Rarely Earth
Earth-like worlds are extraordinarily rare but there are vast numbers of them. That is not a contradiction! Even if worlds as highly habitable as our own are so rare that they are typically separated by billions of light years, that is a tiny step compared to the immensity of our Universe. The existence of vast numbers of near-twins to the Earth in really deep space therefore remains all but inevitable. That’s how I know that Nemesis, or something very like it, almost certainly exists somewhere in the cosmos. This chapter looks at these points in more detail.
One reason Earth-like planets are probably rare is that there are just so many different ways to make a world. Until recently, we greatly underestimated the variety of worlds; but the more we look, the more varied and delightful our cosmos becomes. From the moons of our solar system through to planets circling distant stars, late 20th- and early 21st-century technology has replaced the imaginative fantasies of earlier generations with real, idiosyncratic worlds of previously unimagined variety and splendour. It could be argued that we have been spoiled by all this richness. For example, my university department runs a planetary geology course and, as part of this, we set up telescopes for our students to look at the stars and planets for themselves. With the exceptions of the Moon and Saturn, which always cause a sharp intake of breath when first viewed telescopically, some of our novice stargazers are a little disappointed by the experience. Partly this is because of the limitations of our relatively small telescopes and the quite appalling light pollution in the south-east of England. Mostly, however, it’s because the beautiful images from space probes and from the Hubble Space Telescope have produced unrealistically high expectations. There is nothing quite like seeing things with your own eyes, but, nevertheless, I have to reluctantly admit that a web browser is a more useful tool for studying planetary geology than a telescope. Indeed, I’d recommend surfing the web for images of other worlds as you read this chapter!
For me, the realisation that nothing I could see through a telescope would ever again match the achievements of the space age came when Pioneer 10 flew past Jupiter in December 1973. In those pre-internet days it took months for high-quality pictures to appear in magazines so I’m not sure when exactly in 1974 I first saw them, but I do know that they made a deep impression on me. Pioneer 10’s pictures have since been far surpassed, but in the early 1970s no one had yet seen anything quite like those images of an agitated alien world. For the first time we could see the intricate, colourful and ever-changing patterns of the Jovian weather. Continent-sized ripples and spots looped and curled along violently shearing boundaries between alternating orange and white cloud-belts and ruddy eddies churned inside the centuries-old storm of the Great Red Spot.
Pioneer 10 also took the first space probe pictures of Jupiter’s moons but they were too distant for the images to reveal much. Better photographs had to wait for missions that followed in Pioneer’s footsteps over the next few decades. When they came, the results were astounding. The Jovian satellites form a solar system in miniature with 67 moons, at the last count, orbiting at up to 30 million kilometres from Jupiter. Most of the moons are tiny lumps of rock and ice just a few kilometres across but the biggest exceptions, literally, are the four satellites discovered in 1610 by Galileo and mentioned in the last chapter. These moons were probably the first new worlds to be discovered in at least several hundred thousand years, since our earliest ancient ancestors had become aware of Mercury, Venus, Mars, Jupiter and Saturn. It is hard now for us to imagine the shock, wonder and disbelief that Galileo’s revelation must have caused.
In the three-and-a-half centuries following Galileo’s discovery, astronomers found that three of the Galilean satellites are bigger than our own Moon while the fourth is only slightly smaller. Despite their large size, nothing much else could be made of these worlds from Earth-based telescopes sitting more than 600 million kilometres away and, as a consequence, pre-space-age ideas about their appearance were largely guesswork. It was realised that they differ in size, density and brightness but the expectation was that they would all be dead, dusty and pocked with ancient craters just like our own Moon. Voyager 1, which reached Jupiter in 1979, showed that nothing could be further from the truth. The Galilean moons are astonishingly different from each other and quite unlike our own satellite. Their surprising and diverse nature was reinforced by the subsequent Galileo mission (named of course after the moons’ discoverer) that spent six years surveying Jupiter and its moons beginning in late 1995. With these missions, the solar system had begun to teach us an important lesson about diversity: the variety of possible worlds was far greater than we had dreamed.
Perhaps the biggest surprise was Io, a world slightly larger than our Moon and that orbits Jupiter just a little further away than our own satellite does from us. Despite this similarity in size and separation, the massively greater gravity of Jupiter compared to Earth has produced a very different history for these two worlds. The immense tidal forces produced by Jupiter bend and twist Io’s body during its two-day orbit and create intense heating that makes Io the most volcanically active body in the solar system. In contrast, our Moon has been cold and geologically dead for billions of years. The most obvious result of Io’s volcanic activity is its vivid red and yellow sulphurous colours. Although these are a little exaggerated in typical probe images, Io looks like a moon that Vincent van Gogh might have painted.
Europa is the next moon encountered as we move away from Jupiter, and it is potentially even more interesting because it is one of the most promising locations for life in our solar system. An ocean of liquid water lies beneath the moon’s thin icy crust and this ocean is underlain in turn by a warm rocky sea floor. Europa therefore has the heat, liquid water and minerals that may be all that are needed for life to begin and thrive (sea floor volcanic vents are currently favoured by many experts as the place where life began on our own planet). Many worlds in the outer solar system contain water, but at these distances from the Sun it usually freezes harder than concrete. Tidal heating on Europa, similar to that on Io but less intense, has prevented this by melting a 100 kilometre-thick layer of water that lies a few kilometres beneath the visible surface. This is an underground sea with twice the volume of Earth’s oceans and one that Jules Verne would have been proud to imagine. The evidence for an internal ocean on Europa comes from measurements of how this electrically-conducting, salty sea disturbs Jupiter’s magnetic field. This interpretation was confirmed through observations of the Galileo probe’s deflection by Europa’s gravity as it swung by the satellite, which told us that the outer layers of Europa are not heavy enough to be made of rock. The best evidence of all is that the moon’s appearance strongly supports the existence of a sub-surface sea; Europa’s exterior looks like a partially melted and re-solidified ice pack. The huge, fragmented floes of this moon seem to have been shattered by immense pulses of heat from below or impacts from above and then rapidly refrozen into immobility by the bitter cold of space. The entire moon is also criss-crossed by hundreds of long, brown cracks stained by minerals carried in currents of warm water rising from the bottom of the sea. The best description I have heard of Europa is that, as a result of those coloured cracks, it looks like a huge ball of string.
Moving further out from Jupiter to the next moon, Ganymede, brings us to the largest satellite in the solar system; a moon bigger than the planet Mercury. At first sight, parts of this world look like my idea of a proper moon with a heavily cratered surface. But that impression doesn’t last long; only about 40 per cent of the surface has that appearance. The rest of Ganymede’s surface is a jigsaw puzzle of innumerable plates, each like a massively magnified fragment of a muddy, tyre-track-covered building site. These grooved plates have fewer craters than the un-grooved regions, implying that they are significantly younger, and so it seems that more than half the surface of this moon has been stretched and deformed by huge forces to produce a world that, like the Earth with its oceans and continents, is schizophrenically divided into two very different terrains. At present we have no firm idea about what exactly happened, when it happened or why it happened, but there is little doubt that this moon has experienced intense surface activity at some point in its history. Ganymede probably also has a sub-surface sea like that of Europa, but, since Ganymede is a significantly larger moon with a thicker water layer, the higher pressures at the ocean’s base turn water into an exotic form of ice not found naturally on Earth (and hard to produce even in a laboratory). The liquid part of Ganymede’s ocean is therefore sandwiched between normal ice at the moon’s surface and this weird, high-pressure ice at the sea floor, so that it is not in contact with rocks rich in the mineral ingredients of life. This makes the sea of Ganymede a much less promising site for alien biology than Europa’s ocean.
The outermost Galilean moon, Callisto, finally brings us to a world that really is dead, dusty and pocked with craters, but this moon-like appearance is deceptive and disappears on closer inspection. Callisto has the oldest surface in the solar system, since even our own Moon has been active more recently. The lunar maria, the dark features that give the appearance of a ‘man in the Moon’, are the result of flooding of huge impact basins by basaltic lava long after the craters were created during the violent collisions of the young solar system. In contrast, Callisto shows no signs of any geological activity at all following the intense bombardment of the solar system’s half-billion-year birth pains. Callisto does have one intriguing mystery to offer: a surprising lack of small craters. The heavily cratered surfaces of the solar system usually have many more small craters than large ones and Callisto shows this pattern until you get down to craters about 3 kilometres across. Below this threshold, there are far fewer craters than you’d expect. So Callisto may, after all, have some kind of geological activity on its surface; some exotic process that obliterates smaller features. Alternatively, perhaps the young Callisto had a thick atmosphere that, like the Earth’s, prevented smaller meteorites from getting through to the surface.
The unpredicted variety of the Galilean moons reflects, in miniature, the diversity we see throughout the solar system and beyond. All planets are peculiar in one way or another. In our solar system Mercury is made mostly from iron, Venus may have selenium snow, Earth has continents that move, Mars has huge volcanoes, Jupiter is much bigger than all the other planets put together, Saturn has its stunning rings, Uranus is tilted on its side, and Neptune is the true ‘blue planet’ of the solar system. No two worlds are the same, even when they have similar dimensions and share a similar location. Venus and Earth, like the moons of Jupiter, are close in size, started life with almost identical compositions and are near neighbours in space. Despite this, it would be hard to find a less pleasant and less Earth-like place than the surface of Venus, a planet that has justifiably been called ‘our evil twin sister’. Even Uranus and Neptune, perhaps the closest our solar system comes to a true set of twins, are quite distinct when looked at in detail, with Neptune having a stormier surface and a more uniform internal construction than Uranus.
The unique character revealed by our robot probes of every world they have explored has been emphasised by the discovery in recent decades of exoplanets. The first widely accepted planet orbiting a normal star other than our own Sun was discovered less than twenty years ago by Michel Mayor and Didier Queloz. These Swiss-based astronomers detected the very slight changes in starlight caused by a planet orbiting the star 51-Peg, 45 light years from Earth. On a dark winter’s night, 51-Peg, which sits half way down the west side of the square of Pegasus, is just visible to the naked eye, but nothing would make it stand out to a casual observer. More detailed study has shown that this star is remarkably similar to the Sun although it is slightly heavier, slightly brighter and probably just a little younger than our own star. More remarkably still, Mayor and Queloz were able to show convincingly that a planet half the size of Jupiter revolves around 51-Peg every four days in an orbit so tight that temperatures on the planet’s sun-scorched surface must be close to 2,000°C. For comparison, Jupiter takes twelve years to orbit our Sun, and even Mercury, the closest planet to our star, takes nearly three months to complete each orbit.
The discovery of this, and subsequently many other, ‘hot Jupiters’ was a shock to planet-formation theorists. They thought they understood how the solar system formed and expected the same processes to occur around other stars. Small rocky worlds should form close to a star, with larger planets forming further out since, according to our theories, the near-star environment is simply too hot for gas to condense and make gas giants like Jupiter or Saturn. The now widely accepted explanation is that gas giants do indeed only form further out but that they can migrate inwards once formed. So, thanks to Mayor and Queloz, we now know that planets exist outside of our own solar system and we also know that other planetary systems can be very different from our own.
Detecting a planet, as Mayor and Queloz did, by looking for fluctuations in starlight is like trying to check on the breathing of a patient by listening to the siren of his departing ambulance. This may seem a rather whimsical way of putting it, but it’s actually quite an accurate analogy. The drop in siren pitch as an ambulance rushes past is a familiar experience that results because successive waves of siren-sound crowd together to give a high pitch as an ambulance approaches but are stretched into a deeper tone by a receding ambulance as each successive pulse begins its journey to your ears from further and further away. The strength of this ‘Doppler effect’ depends on the speed of the ambulance and this could, in principle, allow a patient’s vital signs to be detected from the sound of the siren alone. The passenger’s breathing will rock the ambulance very gently and the resulting tiny fluctuations in speed minutely change the siren’s pitch. However, the effect would be unimaginably minuscule, and swamped by the shaking of the ambulance as it trundled over the rough road, so that you’d have great difficulty convincing anyone that you had really detected a breathing patient.
In an almost identical fashion, the colour of light from a star is changed by the star’s motion. The pulses of a light wave are crowded together, if the star is coming towards us, and this gives a tiny blue-shift in its colour. In contrast, there is a slight red-shift if it happens to be moving away. Any planets orbiting such a star will gently rock it back and forth to produce planet-revealing fluctuations in these colour-shifts. As with my ambulance analogy, the effect is tiny and swamped by noise, but very heavy planets with small, rapid orbits will generate the greatest rocking, and this technique therefore tends to find big planets in small orbits: hot Jupiters like the one orbiting 51-Peg. This approach to finding worlds beyond our solar system is called the radial velocity (RV) technique, and though there are other planet-hunting techniques that I’ll describe later in this chapter, RV remains the most successful method of detecting exoplanets.
It’s probably worth saying a few words at this point about planet-naming conventions. The world discovered by Mayor and Queloz is known as 51-Peg b rather than, as you might expect, 51-Peg a. This results from a long-standing convention for naming multiple stars, stars that come in pairs or bigger groups and orbit around each other. The nearest such star system to the Earth is Alpha Centauri, consisting of the Sun-like star Alpha Centauri A and the slightly dimmer Alpha Centauri B, which take 80 years to orbit around each other. There is a third, very faint, member of this system that should be called Alpha Centauri C but more often goes by the name Proxima Centauri since it happens to be the closest star to Earth. Planets are named using this same convention except that lower-case letters are used to indicate that the companion is a planet rather than a star. Hence 51-Peg b because 51-Peg A is the star itself. This naming convention can produce some real mouthfuls, with my favourite, so far, being OGLE-2006-BLG-109L b. I’ll come back to this planet, and its companion OGLE-2006-BLG-109L c, at the end of this book when I try to explain this rather curious and complex name as well as why these particular planets are important.
Many of the newly discovered worlds will probably be given more euphonious names eventually but, for now, the vast majority follow this rather dull convention simply because there are an awful lot of them and there’s been no time to agree on anything more poetic. We now know of hundreds of exoplanets (942 as I write this in September 2013). Five hundred and thirty-two of these have been found using the RV technique and, despite the observational bias of RV towards hot Jupiters, the star–planet separations range from worlds so close to their star that they orbit in not much more than a day, through to planets as far from their star as Neptune is from ours. The biggest two planets yet found are at least 30 times heavier than Jupiter and both orbit the star Nu Ophiuchus – which is itself a bit of a heavyweight, being about three times more massive than our own Sun. These worlds are in fact large enough to be considered stars in their own right, brown dwarfs as they are known, and serve to show that planets’ sizes span the full range right up to that of small stars.
In contrast, the smallest planets discovered so far are Earth-mass worlds such as one that orbits Alpha Centauri B. Sadly, this planet is 25 times closer to its slightly smaller sun than we are to ours and is therefore far too warm to be habitable. If you could stand on this blisteringly hot world, a sun looking twenty times bigger than our own would dominate the sky, and Alpha Centauri A would appear as a second, distant sun with a disc just large enough to be visible by eye. Proxima Centauri, on the other hand, is so faint and so far from Alpha Centauri Bb that it would not stand out in that planet’s night sky, although it would be just barely visible if you knew where to look. Our own Sun would appear in the constellation Cassiopeia as one of the brightest stars in the, otherwise scarcely different, heavens of this near neighbour in space.
The RV method is not the only way in which planets have been found outside our solar system. The first exoplanets were found, a few years earlier than 51-Peg b, by detecting fluctuations in the arrival time of the rapidly pulsating radio signals from a pulsar, a dead star with the mass of a sun but the size of a city. Pulsars are created during supernova explosions, and any planets orbiting stars prior to these cataclysms should have been utterly destroyed. The pulsar planets must therefore have been formed from the debris and, as so often in this story, their existence was a complete surprise unpredicted by anyone before their discovery.
Another technique used to find exoplanets is to look for transits: regular dips in stellar brightness caused when a planet crosses in front of a star and blocks out a small fraction of its light. The effect is small and, for any given planet, unlikely to happen at all since the necessary alignment is quite rare. The chances of an Earth-like world producing a transit each time it orbits its star are about 1 in 700 and the drop in brightness it would produce is less than one-hundredth of a per cent but, nevertheless, Earth-like worlds can be found provided enough stars are looked at with sensitive enough equipment. So far, 333 planets have been detected by transits and there are thousands of candidate worlds awaiting confirmation by repeat observations. Most of these have been found by the Kepler space mission, a space-based telescope that has been continuously monitoring 100,000 stars in the constellation Cygnus since 2009. None of Kepler’s discoveries are yet truly Earth-like but it is probably only a matter of time before it finds an Earth-sized planet circling within the habitable zone of a stellar system. Indeed, announcement of such a world could well come between me finishing this book and its publication (an occupational hazard of discussing topical subjects). Kepler has already found some pretty odd worlds, though, and perhaps the strangest are four circumbinary planets: planets that literally have two suns (note that Alpha Centauri Bb discussed above is orbiting only one of the stars in that triple-star system and therefore has only one sun).
Exoplanets can also be found by directly looking for them, but the enormous brightness difference between a star and any planet orbiting it makes this very difficult, with only 38 worlds having been seen this way by late 2013. However, unlike the RV and transit methods, directly looking for them works best for planets with large separations from their host star. The record separation, so far, is a planet with the mass of twenty Jupiters orbiting 500 billion kilometres from a star seven times the size of ours. Even allowing for its much brighter sun, this must be a very cold world indeed.
One final method for finding exoplanets is called microlensing. This technique detects changes in the brightness of distant stars caused when the gravity of a much nearer star focuses their light onto the Earth. The smooth increase and decrease in brightness, over a few weeks, that this chance alignment causes is distorted when planets are present around the nearer star. Analysis of this distortion allows the size and separation of the planet to be calculated. Microlensing has been used to find just 24 planets so far, but despite this low success rate, the importance of this approach will become clear later in this book.
Of all the techniques for finding worlds orbiting other stars, the transit method has the most potential to provide ground-breaking results in the coming decades. Transits are already providing the best data about the relative frequencies of different sizes of planets and different sizes of orbits. But transiting planets allow an even more thrilling possibility than just exoplanet discovery: the technique allows us to determine the composition of a planet’s atmosphere even though we are hundreds of light years away. During a transit, light from the star passes through any atmosphere that the transiting planet possesses, and this allows its composition to be assessed. The drop in light during the transit is larger for those wavelengths of light where the atmosphere is opaque, and the wavelengths where any such absorption occurs can tell us about the makeup of the atmosphere. Such studies are in their infancy, with only hot Jupiters currently giving a strong enough signal to allow analysis. Nevertheless, two dozen worlds have had their atmospheres investigated and the constituents found include water, methane, sodium, carbon dioxide and carbon monoxide. This is currently the most exciting research area in exoplanet science and fascinating results are expected in the near future, especially once the James Webb Space Telescope (the Hubble Space Telescope’s replacement) is launched in, or after, 2018.
Robotic exploration of the solar system, together with our recent discoveries of planets around other stars, has revolutionised our understanding of the variety of possible worlds. Until recently we divided planets into only two groups, the terrestrial planets (Mercury, Venus, Earth and Mars) and the gas giants (Jupiter, Saturn, Uranus and Neptune). Exploration of the solar system has shown us that this is inadequate and so we generally split the giants now into two groups: the gas giants Jupiter and Saturn and the misleadingly named ice giants Uranus and Neptune. This new category, ice giants, recognises that these worlds have significantly more volatiles (molecules such as water, carbon dioxide and nitrogen) than the gas giants. These volatiles are loosely described as ‘ices’ since they would have originally condensed as ice at these distances from the Sun, although, once incorporated into worlds like Neptune, they are certainly no longer in ice form. Exoplanet discoveries have also forced us to introduce additional planet categories to cover kinds of world not seen in our own solar system such as the hot Jupiters already discussed and super-Earths (rocky planets with diameters approximately twice that of Earth).
Nevertheless, these additional categories of planets still don’t encompass the variety seen when worlds that are not strictly planets are also included. As we saw above, the moons of Jupiter show entirely unexpected ways of making worlds, and I haven’t even mentioned the satellites of Saturn such as Titan with its ethane rain, rivers and lakes, Enceladus with its powerful geysers blasting water into space, or Iapetus with its ‘Great Wall’, a unique ridge running right around its equator. I also haven’t mentioned the new category of ‘dwarf planets’ which had to be reluctantly introduced when it became clear that there are probably hundreds of Pluto-sized objects orbiting in the distant parts of our solar system. Dwarf planets are objects big enough for their gravity to make them spherical but not big enough to have cleared other large bodies from the surrounding space. Thus Ceres, discovered in 1801 as the first of the asteroids that orbit between Mars and Jupiter, is now classified as a dwarf planet, as is Eris, which was discovered in 2005 orbiting our Sun three times further away than the slightly smaller Pluto. Pluto remains a fascinating body regardless of what we artificially choose to call it, and NASA’s forthcoming New Horizons space mission can only further emphasise the restrictiveness of current planet classification schemes when it gets to Pluto and its five (or more!) moons in 2015. I can’t wait to see the pictures.
The study of planets, moons, dwarf planets and exoplanets has revealed an unexpected variety of worlds. Fortunately, order can be brought to this complicated picture. From what we have seen in the solar system, the developmental history of any world largely depends on its size, its composition and its energy sources. Is a planet metal-rich, rocky, gas-rich or volatile-rich? Is it bathed in warm sunlight or does it shiver at the frigid edges of its system? Does it have significant internal heat from radioactivity and tidal forces or primeval heat left over from its cataclysmic formation? Taking six components (metal-mass, rock-mass, volatile-mass, gas-mass, illumination and internal heat), and crudely splitting worlds into small, medium or large in each of these categories, gives 216 different possible types of world. Of course, some combinations will be more common than others but our experience with the variety seen in the solar system and beyond suggests that examples of many of these categories could be found eventually. It seems to me almost inevitable therefore that we will eventually end up with hundreds of types of world as our knowledge of their variety continues to expand. In itself, the existence of many types of worlds does not necessarily mean that life-friendly ones are rare, since several different categories may be suitable, such as Earth-like worlds and Europa-like worlds. On the other hand, if small differences are sometimes important, the crude division discussed above may not capture the full richness of planetary evolution.
Venus, for example, is an Earth-like world in terms of its size, composition and location but it is certainly not life-friendly. The relatively small difference in solar heating may explain this, but other factors also play a role. In particular the Earth’s magnetic field, which is uniquely strong and long-lived among rocky worlds in the solar system, could be important. Planets like the Earth that retain a strong field over billions of years may be more conducive to life because a magnetic field acts as a shield against the solar wind. The solar wind is a million-mile-an-hour stream of energetic particles sent out by the Sun that, if not deflected, can remove water from a planet’s atmosphere as collisions between the wind particles and water split the molecules into their constituent hydrogen and oxygen atoms, allowing the lightweight hydrogen to escape into space. This probably happened to Venus, which has a thick but completely dry atmosphere and no magnetic field. Unlike Venus, our planet has a geomagnetic field that has persisted for billions of years. The reasons for this are still being investigated. It may have to do with disparities in rotation rate, composition or size. Whatever the cause, the existence of our magnetic field illustrates that there are differences in planetary characteristics that are not captured by the crude scheme outlined above but that may still be important to habitability.
Habitability may also be affected by properties not directly related to planet type. For example, location in the galaxy may matter (are there lots of nearby supernovas?), and the lifetime of a star is almost certainly a major consideration (will the planet be habitable for very long?). Furthermore, this book concentrates in particular on the influence of climate and the many factors that affect it, such as a planet’s rate of rotation, the circularity of its orbit, the angle between a planet’s axis and its orbit and, as we’ll see in detail later, even the locations of other planets in the system. This gives another six properties to add to the six I mentioned earlier and, if planets need even moderate fine-tuning of these to be highly habitable, a low probability that any given planet will be life-friendly becomes mathematically inevitable.
Imagine taking all the planets that satisfy one of the properties – say, having the right amount of rock – and, for the sake of argument, imagine that one in ten planets satisfies this criterion. How many of these will also have the right amount of volatiles? Again, for sake of argument, imagine that one in ten worlds already chosen satisfies the volatile criterion too. So, one world in a hundred satisfies both properties. If this argument is carried through all twelve properties then, assuming that at each cut we keep one in ten planets, we end up with only one planet in a trillion satisfying all twelve properties. I should emphasise that the numbers I’ve used here are for illustration only. I do not know how many properties of the Earth are fine-tuned for life and I do not know what is the probability of each of these happening by chance. So, please don’t take my ‘one in a trillion’ too seriously; the true frequency of habitable worlds could be much higher than this or it could be very much lower. Nevertheless, unless there are surprisingly few planetary properties necessary for complex life, habitable worlds are going to be pretty rare.
The flip side of this argument is that there must be huge numbers of planets in the Universe to ensure that, despite poor odds, worlds like ours with the right combination of characteristics will still occasionally appear. So how many planets are there in the Universe? We don’t yet know if the majority of stars have planets orbiting them but it is at least clear that planets are not at all unusual. We also know that some stars have more than one planet. Hence, a first guess is that there are roughly as many planets as there are stars in the Universe. But how many is that?
Our galaxy is a disc about 100,000 light years across and several thousand light years thick, giving it a volume of about 10 trillion cubic light years. In the vicinity of the Sun, there is about one star in every hundred cubic light years. A crude calculation therefore suggests that our fairly typical galaxy holds 100 billion stars. More detailed calculations suggest there are actually about 200 billion stars making up our galaxy. Furthermore, huge numbers of galaxies can be seen through our telescopes. An idea of just how many galaxies there are can be gauged from an extraordinary picture taken by a unique telescope during the Christmas holidays of 1995. Five years after the Hubble Space Telescope was launched, it was pointed continuously at a small patch of sky for ten days just to see what was there. A particularly boring spot was deliberately chosen in the constellation of Ursa Major (Latin for the Great Bear but perhaps better known as the Big Dipper or the Plough). The location needed to be dull; the Hubble Deep Field, as the picture would be called, had to look into the distant Universe without having its view obscured by relatively nearby objects. Over that ten-day period the telescope took several hundred digital pictures, which were added together by computer to produce the final, highly sensitive image. The result was stunning: 10,000 galaxies in a field of view so small that it would take a hundred of them to cover a patch of sky the size of the Moon. So next time you look at the Moon, remember Neil Armstrong but also remember that, wherever it is in the sky, there will be about a million galaxies hiding behind it! How many Moon-sized patches of sky do you think it would take to cover the entire heavens? Hundreds? Thousands? Actually it’s about 20,000 and so, if we got Hubble to survey the entire sky, it would see tens of billions of galaxies. More detailed versions of this calculation imply that there are around 100 billion galaxies in the observable universe.
Hundreds of billions of galaxies each containing hundreds of billions of stars implies more than 10,000 billion billion (10,000,000,000,000,000,000,000) stars in the visible Universe, and the number of planets is probably similar. Numbers this large are notoriously difficult to visualise but, to get some idea, imagine sand so fine that you can barely see the individual grains. A pint glass would hold nearly a billion of them; a number so huge you couldn’t pull them out one at a time even if you were insane enough to dedicate a sleepless lifetime to the job. Now imagine having enough of these sand grains to fill a box a mile long by a mile wide by a mile high. That’ll be about the right number.
Even that doesn’t complete the tally for the number of stars in the heavens. If currently favoured models of cosmology are correct, the part of the Universe that happens to be visible from the Earth, the observable Universe, is an insignificant fraction of the entire Universe. The bit we can see goes out 13 billion light years. More accurately, the furthest visible galaxies were 13 billion light years away from us when the light we now see started its journey, but they’re now more than 20 billion light years away. However, if the latest cosmological theories are right, there are stars and galaxies much further away than this; but there has not yet been enough time, since the creation of the Universe less than 14 billion years ago, for their light to reach us. So, the whole Universe is larger than the observable Universe and it could be much larger. These new cosmological theories, which are very good at explaining many important features of the bit of the Universe that we can see, suggest that the whole Universe is truly vast and extends over scales that make the visible Universe look microscopic by comparison. In such theories the volume of the entire cosmos is at least a billion times larger than the bit we can see – and that’s just the minimum estimate!
So, taking 100 billion stars per galaxy and 100 billion galaxies in a visible Universe that fills only one billionth of the volume of the whole Universe gives a minimum estimate of 10,000,000,000,000,000,000,000,000,000,000 stars and planets. To visualise this truly staggering number you will need to imagine enough fine sand to make an object the size of the Moon. The number of planets in the whole Universe therefore makes even the number of grains in a cubic mile seem like small change, and this unimaginably large quantity of planets allows highly habitable worlds like ours to be both very rare and, at the same time, very numerous. As I said before, this is not a contradiction. One such world on average in a volume the size of the observable Universe is pretty sparse but still allows at least a billion of them in the entire Universe. Even if life-friendly planets are vanishingly rare, the Universe is so enormous that they remain inevitable. Under such circumstances we cannot draw conclusions about habitable worlds flooding the galaxy just because we happen to be sitting on one.
It’s time now to take our eyes off the heavens to look at something much stranger: Earth itself.