Turn Right At Orion (18 page)

What struck me as most strange was that I was suddenly immersed in an environment that was comprehensible to human senses. Everywhere else I had encountered bodies so large (or atoms so small) and speeds so incomprehensible that I had been forced to abstract an idea of their dimensions through the use of my instruments or the conscious interpretation of my observations. I knew that the atoms of air at room temperature were moving 10 times faster than these grains, but because I couldn't see the atoms I had never had to address the reality of that motion. Here I could sense directly what was happening, to the extent of having to put up with the racket of these tiny grains striking
Rocinante's
windows dozens of times per second. They made the same kind of rattling noise that had driven me to distraction during that sandstorm many years ago. And through the windows I could see these particles flying at me from all directions, like flakes in an infernal snowstorm. I found it curious that a terrestrial sandstorm would linger in my memory as having temporarily robbed me of my senses, whereas its cosmic counterpart would suddenly restore them to relevance. I took this as a preliminary, indirect sign that perhaps some order amenable to life could emerge from this formless debris.

Not that I was surprised to see the grains in random motion. I reckoned there was enough unconsolidated dust to construct, maybe, a thousand Earths, spread in a disk that extended out from the central star to at least 100 times the size of the Earth's orbit around the Sun. The dust layer was thin, but not infinitely so. Most of the dust was concentrated along a sheet that was about 1000 times thinner than it was broad. Yet the breadth of the sheet was so great that it was still thick enough to hold a thousand Earths laid side by side. The disk's finite thickness went hand in hand with the random motions that had announced themselves so annoyingly along
Rocinante's
ceramic skin. The motions of the grains were exactly analogous to the motions of molecules that give pressure to a gas. If this pressure
did not exist, the gravitational attraction of the central star would draw a gaseous disk down to a plane of paper thinness, and the same thing would happen to a disk of dust if the particles did not have some random motions superimposed on their orbital circulation. The average speed of the grains was linked to the thickness of the disk via the star's gravitational attraction, just as the speeds of gaseous atoms had been in proportion to the thickness of the disk I had visited in Cygnus X-1. I was comforted to find that this dust disk was a good facsimile of the gaseous disks I had seen time and again, only this time executed in the medium of pulverized stone.

What caused the grains to dance continually? Unlike the disks of gas I had visited before, there were no eruptions of magnetic flares, no bursts of radiation remotely intense enough to stir up these heavy particles of grit. Still, I surmised that this thickening was not accidental. Had the disk been thinner, the dust particles pressed more closely together, the gravitational attractions of the dust grains for one another would have partially overwhelmed their organized attraction to the central star. Once this happened, the smooth structure of the disk would dissolve into striations, braids, and swarms of dust that would not stand still. All of this activity would amount to a stirring action, which would give the dust even more random motions than I had measured. Thus it was gravity that must have stirred up the grains. just enough stirring, and the dust's self-attraction would balance the pull of the star. It seemed like a new kind of equilibrium, one in which two sources of gravitational attraction—dust for dust versus star for dust—had achieved a delicate balance. But I quickly realized that it wasn't completely unfamiliar. A similar competition, in which stars played the role of dust grains and the Galaxy as a whole served as the central star, gave rise to the spiral waves that decorated the Milky Way. I was pleased to see the connection, but a little wistful that true novelties were becoming rarer as my travels continued.

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.

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.