The planetary group in which we live is a varied lot. The Solar System includes 1 star, 8 planets, >150 moons, tens of thousands of asteroids ranging in diameter from ~1 m to 300 km, eight asteroids and >100 Kuiper-belt objects (including Pluto) larger than 300 km, countless comets of kilometer dimensions, and myriad meteoroids <1m across. That list will only grow with future exploration of our home territory in space.

With the Earth-Sun distance of ~150 million km termed an astronomical unit (or A.U., for short), our planetary system extends end to end for nearly 80 A.U. That may sound large, but it’s only about a thousandth (0.001) of a light-year, hardly more than a billionth the size of our Milky Way. Planetary systems are much smaller than the distances separating stars—at least out in the galactic suburbs where we live—making each such system a celestial island unto itself.

The four innermost planets, Mercury, Venus, Earth, and Mars, are often termed Terrestrial Planets because of their physical and chemical similarity to rocky Earth. In contrast, the larger, outer bodies, Jupiter, Saturn, Uranus, and Neptune, are called Jovian Planets because of their resemblance to gassy Jupiter. Between these two groups, in a broad band, or thick “belt,” ~2-3 A.U. from the Sun (roughly between Mars and Jupiter), roam the stony asteroids, sometimes labeled “minor planets” or even “planetoids” for they are actually not starlike at all. Pluto, for decades classified as the outermost planet, doesn't fit well into any of these catagories. Well smaller than Earth's Moon, the literal odd ball Pluto was recently removed from official major planet status; rather, it is now considered one of the most prominent members of the Kuiper belt of trans-Neptunian icy-rock objects some 50 A.U. from the Sun. Figure 4.1 presents the size and scale of the principal members of our Solar System.

FIGURE 4.1 FIGURE 4.1 — This "family portrait" of the Solar System shows the Sun and its planets to scale. Although the Jovian Planets are much larger than the Terrestrial Planets, all are dwarfed by the central Sun. (Prentice Hall)

From a remote vantage point far beyond our home planetary system, the Sun overwhelmingly dominates our cosmic neighborhood, with Jupiter an inferior second. Our star has >1000 times Jupiter’s mass and ~700 times that of the whole rest of the Solar System including Jupiter. The Sun, then, houses >99% of all the matter in the Solar System. Everything else, especially the small Terrestrial Planets and notably Earth, resembles a collection of nearly insignificant debris.

Draw a distinction in Jupiter’s case, however, for this is no ordinary heavenly body. Jupiter in fact just missed becoming a star. The composition and structure of this giant planet—and possibly all the big Jovian Planets—is largely stellar. They are rich in hydrogen and helium, light gases that have long ago escaped from the smaller Terrestrial Planets. But none of the Jovians is quite big enough to ignite—to start a thermonuclear reaction at its core by virtue of its own overlying mass. Had Jupiter gathered several tens of times more matter, its central temperature would equal that needed to commence nuclear fusion, converting it into a dwarf star. Thus, our Solar System almost formed as a binary-star system, an astronomical posture that would have rendered Earth life improbable, perhaps impossible.

We owe a debt of gratitude to the Sun for lighting up, and to Jupiter for not.

Older Models Hitherto and growing complications of a perceived clockwork Solar System—especially the retrograde motions of some of the planets, such as Mars—were greatly simplified in Renaissance times. Looking (observation) and thinking (theory) combined to build more objective mental models than those deduced by the ancients; testing became a vital part of the process of inquiry. The 16th-century Polish cleric Nicholas Copernicus recognized that a heliocentric (Sun-centered) model improved the harmony of the tangled geocentric (Earth-centered) models proposed by the Greeks and Romans of old.

Despite the support of empirical data and a mathematical underpinning by two 17th-century scholars, the German Johannes Kepler and the Englander Isaac Newton, the Copernican model wasn’t easy to accept even as recently as a few hundred years ago. Heliocentricity rubbed against the grain of all previous logic and it violated many religious teachings of the time. Above all, it relegated Earth to a noncentral and undistinguished location within the Solar System and the Universe. Earth became just one of many planets.

Although we now realize that these Renaissance workers were correct, none of them was able to prove to his contemporaries that our system is centered on the Sun, or even that the Earth moves. Unambiguous proof of the latter came only in the mid-19th century when the German astronomer Friedrich Bessel first observed stellar parallax—the yearly to-and-fro artificial motion of a nearby star caused by Earth’s real motion around the Sun. Heliocentricity of the Solar System has been verified repeatedly over the years with an ever-increasing number of experimental tests, culminating with the recent expeditions of our robot space probes that have toured through an obviously Sun-centered planetary system.

Initial motivation for the heliocentric model was simplicity, at least in mind’s eye. Heliocentricity provides a more natural explanation of the observed facts than can any geocentric model. Even today, scientists are often guided by simplicity, symmetry, and beauty in modeling all aspects of the Universe. Those models in science having a measure of elegance are often closer to reality; those that are complicated are usually wrong.

Development and eventual acceptance of the heliocentric model is an awesome milestone in our thinking as human beings. Discovering the framework of our planetary system freed us from an Earth-centered view of the Universe, and enabled us to realize that ours is only one of many planets orbiting the Sun. Surprisingly, it was less than a century ago that the American astronomer Harlow Shapley took the next bold step, in turn proving that, as a resident of the suburbs of the Milky Way, neither was our Sun centralized, unique, or special in any way. The more we look and the more we test, the more mediocre our niche in the Universe seems to be.

Modeling Requirements Any model capable of explaining the origin and architecture of our planetary system must adhere to the known facts. Generally, these facts derive from studies of interstellar clouds, landed meteorites, and Earth’s Moon, as well as from observations of numerous planets both within and beyond our Solar System. The meteorites provide especially useful information, for they contain entrapped traces of solid and gaseous matter uneroded from the early Solar System. Radioactively determined dates of all meteorites uniformly imply that our system formed, with the Sun and Earth as part of it, ~4.5 billion years ago. Laboratory analyses of the oldest lunar rocks generally confirm this date, as does theoretical modeling of the Sun itself.

Among the many observed properties of our Solar System, 7 stand out most boldly:

1. Each planet is relatively isolated in space, none of them being bunched together; most planets reside roughly twice as far from the Sun as its next inward neighbor, implying a certain geometric harmony—the kind of order and elegance alluded to earlier.

2. The orbits of the planets describe nearly perfect circles, with only one exception; Mercury’s noticeable elliptical orbit is surely caused by this innermost planet’s proximity to the neighboring (and tidally wrenching) Sun.

3. The orbits of the planets all lie in nearly the same plane, Earth’s such plane being called the “ecliptic”; each of the planes swept out by the planets’ orbits aligns with the others to within a few arc degrees (excepting again Mercury), the whole system of planets having the shape of a rather flat disk.

4. The direction in which the planets orbit the Sun is the same in which the Sun rotates on its axis (counterclockwise from terrestrial north); virtually all the angular momentum in the Solar System—the planets’ orbits and the Sun’s spin—seems systematized, again implying a high degree of unison.

5. The direction in which most planets rotate on their axes also mimics that of the Sun’s spin (again counterclockwise); the two exceptions are Venus, which spins oppositely (retrograde), and Uranus, whose poles are tipped over so as to lie in the plane of its own orbit.

6. Most of the known moons revolve about their parent planets in the same direction as the planets rotate on their axes; some moons, like those associated with Jupiter, resemble miniature Solar Systems, revolving about their parent planet in roughly the same plane as the planet’s equator, and once more evincing unison throughout our planetary system.

7. The Solar System is highly differentiated; the inner, Terrestrial Planets are characterized by small sizes, rocky makeup, high densities, moderate atmospheres, slow rotations, and few or no moons and rings, whereas the outer, Jovian Planets have large sizes, gaseous makeup, low densities, thick atmospheres, rapid rotations, and many moons and rings.

All these observed properties, when taken together, clearly denote a high degree of order within our Solar System. Although much diversity prevails among individual planets and moons, the whole ensemble is apparently not a random assortment of objects spinning and orbiting this way or that. It hardly seems possible that the Solar System is a pickup team, amassed by the slow accumulation of already-fashioned interstellar bodies casually captured by our Sun over the course of billions of years. The overall architecture of our Solar System is too neat and tidy, and the ages of its members too uniform, to be the result of chaotic events or haphazard circumstances. All signs point toward a single formation, the product of an ancient but one-time event not quite 5 billion years ago.

A comprehensive account of all these properties has been a principal goal of astronomers for well more than a century. The Solar System is, after all, our extended home in space and it would be nice to know, specifically and in detail, how it all came to be.

Nebular Model Though not all these planetary properties were known hundreds of years ago, the crux of the modern theory of our Solar System’s origin dates back at least that far. Called the nebular model, the original idea is often attributed to the German philosopher Immanuel Kant, but he merely elaborated upon an earlier proposal made in the 17th century by the French philosopher Rene Descartes. In this conceptual model, a giant, swirling gas cloud gradually contracts to form a central Sun, and the planets and their moons are assumed to be natural by-products of the star formation process. But these philosophers failed to work out the mathematical details of their models; their proposals amounted to little more than qualitative words and untested ideas.

Later in the 18th century, the French mathematician-astronomer Pierre-Simon de Laplace tried to give this type of model a quantitative basis. Using angular momentum arguments, he showed mathematically that gaseous bodies spin faster as they contract. A decrease in the size of a rotating mass must be balanced by an increase in its rotational speed, much like a pirouetting figure skater who spins faster while closely retracting her arms, or a high diver who somersaults quickly by tightly curling his body. As suggested by Figure 4.2, an interstellar cloud would eventually flatten into a pancake-shaped disk, for the simple reason that gravity can pull matter toward the center of the region more easily along its rotation axis than perpendicular to it—which is why a spinning body tends to develop a bulge around its middle. This model provides a plausible origin of some of the ordered architecture observed in our Solar System today—the planets’ near-circular orbits, their residence in a well-defined disk, and many of the other properties just listed. These properties are among the natural results of simple changes expected in any galactic cloud, a straightforward obedience of a parcel of gas to the known laws of physics.

FIGURE 4.2 FIGURE 4.2 — Conservation of angular momentum demands that initially (a) a contracting, rotating cloud gradually speeds up its rate of rotation. Eventually (b), the primitive Solar System came to resemble a giant spinning pancake. Ultimately (c), the planets that formed inherited its rotation and flattened shape. (Prentice Hall)

Continued contraction of such a primitive Solar System forces the entire cloud to spin more rapidly as time proceeds. Near the fringe, the outward centrifugal push eventually exceeds the inward gravitational pull. This push creates a thin ring of gaseous matter that breaks away from the rest of the system, which in turn contracts a little more until such time as another ring of matter is deposited inward of the first. Progressing in this way, an entire series of rings were imagined by Laplace to form around the central protosun. Each ring is furthermore theorized to condense, over long intervals of time, into a planet. Several outer planets might develop quickly while the interior of the early Solar System continued to shape the inner planets and the Sun.

Figure 4.3 is an artist's illustration of such a scenario. At various distances from the protosun, the effect of rotation overwhelms gravity, implying that rings are created. Each ring then clumps into a protoplanet—a forerunner of a geniune planet. By this scheme, several outer planets might develop while the interior of the primitive Solar System still contracted to shape the inner planets and the central Sun.

FIGURE 4.3 FIGURE 4.3 — The nebular theory envisions the formation of rings (a) of gaseous matter at various distances from the central protosun. Eventually (b), the rings might clump into planets. (Prentice Hall)

As sensible as this nebular model seems, it’s not without difficulties. Detailed analyses show that material in a ring of this sort would not likely assemble into a planet. In fact, computer simulations predict just the opposite. The rings would tend to disperse, owing to both a wealth of heat and insufficient mass within any one ring. Gravitational clumping of interstellar matter is one thing—it works reasonably well when making stars because vast amounts of mass are often housed within a typical, cold galactic cloud. But coagulation of a warm, protoplanetary ring is quite another scheme—not nearly enough matter is present for gravity to best the heat and thereby gather the gas into a planet-sized ball. Instead of coalescing to form a planet, computations predict the ring will break up and drift away.

Don’t be too hard on Laplace. He didn’t have a computer and it’s both tricky and tedious to account for all the statistical aspects of this problem without one. Even today, experts disagree on some of the details in the best computer models, as noted below. That said, this description of the changes experienced by a shrinking interstellar fragment does follow directly from the obedience of a parcel of galactic gas to the known laws of physics. Laplace did get the essential description correct. It’s just that, in modern times, as we learn how to program computers to simulate multitudes of particles and to account for subtleties in this kind of modeling, astronomers have found some fatal flaws among the details.

A second problem further complicates the nebular model for the origin of the Solar System. It’s well known that the Sun spins on its axis once in ~30 days, a good deal more slowly than Earth. This solar sluggishness baffles astronomers for one simple reason: Although the Sun contains >1000 times the mass of all the planets combined, it boasts <2% of the system’s angular momentum. Jupiter, for instance, has a lot more momentum than our Sun. Not merely that it spins on its axis so fast (<10 hours once round), rather that an object with Jupiter’s sizable mass so distant from the Sun carries a great deal of orbital momentum. In fact, Jupiter presently harbors more than half of the Solar System’s total momentum. All told, the four big Jovian Planets account for ~98% of the momentum of our Solar System. By comparison, the lighter Terrestrial Planets have negligible momentum.

The puzzle here is that the nebular model predicts the Sun should command most of the Solar System’s angular momentum. It should be spinning much faster. After all, since the Sun has most of the system’s mass, why shouldn’t it also have most of its momentum? This is especially true since contracted objects are expected to increase their spin rate, again in the manner of the figure skater’s pirouette. Expressed another way: If all the planets, with their large amounts of orbital momentum, were hypothetically deposited inside the Sun, it would spin ~100 times faster than at present. Instead of rotating about once a month, the Sun would spin around once every several hours.

Catastrophic Models These and other problems with the nebular model forced researchers, for a while at least, to consider alternative ideas—ones that are less evolutionary and more catastrophic. One such idea is embodied in the so-called collision model, which does indeed invoke a more violent process. Here, the planets are imagined as end products of hot, streaming debris torn from the Sun during a close encounter with another star. The flaming streamers induced by such a near collision, shown in artist’s conception in Figure 4.4, are surmised to remain gravitationally bound to the Sun, to be captured into orbits about it, and eventually to assemble into planets. Despite the phenomenal tides surely accompanying the near collision of two stars, the predicted aftermath agrees with the common orientation of the planets’ orbits and the Sun’s spin, as well as perhaps the close planar alignment of all the planets in a disk.

FIGURE 4.4 FIGURE 4.4 — A close encounter between two stars would surely cause flaming matter to be torn from each star. According to the collision theory, some of these streamers eventually formed planets. (Prentice Hall)

Although first proposed during the 18th century, the collision model enjoyed its greatest popularity ~100 years ago when astronomers not only began realizing that the nebular model was impractical, but also began discovering a few minor exceptions to the overall harmony of our planetary system—a retrograde moon of Neptune, a sizable tilt of Uranus, among other irregularities. The absolute beauty and ordered architecture of the Solar System diminished somewhat, giving a boost to chancy models that invoke accidental or unlikely celestial events.

However, few astronomers take the collision model seriously today. Though it has some points in its favor, models that depend on stellar collisions also have their pitfalls—some of them seemingly fatal. The high improbability of a near collision between two stars is the foremost problem. Stars are large by terrestrial standards, but minute compared to the distances separating them, as noted in the GALACTIC EPOCH. For example, the Sun is ~106 km in diameter, whereas the distance to Alpha Centauri, the nearest star system, is >1013 km. Probability studies predict that, given the number of stars, their sizes, and their typical separations, not more than a handful of such near collisions ought to have occurred throughout the entire expanse and history of the Milky Way Galaxy (at least outside of the heavily congested galactic central regions). Although galaxy collisions are frequent and clearly seen at many locations on the sky, stellar collisions must be extremely rare; in fact, none has ever been reported in the history of astronomy.

The improbability of stellar collisions doesn’t, of course, disprove the idea. Our Solar System could conceivably be the foremost—even the only—example of such an extraordinarily uncommon phenomenon. Should this idea be correct, we can justifiably conclude that our planetary system is an extremely rare type of astronomical system. Very few stars would be expected to have planets and the prospects for extraterrestrial life would greatly decrease. However, as noted later in this PLANETARY EPOCH, recent discoveries of numerous planets orbiting many nearby stars virtually rule out the collision model. Too many extrasolar planets—those beyond our Solar System—are now being found for all of them to be the result of close encounters among stars.

As if the small likelihood of collision were not enough to badly wound this idea, several other problems plague the notion of planetary origins via encounters of any kind. First, the momentum puzzle besetting the nebular model is again troubling. Second and more formidable, it’s hard to understand how hot matter torn from the Sun could contract; hot gases usually disperse. Consequently, although such a near collision between two stars might happen occasionally, it’s unlikely that the resulting hot fragments would form planets. Some of the hot streamers would surely fall back into the Sun. Others would tend to dissipate even more quickly than the merely warm matter in the purported rings of the nebular model. A third quandary concerns the nearly circular orbits traced by each of the observed planets. If matter were tidally ripped from the Sun to form the planets, why should each of the clumps of debris end up orbiting the Sun in a near-perfect circle? The collision model cannot explain this observed fact even qualitatively.

Condensation Model The model of Solar System formation most embraced by astronomers today is termed the condensation model. Really a sophisticated version of the nebular concept explained earlier, this model mixes all the attractive features of the old nebular model with our recently revised assessment of interstellar chemistry—or “astrochemistry,” that rich and vibrant interdisciplinary area of frontier research noted in the previous STELLAR EPOCH. Theorists can now concoct a modern condensation model that alleviates several of the aforementioned theoretical problems. And what’s more, the new models generally agree with the wealth of observational data now being acquired with today’s telescopes and spacecraft.

Recall that the first problem with the nebular model is its inability to assemble ringed material into a tight-knit ball of protoplanetary matter. Each ring would have likely had too little mass and too much heat to gravitationally contract. However, a new twist has been added only within the past decade or two. We have come to realize the ubiquity and importance of dust in interstellar space. Dust grains—solid microscopic bodies of rock and ice having sizes of ~10-5 cm—are liberally strewn throughout the Galaxy, doubtless the ejected debris from long-dead stars.

Much of our knowledge of interstellar dust comes from meteorite fragments and captured radiation. Ironically, the dust grains within fallen meteorites provides only indirect information even though we can touch those rocks, whereas that information is more direct when analyzing infrared radiation emitted by the dust particles themselves that are far out of reach. The reason is that in rocks the dust is embedded and contaminated, yet in space it’s more pristine. Some exceptions are submillimeter-sized, fluffy dust particles collected by high-flying (U2) aircraft in the stratosphere, but these are probably biased samples of chemically altered dust near the Earth and not representative of native “stardust” formed in the outer atmospheres of ancient red-giant stars or in the debris fields propelled into space by supernovae.

Most space dust comprises rock (rich in silicon and iron) and ice (mostly dirty water), but a good deal of it also includes carbon, especially a class of organic compounds known by the tongue-twisting name of polycyclic aromatic hydrocarbons (or PAHs, for short), similar to the large benzene-ringed molecules found in cigarette smoke and automobile exhaust. Not surprisingly, then, some grains are made of the widespread interstellar molecules noted in the previous STELLAR EPOCH. Ubiquitous in space, the dust particles are sized midway between atoms and planets, indeed they reside on the evolutionary path whereby atoms make planets. What’s more, given its organic nature, the dust might also have been the source material for the origin of life on planet Earth, a topic best addressed in the next CHEMICAL EPOCH.

Such miniature dust grains play an important role in the evolution of any gas. At issue here is the way that thermodynamics works in the presence of gravity. Dust helps to cool warm matter by efficiently radiating away its heat in the form of infrared radiation, thereby reducing the outward pressure of the heat and allowing the inward contraction caused by gravity to proceed more easily. This escaped radiation is detectable with infrared telescopes, granting information about both the emitting dust and the infalling cloud.

The condensation model, then, assumes that dust was peppered throughout the warm gas of the primitive Solar System, helping to cool it by releasing heat from its protoplanetary blobs. Furthermore, dust grains accelerate the clustering of atoms within the gas, acting as miniature condensation kernels (hence the name of this model) around which other atoms can aggregate, in turn forming larger and larger balls of matter. (This resembles the way that raindrops form in Earth’s atmosphere, when dust and soot in the air act as condensation foci around which water molecules cluster.) In short, the presence of dust often guarantees that gravity wins—at least usually, and at least gradually—in the ceaseless tug-of-war between the pressure of heat pushing out and the onslaught of gravity pulling in.

For the particular case of our home in space, by postulating the existence of a dusty interstellar cloud ~5 billion years ago, theorists reason that dust-grain cooling must have occurred before the gas had a chance to drift away. Accordingly, modern observations of sooty interstellar matter suggest, but do not prove, the likelihood of assembly rather than dispersal of protoplanetary matter. Alas, nagging problems still do remain, yet ones that are actively being tackled observationally, not just theoretically.

Observational Evidence Astronomers are fairly confident that the solar nebula formed such a dusty disk long ago because similar disks of loose gas and dust have recently been observed around young stars not too far away. Foremost among them is the naked-eye star Beta Pictoris, a very young object ~60 light-years distant. When the light from this star itself is suppressed (with a suitable instrument that blocks receipt of most of its light), a faint disk of warm matter is apparent—especially in the infrared part of the spectrum where dust radiates most strongly. Although this particular disk is ~10 times the diameter of the Kuiper belt, modeling implies that a star like Beta Pictoris, perhaps as young as 20 million years old, is only now undergoing its earliest, somewhat turgid, evolutionary phase akin to that probably experienced by our own Sun ~5 billion years ago.

Figure 4.5 shows an actual image and an artistic rendering of Beta Pictoris. Note how its warm matter seems spread out in a disk, much of it composed of myriad dust particles mostly millimeter in size and probably partaking of the first stage of planetary formation. The image certainly resembles more or less the early stages of our own Solar System, but there’s currently no way to ascertain whether the particles are primordial matter that could develop into planets or are merely bits of interplanetary matter dispersing outward. Even so, that these particles might soon materialize into a genuine planetary system well beyond our own is an exciting prospect indeed.

FIGURE 4.5 — (a) This computer-enhanced photograph of a disk of warm matter surrounding the star Beta Pictoris was taken from a mountaintop observatory in Chile. Most of the star itself (at center) is blocked by a coronograph, an instrument designed to detect faint halos around bright objects. The full extent of the disk, which is seen edge-on from our perspective, measures ~1000 A.U., or ~30 times the diameter of Neptune's orbit. (b) An artist's conception of the disk of clumped matter depicts what our own Solar System probably looked like in its formative stages ~4.5 billion years ago. (JPL; Dana Berry)

The archetypical star-forming region, namely, the Orion Nebula noted in the STELLAR EPOCH, also provides dramatic confirmation of the above ideas. Direct imagery, such as that in Figure 4.6, shows >100 newborn stars throughout the area, each hardly a million years old and enveloped by disks of gas and dust seen in silhouette against the nebula’s bright background. No planets are seen in the disks, nor are any expected in such juvenile regions. At least several million more years, and perhaps tens of millions more, are needed for genuine planets to emerge from the hodgepodge of gas and dust. This formative sequence has already occurred for the 4 well-known Trapezium stars now illuminating the nebula, although at a distance of ~1500 light-years any planets orbiting them are impossible to discern by any current observational technique—and in any case such planets would be bathed in ultraviolet radiation lethal by human standards.

FIGURE 4.6 FIGURE 4.6 – This high-resolution image of the Orion Nebula reveals, upon close inspection, many young stars surrounded by disks of gas and dust where planets might ultimately form. The whole nebula is a few light-years across. (STScI)

Not all protoplanetary disks give rise to planets. Many of these disks get blown away by young stars that develop fierce winds shortly after nuclear ignition. This is especially likely for congested clusters such as Orion, where many massive stars energize the nebula that probably houses thousands of low-mass, Sun-like stars. The process might well resemble a kind of “survival of the fittest” among disks: Those able to withstand the onslaught of ionizing radiation and the battering by loose gas and dust will at least have a chance to form planets from leftover matter. For many “wannabe” planets, it may be a race against time and a battle against blistering radiation. Those disks that manage to coalesce rocky ice balls quickly in potentially hostile surroundings might give birth to planets, and those that don’t surely won’t. Nature selects some protoplanets to become planets and chokes off the others by destroying their raw materials. As with star formation noted in the STELLAR EPOCH, planet formation is a hazardous process with an unpredictable outcome—a little like life in a changing, challenging environment.

Closer to home, circumstellar disks are now evident at many places in our sector of the Milky Way, wherever the radiation of young stars warms surrounding dust not yet swept away. They include the famous bright stars Vega and Fomalhaut only 20 and 25 light-years distant, respectively, and Epsilon Eridani even closer at only 10 light-years. These and other young stars all emit tell-tale signatures of infrared emission from surrounding disks, some of which are warped perhaps owing to perturbations by unseen giant planets. Such planetary nurseries (namely, the disks) are easier to spot than the toddlers (the planets themselves) for the sole reason that the dusty disks are a few trillion times larger than the suspected planets. During the 1990s, astronomers discovered—and mapped in some detail—dozens of disk-shaped regions of emitting dust around adolescent stars. It is in these regions, many of which span Solar-System dimensions, that the growth of planets is thought to be now underway. In fact, some of the disks seem to show hollow centers, comparable in size to our own planetary system and perhaps carved out by newborn (yet unseen) planets. These new and exciting findings lend support to the specifics of the condensation model now being fine-tuned by planetologists—those pioneering researchers who explore nascent planetary systems beyond the Sun, all the while striving to understand the origin and evolution of our own Solar System.

Supernova Trigger? To trace the formative stages of a planetary system such as ours, the modern condensation model stipulates the following broad scenario, starting with a large, dusty interstellar cloud. The original cloud itself might have extended for 10-100 light-years, but the smaller fragment that would become our home probably spanned no more than ~1 light-year across. Intermingled with the usual plenitude of hydrogen and helium atoms, the cloud was surely sprinkled with some heavy-element gas and dust ejected from many prior supernovae. Gravitational instabilities started the parent fragment contracting, routinely in fact down to a size of ~300 astronomical units—a little less than the size of Beta Pictoris today. All the while, it rotated faster and flattened yet more, after which dense protoplanetary eddies emerged of their own accord.

The initial instability that triggered the infall of our ancestral cloud could have been caused by many events. Perhaps a collision with another interstellar cloud, or maybe the passage of a galactic spiral arm; either type of event is suspected to happen relatively frequently—by cosmic standards—roughly once every 10 million years. However, the view now favored by the astronomical community is that a nearby supernova was probably the culprit. Old, uneroded meteorites found on the Antarctic ice sheets contain overabundances of certain elements (especially the residues of some mildly radioactive metals, such as iron and aluminum), implying that the genesis of our Solar System might have begun with the concussion of a nearby supernova ~5 billion years ago. Dating of the meteoritic grains support this idea, implying that the supernova blazed forth less than a few million years before the meteorites condensed into solid rock. Apparently the ejected debris from the supernova didn’t have time to become completely mixed with the primordial matter of our parent galactic cloud before our planetary system formed, the result being microscopic inclusions embedded within today’s captured meteorites.

To be just a bit more technical, heavy elements in the primitive Solar System were accumulated from matter ejected by many supernovae. By the time the Solar System began taking shape, the various elements and isotopes should have been well mixed, homogeneously peppering the hydrogen-helium gas with supernova debris. Normal elements would be expected to differ from place to place in the Solar System, an is known to be the case when comparing, for example, the composition of the Sun, Earth, and some of the other planets. But the isotopes of a given element should have their same relative abundances whether part of a planet, a moon, a meteorite, or the Sun. In fact, however, this isn’t the case; the relative isotope abundances are not uniform throughout our Solar System. Compared to other matter on Earth, meteorites that have fallen to Earth often contain in their embedded dust grains certain isotopes (of carbon, nitrogen, oxygen, magnesium, neon, and xenon) that are anomalously abundant—presumeably contaminants from a supernova. Since no known chemical process can change one kind of isotope into another, astronomers surmise that some of the meteorite grains must have had a different origin from most of the matter in meteorites as well as in the rest of the Solar System. Such meteorites are among the oldest, unevolved material substances known in our Solar System.

Such supernova explosions inevitably create shock, or blast, waves that compress matter, as noted in the third, STELLAR EPOCH. In the case of our parent interstellar cloud, those shocks would have piled up matter into dense sheets, much like snow swept by the blade of a plow. Calculations show that the shocks race around the thinner exterior of the cloud more rapidly than they penetrate its thicker interior. Such sudden pressures wouldn’t just compress the cloud from only one direction; they would squeeze it all around. Nuclear weapons tests conducted at the Pacific’s Bikini Atoll during the Cold War experimentally demonstrated this squeezing. Shock waves created in the bomb blast literally surrounded buildings, causing them to be blown together (imploded), rather then apart (exploded). In like manner, shock waves can cause the initial compression of an interstellar cloud, after which natural gravitational instabilities divide it into fragments that eventually form stars and planets. Ironically, the demise of old stars may be the trigger needed to conceive not only new stars, but whole new worlds as well.

Once the shock wave passed, turbulent, whirling eddies arose naturally in the loose gas and dust at many locations throughout the primitive, rotating solar nebula, the bulk of which by this time would have flattened into a Frisbee-shaped disk. As in the earlier cases of galaxy and star formation, and as depicted in Figure 4.7, these eddies were nothing more than density fluctuations that came and went at random. It’s partly an issue of statistics, probability, and chance, but it’s also the laws of physics at work—a mixture again of randomness and determinism. The phenomenon is akin to the eddies forming in the wake of a spoon stirring a coffee cup or of a hurricane gathering strength while cruising the Atlantic. Provided an eddy could sweep up enough matter while orbiting the protosun, including a rich enough mixture of dust to cool it, then gravity alone would virtually ensure the formation of a planet.

The process of planet-building can be likened to a snowball thrown through a fierce winter storm; the ball grows fatter while encountering more snowflakes in its path. In this way, individual planetesimals the size of moons grew by accretion—the gradual accumulation of small objects by ongoing collision and sticking—and they in turn became protoplanets at various distances from the protosun. The smaller planets eventually and preferentially formed in the inner disk where the amount of matter was less. The larger planets, in the outer disk where more matter naturally settled, likely formed more quickly, as their greater gravity aided and abetted their growth—the rich got richer in the early Solar System. As in any disk, 75% of its area resides in its outer half, and even if the disk density gradually decreased with distance from the protosun, the ancient disk housed most of its matter well outboard of the young Sun.

FIGURE 4.7 FIGURE 4.7 — This sequence of artistic renderings from top to bottom shows eddies forming and evolving in a flattened, spinning, dusty cloud. According to the condensation model, some of those eddies eventually became planets while the one big one in the middle became the Sun. (Prentice Hall)

The natural satellites, or moons, of the planets presumably formed in similar fashion but on smaller scales, as mini-eddies of gas and dust condensed in the vicinity of their parent planets. Fragmentation, collisions, and accretion would have aided the growth of miniature solar systems in the gravitational fields of at least the big Jovian Planets. Surely, the larger moons formed in this way; the smaller ones may have been chipped off their parent planets during collisions with asteroids; still other small moons may be captured asteroids themselves. Admittedly, the details are lost to times long past, forever irreproducible in our computer simulations, if only owing to the (limited) role played by chance.

Assuming the “sweeping” process of accretion was reasonably efficient throughout the primitive disk, we can appreciate how our present Solar System came to exist as a collection of rather tiny, well-separated planets wheeling around a huge sunny sphere in an otherwise empty region of space. Mathematical modeling and meteorite analyses imply that the bulk of the formation process probably took <10 million years to evolve 8 protoplanetary eddies, scores of protomoons, as well as the big protosolar eddy in their midst. Within ~30 million years, the whole region had come to resemble a dirty version of our present Solar System. Nearly a billion more years would have been needed to sweep the system reasonably clear of interplanetary trash.

Those bodies that didn’t eventually collide with a planet or moon ended up as rocky asteroids in inner belts around the Sun, or as icy comets normally resident far from the Sun. Whether a rare and spectacular Halley’s comet or minute debris that shower the Earth like the Perseids or Leonids each year, these are all vestiges of an erstwhile formative stage. Comets and meteors, then, ought to serve as reminders of birth and construction, not (as in historical lore) as omens of death and destruction.

Uncertainties The weakest link in the condensation model is, once again, the anomalously small momentum of our present Sun. Every quantitative analysis of the young Solar System stipulates the Sun to have spun much faster than at present. Somehow, it must have slowed its rotation dramatically, but no consensus has yet been reached on how it actually managed to do so. Friction alone in the early, congested protosolar disk surely would have been a factor, but unlikely enough to slow this big solar ball appreciably.

Some researchers speculate that the solar wind, discovered only in the 1960s by some of the first robotic satellites, could have helped slow the Sun’s spin ever so gradually over the course of 5 billion years. High-velocity elementary particles (mostly protons and some nuclei) constantly escaping the Sun through flares and other surface storms could conceivably have acted as accruing microscopic brakes to diminish its early rotation. That’s because the particles are charged and tied to the Sun’s extended magnetism, all of which acts as a drag on any spinning star. The Sun’s terrific mass loss rate of roughly a million tons of matter per second could have indeed robbed it of much of its initial spin, as each and every solar particle must carry with it a minute amount of the Sun’s momentum over the course of billions of years. Manned and unmanned space vehicles are now trying to measure the intensity of current solar activity, though it’s controversial estimating the level of that activity billions of years ago.

Other researchers prefer to solve the Sun’s momentum problem by postulating a primitive Solar System much more massive than the present-day system. They argue that the accretion process wasn’t overly successful during the system’s formative stages, especially in its inner parts where the smaller planets never became massive enough to capture lightweight gases. Matter not captured by the Sun or the planets may well have transported some momentum while escaping back toward interstellar space. The matter that was then lost, or nearly so, might now be in the so-called Oort Cloud, a vast reservoir of comets theorized decades ago by a Dutch astronomer but never observed to date. This idea is tough to test because the escaped matter would be currently far beyond the range of today’s spaceprobes, only a handful of which—the Pioneer and Voyager missions—have now passed the orbit of Neptune. What specifically lies at those outer realms of the Solar System—or beyond even that, at ~1000 times the distance to Neptune where interstellar space begins—is frankly unknown.

In building a viable model of our planetary origins, it’s imperative to touch base periodically with reality—to use the traditional, and sometimes sobering, actuality of genuine data. In recent years, astronomers have managed to acquire increasing evidence that all young stars apparently do experience a highly active evolutionary stage known as the T-Tauri phase. (The 20th, or “Tth,” star in the constellation Taurus is the premier example.) It’s at this stage, when the stars have only recently fired up their nuclear fusion, that their brightness is especially great, their winds extremely intense. Nebular particles in the form of opposing jets travel outward along rotational axes, as noted in the previous STELLAR EPOCH, carrying with them much mass and momentum from their spinning source. Such bipolar jets are now observed for many T-Tauri stars, none of which is more than a few million years old.

Although we have no direct information about our embryonic Sun itself, observations of strong stellar winds emanating from young stars elsewhere suggest that much of the nebular gas left over among the planetesimals of our system could have been swept away into interstellar space—and with it some of the system’s “missing” angular momentum. The youthful Sun likely had narrow, fast-moving jets that reached light-years across, beaming upward out of the plane of the Solar System and parallel to the Sun's spin axis. What turned them on is a contentious issue, but what turned them off is simply unknown, other than perhaps the infalling matter simply ran out. In any case, the adolescent Sun’s solar wind, energetic flaring, and radiation pressure would have aggressively, and not so gently, blown away much of the loose nebular disk within a few million years of its formation, maybe even before hydrogen fusion commenced—all of which could have greatly slowed the spin of the massive ball at the center of the disk.

Despite some lingering controversy as to how to solve this momentum quandary, nearly all astrophysicists agree that some version of the condensation model is correct. The details, however, not yet worked out and still under debate, form the essence of a most challenging problem now being addressed at the frontiers of science at several leading observatories around the world. To repeat: The big picture of our Solar System’s origin is in place. At issue are the specifics; we want to know, as best we can, exactly how our home in space did materialize.

Differentiation Diversity of physical conditions in the earliest years of the Solar System is probably responsible for the large contrast in content and structure between the Terrestrial and the Jovian Planets. This is where the adjective “condensation”—as in the condensation model—takes on its true meaning. Again we return to consider thermodynamics operating in a gravitational field.

As the primitive Solar System contracted under the influence of gravity, it heated, spun up, and flattened. Well before even the initial protoplanetary eddies began taking shape, the rising warmth broke apart the interstellar dust gains into simple molecules, and they in turn split into simpler atoms. Since the density, and hence the particle collision rate, were surely greater close to the protosun, matter there would have become hotter than in the outlying portions of the youthful system. A temperature gradient naturally developed—one that surely caused the original dust in the inner regions to incinerate, but not necessarily so in the outer regions where the grains would have probably remained largely intact. While the gas temperature was >1000 K near where the Earth was about to form ~1 A.U. from the contracting core, it would have been well below the freezing point of water ~10 A.U. away, out where Saturn now resides. Figure 4.8 plots this temperature gradient across the primitive solar nebula, prior to the formation of any planets.


FIGURE 4.8 — Theoretically computed variation of temperature across the primitive Solar System, much like that depicted in Figure 4.7(b). It is this thermal gradient in the presence of a gravitational field that caused the early Solar System to differentiate into rocky, metallic inner planets and icy, gaseous outer planets.

Such a gaseous region cannot continue to heat indefinitely, lest the whole blob explode, or at least dissipate away. Like any hot gas, the primitive Solar System must have released some of its newly gained energy. So, even as the central protosun continued heating upon contraction, the outer regions of the primordial system cooled. As a result, heavy elements several A.U. from the protosun began reversing their fate by crystallizing from their hotter gas phase to their cooler solid phase. (Again, the same process occurs today on Earth, though on a much smaller scale, as raindrops, snowflakes, and hailstones condense from moist, cooling air.)

With a further passage of time, the temperature decreased at all locations, except at the very core where the Sun, not yet a genuine star, was still forming. Everywhere beyond the protosun, atoms slowed while returning to their low-energy states, after which some of them collided and stuck to form molecules, which in turn clustered to form dust grains once more. This is the accretion process described above, but here one that would have operated more selectively, creating a compositional gradient in the early Solar System.

We might think it amusing that, although plenty of interstellar dust grains uniformly peppered the area early on, Nature saw fit to destroy them only to rebuild them again later. However, a critical change had occurred in the meantime. Initially, the interstellar gas was evenly sprinkled with an array of all sorts of dust grains. When the dust later reformed, the mixture was much different, for the chemical condensation of solid dust from hot gas depends on the temperature. The act of contractive heating had served to sterilize much of the region, thus setting the stage for a Solar System highly diversified in planetary composition.

Note that these are chemical changes in the lower-temperature solar nebula, not nuclear changes that were about to occur in the higher-temperature core of the newly forming Sun. Yet, they were changes nonetheless--change is ubiquitous in Nature.

In the outer, colder regions of the nascent planetary system, beyond several A.U. from the Sun, where temperatures would have been <500 K, reasonably abundant heavy elements such as carbon, nitrogen, and oxygen combined with the most abundant element, hydrogen, to form some well-known simple chemicals, including ice crystals of water (H2O), ammonia (NH3), and methane (CH4)—to be sure, the primary constituents of the Jovian atmospheres seen today. (Helium is an inert element and doesn’t combine chemically with other atoms.) The ancestral fragments destined to become the Jovian Planets were fashioned under rather cold conditions by gravitational instabilities much like those noted earlier for the formation of galaxies and stars.

Accretion was no doubt at work way out there as well. Microscopic icy grains orbiting throughout the outer nebular disk gradually collided and stuck together, fabricating increasingly larger aggregates of ice in much the same way that fluffy snowflakes can be compressed into snowballs. Together with leftover hydrogen and helium atoms trapped by the strong gravitational pull of these huge protoplanets, gassy and icy compounds are now known to comprise the bulk of the Jovian Planets. Had we been there to see it, the emergence of these massive planets probably resembled the formation of our Sun, in miniature. None of them is quite massive enough, though, to kindle nuclear fusion, the hallmark of any star.

By contrast, in the inner, warmer regions of the young Solar System, the average temperature would have been ~1200 K at the time when condensation from gas to solid began. The environment there was simply too hot for ices to survive. Instead, many of the abundant heavier elements such as silicon, iron, magnesium, and aluminum would have combined with oxygen in order to make iron oxides, crusty silicates, and a variety of other rocky minerals. Planetesimals in the inner system were therefore predominantly rocky in nature, as were the protoplanets and planets they ultimately formed.

The orbiting rocky grains gradually coalesced into objects of pebble size, boulder size, kilometer size, and larger—another bottom-up scenario. The bigger they grew, the quicker gravity helped them coalesce, sweeping more and more matter from the surrounding regions of the flattened nebular disk, and eventually fabricating planet-sized objects. That the Terrestrial Planets are smaller than their Jovian counterparts owes mostly to the relative lack of material in the inner disk; their difference in chemical composition owes mostly to the formative system’s temperature gradient. The very abundant light elements of hydrogen and helium, as well as many other gases that failed to condense into solids, would have surely escaped from the smaller protoplanetary objects. Their temperature was too high, and their gravity too low, to prevent light gases from escaping the inner planets. What little hydrogen and helium did manage to stick around was probably blown away by the wind and radiation of the newly ignited Sun. What remained, so say the theoretical models, were a few rocky planets, each relatively cool, hostile, and devoid of an atmosphere.

Thus emerged early Earth, third rock out from the Sun—a lump of mostly heavy elements orbiting in space, barren and alien, yet on which the most wondrous things would arise.

Why the myriad rocks of the asteroid belt between Mars and Jupiter failed to coalesce into a planet remains a mystery. Perhaps one did exist, after which it blew up, the puzzle then being for what reason. If a planet did once reside there, it must have been a small one; the total asteroid belt now contains only about a tenth the mass of the Moon or a thousandth the mass of Earth. More likely, these old, uneroded rocks (most <3 m across) never did manage to clump together to form a planet, given the incessant tug of Jupiter’s gravitational tides that caused the asteroids to collide destructively rather than assemble constructively. That destructive process is probably still underway today, preventing the development of any protoplanet in the belt. If so, then the asteroids are perhaps the sole surviving witnesses that must hold primal clues to the grand event that did occur here nearly 5 billion years ago.

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