Shadows of Forgotten Ancestors

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.