Turn Right At Orion (19 page)

I sped across the distance that separated me from this banded disk and hovered just above it. I knew instantly that it provided me with a new option, because it was obviously a planetary system caught at a later stage of development—just what I was looking for. The dark bands were indeed narrow gaps, and I counted on each of them being kept clear by a sizable shepherding
body, in this case a small planet. The spaces between the gaps were still filled with debris, including some fine dust, although this time the dominant contents seemed to consist of an assortment of bodies ranging from millimeter-sized pebbles up to large rocks and mountainous fragments several kilometers across. These must have been built up by the same coagulation process I had witnessed earlier among much smaller particles. As these bodies grew, they swept up material at an ever-increasing pace, yet the time it took to accumulate a meter-sized boulder was impressive: tens of thousands of years. To build a smallish mountain required millions of years. The spaces filled with rubble looked as dangerous as ever, but now I had the option of riding in the clear area of a gap. However, before I risked my neck again by descending into the plane of the disk, I wanted to see the body that was alleged to be keeping the gap clear.

It took a while to find it. Avoiding the most concentrated layers of the disk, just in case, I selected a particularly broad and well-formed gap and circled above it. It wouldn't have worked for me to synchronize my orbit around the star with that of the nearby orbiting debris, because the purported “shepherd planet” would be orbiting at the same rate. Without exceptional luck I could get locked into a perpetual game of cat and mouse (or mouse and cat, given that my prey was much larger than I) and might never encounter the planet. I knew that tiny moons could shepherd large, prominent rings. But it is hard to appreciate just how powerful the shepherding effect can be until you see for yourself the discrepancy between the width of the gap and the size of the planet required to clear it. In fact, I passed by the planet twice before I even noticed it. I had expected at least to sense the tug of the planet's gravity as I passed by, but I must have been traveling too fast or have been too far above the disk for this to be noticeable. On my third time around, I spotted it visually. If it hadn't been for the corroboration of my calculations, I would have ignored it and kept on looking. From all appearances this was an insubstantial body, slightly more than a thousand kilometers across and weighing (by my estimates) only
one-thousandth as much as the Earth. Yet the gravitational attraction of this planet, exerted steadily over thousands of years, had cleared the debris from a gap that was a thousand times wider than the body itself!

I closed in on the body and synchronized my orbit with its motion. It was not the kind of object I envisaged when I thought of a rocky planet. Planets like Earth, Mars, and Venus are highly structured. They are layered like an agate, their centers rich in iron and the heavier minerals, and their outer layers consisting of successively lighter materials until (in the case of Earth, at least) the top layer, the lithosphere, literally floats on the heavier mantle. From the gravitational tugs I finally measured as I approached, I could tell that this was still a mainly undifferentiated lump, reflecting its heritage as the sum of the countless agglomerations of dust, pebbles, and rocks that had gone into constructing it.

The collisions that had built up this body had not been entirely random, however. Objects as large as a few kilometers, or less, grow in a haphazard way, accreting whatever they happen to run into. But when any concretion within the disk reaches the size of a mountain, a dramatic change occurs in the way it interacts with its surroundings. The growth becomes more directed, and inexorable. Whereas before, the concretion merely ran into neighboring, smaller bodies by chance, its gravity now begins actively to focus nearby debris onto collision courses with it. Like some kind of inanimate Pied Piper, it develops a retinue of smaller bodies that slavishly trail behind it. As they jostle one another for position, some of them move too close and merge with the leader. This focusing effect greatly accelerates the rate at which a protoplanet can grow: To double a mountain-sized body would take millions of year, but To bring this shepherd planet from half its size to its present dimensions might have taken only a few hundred years, if that.

There is another intriguing aspect to this runaway process of accretion. Once a body has become the dominant protoplanet of its domain, the growth of competitors to this anointed one dies
back. I ventured above the disk to look for other bodies of similar size: None were visible in the vicinity of this gap, though there would have been enough matter to create them. I saw plenty of objects measuring tens, even hundreds, of kilometers, but there was nothing in sight to challenge the dominance of my shepherd planet over the domain of its own gap.

Such a breakneck rate of growth cannot continue indefinitely. Eventually the protoplanet swallows everything within its reach and runs out of matter to accrete, even given its ever-increasing powers to attract smaller particles. This happens precisely when it has cleared out its gap, a threshold that depends on the mass of the planet and the thickness of the disk. This is why I found this planet to have grown tantalizingly close to an. Earth-like mass (a factor of 1000 seemed awfully close, compared to the cosmic dust bunnies I had encountered earlier), but no closer.

Having convinced myself that it was safe, I maneuvered
Rocinante
into the gap and flew, in formation, ahead of the shepherd planet. It was thrilling to see the seething boulders, mountains, and pebbles held at bay along either wall of the gap. I thought of Moses parting the Red Sea and wondered whether the parallel walls of water had given a similar impression to the Israelites. I fantasized that somehow it was my gravity that had cleared the way, exploiting some amazingly subtle interplay of forces. If this sounds like hubris, it probably was, for at that moment I suffered the kind of shock that often befalls those who overestimate their ability to predict (much less to control) the forces of nature.

I noticed a swishing motion in the particles lining the starboard wall of the gap, like a wave in a curtain. Without warning, a large jagged body, at least several tens of kilometers across, parted the wall and came barreling toward me at about 10 kilometers per second—that's more than 30,000 kilometers per hour. This was only a third the speed of my orbit around the star, but because everything in my vicinity had been orbiting together, it was enormous compared to any of the speeds I had encountered lately. This body—let's call it an asteroid, it had that kind of shape and size—must have swung tightly around the
back of another, much larger body somewhere beyond the gap and on the other side of the disk. Perhaps there was a planet in the outer reaches of this system that had already grown to the dimensions of an Earth, or even larger. Sudden, close-up encounters like this can act as slingshots. Two bodies, swinging close together because of a slight fluctuation in the disk's orbital regularity, suddenly find themselves pulled toward one another by immense, rapidly changing gravitational forces. More often than not, they are moving too fast to linger; instead, the lighter one shoots off at high speed on a new and reckless trajectory. These kinds of interactions can easily override the delicate balance established by the perennial but predictable tugs of a shepherd planet. I had been lulled into a false sense of security, thinking that the order imposed on the gap by my shepherd planet could not be disturbed by events occurring elsewhere in the disk.

The asteroid seemed to be heading straight toward me. I grabbed
Rocinante's
controls, planning to accelerate away once I determined whether the best escape route lay ahead of me, to the rear, or sideways. But before I could make a decision, the asteroid had crossed into the central gap, and I could see that it would pass well behind me. I instinctively relaxed, but I shouldn't have. The asteroid made a beeline for the shepherd planet (it was moving too fast for its path to have been curved noticeably by the planet's gravity) and then hit it squarely. A brilliant spot of light spread from the impact point. The patch just below the impact had been heated to nearly 3000 degrees and had vaporized. As the pressure under the impact point was released, a huge dome of vapor erupted, with a jet of luminous gas squirting outward from its highest point. Farther away from the impact, the planetary material had liquefied, and I could see globules of molten rock being thrown up from the surface. Within a few minutes the shock wave had spread halfway around the planet. No longer intense enough to melt the rock, it was cracking and buckling the planet's surface, sending shards of rock hurtling into space at high speed. A spray of these shards, mixed with flash-frozen molten globules, approached my craft, and I knew it was time to flee.

It was easier to outrun the spreading debris than to outpace my frustration, but I managed to do both. I was disappointed to find that a disk this far along in forming planets was still too dangerous a place for me to go poking around. I was henceforth to be relegated to a longer view of these systems, just as I had been banished from getting close to black-hole remnants of stars by the sickening effects of their tidal forces. This kind of frustration was beginning to seem dismayingly routine; I had no choice but to take my disappointment in stride.

The process of planet formation had proved to be much more gradual than I had anticipated. It was really a complex series of processes, with many steps. How different this was from star formation, in which the main body of the star came together all at once, leaving behind a disk that provided additional nourishment in what seemed a straightforward, two-step process. I recalled how difficult it had been, in the first planetary disk, to spot the evidence that tiny dust grains were starting to stick together and accumulate. Now, in the second disk, I found bodies that were almost but not quite the size of planets. I knew that they had to grow another 100- or 1000-fold in mass, yet the gaps that they had carved out around themselves had put the brakes on their rapid accumulation of additional matter. To go
beyond this stage, the miniature planets had to latch on to a new mode of growth, and I had to decipher it without being able to explore the disk from within.

As I puzzled over how the protoplanets would get beyond the hiatus in their growth, I pulled far enough above the disk to survey several gaps at once. Some of the gaps were much wider than the one I had studied close-up, which indicated that their shepherd planets had greatly surpassed mine in mass. Errant asteroids, like the one that had slammed into my shepherd planet, must have etched the numerous faint trails that cut across the debris-filled zones between gaps. In some places, the gaps had wavy or indistinct walls, in contrast to the sharp boundaries I had considered to be the norm. I suddenly realized how naïve I had been to consider each gap, along with its shepherding planet, an isolated part of the disk. They all communicated, through the weak gravitational tugs that destabilized portions of the gaps (causing them to fill in) or changed the planets' orbits, or via the occasional asteroid hurled on a collision course from one to another of the shepherds' domains. The hope that I could find safety by attaching myself to a shepherd planet and accompanying it along its track had seduced me into assuming that each miniature proto-Earth would eventually grow into a full-sized version, while occupying much the same orbit it had been so zealously keeping clean. The gaps in the second disk had seemed like such nicely grooved tracks that I had thought the planets would stay put. In fact, the gaps, and the mini-planets that shepherded them, were ephemeral.

The asteroid-planet collision that had driven me from the second disk was my key to understanding the next stage of planetary growth. True, the particular collision that I had witnessed had been more destructive than constructive. On balance, the planet had lost a little mass in the encounter, but only because the asteroid had come in uncharacteristically fast and the planet was on the small side. The more common encounters would be less violent, and the planet would gain, on average. When the planet approached Earth size, even fast collisions like the one I
had seen would be hard pressed to liberate a large fraction of the matter involved. At worst, shattered chunks of both colliding bodies would be thrown into orbit about the planet; most of the debris would rain back down on it, the remainder coagulating to form a satellite. Before I left Earth, many scientists had become convinced that our home planet had had at least one such collision, an impact with a Mars-size body (proto-Mars itself?) that had gouged out enough matter to form the Moon.

It seemed that protoplanets were susceptible to many possible fates. A lot of chance was involved. Sometimes a planet would grow mainly by repeated collisions with. much smaller bodies. Asteroids might be thrown into the planet's path through the gradual disintegration of the gap walls. When this happened the collision would be slow, and the planet would absorb most of the asteroid's matter. In other cases the asteroid would be hurled into the planet's path, slingshot-style, following an encounter with another planet. The planets themselves also engaged in a very complex dance with its own internal logic. The planets' orbits would become locked into syncopations that were very hard to break once they formed. In such a “resonance,” one planet might execute three orbits in exactly the same amount of time that it took a second planet to complete two. I remembered how hard it had been to learn to beat three notes against two when I studied piano as a child, but Nature seemed to do it effortlessly. A third planet might get locked to the first two by beating five orbits against three against two, a feat I had never even attempted on the piano. Four versus three versus two was also possible; the combinations were endless. Once locked in a resonance, planets could exert powerful influences on each other's orbits. Gaps and their attendant shepherds could drift, separate, or come together and merge. Sometimes, the planets encountered and even collided with one another. The motion didn't always look orderly. Smaller planets, especially, sometimes moved onto orbits that seemed chaotic. But the results of all these processes were greater variety, unexpected change, and, of course, the growth of planets toward their final dimensions.