This spinning disk out of which worlds are forming has fallen together from the sparse matter that punctuates a vast region of interstellar vacuum within the Milky Way galaxy. The atoms and grains that make it up are the flotsam and jetsam of galactic evolution—here, an oxygen atom generated from helium in the interior inferno of some long-dead red giant star; there, a carbon atom expelled from the atmosphere of a carbon-rich star in some quite different galactic sector; and now an iron atom freed for world-making by a mighty supernova explosion in the still more ancient past. Five billion years after the events we are describing, these very atoms may be coursing through your bloodstream.
Our story begins here in the dark, pullulating, dimly illuminated disk: the story as it actually turned out, and an enormous number of other stories that would have come to be had things gone just a little differently; the story of our world and species, but also the story of many other worlds and lifeforms destined never to be. The disk is rippling with possible futures.3
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For most of their lives, stars shine by transmuting hydrogen into helium. It happens at enormous pressures and temperatures deep inside them. Stars have been aborning in the Milky Way galaxy for ten billion years or more—within great clouds of gas and dust. Almost all the placenta of gas and dust that once surrounded and nourished a star is quickly lost, either devoured by its tenant or spewed back into interstellar space. When they are a little older—but we are still talking about the childhood of the stars—a massive disk of gas and dust can be discerned, the inner lanes circling the star swiftly, the outer ones moving more stately and slowly. Similar disks are detectable around stars barely out of their adolescence, but now only as thin remnants of their former selves—mostly dust with almost no gas, every grain of dust a miniature planet orbiting the central star. In some of them, dark lanes, free of dust, can be made out. Perhaps half the young stars in the sky that are about as massive as the Sun have such disks. Still older stars have nothing of the sort, or at least nothing that we are yet able to detect. Our own Solar System to this day retains a very diffuse band of dust orbiting the Sun, called the zodiacal cloud, a wispy remake of the great disk from which the planets were born.
The story these observations are telling us is this: Stars formed in batches from huge clouds of gas and dust. A dense clump of material attracts adjacent gas and dust, grows larger and more massive, more efficiently draws matter to it, and is off on its way to stardom. When the temperatures and pressures in its interior become high enough, hydrogen atoms—the most abundant material in the Universe by far—rare jammed together and thermonuclear reactions are initiated. When it happens on a large enough scale, the star turns on and the nearby darkness is dispelled. Matter is turned into light.
The collapsing cloud spins up, squashes down into a disk, and lumps of matter aggregate together—successively the size of smoke particles, sand grains, rocks, boulders, mountains, and worldlets. Then the cloud tidies itself up through the simple expedient of the largest objects gravitationally consuming the debris. The dust-free lanes are the feeding zones of young planets. As the central star begins to shine, it also sends forth great gales of hydrogen that blow grains back into the void. Perhaps some other system of worlds, fated to arise billions of years later in some distant province of the Milky Way, will put these rejected building blocks to good use.
In the disks of gas and dust that surround many nearby stars, we think we see the nurseries in which worlds, far-off and exotic, are accumulating and coalescing. All over our galaxy, vast, irregular, lumpy, pitch-black, interstellar clouds are collapsing under their own gravity, and spawning stars and planets. It happens about once a month. In the observable Universe—containing as many as a hundred billion galaxies—perhaps a hundred solar systems are forming every second. In that multitude of worlds, many will be barren and desolate. Others may be lush and fertile, on which beings exquisitely adapted to their several circumstances are growing up, coming of age, and attempting to piece together their beginnings. The Universe is lavish beyond imagining.
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As the dust settles and the disk thins, you can now make out what is happening down there. Hurtling about the Sun is a vast array of worldlets, all in slightly different orbits. Patiently you watch. Ages pass. With so many bodies moving so quickly, it is only a matter of time before worlds collide. As you look more closely, you can see collisions occurring almost everywhere. The Solar System begins amid almost unimaginable violence. Sometimes the collision is fast and head-on, and a devastating, although silent, explosion leaves nothing but shards and fragments. At other times—when two worldlets are in nearly identical orbits with nearly identical speeds—the collisions are nudging, gentle; the bodies stick together, and a bigger, double worldlet emerges.
In another age or two, you notice that several much larger bodies are growing—worlds that, by luck, escaped a disintegrating collision in their early, more vulnerable days. Such bodies—each established in its own feeding zone—plow through the smaller worldlets and gobble them up. They have grown so large that their gravity has crushed out the irregularities; these bigger worlds are nearly perfect spheres. When a worldlet approaches a more massive body, although not close enough to collide, it swerves; its orbit is changed. On its new trajectory, it may impact some other body, perhaps smashing it to smithereens; or meet a fiery death as it falls into the young Sun, which is consuming the matter in its vicinity; or be gravitationally ejected into the frigid interstellar dark. Only a few are in fortunate orbits, neither eaten, nor pulverized, nor fried, nor exiled. They continue to grow.
Beyond a certain mass, the bigger worlds are attracting not just dust, but great streams of interplanetary gas as well. You watch them develop, eventually each with a vast atmosphere of hydrogen and helium gas surrounding a core of rock and metal. They become the four giant planets, Jupiter, Saturn, Uranus, and Neptune. You can see the characteristic banded cloud patterns emerge. Collisions of comets with their moons splay out elegant, patterned, iridescent, ephemeral rings. Pieces of an exploded world fall back together, generating a jumbled, odd-lot, motley new moon. As you watch, an Earth-sized body plows into Uranus, knocking the planet over on its side, so once each orbit its poles point straight at the distant Sun.
Closer in, where the disk gas has by now been cleaned away, some of the worlds are becoming Earth-like planets, another class of survivors in this game of world-annihilating gravitational roulette. The final accumulation of the terrestrial planets takes no more than 100 million years, about as long compared to the lifespan of the Solar System as the first nine months is relative to the lifetime of an average human being. A doughnut-shaped zone of millions of rocky, metallic, and organic worldlets, the asteroid belt, survives. Trillions of icy worldlets, the comets, slowly orbit the Sun in the darkness beyond the outermost planet.
The principal bodies of the Solar System have now formed. Sunlight pours through a transparent, nearly dust-free interplanetary space, warming and illuminating the worlds. They continue to course and careen about the Sun. But look more closely still and you can make out that further change is being worked.
None of these worlds, you remind yourself, has volition; none intends to be in a particular orbit. But those that are on well-behaved, circular orbits tend to grow and prosper, while those on giddy, wild, eccentric, or recklessly tilted orbits tend to be removed. As time goes on, the confusion and chaos of the early Solar System slowly settle down into a steadily more orderly, simple, regularly spaced, and, to your eyes, increasingly beautiful set of trajectories. Some bodies are selected to survive, others to be annihilated or exiled. This selection of worlds occurs through the operation of a few extremely simple laws of motion and gravity. Despite the good neighbor policy of the well-mannered worlds, you can occasionally make out a flagrant rogue worldlet on collision trajectory. Even a body with the most circumspect circular orbit has no warrantee against utter annihilation. To continue to survive, an Earth-like world must also continue to be lucky.