OUR COSMIC HABITAT I: PLANETS, STARS AND LIFE
Damn the Solar System. Bad light; planets too distant; pestered with comets; feeble contrivance; could make a better one myself.
Lord Jeffrey
PROTOPLANETS
There is a great cloud in the constellation of Orion, containing enough atoms to make ten thousand Suns. Part of it is a glowing nebula, heated by bright blue stars; the rest is cold, dark and dusty. Within it are warm blobs, emitting no light but generating heat that can be picked up by telescopes fitted with infrared detectors. These blobs are each destined to become stars but are at present ‘protostars’, contracting under their own gravity. Each is encircled by a disc of gas and dust.
These discs are not unexpected. The dusty cloud in Orion, though denser than most of the expanses between the stars, is still very rarefied, and to form a star some of this gas must contract so much that its density rises a billion billion times. Any slight spin would be amplified during a collapse (a cosmic version of the ‘spin-up’ when ice skaters pull in their arms) until centrifugal forces prevent all the material from joining the star. Surplus material would be left behind, spinning around each newly formed star. The resultant discs are the precursors of planetary systems: dust particles would collide frequently, sticking together to build up rocky lumps; these in turn would coalesce into larger bodies, which merge to make planets. Our Solar System formed in this way, from a ‘protosolar disc’; other stars formed similarly to our Sun, and there is every reason to expect them also to be orbited by retinues of planets.
This scenario, supported by the actual evidence of discs around newly formed stars, has superseded the ‘catastrophist’ theories popular at the beginning of the twentieth century, which envisaged planetary formation as a rare and special accident. It was thought that our Sun underwent a close encounter with another star – a freakishly rare event because the stars are, on average, so widely dispersed from each other – and that the star’s gravitational pull extracted a plume of gas from the Sun; the plume supposedly condensed into ‘beads’ that each became a planet.
Astronomers in earlier centuries, however, were no more averse than we are today to the idea of other Solar Systems. Back in 1698 Christiaan Huygens, a Dutch scientist who did pioneering work in optics, wrote ‘Why [should] not every one of these stars and suns have as great a retinue as our sun, of planets, with their moons to wait upon them?’
OTHER SOLAR SYSTEMS?
Fully formed planets orbiting other stars are harder to detect than their precursor discs. A real highlight of the late 1990s has been the first compelling evidence that planets are indeed common. The principle here is very simple. An observer viewing our Sun from a distance of (say) forty light-years couldn’t see any of the planets orbiting it, even with the use of a telescope as powerful as the largest we have on Earth. Nevertheless, the existence of Jupiter (the heaviest planet) could be inferred by careful measurements of the Sun’s light. This is because the Sun and Jupiter are both pivoting around their centre of mass, the so-called ‘barycentre’. The Sun is 1,047 times more massive than Jupiter. The barycentre is closer by just that factor to the Sun’s centre than to Jupiter’s (it actually lies beneath the Sun’s surface); the Sun consequently moves about a thousand times more slowly than Jupiter does. Its actual motion is more complicated, because of extra wobbles induced by the other planets, but Jupiter is much the heaviest planet and exerts the dominant effect. By analysing the light very carefully, astronomers have detected small ‘wobbles’ in the motion of other stars, which are induced by orbiting planets, just as Jupiter induces such motions in our Sun.
The spectrum of starlight reveals patterns due to the distinctive colours emitted or absorbed by the various kinds of atom (carbon, sodium, etc) that stars are made of. If a star moves away from us, its light shifts towards the red end of the spectrum, as compared with the colours emitted by the same atoms in the laboratory – this is the well-known Doppler effect (the analogue, for light, of the way the sound from a receding siren shifts to a lower pitch). If the star is approaching, there is a shift to the blue end of the spectrum. In 1995, two astronomers at the Geneva Observatory, Michel Mayor and Didier Queloz, discovered that the Doppler shift in 51 Pegasi, a nearby star resembling our Sun, was going up and down very slightly as though it was moving in a circle: coming towards us, then receding, then approaching again, and so on in a regular fashion. The implied speed was about fifty metres per second. They inferred that a planet about the size of Jupiter was orbiting it, causing the star to pivot around the centre of mass of the combined system. If the invisible planet were one-thousandth of the star’s mass, its orbital speed would be fifty kilometres per second – a thousand times faster than the star is moving.
Geoffrey Marcy and Paul Butler, working in California, have been the champion planet-hunters of the late 1990s. Their instruments can record wavelength shifts of less than one part in a hundred million; they can thereby measure the Doppler effect even when the speeds are only one hundred-millionth of the speed of light – three metres per second – and they have found evidence for planets around many stars. These inferred planets are all big ones, like Jupiter. But this merely reflects the limited sensitivity of their measurements. An Earth-like planet, weighing a few hundred times less than Jupiter, would induce motions of only a few centimetres per second, and the Doppler shift would then be only about one part in ten billion – too small to be discerned by the techniques that have discovered the bigger planets.1
It should be noted in passing that the telescopes used by the planet-seekers are of moderate size, with mirrors only about two metres in diameter. It is gratifying – and sometimes obscured by the hype that accompanies the biggest projects – that not all important discoveries demand the largest and most expensive equipment. Persistent and ingenious scientists can still achieve a lot with innovative but modest instruments on the ground.
The actual layout of our Solar System is the outcome of many ‘accidents’. Rocky asteroids whose orbits cross the Earth’s still pose a genuine threat. For example, the impact of a ten-kilometre asteroid, leaving a huge undersea crater near Chicxulub in the Gulf of Mexico, had worldwide climatic effects that probably sealed the dinosaurs’ fate sixty-five million years ago; and smaller impacts, still severe enough to cause local devastation, have been more common. But impacts were far more frequent when the Solar System was young, because most of the original protoplanetary bodies within it have by now either been destroyed or kicked out. Our Moon was torn from Earth by a collision with another protoplanet – the intense cratering on its surface bears witness to the violence of its early history. Uranus probably underwent a shattering oblique collision soon after it formed; it is otherwise hard to understand why it spins around an axis almost in the plane of its orbit, in contrast to the other planets, whose axes are more or less aligned perpendicular to that plane. Pictures beamed back by artificial space probes reveal that all the planets of our own Solar System (and some of their larger moons) are highly distinctive worlds.
It’s unlikely that other planetary systems would have the same number of planets, in the same configurations, as our Solar System. Several of those already found have a large Jupiter-like planet closer to the parent star than Mercury (the innermost member of our Solar System) is to our Sun. This is partly an observational bias: heavy planets in fast short-period orbits are easier to detect. The heavy planets already detected may well be accompanied by smaller Earth-like ones.
Only rather special planets could harbour life that in any way resembled what we have on Earth. Gravity must pull strongly enough to prevent their atmosphere from evaporating into space (as would have happened to an atmosphere on our Moon, if it ever had one). For water to exist on their surfaces, planets must be neither too hot nor too cold, and therefore the right distance from a long-lived and stable star. Their orbits must be stable (which they would not be if, for instance, their path was repeatedly crossed by a Jupiter-like planet in an eccentric orbit). The high ‘hit rate’ of the planet-seekers suggests that there are planets around a high proportion of Sun-like stars in our galaxy. Among these billions of candidates, it would be astonishing if there were not many planets resembling the young Earth.
In the US, NASA’s somewhat messianic, chief executive, Dan Goldin, has urged that the quest for Earth-like planets – a quest to actually make an image of them rather than just infer them indirectly – should become a main thrust of the space programme. Mere detection of such a faint speck – in Carl Sagan’s phrase, a ‘pale blue dot’ – is a challenge that may take fifteen years to meet. Large arrays of telescopes would have to be deployed in space.
The dim light from a distant world conveys information about cloud cover, the nature of its surface (land or oceans), and perhaps daily or seasonal changes. From the spectrum of the planet’s light, we could infer what gases existed in its atmosphere. Our Earth’s atmosphere is rich in oxygen; it didn’t start out that way, but was transformed by primitive bacteria in its early history. The most interesting question, of course, is whether this may have happened elsewhere: even when a planet offers a propitious environment, what is the chance that simple organisms emerge and create a biosphere?
FROM MATTER TO LIFE
Only in the last five years of this millennium have we learnt for sure that there are worlds in orbit around other stars. But we are still little closer to knowing whether any of them harbours anything alive. This question is one for biologists, not for astronomers. It is much more difficult to answer, and there seems no consensus among the experts.
Life on earth has occupied an immense variety of niches. The ecosystems near hot sulphurous outwellings in the deep ocean bed tell us that not even sunlight is essential. We still don’t know how or where life got started. A torrid volcano is now more favoured than Darwin’s ‘warm little pond’; but it could have happened deep underground, or even in dusty molecular clouds in space.
Nor do we know what the odds were against it happening here on Earth – whether life’s emergence is ‘natural’, or whether it involves a chain of accidents so improbable that nothing remotely like it has happened on another planet anywhere else in our galaxy. That’s why it would be so crucial to detect life, even in simple and vestigial forms, elsewhere in our Solar System. Mars is still, as it has been since the nineteenth century, the main focus of attention: during the coming years, an armada of space probes is being launched toward the ‘red planet’ to analyse its surface, to fly over it, and (in later missions) to return samples to Earth. Life could also exist in the ice-covered oceans of Jupiter’s frozen moons, Europa and Callisto, and there are plans to land a submersible probe that could explore beneath the ice.
If life had emerged twice within our Solar System, this would suggest that the entire galaxy would be teeming with life, at least in simple forms. Such a momentous conclusion would require that the two origins were independent. That is an important proviso – for instance, if meteorites from Mars could impact the earth, maybe we are all Martians; conversely. Mars could have been seeded by reverse traffic from Earth!
FROM SIMPLE LIFE TO INTELLIGENCE
We know, at least in outline, the elaborate history and the contingencies that led to our emergence here. For a billion years, primitive organisms exhaled oxygen, transforming the young Earth’s poisonous atmosphere and clearing the way for multicellular life. The fossil record tells us that a cornucopia of swimming and creeping things evolved during the Cambrian era 550 million years ago. The next 200 million years saw the greening of the land, offering a habitat for exotic fauna – dragonflies as big as seagulls, millipedes a metre long, scorpions and amphibians. And then the dinosaurs, whose traditional dim and torpid image has been replaced by the dynamism portrayed (in accordance with current scientific opinion) in films such as Jurassic Park. They were wiped out in the most sudden and unpredictable of all extinctions: an asteroid crashed onto Earth, causing huge tidal waves and throwing up dust that darkened the sky for years. This opened the way for the line of mammalian descent that led to humans.
Even if we knew that primitive life was widespread, the issue of intelligent life would still remain open. An extraordinary procession of species (almost all now extinct) have swum, crawled and flown through our biosphere during its long history. We are the outcome of time and chance: if evolution were rerun, the outcome would be different. Nothing seems to pre-ordain the emergence of intelligence; indeed, some leading evolutionists believe that, even if simple life were widespread in the cosmos, intelligence could be exceedingly rare. We still understand far too little to assess the odds, but there is no reason for obdurate scepticism.
The amazing and fascinating complexity of biological evolution, and the variety of life on Earth, makes us realize that everything in the inanimate world is, in comparison, very simple. And this simplicity – or, at least, relative simplicity – is a feature of the objects that astronomers study. Things are hard to understand because they are complex, not because they are big. The challenge of fully elucidating how atoms assembled themselves – here on Earth, and perhaps on other worlds – into living beings intricate enough to ponder their origins is more daunting than anything in cosmology. For just that reason, I don’t think it’s presumptuous to aspire to understand our large-scale universe.
The concept of a ‘plurality of inhabited worlds’ is still the province of speculative thinkers, as it has been through the ages. The year 2000 marks the fourth centenary of the death of Giordano Bruno, burnt at the stake in Rome. He believed that:
In space there are countless constellations, suns and planets; we see only the suns because they give light; the planets remain invisible, for they are small and dark. There are also numberless earths circling around their suns, no worse and no less than this globe of ours. For no reasonable mind can assume that heavenly bodies that may be far more magnificent than ours would not bear upon them creatures similar or even superior to those upon our human earth.
Ever since Bruno’s time, this belief has been widely shared. In the eighteenth century, the great astronomer William Herschel, discoverer of the planet Uranus, thought that the planets, the Moon, and even the Sun were inhabited. In the 1880s, Percival Lowell, a wealthy American, built his own observatory in Flagstaff, Arizona, primarily to study Mars. He believed that the ‘canals’ (now recognized to be no more than a combination of wishful thinking and optical illusion) were an irrigation project to channel water from the frozen polar caps to the ‘deserts’ of its equatorial zones. In 1900, a French foundation offered the Guzman Prize of 100,000 francs for the first contact with an extraterrestrial species; but prudence led them to exclude Mars – detecting Martians was thought to be too easy!
A COMMON CULTURE WITH ALIENS?
Searches for extraterrestrial intelligence (SETI) are being spearheaded by scientists at the SETI Institute in Mountain View, California. The efforts have concentrated on searches for radio transmissions that could be artificial in origin, and have used various large radio telescopes around the world. This option is familiar also from fictional depictions such as Carl Sagan’s Contact (in which it generally pays off). But radio is not the only conceivable channel: narrow-beamed lasers could span interstellar distances with a modest power consumption. We already have the technology, if we so wish, to proclaim our presence many light-years away by either of these methods; indeed, the combined effects of all radio transmitters, radars and so forth would in any case reveal us to any aliens with sensitive radio telescopes. We know so little about the origin and potentialities of life that it is hard to assess what method for detecting it is best. So it is sensible to use every available technique and be alert to all possibilities. But we should be mindful of ‘observational selection’: even if we do discover something, we can’t infer that it is ‘typical’, because our instruments and techniques restrict us to detecting a biased and incomplete selection of what may actually be out there.
There may be no other intelligent life elsewhere. Even if there is, it may be on some water-covered world where super-dolphins enjoy a contemplative oceanic life, doing nothing to reveal themselves. There are heavy odds against success, but systematic scans for artificial signals are a worthwhile gamble because of the philosophical import of any detection. A manifestly artificial signal – even if it were as boring as lists of prime numbers, or the digits of ‘pi’ – would imply that ‘intelligence’ wasn’t unique to the Earth and had evolved elsewhere. The nearest potential sites are so far away that signals would take many years in transit. For this reason alone, transmission would be primarily one-way. There would be time to send a measured response, but no scope for quick repartee!
Any remote beings who could communicate with us would have some concepts of mathematics and logic that paralleled our own. And they would also share a knowledge of the basic particles and forces that govern our universe. Their habitat may be very different (and the biosphere even more different) from ours here on Earth; but they, and their planet, would be made of atoms just like those on Earth. For them, as for us, the most important particles would be protons and electrons: one electron orbiting a proton makes a hydrogen atom, and electric currents and radio transmitters involve streams of electrons. A proton is 1,836 times heavier than an electron, and the number 1,836 would have the same connotations to any ‘intelligence’ able and motivated to transmit radio signals. All the basic forces and natural laws would be the same. Indeed, this uniformity – without which our universe would be a far more baffling place – seems to extend to the remotest galaxies that astronomers can study. (Later chapters in this book will, however, speculate about other ‘universes’, forever beyond range of our telescopes, where different laws may prevail.)
Clearly, alien beings wouldn’t use metres, kilograms or seconds. But we could exchange information about the ratios of two masses (such as the ratio of proton and electron masses) or of two lengths, which are ‘pure numbers’ that don’t depend on what units are used: the statement that one rod is ten times as long as another is true (or false) whether we measure lengths in feet or metres or some alien units. As Richard Feynman noted, he could tell extraterrestrials that he was ‘seventeen billion hydrogen atoms high’ and they should understand him.
Some ‘intelligences’ could exist with no intellectual affinity to us whatsoever. But any beings who transmitted a signal to us must have achieved some mastery over their physical surroundings. If they had any powers of reflection, they would surely share our curiosity about the cosmic ‘genesis event’ from which we’ve all emerged. They would be likely to be interested in how our universe is structured into stars and galaxies, what it contains, how it is expanding, and its eventual destiny. These things would be part of the common culture that we would share with any aliens. They would note, as we do, that a few key numbers are crucial to our shared cosmic environment.
Six of these numbers are the theme of the present book. They determine key features of our universe: how it expands; whether planets, stars and galaxies can form; and whether there can be a ‘chemistry’ propitious for evolution. Moreover, the nature of our universe is remarkably sensitive to these numbers. If you imagine setting up a universe by adjusting six dials, then the tuning must be precise in order to yield a universe that could harbour life. Is this providence? Is it coincidence? Are these numbers the outcome of a ‘theory of everything’ that uniquely fixes them? None of these interpretations seems compelling. Instead, I believe that the apparent ‘tuning’ intimates something even more remarkable: that our observable universe – all we can see out to the limits of our telescopes – is just one part of an ensemble, among which there is even a diversity of physical laws. This is speculation, but it is compatible with the best theories we have.
We know that there are planets orbiting other stars, just as the Earth orbits our own star, the Sun. We may wonder what habitats they offer. Is their gravity too weak to retain an atmosphere? Are they too hot, too cold, or too dry to harbour life? Probably only a few offer an environment conducive for life. So, on a much grander scale, there may be innumerable other universes that we cannot observe because light from them can never reach us. Would they be propitious for the kind of evolution that has happened on at least one planet around at least one star in our ‘home’ universe? In most of them, the six numbers could be different: only a few universes would then be ‘well tuned’ for life. We should not be surprised that, in our universe, the numbers seem providentially tuned, any more than we should be surprised to find ourselves on a rather special planet whose gravity can retain an atmosphere, where the temperature allows water to exist, and that is orbiting a stable long-lived star.