selfdefined centre, the more ‘extreme’ things become.

I remember a museum exhibit about deep-sea fish, which said something to the effect that ‘their strange shapes reflect the strange conditions under which they live’. It seems to make sense: strange conditions imply strange shapes. Not like normal conditions, which imply normal shapes. Like us. But it’s all back to front. Normal conditions, in this sense, are the ones we are accustomed to. So are normal shapes. But we are as different from the fish as they are from us, in both shape and habitat. To them, we would be strange and they would be normal.

To evolution, we would both be normal – relative to our habitat.

A more imaginative reading of the Goldilocks tale makes the same point, and raises a far more interesting set of questions. Mummy Bear’s wimpy porridge was too cold for Goldilocks, and Daddy Bear’s macho male porridge was too hot, while Baby Bear’s intermediate porridge was just right. And so they were – for Goldilocks.

For Mummy Bear, however, the intermediate porridge was too warm. For Daddy Bear, it was too cold. Goldilocks’ point of view is not privileged. Woolly-minded social relativism though it may be, I think that Mummy and Daddy Bear both had valid opinions too.

Discussions of such things as Jupiter’s alleged importance in protecting the Earth from comet impacts often run along the following lines: ‘Without Jupiter, the Earth would be hit by a comet every twenty years.’ There’s a sense in which such statements are true, but a closer look reveals that they don’t address anything of substance. They’re like sports commentators saying, ‘If only he hadn’t been offside, that goal he just scored would have won the match.’ But if the player had not been offside, he would have been in a different place. To score a goal, he would have to have kicked the ball differently. You can’t just change one factor, offside, and keep the rest exactly as before.

It’s the same with Jupiter. Yes, if you took the Solar System as it is today, and magically spirited Jupiter away, comets would rain in upon an unprotected Earth. But if the Solar System had evolved without Jupiter, it would not be the same – in all other respects – as it is. It would have been quite different. More comets would have hit the Earth in the past, for instance, leaving fewer to hit it now.

The mathematics of many bodies moving under gravity, called celestial mechanics, is revealing an unsuspected aspect of planetary systems. Namely, that they are systems. Over billions of years, they organise themselves in complex ways. The biggest planets, the Jupiter-like gas giants, have the biggest influence. Other, lesser worlds, and even those only slightly less massive, get rearranged until the entire system fits together and acts as a whole. This is celestial Gaia.

Very recently, it has been discovered that Jupiter’s influence on the Solar System has created a kind of celestial subway, a network of gravitational ‘tubes’ that can be perceived mathematically but consist of empty space.18 These tubes are pathways along which matter can move more efficiently. The arrangement has come about as a result of subtle feedback effects, caused by gravitation. The equations for gravity are nonlinear, meaning that effects are not proportional to causes. Nonlinear systems have a tendency to behave in surprisingly complicated ways, and they tend to organise themselves by settling into special forms of behaviour.

Considering a Solar System without Jupiter, and arguing that it wouldn’t be so hospitable for life, makes the same mistake as the sports commentator. It forgets that if you change one thing, you change everything. The evidence to date shows that most solar systems have huge planets like Jupiter. It seems likely that most of them also have smaller planets, though these are very difficult to spot at the moment. If so, then the Jupiters will organise their lesser brethren, and often enough there will be a few small worlds closer to the star, and some big ones further out. So even if it is indeed true that planets like Jupiter provide overall protection against comets and the like, it is no huge coincidence if one of them exists in roughly the right orbit. Nature does not simply build solar systems by plonking down planets at random. They have a selfconsistent structure.

This is not to say that habitable planets are inevitable. There are many ways to fail to be habitable. But there are many stars – at least 4×1022 of them – in the universe, and probably even more planets. There are many ways to be habitable too, and habitable planets will not all be carbon copies of the Earth. Rerun the Solar System again, from different beginnings, and there’s a fair chance that at least one world might still be suitable for life.

At some point in its history. Mars may have been suitable for Earth-like life, a billion or more years ago. In fact, it has been suggested that Earthly life was originally seeded from Mars. The current consensus is that this is probably wrong, but it’s not totally out of the question. It will take a close look at Mars to decide the matter.

In contrast to the Rare Earth story, I will describe some mathematical simulations and models devised by Harvard astrophysicists Dimitar Sasselov, Diana Valencia and Richard J. O’Connell, which suggest that planets capable of supporting Earth-like life may be far more common than has previously been thought.19 Their results also call into question the common view that Earth is the ideal kind of world for the kind of life that we find here.

The starting point for their work is the realisation that the conditions that make our kind of life possible do not necessarily require the planets concerned to be of a similar size to our own. What matters is that they should resemble our own world in one key respect: the occurrence of plate tectonics. There is a growing suspicion that the dynamic movement of continents helps to stabilise the Earth’s climate. In particular, carbon dioxide is recycled from the atmosphere to the ocean floor, where it is taken up by marine microorganisms and turned into carbonate; then the subducted carbonate is turned back into carbon dioxide by volcanoes. A stable climate helps liquid water to exist for geologically long periods of time, and water is required for our kind of life, even if other kinds might exist without it. That, in turn, enables evolution to generate complex water-dependent life forms.

It had been assumed that plate tectonics is rare, and that it requires a world of comparable size to our own. The crust of a much smaller world could not break up into suitable plates, while a much larger world would be a gas giant and not have a surface as such anyway. Sasselov and colleagues have shown that both these assumptions are false. Plate tectonics may actually be very common, and could occur on planets much larger than the Earth. The reason is the possibility of ‘super-Earths’: rocky worlds with a similar geological composition to ours, but having much greater mass. No one had previously investigated the internal geological processes of such a world, probably because no such exoplanets were then known. Indeed, virtually all known exoplanets were so large that they had to be gas giants, and this was still the case when the team began their modelling and published their first paper.

By 2005, however, the picture had already started to change with the discovery of the exoplanet GJ 876d, which orbits the star Gliese 876. This was smaller than the typical gas giant exoplanets then known, though still much larger than the Earth; there were hints that it might be mostly rock, rather than gas. However, there was no good way to measure the planet’s density, which would decide the issue, because the only known method required the planet to cross the face of its parent star when viewed from Earth. In 2009 a new exoplanet was found, CoRoT-7b, which did cross the face of its star. Now a density estimate was feasible, and the result was definitive: CoRoT-7b is made from rock. It has about 4.8 times the mass of the Earth and 1.7 times its radius. By 2010 a second super-Earth that transits its star had been located, known as GJ 1214b, with a density closer to that of water than rock, suggesting that it has a thick gaseous atmosphere. This planet has 6.5 times the mass of the Earth and 2.7 times its radius.

Now there were real planets to supplement the theoretical analysis made by Sasselov and his colleagues, adding to the interest of such calculations. They first showed that there are two main kinds of super-Earth: those with a lot of water, and those with much less. The first kind would have formed quite a long way out from the parent star, where they would pick up large amounts of ice. The second kind would have formed further in, and be relatively dry. Both kinds would acquire a large iron core as the denser parts of their molten material sank towards the centre, and a silicate mantle as the lighter materials rose. The water-rich super-Earths would have very deep oceans above the mantle; the drier ones would have thin oceans, or none.

Because the pressure at the centre of a large super-Earth is higher than it is for our own world, the iron core will solidify faster. This probably implies that such a planet will have little or no magnetic field, and that might be bad for the occurrence of life, because magnetic fields shield the surface from radiation. However, so do deep oceans, and in any case we don’t know how necessary a magnetic field really is. Some Earthly bacteria are radiationresistant, for example.

The interior of a large super-Earth should contain the radioactive elements uranium and thorium, which generate most of the heat that keeps our own planet’s core molten. Because these elements occur in much the same proportions throughout the Galaxy, the large super-Earth would have more of them than our own world does, and its core would be considerably hotter. The extra heat would cause convection in the mantle to be more vigorous, and this in turn would drive the movement of large plates at the boundary of the rock, much as it does on Earth. It turns out that these plates would be thinner than they are on Earth, because they move more rapidly and so have less time to thicken up by cooling. They would be easier to deform, except that the planet’s greater gravity exerts more pressure on fault lines, so the plates don’t slide as easily as they do here. These two effects tend to cancel out, so overall the frictional resistance when the plates slide past one another is much the same, regardless of size.

In short: plate tectonics is likely to be more common on large super-Earths than it is on Earth-like terrestrial planets that are similar in size to our own world. It also happens faster, which means that the cycle of subduction and volcanic activity that tends to keep the carbon dioxide concentration fairly stable would, if anything, work better. So a super-Earth that is considerably larger than our own world would probably have a more stable climate than ours, on geological timescales, making it easier for complex life to evolve.

This analysis completely changes the ‘rare Earth’ picture. Terrestrial planets, roughly the same size as our own, should occur fairly often, but comparatively speaking they are probably fairly rare. But the likely number of super-Earths in the Galaxy is far greater than the number of terrestrial planets, so the prospects for life are much better than they would appear if we were to focus solely on terrestrial planets. It also casts serious doubt on Goldilocks arguments, because it turns out that the Earth, far from being ‘just right’ for plate tectonics to arise, is very close to the lower extreme of the range of sizes for which such effects can happen. If the Earth were slightly smaller, it would not have plate tectonics, and that might have caused it not to evolve complex life.

The ideal Earth-like planet, it seems on this analysis, is considerably larger than the Earth. We just scraped into the acceptable range.

There is a general message in this work, and it is one that needs to be far more widely appreciated. The way to understand how likely alien life might be is not to focus on conditions that are virtually identical to those found on this world, and then argue – typically confusing sufficiency with necessity – that only those conditions are suitable for life. What really matters is just how different a planet can be from ours, and still support its own form of life, adapted to its prevailing conditions by evolution.

How diverse can living creatures, and their worlds, be? You won’t find out if you start by assuming they all have to be just like us.

Perhaps alien life has already been discovered.

In 1997 NASA launched the Cassini – Huygens spacecraft, a mission to Saturn. Seven years later the craft reached its destination. The Huygens probe landed on one of Saturn’s moons, Titan. The Cassini spacecraft went into orbit round the planet. One of the early discoveries – dramatic, though to some extent expected – was that Titan has lakes. Because of the deep cold at that distance from the Sun, the lakes are not of water, but liquid methane and ethane.

Now some scientists are wondering whether Cassini has found signs of an exotic kind of life. This is one possible explanation for the strange behaviour of two gases on Titan: hydrogen and acetylene. There ought to be quite a lot of hydrogen, spread fairly uniformly in the moon’s atmosphere. Darrell Strobel, working at Johns Hopkins University, has discovered that the hydrogen streams downwards through the atmosphere and disappears near the surface. Astronomers had expected acetylene to be fairly common too, produced by simple chemical reactions in Titan’s atmosphere and deposited on the surface.

But there isn’t any.

In 2005 Chris MacKay, a planetary scientist at NASA, realised that hypothetical methane-based microbial life would be very likely to get its energy by reacting hydrogen with acetylene, in the same way that most Earthly life reacts oxygen with molecules that contain carbon. The new observations are consistent with this kind of life inhabiting Titan’s surface, and using up all of the missing hydrogen and acetylene.

Of course this doesn’t come close to a proof that Titan harbours such exotic life forms, and Mark Allen (also at NASA) has suggested that non-living processes are a more likely explanation. Cosmic rays could convert acetylene to more complex substances, for instance, when they collide with its molecules. But it does illustrate the value of not assuming that life everywhere must be very much like life here. By doing so, we could have stood a small but significant chance of missing an alien life form in our own backyard.

Watch this space.