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Forging the planets: chaos, collision, fire and ice - the sun and its retinue of planets had wild beginnings.

For more than two centuries, scientific ideas about how the planets came to be were based almost entirely upon theory. There were few constraints on such speculation: astronomers knew that stars rotated and that the Sun's planets are nested in almost-circular orbits. But beyond these meager knowns, those who pondered the solar system's origin had nowhere to turn except to their own visions of how a gigantic amount of cosmic matter, obeying the laws of physics as then understood, might have organized itself into a Sun and planetary system. Theorists continued to offer variations on the same answer: the Sun and planets were born from a rotating disk of cosmic gas and dust. The flattened form of the disk constrained the planets that formed from it to have orbits lying in the same plane, or nearly so, all moving in the same direction the disk had turned. This hypothetical disk, the solar nebula, is where any discussion of the origin of our solar system must begin.

The idea of a solar nebula was first formulated in 1755 by the Prussian philosopher and physicist Immanuel Kant. Although his treatment of the problem was only qualitative, its precepts were remarkably similar to those considered fundamental today. Kant pictured an early universe evenly filled with thin gas. He thought such a configuration would have been gravitationally unstable, so it must have drawn itself together into many large dense clumps of gas. Kant correctly attached a great deal of importance to rotation: he assumed these primordial clumps of gas were rotating, and as they shrank the rotation spun them out into flattened disks. One of these disks became our solar system.

Kant's validation was long in coming, as generations of telescopes proved unable to resolve a disk associated with another star in the heavens. The first indirect evidence for disks came from studies of T Tauri stars, which are similar in mass to the Sun but very young - roughly a million years old. In the 1980s astronomers realized that about a third of T Tauri stars have "infrared excesses," that is, the amount of infrared radiation they emit is too great to be consistent with their output at visible wavelengths. This can be understood if the stars in question are surrounded by halos of dust kept warm by short-wavelength radiation from the stars; the dust then reradiates the energy it receives at longer (infrared and radio) wavelengths. However, the strong infrared signatures implied the presence of enough dust, if distributed evenly in a sphere, to completely block our view of such a star at visible wavelengths. Only if the dust were arranged in a flattened disk, tilted somewhat to our line of sight, could we expect to see the star itself.

Our first direct views of these disks came a few years later, when astronomers used radio telescopes to see through the dark dusty clouds where stars are forming. Most recently the sharp eye of the Hubble Space Telescope (HST) has resolved several dozen disks at visible wavelengths in the Orion Nebula, a giant stellar nursery about 1,600 light-years away (facing page). They have been given the colorful name "proplyds" a contraction of the term protoplanetary disks. Some of these are visible as silhouettes against a background of hot, bright interstellar gas; others have been illuminated by nearby bright stars. The Orion disks are large compared with the solar system, and they contain more than enough gas and dust to provide the raw material for future planetary systems. The stars associated with these disks are very young, at most a few million years old.


Today we realize that Kant, by and large, got it right. Stars and their disks form in much the way he pictured, by the gravitational collapse of huge volumes of thinly dispersed interstellar gas and dust onto appropriate nuclei. Many theoretical studies have attempted to model stellar collapse and its aftermath, but a completely realistic simulation remains an elusive goal. The setting for star formation is a huge cloud, a particularly dense concentration of interstellar matter with roughly 10,000 gas molecules per cubic centimeter - vanishingly small compared to Earth's atmosphere, yet much denser than most of interstellar space. The cloud is dark, cold (10 to 50 Kelvin), turbulent, and threaded by magnetic fields. By chance, it contains concentrations (cloud cores) that become the favored sites for gravitational collapse. The Orion Nebula provides just such an environment.

The magnetic fields in cloud cores tend to resist collapse, but sometimes gravity prevails, drawing cloud material through the field and concentrating it near the center of a core. As this nucleus grows, its attractive force overwhelms magnetic resistance, and core material begins to pour onto the nucleus or protostar at free-fall velocity. Pressure and temperature mount in the growing object, and it begins to radiate energy from its surface.

Rotation adds a complication to the picture. The original cloud core that produces a given star inevitably has some small amount of angular momentum, perhaps inherited from the slowly roiling turbulence of its parent cloud. Initially it may rotate only once every few million years. But as the core's substance collapses to the dimension of a star, the conservation of angular momentum dictates that it spin faster and faster.

Most of the collapsing core material has too much angular momentum to fall directly onto the protostar. Instead it attempts to follow an orbit (much like the orbit of a comet) around the protostar. However, the flows of material falling in from the northern and southern hemispheres of the rotating cloud core collide at its equatorial plane. The vertical motions of both flows cancel out, and their material is added to a disk that orbits about the protostar. This disk, not the protostar, carries most of the angular momentum inherited from the cloud core. But for the protostar to grow, matter must flow it. Thus the disk's angular momentum must be redistributed outward, which allows the inner-disk material to join the protosun while outlying matter is spun to greater radii.

How disks accomplish this is not understood, but gravitational instability may again be responsible. Once the accumulating disk achieves a mass roughly one-third that of the protosun, it becomes gravitationally unstable and changes from an elegant, symmetric item of cosmic dinnerware to something less regular, perhaps resembling a miniature galaxy. The dense lumps in such an asymmetric structure exert tidal forces on one another, which have the effect of moving angular momentum outward. This would feed some disk material into the protostar, decreasing the disk-protostar mass ratio and perhaps restoring gravitational stability. It may be that star/disk systems grow by a series of excursions into instability of this sort.

Real stars display phenomena, some very dramatic, that correlate with the processes pictured by theorists. In the earliest stages of collapse, protostars are deeply embedded in their dusty parental clouds. Thus hidden, they can be observed only at radio and infrared wavelengths. Radio telescopes have detected powerful winds blowing away from such embedded objects in two opposing directions. These bipolar outflows are held responsible for turning back the inflow of dusty gas that feeds and conceals protostars. In time the outflows dominate, revealing the central masses as T Tauri stars. When seen in telescopes at visible wavelengths, such stars are girdled by disks (proplys) and are still ejecting intense bipolar outflows. It takes about a million years to reach this stage of stellar evolution.

A solar-mass star remains in the T Tauri stage for another 10 million years, more or less, before it attains core temperatures high enough to initiate hydrogen burning and evolves further into an ordinary main-sequence star. During this time the associated dusty disk becomes less and less evident. Some of its substance probably continues to migrate into the central star; much of the residual gas may be heated so greatly by the star's ultraviolet radiation that it evaporates to interstellar space. A very small fraction of the dust and gas that pass through the disk may collect into discrete lumps that remain in orbit around the star after the disk is gone. These serve as the seeds of its planetary family.


Before considering the formation of planets, let us take stock of the interstellar raw material from which the solar nebula emerged. The bulk of the matter in the solar system consists of atoms of hydrogen and helium created in the Big Bang some 12 to 16 billion years ago. Other chemical elements, which are less than one thousandth as abundant as hydrogen, were formed by nuclear reactions in the interiors of earlier generations of stars that existed between the time of the Big Bang and the origin of the solar system 4.55 billion years ago. These heavier, later-formed nuclides were released from their parent stars into the interstellar medium by stellar winds or stellar explosions.

Surprisingly little can be said about the physical state of these elements before and during the collapse process that formed the solar system. Extrapolating from our knowledge of the present interstellar medium, the most abundant element, hydrogen, existed chiefly as the diatomic gas molecule [H.sub.2] - thus justifying the often-used term "molecular cloud" Metallic elements (most notably magnesium, silicon, and iron, the principal ingredients of rocky planets) condensed into solids at the highest temperatures and are termed refractory for this reason. They combined with oxygen and other elements while still in interstellar space to form tiny grains roughly 0.1 micron across - only about 1,000 atoms wide. The physical state of the elements lying between H (and He) and Mg, Si, and Fe in atomic mass and abundance is very poorly known. In part they occurred as a variety of molecules in the gas phase, such as CO, [N.sub.2], N[H.sub.3], and free oxygen. In part they condensed into solid grains, as graphite (carbon, C) and silicon carbide (SIC), for example. They also formed coatings of complex organic compounds and mantles of frozen ices on more refractory grains, though the ices would have evaporated when the grains were warmed as they fell toward the protosun.

Most of the matter present in the original solar nebula is gone now. It was drawn into the Sun, expelled into interstellar space, or incorporated into planets whose internal activity has reprocessed it into some new form. However, some of the primordial nebular material has survived and thus provides a crucial key to learning the details of how our solar system formed. The most abundant reservoir of unchanged nebular matter is in the form of comets. Because they remained small and far from the Sun, effectively in deep freeze for eons, these icy planetesimals retain most or all of the properties they had when they accreted in the outer nebular disk.

We do not yet have direct access to comets for study, but some of their ingredients are in our laboratories. Comets warmed by the Sun's heat release dust particles into space. Some of these are swept up by Earth, along with other interplanetary dust particles, at which point they can be collected by research aircraft flying high in the stratosphere. Exactly which particles in these collections are cometary remains a puzzle. Viewed by electron microscope (as below), many particles consist of dusters of tiny grains of minerals, organic compounds, and nondescript amorphous materials. Notably, these component grains tend to have roughly the same dimension, 0.1 micron, attributed to interstellar grains. Some fraction of them probably are just that, having fallen into the nebula 4.55 billion years ago. They then became embedded in the snowflakes that joined growing icy planetesimals.

Samples of more refractory primitive material, from the inner solar nebula, are preserved in the form of meteorites known as chondrites. These are fragments of asteroids, bodies that were not large and geologically active enough to completely reprocess the primitive nebular material. Although all chondrites were affected to some degree by thermal or hydrous metamorphism in their parent asteroids, the least-altered ones contain bona fide interstellar grains. We conclude such an origin based on the grains' anomalous isotopic compositions, each of which records the particular nuclear reactions occurring in the unknown star that gave rise to it, long before our solar system formed. Collisions between asteroids release a shower of chondritic debris into space, some of which eventually reach Earth's surface as meteorites.

The solar nebula was hot near its center, tapering off to cold, then very cold, at its outermost margins. Of course, the environment near the infant Sun was warmed by its radiant energy. More important than this heat, however, were the Sun's mass and gravitational attraction. Close to the Sun, the nebula was its thickest and densest, and all the mechanical processes affecting the nebula - infall of molecular-cloud material, relative motions of nebular gas, turbulence, shocks - were stronger there and generated more heat than they did farther from the Sun.

Think of the nebula's falloff in temperature with heliocentric distance as defining three radial zones, like rings in a target. The innermost zone was too warm for water to condense as ice; objects forming there consisted entirely of silicate minerals and other refractory materials, ultimately becoming the terrestrial planets. The next zone was colder, water ice was stable, and a vast blizzard of snowflakes gave rise to the much larger Jovian planets. In the outermost and thus coldest zone, condensed matter was also icy. But it was too sparsely distributed to accrete into sizable planets; instead it remained dispersed in small icy planetesimals - comet nuclei - in what we now call the Kuiper Belt. Remarkably, the planets assembled themselves very quickly. Although the process differed in detail from zone to zone, virtually everything was in place within 10 million years, by which time the solar nebula had largely dissipated.


Terrestrial planets were not made by simply sweeping up the refractory interstellar dust grains that fell into the nebula. We know this from the makeup of chondritic meteorites, which are samples of terrestrial planetary material as it first accreted. Chondrites consist in large part of small igneous spheroids called chondrules (seen at lower left). These were once molten, which required temperatures in excess of 1,500[degrees] to 1,900[degrees] K. When the chondrules cooled and Solidified they must have been dispersed in space, not aggregated as we see them now, in order to have maintained their droplike shapes. Over a century ago the English microscopist Henry Clifton Sorby argued that chondrules must have formed as "detached glassy globules, like drops of a fiery rain." Sorby further surmised that chondrules were "residual cosmic matter, not collected into planets, formed when conditions now met with only near the surface of the Sun extended much further out from the centre of the solar system" - a remarkably prescient assessment.

Efforts to simulate chondrules in laboratory furnaces have shown that a sample must be cooled relatively rapidly, in roughly an hour, to reproduce the properties of meteoritic chondrules. This means the chondrules could not have been melted by the ambient temperature of the innermost nebula, because there is no way to cool matter so quickly in that setting. Instead the workings of the inner nebula must have involved pervasive, local, impulsive, high-energy events that drastically - but very briefly - affected its dispersed silicate material. We have little clue to the nature of these energetic pulses. Nor is it clear whether they occurred during the violent first million years of disk history, while interstellar material was still falling into the nebula, or in the 10 million years thereafter, when the nebula was thinner and more quiescent. Most researchers favor the latter period.

Severe thermal processing is capable of changing the chemical composition of planetary material, by selectively boiling off the more volatile chemical elements; these volatile elements are then free to recondence perhaps somewhere else. chemical fractionations of this sort must have occurred in the solar nebula; we see the evidence in individual chondrules, in their bulk aggregations (such as chondrites), and even in the planets themselves. For example, potassium, which is moderately volatile, exists in the Earth at only about one-fifth the abundance (relative to nonvolatile elements) that is present in average solar-system material.

Eventually, chondrules and dust began to stick together, creating larger and larger clods of chondritic material. This is a crucial moment in the history of planet formation, but like much else in the story it is very poorly understood. The gravitational attraction between such small objects is much too feeble to get the process of accretion started. Small particles that collided at less than about 1 meter per second might have stuck together because of van der Waals attraction (weak, short-range forces caused by the uneven distribution of electrostatic charge). However, clumps bonded by van der Waals forces alone would not be strong enough to survive mutual collisions in the nebula's turbulent zones. The onset of accretion would be easier to understand if some stronger "glue" were available.

There is no question that sticking did occur. Many chondrules in chondrites are coated with rims of dust particles gathered before they became grouped with other chondrules. Once the clusters exceeded a few centimeters across they became too heavy to be pushed around by turbulence in the gas, whereupon they settled and concentrated near the midplane of the nebula.

They also began spiraling in toward the Sun. An orbiting solid object, like a bit of chondrite, maintains a simple balance between centrifugal force (directed outward) and the Sun's gravity (inward). However, a parcel of gas in the nebula would have been pushed outward not only by centrifugal force but also by the gradient of gas pressure, which decreased outward in the disk. Consequently, gas required less centrifugal force and orbital velocity than solids did to remain in a given orbit, so any solid object in the same orbit traveled a little bit faster than the gas surrounding it. The resulting drag continually slowed the particle down, thus forcing it to spiral inward through the nebula. As chondritic clusters crept inward they swept up and accumulated dust, loose chondrules, and smaller aggregations. Once such an object grew to a dimension of a kilometer or so, the gas-drag effect became relatively small and the object's motion toward the Sun ceased. At about this size we dignify masses that were growing in the nebula with the name planetesimal.

Imagine being a passenger on the surface of one of these 1-km planetesimals. At the stage of evolution just described, the nebula teems with countless other planetesimals of similar size, but they are so widely dispersed that slight differences in their orbit, the planetesimal move relative to one another: a combination of up-and-down motion relative to the ecliptic plane, due to varying orbital inclinations, and in-plane motion with respect to one another due to orbital eccentricity. Because of these, every so often one of the other planetesimals zooms past us, then quickly disappears in the distance. During the closest brushes, each planetesimal gives a tiny gravitational tug to the other, which changes the orbits of both slightly. And if we stay aboard our planetesimal long enough (maybe 1,000 years), eventually it will hit something of comparable size. If the collision velocity is slow, the two objects will merge into a single, larger mass; if fast, they knock each other apart. Either way, our ride is over!

High-speed computers allow us to study how planetesimals grow in such a system, and how fast it happens. Early on, planetesimal growth probably does not occur uniformly; instead, something like "the rich get richer and the poor get poorer" occurs. The largest objects experience runaway growth at the expense of their smaller cousins, and in some 20,000 years hundreds of bodies roughly the size of the Moon have been produced. Computer simulations have shown that such a population of bodies, through mutual orbital perturbations, coalescences, and catastrophic collisions, will eventually evolve into a family of bodies similar to the terrestrial planets. These simulations are limited in many ways, which make them incapable of exactly reproducing Mercury, Venus, Earth, and Mars. But they do show that the terrestrial planets reached nearly their full size in about 10 million years, then continued to sweep up large planetesimals for another 100 million years.

The impacts of accreting planetesimals deposited huge amounts of kinetic energy in the planets being formed, enough to partly melt them. The earliest history of planetary surfaces was a chaos of solidifying crustal slabs, erupting lava, and giant explosions caused by the arrival of more planetesimals.


The border between zones 1 and 2, the asteroid belt, is an untidy place. There is no object large enough to be called a planet in the asteroid belt; the aggregate amount of mass there is tiny, less than that in Earth's Moon. Yet we presume there was a smooth distribution of mass in the solar nebula before the planets assembled, and the Titius-Bode "law" suggests that a planet should be in the belt's location (shown at lower left). So what happened?

Planetary scientists believe the early appearance of Jupiter, with its mighty gravitational field, disrupted the orderly accretional assembly of a planet in this region. It may also have stunted the growth of Mars. Perhaps planetesimals in this transition zone never grew any larger than the asteroids are now, or it may be that for a time it was occupied by objects as big as the Moon or larger. If the latter, Jupiter eliminated them in the same way it removes material from the belt today. Those objects whose orbital periods are a simple integral fraction of the period of Jupiter's orbit (like 1:2, 2:5, 1:3) suffer resonant perturbations by Jupiter. The process might be likened to using a child's swing: small pushes, applied at exactly the right moment during each cycle of the swing, make it go higher and higher. Jupiter's resonant perturbations cause the orbital eccentricity of an asteroidal object to vary chaotically. In time it may collide with another asteroid, dive into the inner solar system (often hitting a planet or the Sun itself), or soar out to Jupiter's orbit or beyond. Ultimately, something almost certainly happens to remove it from the asteroid belt.

This may seem an ineffective way of removing things from the asteroid belt, because only a small percentage of its objects have orbital periods close to being integral fractions of Jupiter's period. However, asteroid orbits change periodically because of the mutual gravitational interactions that result when they pass close to one another; sometimes one of these encounters redirects an object into a resonant relationship with Jupiter. At that point it is destined to leave the asteroid belt.


Farther out in the solar nebula, it was cold enough for its water to exist as ice. In this second zone, snowflakes were 10 times more abundant, by volume, than the silicate dust particles. This follows because oxygen - the main ingredient in water - is more abundant in the solar system than magnesium, silicon, and iron combined. Clearly the planetesimals that collected in the outer nebula, and ultimately the planets that formed from them, would have very different compositions from those of the terrestrial planets. However, the largest worlds, Jupiter and Saturn, do not have water as their main ingredient; instead they consist mostly of hydrogen and helium - a composition closer to the Sun's than to an icy planetesimal's. (Actually, in December 1995 the Galileo probe found the outermost Jupiter atmosphere contains much less oxygen than average solar-system matter, not more. But this superficial layer may not be representative of the planet as a whole.)

Jupiter and Saturn's compositions were not established by accreting snowflakes of pure hydrogen and helium, because temperatures in the solar nebula were not nearly cold enough to permit either of these gases to condense. Instead, these planets most likely gathered the bulk of their mass directly from the nebula, wholesale, without discriminating between solids and gases. Thus the compositions they have today are essentially that of the solar nebula and, in turn, that of the Sun.

There are two ways in which they might have gathered in nebular material. Large cores or nuclei of ice and dust may have accreted first, much as terrestrial planetesimals did. When they became massive enough, their gravity began to attract and hold the nebular gas. The more gas these icy nuclei collected the heavier they became, and the greater their attraction for even more gas. The other possibility is that the very early solar nebula was massive enough to go through periods of gravitational instability, as described earlier. One form this instability might have taken, especially in the outer disk, was the separation and pulling together of gaseous, self-gravitating protoplanets massive enough to resist being dispersed by later tidal forces. In this case, nuclei of solid material would not have been required to get the process started.

Any good model for formation of the giant planets must explain why they differ in composition with radial distance from the Sun (see above). Although hydrogen and helium dominate the compositions of Jupiter and Saturn, Uranus and Neptune consist mostly of the elements that form ices: oxygen, carbon, and nitrogen. In the icy-nuclei model, this trend could have occurred if the planetary nuclei grew so slowly out beyond Saturn that, by the time they were massive enough to attract gases, the nebula had largely dissipated. Another possibility is that Jupiter and Saturn started out as gaseous protoplanets, but Uranus and Neptune were formed by the accretion of icy planetesimals.

These various planet-forming processes are interdependent and must have taken place in a particular sequence. However, their exact timing is not well understood. The facing page (lower left) shows two possible sequences as they relate to the chronology of star and disk formation laid out earlier. Other scenarios are also possible.

According to the upper scenario, things happened - or began to happen - very early, while interstellar material was still actively falling into the nebular disk. This option is discounted by meteorite researchers, however, because the isotopic record in meteorites seems to argue against such an early beginning. Specifically, some inclusions present in chondrites once contained the short-lived radionuclide aluminum-26, whereas other inclusions in the same chondrites never did. If the latter inclusions did not form until the early solar system's 26Al had decayed to indetectability, and if chondrite accretion had to wait until these inclusions became available, the sequence would have been delayed for several million years.

The lower scenario adheres to the 26Al constraint, in that chondrites and terrestrial planets do not begin to form until well after interstellar collapse has ended and the Sun and protostellar disk are in place. Another constraint assumed in this latter case (one that cannot be proved with absolute certainty) is that the terrestrial planets were made of chondritic material and their formation followed chondrite formation.


Somehow, the process of accretion imparted rotation to the planets. Planetesimals that struck a growing world on the right side, as viewed from the direction of their approach, increased the planet's spin in a prograde sense. However, an approximately equal number of planetesimals might be expected to strike the planets' left sides too, such that the spins imparted by the two families of accreting bodies would tend to cancel one another. If the planets grew from a very large number of small bodies, the averaging and cancellation of their many contributions should have left the planets with similar, rather slow rotation rates, all in the same direction and all about axes that were nearly perpendicular to the ecliptic plane.

However, this is not what we observe today. As depicted at lower right, the planets spin at a wide variety of rates, two of them turn in a retrograde sense, and most of their rotation axes are tipped at substantial angles to the ecliptic perpendicular. This is consistent with accretion not from a large number of small bodies but from a small number of large bodies, some of which were quite large. In this case the sum of all the planetesimals' contributions would have been unequal, leaving residual spins and tilts (obliquities) like those that characterize our planetary system. The evidence argues for such hierarchical growth, with the accretion of relatively large planetesimals occurring in the final stage.

We realize that Jupiter and Saturn did not accrete from solid planetesimals but instead mostly gases gathered directly from the nebula. Both planets spin quite rapidly, yet the manner in which they acquired their angular momenta is not well understood. But they too were subject to having their spins modified by the late addition of large planetesimals. The tilt of Saturn's rotation axis probably requires an oblique collision, near one of its poles, from something more massive than all of the terrestrial planets put together.

The giant planets were hot when they accreted, just as the terrestrial planets were, because as nebular material fell onto them its kinetic energy was converted to heat. This heat expanded the atmospheres of the giant planets to vastly larger dimensions than they have today. Thereafter they radiated heat, cooled, and shrank. According to one popular model, as each planet shrank a disk of gas, ice, and dust was left in orbit around it - a small analogue of the solar nebula. From these disks emerged the regular satellites and ring systems, by means more or less analogous to the formation of planets and asteroids in the solar nebula. The irregular satellites, which have high eccentricities or inclinations (or both) relative to the equatorial plane of their primary planet, are thought to have been captured from the solar system at large. These include Phoebe, Triton, and many very small satellites, among them Mars's Phobos and Deimos.

Earth's Moon is another story. As best we can surmise, after our planet had grown to most of its present size, it was struck off-axis by a relatively large, fast-moving planetesimal. The energy of the impact heated Earth and the impactor to very high temperatures, and it spalled off a plume of molten rock and vapor. Some of this debris fell back onto Earth and some escaped to interplanetary space, but a portion of it settled into an incandescent disk orbiting the planet. We believe that such a disk would spread outward very quickly, and that once material was beyond the Roche limit, about 10,000 km above Earth's surface, gravitational instabilities would quickly gather it into a number of moonlets. (Inside the Roche limit individual objects cannot pull together because tidal forces induced by the central planet exert dispersing forces stronger than the objects' self-gravity.) Over a longer period of time, the hot moonlets coalesced into our Moon.


Where does the solar system end? The traditional answer has been at the orbit of Pluto (mean heliocentric distance: 39.5 astronomical units). However, we now know the Sun's family extends to much greater distances. It was long suspected that large planets failed to form beyond Neptune not because there were no icy planetesimals from which to accrete them, but only because the planetesimals were dispersed too thinly. Some 50 years ago Kenneth Edgeworth and Gerard Kuiper independently predicted that such a belt of remnant building blocks still exists beyond Pluto, and recent telescopic observations have confirmed the existence of these objects. Extrapolating from the sample observed, there must be more than 200 million objects, each a few kilometers or more across, orbiting in the inner edge of what has come to be called the Kuiper Belt. (Pluto itself, its satellite Charon, and the Neptunian moon Triton are probably the largest examples of Kuiper Belt objects.) The presence of all this solid matter extending the solar system's realm beyond Pluto is consistent with the large disk found in association with young solar-type stars.

Short-period comets are very likely Kuiper Belt planetesimals that have been perturbed into the inner solar system by mutual interactions or by (as yet undiscovered) Pluto-size objects. Long-period comets are thought to have had a different origin, from within zone 2. As they neared their final sizes, the giant planets gravitationally "stirred" the icy planetesimals around them into very eccentric orbits. (NASA refers to such an interaction with a planet as a "gravity assist" and employs the technique to pump up the orbital energy of its spacecraft.) Most of the redirected planetesimals were ejected from the solar system altogether, doomed to wander "forever" in interstellar space. However, a great many did not quite reach the solar ape velocity; these hangers-on, still feebly e Sun at vast distances from it, make up the whose representatives occasionally revisit the planetary system in the form of long-period comets.

So where does the solar system effectively end? The Oort Cloud, which is legitimately part of the solar system, probably tapers off to nothing roughly 1 light-year from the Sun. Only there does interstellar space truly begin.

Nearly four centuries of telescopic observation, combined with four decades of space exploration, have taught us this essential truth about the solar system: While the Sun and its planetary system surely arose from one grand spiral of gas and dust in a flurry of collective activity, the results are hardly a homogeneous set of characterless orbs. Instead, this grand scheme of formation has yielded amazing diversity. It is humbling to realize that still other totally different kinds of planets, beyond our imagining, must be circling stars elsewhere in the galaxy.


diameter 1,391,020 km(*) (109 Earth diameters) mass 1.989 x [10.sup.33] g (333,000 Earth masses) volume 1.412 x [10.sup.33] [cm.sup.3] (1.3 million Earths) density: center 151.3 g/[cm.sup.3] mean 1.409 g/[cm.sup.3]

rotation period 25.4 days (sidereal, at equator) escape velocity 617 km/second

temperature: center 15,557,000 [degrees] K photosphere 5,780 [degrees] K corona 2,000,000 [degrees] to 3,000,000 [degrees] K solar constant (at Earth) 1,368 watts/[meter.sup.2]

* All solar and planetary data in this article are from The New Solar System, fourth edition.


moons none

diameter 4,880 km mass 3.302 x [10.sup.26] g mean density 5.43 g/[cm.sup.3] rotation period 58.6462 days escape velocity 4.4 km/second

mean distance from Sun 0.3871 a.u.(*) obliquity 0.1 [degrees] orbital eccentricity 0.206 orbital inclination 7.00 [degrees]

* 1 a.u. is 149,597,870 km


moons none

diameter 12,104 km mass 4.865 x [10.sup.27] g mean density 5.20 g/[cm.sup.3] rotation period 243.0185 days escape velocity 10.4 km/second

mean distance from Sun 0.7233 a.u. obliquity 177.4 [degrees] orbital eccentricity 0.007 orbital inclination 3.39 [degrees]


moons the Moon 3,476 km in diameter

diameter 12,756 km mass 5.974 x [10.sup.27] g mean density 5.52 g/[cm.sup.3] rotation period 23.9345 hours escape velocity 11.2 km/second

mean distance from Sun 1.000 a.u. obliquity 23.45 [degrees] orbital eccentricity 0.017 orbital inclination 0.00 [degrees]


moons Phobos 26.2 x 18.6 km Deimos 15.6 x 10.2 km

diameter 6,792 km mass 6.419 x [10.sup.26] g mean density 3.91 g/[cm.sup.3] rotation period 24.6230 hours escape velocity 5.0 km/second

mean distance from Sun 1.5237 a.u. obliquity 25.19 [degrees] orbital eccentricity 0.093 orbital inclination 1.85 [degrees]


moons Pan 20 km Atlas 36 x 28 km Prometheus 148 x 68 km Pandora 110 x 62 km Epimetheus 138 x 106 km Janus 198 x 152 km Mimas 398 km Enceladus 498 km Tethys 1,058 km Telesto 30 x 16 km Calypso 30 x 16 km Dione 1,120 km Helene 32 km Rhea 1,528 km Titan 5,150 km Hyperion 370 x 226 km Iapetus 1,440 km Phoebe 230 x 210 km

diameter 120,536 km (at equator) mass 5.685 x [10.sup.29] g mean density 0.69 g/[cm.sup.3] rotation period 10.6562 hours escape velocity 35.5 km/second

mean distance from Sun 9.5549 a.u. obliquity 26.73 [degrees] orbital eccentricity 0.056 orbital inclination 2.49 [degrees]


moons Metis 60 x 34 km Adrastea 20 x 14 km Amalthea 250 x 128 km Thebe 116 x 84 km Io 3,642 km Europa 3,130 km Ganymede 5,268 km Callisto 4,806 km Leda 10 km Himalia 170 km Lysithea 24 km Elara 80 km Ananke 20 km Carme 30 km Pasiphae 36 km Sinope 28 km

diameter 142,984 km (at equator) mass 1.898 x [10.sup.30] g mean density 1.33 g/[cm.sup.3] rotation period 9.925 hours(*) escape velocity 59.5 km/second

mean distance from Sun 5.2026 a.u. obliquity 3.12 [degrees] orbital eccentricity 0.048 orbital inclination 1.30 [degrees]

* Outer-planet rotation periods are for their deep interiors


moons Cordelia 26 km Ophelia 32 km Bianca 44 km Cressida 66 km Desdemona 58 km Juliet 84 km Portia 110 km Rosalind 58 km Belinda 68 km Puck 154 km Miranda 480 x 466 km Ariel 1,159 km Umbriel 1,170 km Titania 1,580 km Oberon 1,520 km (Caliban) 60 km (Sycorax) 120 km

diameter 51,118 km (at equator) mass 8.683 x [10.sup.28] g mean density 1.318 g/[cm.sup.3] rotation period 17.240 hours escape velocity 21.3 km/second

distance from Sun 19.2184 a.u. obliquity 97.86 [degrees] orbital eccentricity 0.046 orbital inclination 0.77 [degrees]


moons Naiad 58 km Thalassa 80 km Despina 150 km Galatea 160 km Larissa 208 x 178 km Proteus 436 x 402 km Triton 2,706 km Nereid 340 km

diameter 49,552 km (at equator) mass 1.024 x [10.sup.29] g mean density 1.638 g/[cm.sup.3] rotation period 16.110 hours escape velocity 23.5 km/second

distance from Sun 30.1100 a.u. obliquity 29.56 [degrees] orbital eccentricity 0.009 orbital inclination 1.77 [degrees]


moon Charon 1,250 km(*)

diameter 2,300 km(*) mass 1.32 x [10.sup.25] g(*) mean density 2.0 g/[cm.sup.3] rotation period 6.3872 days escape velocity 1.1 km/second

distance from Sun 39.5447 a.u. obliquity 119.6 [degrees] orbital eccentricity 0.249 orbital inclination 17.14 [degrees]

* Diameters, masses, and densities for Pluto and Charon are uncertain

JOHN A. WOOD studies meteorites, and the larger question of the origin of the planets, at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts. This article has been adapted from The New Solar System, fourth edition.
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Author:Wood, John A.
Publication:Sky & Telescope
Article Type:Cover Story
Date:Jan 1, 1999
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