In 1796 the French mathematician-astronomer Pierre Simon de Laplace tried to develop the nebular model in a quantitative way. He was able to show mathematically that the conservation of angular momentum (see More Precisely 15-1) demands that an interstellar cloud like the hypothetical solar nebula must spin faster as it contracts. A decrease in the size of a rotating mass must be balanced by an increase in its rotational speed.
The increase in rotation speed, in turn, must have caused the nebulas shape to change as it collapsed. In Chapter 11 we saw how a spinning body tends to develop a bulge around its middle. (Sec. 11.1) The rapidly spinning nebula behaved in exactly this way. As shown in Figure 15.1, the fragment eventually flattened into a pancake-shaped primitive solar system. If we now suppose that planets formed out of this spinning material, we can already begin to understand the origin of some of the large-scale architecture observed in our planetary system today, such as the circularity of the planets orbits and the fact that they move in nearly the same plane.
Astronomers are fairly confident that the solar nebula formed such a disk because similar disks have been observed (or inferred) around other stars. Figure 15.2(a) shows visible-light images of the region around a star called Beta Pictoris, lying about 50 light-years from the Sun. When the light from Beta Pictoris itself is suppressed and the resulting image enhanced by a computer, a faint disk of warm matter (viewed almost edge-on here) can be seen. This particular disk is roughly 500 A.U. acrossabout 10 times the diameter of Plutos orbit. Astronomers believe that Beta Pictoris is a very young star, perhaps only 100 million years old, and that we are witnessing it pass through an evolutionary stage similar to the one our own Sun experienced some 4.6 billion years ago. Figure 15.2(b) shows an artists conception of the disk.
THE ROLE OF DUST
Laplace imagined that as the spinning solar nebula contracted, it left behind a series of concentric rings, each of which would eventually become a planet orbiting a central protosuna hot ball of gas well on its way to becoming the Sun. Each ring then clumped into a protoplaneta forerunner of a genuine planet. The description of the collapse and flattening of the solar nebula is essentially correct, but when modern astronomers began to study the more subtle aspects of the problem, some fatal flaws were found in Laplaces nebular picture.
Calculations show that rings of the sort envisaged in Laplaces theory would probably not form, and even if they did, they would not in most cases condense to form a planet. In fact, computer calculations predict just the opposite: over most of the solar system, the rings would tend to disperse. The protoplanetary matter would be too warm, and no one ring would have enough mass to bind its own matter into a ball. Only in the cool outer regions of the nebula might it be possible for a sufficiently large clump of matter to form and survive. As we will see, such a possibility remains an important part of the modern theory.
The model currently favored by most astronomers is a more sophisticated version of the nebular theory. Known as the condensation theory, it combines the good features of the old nebular theory with new information about interstellar chemistry to avoid most of the old theorys problems. The key new ingredient in the modern picture is the presence of interstellar dust in the solar nebula. Astronomers now recognize that the space between the stars is strewn with microscopic dust grains, an accumulation of the ejected matter of many long-dead stars (see Chapter 22). These dust particles probably formed in the cool atmospheres of old stars, then grew by accumulating more atoms and molecules from the interstellar gas within the Milky Way Galaxy. The end result is that our entire galaxy is littered with miniature chunks of icy and rocky matter having typical sizes of about 10-5 m. Figure 15.3 shows one of many such dusty regions found in the vicinity of the Sun.
ACCRETION AND FRAGMENTATION
Modern models trace the formative stages of our solar system along the following broad lines. Imagine a dusty interstellar cloud fragment measuring about a light-year across. Intermingled with the preponderance of hydrogen and helium atoms in the cloud are some heavy-element gas and dust. Some external influence, such as the passage of another interstellar cloud or perhaps the explosion of a nearby star, starts the fragment contracting, down to a size of about 100 A.U. As the cloud collapses, it rotates faster and begins to flatten (just as described in the old nebular theory). By the time it has shrunk to 100 A.U., the solar nebula has already formed an extended, rotating disk (Figure 15.4a; see also Figures 15.1b and 15.2b).
Eventually, this process of accretionthe gradual growth of small objects by collision and stickingcreated objects a few hundred kilometers across (Figure 15.4c). By that time, their gravity was strong enough to sweep up material that would otherwise not have collided with them, and their rate of growth became faster still. At the end of this first stage, the solar system was made up of hydrogen and helium gas and millions of planetesimalsobjects the size of small moons, having gravitational fields just strong enough to affect their neighbors.
In the second phase of the accretion process, gravitational forces between the planetesimals caused them to collide and merge, forming larger and larger objects. Because larger objects have stronger gravity, the rich became richer in the early solar system, and eventually almost all the planetesimal material was swept up into a few large protoplanetsthe accumulations of matter that would eventually evolve into the planets we know today. Figure 15.6 shows a computer simulation of accretion in the inner solar system. Notice how, as the number of bodies decreases, the orbits of the remainder become more widely spaced and more nearly circular.
As the protoplanets grew, another process became important. The strong gravitational fields produced many high-speed collisions between planetesimals and protoplanets. These collisions led to fragmentation, as small objects broke into still smaller chunks, which were then swept up by the protoplanets. Not only did the rich get richer but the poor were mostly driven to destruction! Some of these fragments produced the intense meteoritic bombardment we know occurred during the early evolution of the planets and moons, as we have seen repeatedly in the last few chapters. The fragments that escaped capture by a planet or a moon would later become the asteroids and comets.
Mathematical modeling, like the calculation shown in Figure 15.6, indicates that after about 100 million years, the primitive solar system had evolved into nine protoplanets, dozens of protomoons, and the big protosolar mass at the center. Computer simulations generally reproduce the increasing spacing between the planets (Bodes law), although the reasons for the regularity seen in the actual planetary spacing remain unclear. (Discovery 6-1) Roughly a billion years more were required to sweep the system clear of interplanetary trash. This was the billion-year period that saw the heaviest meteoritic bombardment, tapering off as the number of planetesimals decreased. (Sec. 8.5)
MAKING GIANT PLANETS
The accretion picture just described has become the accepted model for the formation of the inner solar systemthe terrestrial planets. However, the origin of the giant jovian worlds is decidedly less clear. Two somewhat different views, with important consequences for our understanding of extrasolar planets, have emerged.
In the first, conventional scenario, the four largest protoplanets became massive enough to enter a third phase of planetary development, their strong gravitational fields sweeping up large amounts of gas directly from the solar nebula. As we will see in a moment, there was more raw material available for planet building in the outer solar system, so protoplanets grew fastest there. Ultimately they would become the cores of the jovian worlds. (Sec. 11.1, 12.1, 13.5) The smaller, inner protoplanets never reached that stage, and as a result their masses remained relatively low. This is the chain of events depicted in Figures 15.4(c) and (d).
In either case, once the jovian protoplanets reached the critical size at which they could capture nebular gas, they grew rapidly, their capture rate increasing as their gravitational fields intensified. Their large size today reflects the head start they received in the accretion process. If they did not start off with ready-made rock and ice protoplanet cores, these giant worlds had ample time to sweep up rocky and icy planetesimal material after they formed.
Many of the moons of the jovian planets presumably also formed through accretion but on a smaller scale, in the gravitational fields of their parent planets. Once the nebular gas began to accrete onto the large jovian protoplanets, conditions probably resembled a miniature solar nebula, with condensation and accretion continuing to occur. The large moons of the outer planets almost certainly formed in this way. Some of the smaller moons may have been chipped off their parent planets during collisions with asteroids; others may be captured asteroids themselves.
Many aspects of the formation of the giant planets remain unresolved. Interactions among the growing planets, and between the planets and their environment, probably played critical roles in determining just how and where the planets formed. One particularly intriguing scenario, believed by somebut certainly not allplanetary scientists, is the possibility that Jupiter, and maybe all four giant planets, formed considerably farther from the Sun than its present orbit, and subsequently migrated inward to its current location. This supposed migration is indicated schematically in Figures 15.4 and 15.7 by the locations of the jovian protoplanets.
The idea of planet migration has been around since the mid-1980s, when theorists realized that interactions between massive planets and the nebula in which they moved would have caused just such an inward drift. Observational support came in 1999, when Galileo scientists announced that reanalysis of data from the mission's atmospheric probe showed much higher than expected concentrations of the gases nitrogen, argon, krypton, and xenon. (Sec. 11.2) These gases, which are thought to have been carried to the planet by captured planetesimals, could not have been retained in the planetesimal ice at temperatures typical of Jupiters current orbit. Instead, they imply that the planetesimals, and presumably Jupiter too, formed at much lower temperatures. Either the nebula was cooler than previously thought, or Jupiter formed out in what is now the Kuiper belt!
*A good example of such a theory is the collision hypothesis, which imagines that the planets were torn from the Sun by a close encounter with a passing star. This hypothesis enjoyed some measure of popularity during the nineteenth century, in large part due to the inability of other theories to account for the observed properties of the solar system, but no scientist takes it seriously today. Aside from its extreme improbability, it is completely unable to explain the orbits, the rotations, or the composition of the planets and their moons.