Excerpts

Chapter One different worlds From out of nowhere, through the swirling fog, a world the size of the Earth's Moon looms into view. Its surface is glowing with lakes of bubbling lava, and a torrent of impacting meteorites sends white-hot rock spraying high up into its wispy jacket of sulphur-green clouds. The bombardment is relentless and, stone by stone, rock by rock, boulder by boulder, the growing planet absorbs anything that dares stray into its path. Originally just one of a huge army of rocks circling an embryonic Sun, this world has grown large. It has swallowed up countless smaller rivals. Others, which narrowly escaped its draw, it threw off course and sent to a fiery death in the heart of the Sun. The odds of this young planet surviving are slight -- at any moment it could be torn apart in a cataclysmic collision with another large ball of rock. There are scores of others like it in this young Solar System, and only a handful will make it. Yet somehow it does survive. In 100 million years it will grow large enough to hang on to a thicker layer of gas and cloud. Its surface, now hot enough to melt rock, will cool to a temperature where water will condense into oceans and, one day, it will develop life. In 4.6 billion years' time a late-blooming species will call it the Earth. Their small robots, honed from remnants of the exotic metals that now rain down from the heavens, will rocket out of this planet's gravitational pull and venture out into the void to visit the other survivors of this battle zone. Bit by bit, the little spacecraft will send back news of these veterans, and the story of the creation of the planets will unfold.                            Clyde Tombaugh arrived at the Lowell Observatory in Flagstaff, Arizona, on 15 January 1929. A Kansas farm-boy turned amateur astronomer, he was itching to get his professional career under way. He brought with him a suitcase and not much else, not even the money for a return ticket home. Tombaugh was greeted at the station by the observatory director, Vesto Slipher, and given a brief tour of the institution that would be his home for the next few years. On the way to his quarters, Slipher introduced him to a 13-inch photographic survey telescope still in the process of being built. Over the next year, the young man was going to get to know every nut, bolt, cog and gear on that telescope. With its help he would discover a planet and define the limits of the Solar System.     Although it probably would not have occurred to him at the time, Tombaugh was joining the ranks of an ancient profession, the latest in a line of stargazers dating back more than three centuries. In 1609 a Florentine genius called Galileo Galilei had become the first human being ever to point a telescope towards the night sky. He had found that the stars, even magnified many times, could still only be seen as pinpoints of light. But to his delight Galileo also saw a different breed of object up there, which appeared not as dots but as tiny bright discs against the blackness of space. He was seeing up close the objects the Ancient Greeks had called planetos, or `wanderers'. The planets were not fixed in constellations, like the stars, but from night to night, month to month, they could be seen slowly changing their positions in the firmament.     Apart from the Sun and the Moon, the Ancients saw five different bodies wandering through the heavens. So important did they seem that they named them after their gods. The bright one gently sweeping across the night sky was named Marduck by the Babylonians, Odin by the Norse, and Zeus by the Greeks. It was the Romans who called it Jupiter. The faint, fastmoving point of light, never far from the Sun, the Romans named Mercury, after the messenger of the gods. The most brilliant they named Venus, after the goddess of love and beauty; the blood-red one was Mars, in honour of the god of war; and the slowly drifting one was named Saturn, after the god of time. At the time Galileo spied them through his telescope, an idea that is universally accepted today was just taking hold: these wandering discs were worlds just like the Earth. To Galileo the Solar System was a family of six planets orbiting the Sun. Mercury and Venus were the closest in; out beyond the Earth lay Mars, Jupiter and finally Saturn.     By the time Clyde Tombaugh began working in Flagstaff, there were already two additions to the family. In 1781 a telescope many times more powerful than Galileo's had spotted Uranus, moving in an orbit twice as far away as Saturn. Then, in 1846, came the sighting of Neptune, hiding in the dark fringes of sunlight, 30 times as far from the Sun as the Earth. Neptune marked the end of the known Solar System and was the reason Tombaugh had come to the observatory. Soon after its discovery, astronomers had thought the planet seemed to be lurching from its expected path around the Sun. The possibility that the gravitational pull of an even more distant world might be tugging Neptune off course had instigated a search for a new, even more distant planet -- code-named Planet X. The search had been going on for 40 years when the boy from Kansas came on to the scene.     Tombaugh had no formal training in astronomy, he taught himself about the planets from a book that his uncle had lent him. During his teenage years he built several telescopes out of bits and pieces from around his parents' farm and would spend every spare moment observing the sky and sketching the planets. He was 22 when his father's crop was ruined and he determined to look for work to help his family through the coming winter. He wrote to the Lowell Observatory because it was the only one he'd heard of and to his utter surprise he got the job. By February 1929 the new telescope had been finished and Slipher had taught Tombaugh the rudiments of astronomical photography. From then onwards, Clyde Tombaugh was left alone with the telescopic camera and a sky full of stars. Blink-blink The principle of the search for Planet X was simple but laborious. The first stage was to take pictures of a tiny patch of stars several nights apart. The two photographic plates were then to be viewed in a blink comparator. This machine allowed the astronomer to compare the plates by flicking, or blinking, between the two supposedly identical images. Only by subjecting every single star to this scrutiny would it be possible to see if, over the course of a few days, one of the stars had wandered from its original position, signalling that it was in fact a planet.     By the summer, Tombaugh had generated a mountain of plates awaiting this painstaking search and Slipher, rather than tying down a more experienced astronomer, decided to offer the young man the opportunity to clear the backlog himself. Tombaugh was overwhelmed. He realized that the person studying the plates in the blink comparator was, in effect, the person entrusted with discovering a new world, the ninth planet.     Nevertheless, no one said it would be easy. Each photographic plate could include anything from 50,000 to a million stars: it could take a week or more to complete a comparison of just one pair. Tombaugh worked in sessions of three to six hours; any longer and he would become exhausted. As if hours of intense scrutiny of sections of plates containing hundreds of thousands of stars weren't difficult enough, the young astronomer had to be continually alert to false alarms: smudges on the plates caused by drifting asteroids and comets, even adjacent stars whose brightness would inexplicably change. Then, of course, there were the known planets -- the seven discs of light, varying in size and brightness from a dim speck to a glowing ball, which would appear from time to time across his plates.     By January 1930 Tombaugh's photographic plates formed a patchwork of images that together made up a 360-degree strip that circled the entire night sky. On 15 February he placed another two plates in the blink comparator. They were of a region around the star Delta Geminorum that had been taken four weeks earlier. For three days he blinked one plate against the other until he had confirmed a quarter of the points of light as stars. Then he found it. Despite tired eyes and months of false alarms, when Tombaugh saw a faint speck of light changing position as he flicked backwards and forwards between the two plates, he had no hesitation -- it was Planet X. He later recalled, `A terrific thrill came over me. I switched the shutter back and forth, studying the images. [I thought] I had better look at my watch and note the time. This would be an historic discovery' Tombaugh's watch told him it was 4.00 p.m. on 18 February 1930. That night he went into Flagstaff and celebrated by watching Gary Cooper in The Virginian .     As soon as Vesto Slipher announced the discovery, telescopes around the world were trained on Tombaugh's planet, now called Pluto. What they saw was a surprise. Pluto didn't meet anyone's expectations. Planet X was supposed to have been a large world, capable of pulling Neptune off track. Instead, Pluto was tiny, smaller even than the Earth's Moon. It was several years after Tombaugh's discovery when astronomers realized that the apparent deviation of Neptune's orbit was an illusion -- a result of it having been studied for much less than one Neptunian year. The mysterious Planet X did not exist at all. But in discovering Pluto and confirming that there were no more large planets out there waiting to be discovered, Tombaugh had set the scale of the Solar System. The family was complete. Meet the family The Solar System's first world is about 100 times closer to the Sun than Pluto. A close study of Mercury was impossible until the space age -- the planet never wanders far enough from the glare of the Sun for astronomers to be able to see it clearly. Mercury is a rocky planet just twice the size of Pluto, but what it lacks in size it makes up for with speed. It takes a mere 88 Earth days to complete its orbit of the Sun: a year on Mercury therefore lasts about three Earth months. In 1974 the American space probe Mariner 10 sent back the first images of Mercury's cratered surface and revealed a planet that looked like a larger sister to our Moon.     Halfway between Mercury and the Earth lies Venus, the second of the four rocky worlds in the Solar System, and the brightest planet in the night sky. Like all the planets, Venus spins as it travels around the Sun. The time it takes for a planet to complete one revolution on its axis is known as its day; the Earth takes 24 hours to do this, but Venus spins incredibly slowly. A Venusian day takes nearly 243 Earth days -- longer than it takes the planet to orbit the Sun -- so a day on Venus lasts longer than its year. Venus was once believed to be our heavenly twin, since it is not just the closest planet to the Earth but is also nearly identical in size. But in October 1967 a robotic Soviet probe plunged beneath the cloud tops to reveal the terrible truth about our twin: beneath its bright clouds lies a searing, lava-filled hell-hole.     The next planet lies about 80 million kilometres beyond the Earth -- 230 million kilometres from the Sun. Through a telescope Mars is a faint, reddish disc, half the size of the Earth. It's the last of the four rocky planets -- a desert world swept by dust storms. Although it is much smaller than the Earth, it shares some close ties with our world. Mars spins once every 24 hours and 36 minutes, so a day on Mars is almost the same as ours. It also has similar seasons because of the way it is tilted. As they move around the Sun, some of the planets rotate bolt upright (with the poles at due north and south), but most of them are tilted. The Earth, for example, is tilted by some 23 degrees from the vertical and it is this tilt which is responsible for our varying seasons as we travel around the Sun. Mars has an almost identical axial tilt, 25 degrees off vertical. Mars' much longer journey around the Sun, however, means that its year lasts 687 days.     On nights when the Moon and Venus are not visible, Jupiter is the brightest object in the sky. This massive planet, circling nearly 800 million kilometres from the Sun, could swallow all the other planets in the Solar System with room to spare. In the late 17th century, Italian astronomer Gian Domenico Cassini made out the ghostly traces of banded clouds on Jupiter and a giant storm that survives to this day -- the Great Red Spot. This was mankind's first inkling of the true nature of the planets beyond Mars. Jupiter, big enough to contain 1,300 planets the size of the Earth, isn't made of rock and metal -- it is a gas giant, a huge ball of swirling vapours with an atmosphere thousands of kilometres thick.     Even the feeblest telescope will reveal that Saturn has the most eye-catching adornment in the entire Solar System: a set of magnificent pale rings that circle its equator. Beneath the rings lies a planet that is a smaller cousin of Jupiter, covered with storms of cream, ochre and sepia-coloured clouds. Far beyond are the other two giant planets, much smaller than Saturn but still many times bigger than the rocky planets. Uranus is so far from the Sun -- nearly 3 billion kilometres -- that it takes 84 Earth years to complete a single orbit. Seen from Earth, the moons of Uranus revolve around it like the lights on a Ferris wheel. That is because Uranus spins on its side, rolling around the Sun like a barrel. Blue Neptune, 30 times as far from the Sun as the Earth, is so incredibly distant that from its cloud tops the Sun would seem little more than a bright star. No one could live to watch a complete orbit of this twilight world, as a year on Neptune spans two human lifetimes: 165 years.     And what of Pluto? The best telescopes show it to be a solid planet made partly of rock, partly of ice, but no spacecraft has ever seen it close up. From the moment of its discovery, Pluto was a mystery, spurring astronomers to ask the question that had been in the back of their minds for decades. How did the planets all end up so different? Why are the four planets in the inner Solar System small and rocky, and the next four planets giant and gassy? And then there is Pluto which doesn't fit into either category. The story of our attempts to understand the birth of the planets spans several centuries, and is, in truth, a detective story whose last chapter has not yet been written. As the 20th century draws to a close, there are suggestions that Pluto, far from confusing our efforts to unravel the evolution of the planets, may turn out to be the Rosetta Stone of planetary formation -- a tiny, distant clue to how these different worlds came to be. But wherever the story of the creation of the planets might end, the beginning of it, like the Solar System itself, starts with the creation of our Sun. Four billion years BC On 24 April 1990 the exhaust gases billowing out from launch pad 39A at Cape Canaveral in Florida signalled the take-off of space shuttle STS-31. Its mission was to place the Hubble Space Telescope in orbit around the Earth. Free from the atmospheric disturbances that have bedevilled Earth-based astronomers, Hubble would be able to see deep into space. In 1994 it was peering into the distant Orion Nebula when, through a thick cloud of gas and dust hundreds of billions of kilometres across, it saw an arresting sight. Bright points of light were shining through the dust -- as pockets in the cloud collapsed under their own weight, stars were being born. Within this stellar nursery, Hubble found evidence that the formation of a star was not an isolated event. Stars, it seems, are born in clusters, the gravitational instability created by the birth or the death of one star causing nearby pockets of gas and dust to collapse. Stars form just as dominoes fall -- one influencing the next. The spectacular Hubble images captured dramas similar to those played out more than 4 billion years ago and much closer to home when our own Solar System formed.     On the Orion arm of the Milky Way, 23,000 light-years from the centre of our galaxy, a massive cloud was gently drifting through space. At least 8 billion years earlier, the Universe and all the matter in it had been created in the event known as the Big Bang. Soon after that matter had begun to collapse and clump together to form galaxies. By now our own galaxy was already billions of years old and full of stars. As this massive cloud floated between the distant star fields, being pulled first this way then that by gentle gravitational tugs of other stars, pockets of gas in the cloud started to collapse inwards on themselves. As these imploding pockets reached a critical density, their cores heated up and ignited: stars were born. If we could watch the process of aeons speeded up into minutes, we might see small points of light flickering into life as stellar neighbourhoods sprang up throughout the cloud. Most of the smaller lights might glow steadily for several tens of seconds before slowly burning themselves out. But as the rash of stars spread throughout the cloud, a few of the lights -- the largest and brightest -- would flare for just a few brief seconds before disappearing. These are giant stars that burn for just a fraction of the lifetimes of their smaller, steadier siblings. When these stars die, their fate is to explode into supernovae. So violent are these supernovae that they send plumes of white-hot plasma bursting out into surrounding clouds, disrupting their uneasy equilibrium and nudging them into collapse. In one corner of that cloud on the Orion arm of the Milky Way, about 4.6 billion years ago, one of those supernovae sent a shock wave surging out at over 32 million kilometres per hour. Almost immediately a pocket of gas next to it began to contract and rotate, soon flattening out into a swirling disc of debris. Deep in the twinkling parent cloud another light flickered on -- it was our Sun. (Continues...)