15.2 The Condensation Theory

NEBULAR CONTRACTION

Figure 15.1 Nebular Contraction (a) Conservation of angular momentum demands that a contracting, rotating cloud (a) must spin faster as its size decreases. (b) Eventually, the primitive solar system came to resemble a giant pancake. The large blob at the center would ultimately become the Sun.
One of the earliest heliocentric models of solar system formation is termed the nebular theory, and may be traced back to the seventeenth-century French philosopher René Descartes. In this model, a large cloud of interstellar gas began to collapse under the influence of its own gravity. As it contracted, it became denser and hotter, eventually forming a star—the Sun—at its center. While all this was going on the outer, cooler, parts of the cloud formed a giant swirling region of matter, creating the planets and their moons as by-products of the star-formation process. This swirling mass destined to become our solar system is usually referred to as the solar nebula.

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 nebula’s 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. across—about 10 times the diameter of Pluto’s 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 artist’s conception of the disk.

Figure 15.2 Beta Pictoris (a) A computer-enhanced view of a disk of warm matter surrounding the star Beta Pictoris. Both images show actual data taken at visible wavelengths, but are presented here in false color to accentuate the details; the bottom image is a close-up of the inner regions of the disk, implying a warp in the disk possibly caused by the gravitational pull of unseen companions. In both images the overwhelmingly bright central star has been removed to let us see the much fainter disk surrounding it. The disk is nearly edge-on to our line of sight. It is made mostly of microscopic dust grains of ices and silicate particles (see Section 15.3) and is illuminated by the reflected light of the central star. For scale, the dimension of Pluto’s orbit (78 A.U.) has been drawn adjacent to the images. (b) An artist’s conception of the disk of clumped matter, showing the warm disk with a young star at the center and several comet-sized or larger bodies already forming at large radii. The colors are thought to be accurate—generally speaking, if you know the temperature and the density of the nebula, then you can derive its color. At the outer edges of the disk the temperature is low, and the color is a dull red. Progressing inward, the colors brighten and shift to a more yellowish tint as the temperature increases. Mottled dust is seen throughout—such protoplanetary regions are probably very dirty. (NASA; D. Berry)
The nebular theory is an example of an evolutionary theory, which describes the development of the solar system as a series of gradual and natural steps, understandable in terms of well-established physical principles. Evolutionary theories may be contrasted with catastrophic theories that invoke accidental or unlikely celestial events to interpret observations.* Scientists generally try not to invoke catastrophes to explain the universe. However, as we will see, there are instances where pure chance has played a critical role in determining the present state of the solar system.

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 protosun—a hot ball of gas well on its way to becoming the Sun. Each ring then clumped into a protoplanet—a 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 Laplace’s nebular picture.

Calculations show that rings of the sort envisaged in Laplace’s 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 theory’s 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.

Figure 15.3 Dark Cloud Interstellar gas and dark dust lanes mark this region of star formation. The dark cloud known as Barnard 86 (left) flanks a cluster of young blue stars called NGC 6520 (right). Barnard 86 may be part of a larger interstellar cloud that gave rise to these stars. (D. Malin/Anglo-Australian Telescope)
Dust grains play an important role in the evolution of any gas. Dust helps to cool warm matter by efficiently radiating its heat away in the form of infrared radiation, reducing the pressure (which is just proportional to the gas temperature) and allowing the gas to collapse more easily under the influence of gravity. Furthermore, the dust grains greatly speed up the process of collecting enough atoms to form a planet. They act as condensation nuclei—microscopic platforms to which other atoms can attach, forming larger and larger balls of matter. This is similar to the way that raindrops form in Earth’s atmosphere; dust and soot in the air act as condensation nuclei around which water molecules cluster.

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).

Figure 15.4 Solar System Formation The condensation theory of planet formation (not drawn to scale; Pluto is not shown in part e). (a) The solar nebula after it has contracted and flattened to form a spinning disk (Figure 15.1b). The large blob in the center will become the Sun. Smaller blobs in the outer regions may become jovian planets. (b) Dust grains act as condensation nuclei, forming clumps of matter that collide, stick together, and grow into moon-sized planetesimals. The composition of the grains depends on location within the nebula. (c) Strong winds from the still-forming Sun will soon expel the nebular gas. By this time, some large planetesimals in the outer solar system have already begun to accrete gas from the nebula. (d) With the gas ejected, planetesimals continue to collide and grow. The gas giant planets are already formed. (e) Over the course of a hundred million years or so, planetesimals are accreted or ejected, leaving a few large planets that travel in roughly circular orbits.

Figure 15.5 A Disk of Planetesimals? This far-infrared image of a star, HR4796A, more than 200 light-years from Earth is thought to show a circumstellar disk in which the process of planetesimal growth is underway. The star itself is at the position of the cross, and the disk, which is falsely colored to match the dust emission, measures about five times the diameter of our solar system. (NOAO)
According to the condensation theory, the planets formed in three stages. Early on, dust grains in the solar nebula formed condensation nuclei around which matter began to accumulate (Figure 15.4b). This vital step greatly hastened the critical process of forming the first small clumps of matter. Once these clumps formed, they grew rapidly by sticking to other clumps. (Imagine a snowball thrown through a fierce snowstorm, growing bigger as it encounters more snowflakes.) As the clumps grew larger, their surface areas increased and consequently the rate at which they swept up new material accelerated. They gradually grew into objects of pebble size, baseball size, basketball size, and larger. Figure 15.5 shows an infrared view of a relatively nearby star whose protostellar disk is believed to be in just this state.

Eventually, this process of accretion—the gradual growth of small objects by collision and sticking—created 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 planetesimals—objects 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 protoplanets—the 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.

Figure 15.6 Making the Inner Planets Accretion in the inner solar system: Initially, many moon-sized planetesimals orbited the Sun. Over the course of a hundred million years or so, they gradually collided and coalesced, forming a few large planets in roughly circular orbits.

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 (“Bode’s 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 system—the 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).

Figure 15.7 Jovian Condensation As an alternative to the growth of massive protoplanetary cores followed by accretion of nebular gas, it is possible that some or all of the giant planets formed directly via instabilities in the cool gas of the outer solar nebula. Part (a) shows the same instant as Figure 15.4(a). (b) Four gas giants have formed, circumventing the accretion process sketched in Figure 15.4. (c) The giant planets have taken their place in the outer solar system.
In the second scenario, the giant planets formed through instabilities in the cool outer regions of the solar nebula—not so far removed from Laplace’s basic idea—mimicking on small scales the collapse of the initial interstellar cloud. In this view, the jovian protoplanets formed directly, skipping the initial accretion stage and perhaps taking less than a thousand years to acquire much of their mass. These first protoplanets already had gravitational fields strong enough to scoop up more gas and dust from the solar nebula, allowing them to grow into the gas giants we see today. Figure 15.7 illustrates this alternative path to the formation of the jovian worlds.

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 some—but certainly not all—planetary 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 Jupiter’s 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.


Concept Check

Why was interstellar dust so important to the formation of our solar system?