At this point I was beginning to weary of the lack of texture in the background. Where were the planetary structures that were supposed to emerge from this soup? I didn’t see anything of even the size of a marble. I peered more intently into the abyss, trying to spot some clue. Amid the dust I could see a distance of perhaps 10 Earth diameters, a mere hundred-thousandth of the disk’s breadth and barely 1 percent of the distance to the disk’s surface. By astronomical standards, this was the equivalent of pea-soup fog. Most of the dust particles in my near field of vision seemed to be only a little bit coarser than the dust I had seen elsewhere. It was still like fine soot, the typical grains having sizes only slightly larger than a micron, a ten-thousandth of an inch.
It suddenly struck me what was so puzzling about this aggregation of dust. Where was the gas that usually went along with it? I was well acquainted with dust as a routine component of the matter between the stars. Dust had accompanied me nearly everywhere I had visited. At the outset of my first journey, it had frustrated my attempts to see the center of the Milky Way. Only in the superheated regions of explosive bubbles had it been scarce. There I could understand that it had been evaporated by the impacts of the energized ions. Nearly everywhere else it was a trace component, barely 1 percent by mass. But here, it was the dominant material.
Those among you who have never left your home planet may be surprised to learn that one grows accustomed to thinking of all elements other than hydrogen and helium as luxuries. As I have already noted, from the inanimate perspective of the cosmos, planets are most distinguished by their immense concentrations of chemical elements such as oxygen, carbon, silicon, and iron. Nearly everywhere except in planets (and the interiors of some weird bodies known as white dwarfs), these elements taken together make up barely a percent of all matter by mass. It is no accident that this elemental fraction is similar to the fraction of matter that makes up the grains in the spaces between the stars. In those relatively cool environments, the heavy elements are able to exercise their natural tendencies to combine in certain ways and to condense into solids. On Earth, even those elements considered scarce—tin, platinum, and uranium, for example—are enormously concentrated compared to their abundances in space.
In this disk, it seemed that I was witnessing the early stages of this purification process. Somehow, the dust had been winnowed from the gas, and much of the latter blown away. Had the gas been pushed outward through the dusty matrix by gusts of wind from the youthful star? Or had the dust, striving to orbit against the friction of a warm, gaseous environment, drifted inward, leaving the gas behind? It was too late to reconstruct that episode of the story. Anyway, it didn’t matter much where the gas had gone: It would quickly mix with and become indistinguishable from the ordinary matter of interstellar space. The important thing is that the planetary raw material had purified itself, concentrated just those elements that could provide the rigid framework of a habitable body if other conditions were right. I counted this as another sign that I was on a fruitful track.
I scooped up a bagful of grains and drew it inside Rocinante for closer examination. The interstellar dust I had encountered earlier, virtually everywhere except in the hottest environments, had been a fine, solid substance. There had been the odd clumps of particles stuck together, the very rare speck approaching the size of the point of a pin. This dust was noticeably different. It was crumbly. But that wasn’t because it consisted of a different material. When I magnified it and examined its structure, I saw delicate filigrees that looked fragile and broke apart easily when flexed and yet exhibited a surprising toughness when I tried to crush it in my microscopic vise. These were composites of the ordinary grains I had seen before. An observation I had made casually, and to which I had paid little attention, suddenly assumed immense importance. These grains were bigger than the grains of interstellar space. They had started down the long path toward forming planetary structures, after all. I had arrived near the beginning of the process, when they had just begun to stick together. One would have thought that they would shatter, colliding at their random speeds of more than 100 kilometers per hour. Yet the successive impacts not only left them intact but seemed to cement them together. I laughed aloud in astonishment, but the main emotion I felt was awe at the thought that mighty planets like Jupiter could have grown from these tiny smudges of interstellar soot.
At the rate they were scooting across the disk, the smaller grains would run into a mate every year or so. Not every collision would lead to coalescence. Some glancing encounters would leave the grains little scathed. And a good fraction of the collisions, especially those involving the less consolidated clumps, would shatter the participants. But it was clear that things would develop quickly, and the grains would continue to grow, in spite of these inefficiencies. I started to pull Rocinante slightly out of the densest layer to get a longer view, at the same time edging closer to the central star where I thought events would be happening more rapidly. I settled in to watch, then . . . Bang! Rocinante was struck a blow out of all proportion to the annoying but harmless rattle I had endured up to that point. I checked for damage . . . negative. Then . . . Bang! again, only this time I was ready. I trapped the offending projectile: a small pebble barely a millimeter across. This was something you might not stop to remove from your shoe, but here it was a truly extraordinary creation, an object containing a billion of the granular building blocks. Stick a billion of these together and you’d get a meter-sized boulder. Another billion-fold and you would have a mountain. And an amalgam of a billion mountains . . . that would be enough to make an Earth.
Alerted by this harbinger of growth and change, I began to see things I had missed before. These much bigger grains were not so rare as I had thought, although they were much rarer than the tiny dust particles. Whereas the dust grains had peppered every inch or so of the space surrounding my craft, the pebbles were hundreds of meters apart. Their relative rarity explains why I had not noticed them before—that, and my good luck in having avoided a frontal collision before now. The pebbles already contained a few percent of the solid matter in the disk, and that percentage was destined to increase over the next thousand years or so, until they had swept up and incorporated most of the dust. Then, a traveler visiting the central layers of the disk might even be able to see out, to appreciate at once this proto-Solar System and its spectacular setting within the Trapezium’s bubble.
Would I be that traveled? I had a choice. I could rev up Rocinante’s thrusters to accelerate away, thus slowing down the passage of my time, with the intent of returning to witness the development of this system at a later stage. Or I could give up and return to the intense gaseous sprays of newly formed stars, which now looked surprisingly benign by comparison. One thing was clear: I could not remain embedded in this disk. It would be 10,000 years, or more, until enough of these small particles had coagulated so that I could navigate safely among the larger chunks, avoiding dangerous collisions. I pulled above the disk, back into the welcoming pink glow of Orion, to decide what to do.
18
The Shepherd
Serendipity—finding important things by chance—has always been one of the astronomer’s best friends. By definition it is unpredictable, and its impact is often most poignant when conditions look least hopeful. It is safe to say that the most significant discoveries made by my generation of astrophysicists were serendipitous; the jets of SS 433 offer a good example. Astronomers looking for one thing found something entirely different, and in many cases they weren’t even looking. The discovery figuratively descended from the sky and bopped them on the head. My brush with serendipity was not so dramatic. Faced with two unpalatable choices in my search for planets in formation, I unexpectedly found a third way.
Across a gap of what couldn’t have been more than 2 or 3 light-years, and partially embedded in the wall of molecular gas behind the luminous façade of the Orion Nebula, I spied another disk that had the earmarks of dust but also exhibited
some interesting features that were missing from my present venue. This disk was banded with dark and narrow concentric rings, where dust seemed to be absent. Like the rings of Saturn, I thought. During my childhood days as an amateur astronomer, it had been considered an easy test of visual acuity to spot the “division” in the rings that had first been noticed by Cassini in the seventeenth century. This could be done with a small telescope. If you had a bigger scope and a steady eye, you could find hints of the many other narrow gaps that observers had discovered over the years and that stood out prominently when the first close-up pictures came back from Pioneer and Voyager. It later turned out that Uranus, Neptune, and even Jupiter had rings, although these were far beyond the detection capabilities of amateurs with small telescopes. The latter systems were like negative images of Saturn’s rings. Instead of dark gaps between bright annuli, the divisions consisted of narrow, bright rings of reflective particles separating broad, empty spaces.
A theory had been developed to explain both the gaps and the narrow rings. Neither had been expected, because even the small random motions of the particles (which would necessarily arise from the same kinds of gravitational effects that had generated them in the disk I had just visited) would cause narrow rings to spread and merge and gaps to fill in quickly. It was hypothesized that the rings were held in trim, and the gaps kept open and sharp, by small moons orbiting the planet. To clear out a gap, a single moon would have to orbit within it. And to channel particles into narrow rings, there would have to be a pair of moons, locked in synchronized orbits on either side. These moons didn’t have to be very big; in fact, their existence had been predicted simply because their presence would explain the gaps and rings, long before they were seen. When they finally were discovered, it caused a sensation in the world of planetary studies. They were called “shepherd” moons, because they guided the streams of dust and debris and prevented them from getting out of line. “Sheepdog moons” might have been more appropriate, but the anthropomorphic name stuck.
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.
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