19
Finishing Touches
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.
Thus the final stage of planet growth was one of distant encounters, rhythmic orbital changes driven by the syncopated beat of the resonances, and bouts of chaos. The smaller planets gradually disappeared, their gaps merged into the larger bands cleared by bodies that were now truly becoming planet-size. The final growth itself was driven by much more violent collisions than I had hitherto encountered. This growth was not a clean process. Even as the larger bodies did their best to sweep up the finer particles of grit and dust, new debris was created continually as fast impacts nicked and ground down these bodies. Up until the very last stages—almost indefinitely, it seemed—planetary systems in formation were dusty construction sites.
The constant battering of the young planets led to gradual changes in their internal structures. As one impact after another flash-heated the planets’ surfaces, their interiors began to warm. Brittle mineral amalgams became plastic and, in some locations, liquefied. Heavier materials sank to the planet’s core, and the lighter minerals floated to the surface. This internal rearrangement was accelerated by dramatic deviations in the planets’ trajectories. When a planet was knocked off its circular course by big impacts or by a strong gravitational tug from another planet, it could go into an oblong orbit around the star. In this case, the continuously varying pull of the star’s gravity—the star’s tidal force as the planet got nearer and then farther away—would also heat the planet’s insides. It was comforting to know that a planet could get sick from being stretched by tides, just as I did near a black hole. Through the two effects, impacts and tides, the planets’ interiors were cooked, and the undifferentiated conglomerates of dust, pebbles, and boulders evolved into the layered structure—core, mantle, crust—that we associate with home.
20
Virgin Worlds
I felt that I could now tell myself a story of how a planet like Earth might have formed, from start to finish. But I wanted to see whether the story was true. Even if I could not sail safely amid half-formed planets, I was determined to complete my study of planet formation from afar, if necessary. In a third system, only 4 light-years from the second, I hoped to catch the final phase of planetary gestation. Again I found the system first by spotting its dusty swath. This disk was even more open than the second one, and in fact the veil of dust and debris was so thin that you could see through it in most places. The dust was also glowing more warmly, in infrared light, which indicated that much of the starlight was getting through and warming it. There were fewer noticeable gaps and, on closer (but not too close!) inspection, fewer planet-size bodies. But each of the planets I saw was larger.
I could not resist playing time traveler,
heading first for that zone, 150 million kilometers from the Sun-like star, where an incipient Earth might be preparing to host life. Here, the temperature could be just right to allow a planet to retain water and a thick atmosphere. Not that the atmosphere would be in place yet. The accumulation of water on the surface would have to wait for volcanic activity—an outcome of all that interior heating—or a rain of comets—sent into the inner reaches of the planetary system from its outer limits. If the chemistry went awry (if there was too much volcanic activity, for example), then the atmosphere might settle into a steaming sulfurous soup, in the manner of Venus. Too little volcanic activity, perhaps, and the planet might become a cold and arid world, like Mars, with most of its atmosphere evaporated or frozen. I was not expecting life, or even the setting for life, yet. But I was expecting to see a smooth, virgin world, ready for a completely new history to be written on it.
The planet was there, one of four or so, about the size of an Earth, but what I found was a brutally scarred body. This world was very close to spherical—it was so big, its gravity so strong, that any serious indentations would have filled in quickly—but its imperfections were just enough to remind me of a ball of clay shaped by hand, still bearing a child’s thumbprints. Sheets of half-congealed lava, bounded by giant cracks and rifts, covered vast areas. The thick ring of debris that surrounded this planet, a mixture of jagged fragments and smooth globules, betrayed its recent history. This world had suffered a huge bombardment, perhaps of a magnitude similar to the impact that might have created the Earth’s moon, and it had not healed yet. Had it been on the brink of hosting a flourishing ecosystem before its recent encounter? If so, how long before it would ready itself again? Or would some other fate intervene first?
That fate could lie in the hands of processes going on much farther away from the star. In the habitable zone around this star, a billiards game of planetary collisions was under way, but in the outer reaches of the disk, another game entirely was being played out. I had neglected the farther reaches of the other disks, partly out of impatience. Everything happened more slowly there, making it that much more difficult to see the processes of planet growth in action. I suppose that there was also an element of terrestrial chauvinism in my attitude, a bias toward regions and environments with Earth-like dimensions and conditions. Now that I understood the interdependence among vastly different regions of the disk, I knew this was an oversight I had to correct.
I headed outward, not perpendicularly away from the disk as when I had jetted away from the other disks, but this time skimming just above the surface. How different this scene was from the views I had enjoyed while skimming above the disk around a black hole! There the overwhelming impression had been one of luminescence and energy; here it was one of solidity and mass. I realized that this was mainly an illusion: The black hole had contained more mass than the star and planets combined, and I had been so much closer to the black hole that all aspects of the black hole’s gravity had been vastly stronger. But just as we breathe an involuntary sigh of relief when setting down on firm land after a flight or a sea voyage, so I responded to a disk made out of rock and ice instead of superheated gases.
I haven’t mentioned ice before because it was not a prominent constituent of the regions of disks I had been exploring. Ices—not just water ice but also those of ammonia and methane—had a pronounced tendency to condense on the surfaces of dust grains, but they could not do so if the grains were too warm. I had encountered ice-coated grains before, in the cooler regions of interstellar space, and I remembered likening some of these environments to snowstorms. Despite the ample shielding from starlight, ice could not condense on the grains that I had found in the inner regions of protoplanetary disks because too much warmth, mostly in the form of infrared rays, still managed to filter through.
But now, as I headed out to 3, 4, and more than 5 times the distance of the Earth from the Sun, conditions were growing positively chilly, and the dust grains were covered with rime. What was more striking was that the gas had not been winnowed from the dust at these distances. I had to keep adjusting my course to avoid flying through the disk, because the disk itself was flaring upward as the concentration of nimble light gases, mainly hydrogen and helium, first matched and then surpassed the concentration of solid dust.
While I was busily tracing the upward slope of the disk’s gaseous atmosphere, I had a sudden sensation that the bottom had dropped out from beneath my feet. It was like that eerie feeling I once had on Earth when, having climbed steeply to avoid the turbulence in a thunderstorm, I cleared the edge of the anvil and suddenly looked down on a flat cloud deck tens of thousands of feet below. My craft was flying in trim, then as now, but it was impossible to avoid the momentary sensation of falling off a cliff. I looked toward where I thought the disk should be and found that it was gone. I was in the biggest gap I had ever seen, so big that I couldn’t make out the other side. If it had been cleared by the gravitational effects of a shepherd, this had to be one enormous planet.
This time, it did not take long to find the shepherd planet. Although still much smaller than the gap it had cleared, this substantial body was shining brilliantly with its highly reflective clouds. I estimated it to be 3 times heavier than the planet Jupiter—making it about 1000 times heavier than Earth—and of similar architecture. Its rocky core would have been perhaps 10 times heavier than Earth, perhaps a bit more. But the vastness of its bulk would have been contained in its deep, dense, gaseous atmosphere. In these outer reaches of the disk, where the gas had not yet been separated from the dust, there was 100 times more raw material to incorporate into the body of a planet. This proto-Jupiter had greedily sucked up the gas. It had sucked up the grains, too, complete with their icy coatings. Stripped from their grains, the ices formed crystalline suspensions of ammonia and methane as clouds floating in the planet’s envelope; these are what made the planet shine so brightly.
This proto-Jupiter had not yet finished growing. Unlike the young Earth-like planet that I had just visited, which had relied on the complex dance of orbits and random collisions to throw raw materials in its way (and was paying the price in its tortured and scarred visage), this monster planet was skimming a steady supply of gas off the outer wall of its gap. Gas, unlike pebbles and boulders, was not so easily herded by gravitational forces alone. Its pressure was perpetually leaning on it to fill in the gaps, squeezing it toward any vacuum. And when proto-Jupiter passed by in its orbit, the wall of gas was unable to resist the temptation. A gaseous wave erupted from the wall of the gap and licked out toward the planet, pulled by the latter’s gravity. I followed along in the planet’s orbital wake, admiring the swirling eddies that were excited by the passing behemoth, along with the graceful stream of gas that encircled the planet and disappeared into its thick atmosphere.
Moving farther outward in the disk, I found three more giant gaseous planets. They lorded over much larger domains than their Earth-size counterparts but were smaller than the proto-Jupiter I had just visited. Each one was still growing by stripping gas off the walls of its gap, but at a much lower rate than proto-Jupiter. The inability of the gaps to stay completely clear of their shepherding planets made me realize that there was yet another dynamic at work here, one that could endanger the Earth-like planets closer to the star. When each giant planet disturbed the walls of its gap, the interaction was not entirely one-sided. Newton taught us that every action elicits a reaction; thus the pull of a proto-Jupiter on the gas and dust that surrounded it would, in turn, affect the planet’s orbit. In the situations I had observed, each of the planets was dragging the gas along with it, speeding the movement of the gas around the star. Newton’s law implied that the gas would pull back on the planet, with equal and opposite force, thus slowing it down a bit in its orbit. As a result these enormous planets were spiraling, ever so gradually, toward the star. I knew immediately that this spelled danger for proto-Earth and its companions. Already, the gravitational tugs of thes
e distant but massive worlds could be measured in the subtle motions of asteroids close to the Earth-like planets, and even in the motion of proto-Earth itself. Once these giant bodies got much closer, the effects would be drastic. If they didn’t come so close to proto-Earth as to send it careening out of the planetary system altogether, they might fling a fatal barrage of asteroids or planets into its path or destabilize its orbit so that it drifted into the star and vaporized. There was no telling when or if proto-Jupiter or the other giants would tire of their inward march. And there was no predicting what would become of the giants themselves. Would they march inward in lockstep until they plummeted into the star? Would they spread themselves out in a stable configuration that would leave them intact, whether or not there was room for a few Earth-like planets as well? Or would they lose the ability to coexist, invade one another’s territory, and fight a gravitational duel to the death of all of them but one?
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