Astronomy Supplement 11.

Evolution of the Solar System


Latest Modification: October 26, 1998

Table of Contents


Now that we have acquainted ourselves with the nature of the Solar System--with the exception of the Sun--it seems a reasonable step to inquire into what astronomers think that they understand as to the origin of the Solar System and the reason for the diversity of its members. In the preceding chapters we have touched at several points on the origin and evolution of the Solar System. It is now time to bring these scattered ideas together into a coherent theory of that momentous event which occurred some 4.6 billion years ago.


11.1. Architecture of the Solar System

As to the question of how the Solar System began, there are roughly two categories of theories that have been proposed to explain that event. One such category of theories is those theories that invoke an accidental catastrophic event, such as the near collision between the Sun and a star, while the other category of theories is those theories that involve a natural, non-catastrophic event, such as might occur in conjunction with the birth of star. Chapter 17 covers the story of the birth of stars, so here we need only say that we know that stars are born, live out their lives, and die--just as the things of Earth are not eternal. A historical approach to the explanations of planetary genesis is a good beginning, but first we should summarize the major characteristics of the Solar System for which any theory of origin should account.

A sequence of natural forces evidently created and shaped the Solar System somewhat along the lines revealed by the following clues, which suggest that the design most likely possessed a continuity in its processes and did not materialize through a sequence of unrelated, random events:

The planets are isolated from each other without bunching, and they are placed at orderly intervals (Table 5.2).

The planets' orbits are nearly circular, except for those of Mercury and Pluto.

Their orbits are nearly in the same plane; Mercury and Pluto are again exceptions.

All the planets and asteroids revolve around the Sun in the same direction that the Sun rotates (from west to east).

Except for Venus, Uranus, and Pluto, the planets also rotate around their axes from west to east.

A planet's system of satellites can be divided into either a regular system with direct orbits approximately in the planet's equatorial plane or an irregular system with irregular orbits inclined at various angles.

The Terrestrial planets have high mean densities and relatively thin or no atmospheres, rotate slowly, and possess few or no satellites--points that are undoubtedly related to their smallness and closeness to the Sun.

The giant planets have low mean densities, relatively thick atmospheres, and many satellites, and they rotate rapidly--all related to their great mass and distance from the Sun.

Studies of chemical composition suggest that the small, dense Terrestrial planets are rocky bodies that are poor in hydrogen; the large, low-density Jovian planets are fluidlike bodies that are rich in hydrogen; and most of the outer planets' satellites, comets, and Pluto are icy bodies.

11.2. Nebular Hypothesis

The German philosopher Immanuel Kant speculated in the middle of the eighteenth century that the Solar System had been formed out of a huge rotating gaseous nebula slowly contracting and condensing. A nebula is a large cloud of gas, and possibly dust particles, held together by the mutual gravitational attraction of the particles composing it. Such nebula as we see them elsewhere in our Galaxy are immensely larger than the Solar System. Pierre Laplace (1749-1827), a celebrated French mathematical astronomer, expanded the idea in 1796, and it became known as the nebular hypothesis.

Laplace theorized that as the large, slowly rotating solar nebula of hot gaseous matter contracted, it rotated faster and faster, flattening into an equatorial ring. The physical principles involved here are the action of gravity and the conservation of angular momentum, which requires a spinning body to rotate faster as it shrinks. The angular momentum of a rotating body, a measure of its quantity of rotation, remains constant unless energy is taken out of rotation and put into some other form. If the radius of the body decreases, the rotational velocity must increase to compensate for the reduced radius; this is what we observe when we see a spinning ice skater rotate faster as he or she brings his or her outstretched arms closer to their body--his or her angular momentum is thus conserved.

Laplace supposed that when the centrifugal force acting on the outer rotating edge of the solar nebula exceeded the inward gravitational force of the nebular mass, a ring of gaseous matter was split off, eventually coalescing into a planet, as shown in Figure 11.1. The splitting process repeated itself, making concentric rings that formed into planets, whereas the central portion condensed to become the Sun.

[Figure 11.1]

The theory has two major defects. First, whereas 99 percent of the Solar System mass resides in the Sun, 99 percent of the angular momentum of the system resides in the planets' orbital and rotational motions. We might guess intuitively that the distribution of angular momentum in the forming Solar System ought to roughly match the distribution of mass; the central mass could not have transferred this much momentum to the planets. Second, a hot gaseous ring of the type postulated would disperse into space and not pull itself together gravitationally to form a planet.

11.3. Encounter Theories

At the beginning of this century, attempts to reconcile the nebular hypothesis with physical principles were temporarily abandoned. A different approach, the so-called encounter theory--which had been conceived in 1745 by the French naturalist Georges Buffon (1707-1788) when he proposed that material ripped off from the Sun by collision with a comet had condensed into the planets--was taken by the American geologist Thomas Chamberlin (1843-1928) and the American astronomer Forest Moulton (1872-1852). They suggested that giant eruptions were pulled off the Sun by the gravitational attraction of a passing star.

Somewhat later another geologist-astronomer pair in England, Harold Jeffreys (b. 1891) and James Jeans (1877-1946), theorized that a cigar-shaped gaseous filament was pulled from the Sun by the sideswiping action of a passing star, as in Figure 11.2. The middle section condensed into the Jovian planets, and the ends condensed into the smaller planets.

[Figure 11.2]

The encounter theory accounts for the common direction of the planets' orbital motion and the Sun's rotation as well as for the planets' nearly circular and coplanar orbits. In either version, however, this theory has serious failings in that solar matter, whether pulled or ejected, could not have acquired sufficient angular momentum nor could hot gas have condensed into planets. Besides, the probability of a near encounter in our region of the Galaxy is vanishingly small, less than one in many millions.

11.4. Protoplanet Theory, the Solar Nebula

By mid-century, astronomers once more turned their attention to possible improvements in the nebular hypothesis. A new factor was introduced in the form of the existence in the cool gaseous nebula of a small amount of dust, providing nuclei for the condensation of gas particles into larger aggregates that could accrete and solidify into the embryo planets. (The existence of dust particles in the interstellar gas clouds out of which stars are formed was accepted in the 1930s.) This modern version of the nebular hypothesis is called the protoplanet hypothesis, and it owes much of its recent revival to the power and scope of computer analysis. It was first formulated independently by Carl von Weizsacker (b. 1912) and by Gerard Kuiper (1905-1973) in 1945 and then extended and modified over the years by others.

The hypothesis begins with a fragment separating from an interstellar cloud composed mainly of hydrogen and helium, with trace amounts of the other elements. With other fragments of the interstellar cloud presumably following a similar evolution, its central region, being somewhat more dense, collapsed more rapidly than its outlying parts. This formed the central portion of the solar nebula, whose outer portion contained a thin disk of solids within a thicker disk of gases. The original interstellar cloud must have been rotating, and as it fragmented, rotation was imparted to each fragment. Thus as the solar nebula contracted, it rotated more rapidly, conserving angular momentum.

The solar nebula grew by accretion as material continued to fall inward from its surroundings (Figure 11.3). Large-scale turbulence from gravitational instabilities ruptured the thin disk into eddies, each containing many small particles. These particles gradually built up into larger bodies by some combination of adhesive forces. Repeated encounters among them resulted in the accretion of literally billions of still larger asteroid-sized aggregates called planetesimals, which orbited the center of the solar nebula. Mutual gravitational attraction led to further encounters and gradual coalescence into many roughly Moon-size bodies, which in turn coalesced to form the planets.

[Figure 11.3]

Planetesimals must have differed in chemical composition, depending primarily on their initial distance from the Sun as it formed. That is, as the central portion of the solar nebula contracted, the temperature rose to around 2000 K, hot enough to vaporize all compounds in the dust except the "high-temperature" metallic and silicate minerals in the inner portion of the disk, while the outer disk remained relatively cool. Planets that formed close to the young Sun, such as the Terrestrial planets, would be expected to contain less of the volatile icy and gaseous materials and thus be richer in the rocky materials, as sketched in Figure 11.4.

[Figure 11.4]

11.5. Formation of the Planets

11.5.1. Terrestrial Planets' Formation

During and following the formation of the Terrestrial planets, there was a catastrophic bombardment by the remaining rocky planetesimals that cratered the surfaces of these planets. The impacting material, coupled with intense radioactivity and subsequent gravitational concentration, produced sufficient heat to melt and chemically differentiate the planets into their presently layered structure (core, mantle, and crust). The atmospheres of the Terrestrial planets were formed during this process and afterward by outgassing from impacting material and from the hot interiors of the planets.

In the asteroid belt between Mars and Jupiter, the temperature of the solar nebula was lower so that carbon- and water-rich minerals could coalesce in the forming planetesimals. From about Jupiter outward, temperatures were even lower, so that huge amounts of frozen water could accumulate with the rocky material in the planetesimals. At still colder temperatures, other ices would have formed, such as ammonia and methane, giving those distant planetesimals a mixed composition of water, ammonia, and methane ice impregnated with a small amount of rocky matter.

11.5.2. Formation of the Earth-Moon System

Within a relatively short time after contraction of the solar nebula began the young Earth had collected most of the matter that composes it today. Matter attracted by the growing Earth collided with it, giving up its kinetic energy as heat. This energy, along with the energy resulting from Earth's gravitational contraction and emissions by radioactive nuclei, heated Earth's interior.

In a few tens of millions of years, the Earth became molten; chemical differentiation followed. The heaviest elements, iron in particular, separated from the lighter elements, such as oxygen and silicon (primarily in the form of silicates and oxides of iron and magnesium) and sank toward the center. The silicates and oxides rose to form the mantle surrounding an iron-rich core. The lightest materials rose to the top and solidified as the crust.

About 4.0 to 4.5 billion years ago, Earth as a whole was cooling even though volcanic activity on the surface was intense. We believe that during this period an atmosphere of whose composition we are not certain was formed, probably from the gases carbon dioxide, carbon monoxide, nitrogen, water vapor, and possibly some hydrogen sulfide and hydrogen. As Earth cooled, these gases escaped from the interior during volcanic activity and water condensed, forming the oceans. But what of the Moon during this period?

Even with all our new information from the Apollo program, the Moon's origin, like that of the Solar System itself, is still shrouded in mystery. That is not to say that having astronauts trudging over the surface of the Moon was of no benefit. Quite the contrary, the information derived from the astronaut landings on the lunar surface were of inestimable value. However, we now know that missing from the Apollo samples are those oldest of rocks, pristine remnants form our satellite's formation 4.6 billion years ago, that conceivably could settle the question of the Moon's origin forever. As it stands now, no proposal for a beginning for the Earth-Moon system is without some objections.

Prior to the Apollo program, there were three concepts that dominated lunar-origin theories. The earliest concept, the fission theory (Figure 11.5), was that Earth was spinning rapidly and flattened to a dumbbell shape perhaps because of movement in the Earth's molten core. The smaller end of the dumbbell tore away from the primitive Earth to become the Moon, separating ever more from the Earth because of tidal forces. The major objection to this theory is that the primitive Earth could not have spun rapidly enough to promote fission through rotational instability. A second objection is that, compared with Earth rocks, lunar rocks have slightly greater proportions of those elements that are difficult to vaporize and slightly less of the easily-vaporized elements. This suggests that the Moon formed from material somewhat hotter than that from which the Earth formed.

[Figure 11.5]

The second concept is that the Earth-Moon system formed by accretion from chemically related primordial planetesimals condensing out of the gas and dust of the solar nebula; this is the condensation or co-accretion theory (Figure 11.5). The essence of this idea is the Earth and Moon are actually a double planet system with colliding debris trapped in orbit around the growing Earth accreting to form the Moon. A fact which supports this theory is that the two bodies are of comparable ages (4.6 billion years). Also, since chemical analysis of lunar rock samples shows some chemical disparities between the Earth and the Moon, the Earth and the Moon may or may not have evolved from the same parent material. At any rate, if it was the same parent material for both bodies, then some aspect of the formation process permitted the chemical disparities to arise.

Finally, a third concept is that a proto-Moon originally was moving in a highly eccentric orbit around the Sun. As it approached Earth almost on a collision course, it was disrupted by strong tidal forces, and most or some portion of the fragmented body became Earth's satellite--the capture theory. The problem with this idea is that capture is not easy to accomplish. Although it apparently can happen, some means must occur to remove some of the kinetic energy of the captured body so that it moves from a solar to an Earth orbit. Just passing by a more massive body does not automatically lead to capture.

In the post-Apollo era, the Apollo samples impose strict restrictions on the three classical lunar-origin theories such that all three are found wanting. It is unfortunate that the Apollo samples in themselves do not provide the answer. However, such is not the case and these samples can only serve to provide boundaries within which any seriously considered theory is constrained to lie. A post-Apollo theory, which is called a collisional ejection theory, is to assume that a Mars-sized planet struck the primitive Earth in part coalescing with Earth and in part ejecting a cloud of material to orbit Earth (Figure 11.6). This cloud of hot material eventually cools over many centuries and forms the Moon. The chief advantages of this theory is its ability to account for the near similarities, but distinct differences, in the chemical composition of the Earth and Moon. Its disadvantages are mostly dynamical problems in that the lunar orbital plane is not coincident with the equatorial plane of Earth. Thus the collisional ejection theory, like its classical predecessors, is unable to provide all the answers to the long-studied mystery of the origin of the Earth-Moon system.

11.5.3. Jovian Planets' Formation

Within the outer, cooler regions of the solar nebula, the icy planetesimals collided, building larger bodies of ice and rock. As these bodies grew to a mass a few times that of the Earth, then they drew in more hydrogen and helium from the surrounding interplanetary gas. Naturally, capture and retention of gas were easier far from the Sun, where the temperature was lower. Because of their great masses, they have kept very nearly the same relative proportion of hydrogen and helium to the heavier elements as the Sun and the interstellar medium have. This is the most likely mode of formation for Jupiter and Saturn and why they are hydrogen-rich bodies. Uranus and Neptune were simply never massive enough to accrete hydrogen and helium to the extent that Jupiter and Saturn did. Thus carbon, nitrogen, oxygen, silicon, and iron dominate their compositions. The comets are probably a fossil relic of the primordial icy planetesimals that existed in the outermost regions of the solar nebula.

In the regular satellites of Jupiter and Saturn we probably have a miniature version of planet formation. Contraction by Jupiter and Saturn at the time of their formation released a great deal of gravitational potential energy, heating them significantly. Jupiter was some 10 times brighter than at present, so that the contracting planet raised the temperature of matter close to it, accounting for why the two inner Galilean satellites are rocky bodies and the two distant ones are primarily icy bodies. This parallels the decline in mean density with distance from the Sun found in the planets themselves.

11.6. Formation of the Sun

When the protoplanets were all formed, the solar nebula's central bulge rapidly collapsed into the protosun. Continued contraction raised its internal temperature from a few tens of thousands of degrees to several million degrees when the first stages of nuclear burning were initiated. (The nuclear burning processes will be discussed in Chapter 17.) In the last stages of formation, the Sun may have had a much more intense solar wind, which presumably blew away much of the primordial gas and dust left over from the original interstellar cloud. But this point is still pretty much of a mystery.

A weakness in the protoplanet hypothesis is that it does not provide a completely satisfactory explanation for the observed distribution of angular momentum in the Solar System. If the angular momentum of the planets could somehow be returned to the Sun, its present slow rotation (like that of stars similar to it) of 2 km/s would be increased to about 100 km/s. The primitive Sun apparently transferred most of its angular momentum to the planets as they were forming.

To explain this transfer of angular momentum, astronomers have proposed a braking action caused by magnetohydrodynamic forces on the Sun as its magnetic field interacted with the ionized nebular gas in the disk. The magnetic lines of force spiraling outward from the rotating Sun into the surrounding nebula would act as a magnetic drag on the spinning Sun and serve as conduits, transferring angular momentum to the planetary disk.

There is a recent discovery of a relatively high abundance of some rare--by Earth standards--isotopes in primitive meteorites. It has been proposed that the isotopic anomalies are due to the injection of matter from a supernova explosion into the Solar System a few million years before the meteorites solidified. (A supernova is the explosion of a star in the last stages of its life.) Possibly the concussion from the explosion triggered the collapse of the interstellar cloud to form the solar nebula.

11.7. Evidence for Nebulae About Other Stars

Regardless of the means of starting the formation process, planetary systems are believed to grow naturally from physical events that develop after an interstellar cloud has begun to contract. In just the last few years astronomers, have discovered large, cool, dust envelopes around infrared stars in the interstellar clouds of the Milky Way. Some of these objects may be in the early stages of nebular condensation visualized in the protoplanet theory.

However, one of the most exciting events in recent years is the discovery by the IRAS satellite of disks of small, solid particles surrounding: Vega, in Lyrae, Fomalhaut, in Piscis Austrinus, Epsilon Eridani, in Eridanus, and Beta Pictoris (Figure 11.6), in Pictor and about eight other Sunlike stars. The IRAS findings have been supplemented by ground-based discoveries of circumstellar disks around three other stars. The disk surrounding Vega has a radius of some 80 to 90 AU, or about twice Pluto's distance from the Sun (Table 11.1). The sizes of the other circumstellar disks is comparable to that for Vega.

[Table 11.1]

[Figure 11.6]

Although observing circumstellar shells and disks is neither the same as finding evidence for planetlike bodies, nor does it mean that planets will eventually form around these stars, it does lend support to the contention that stellar nebulae, similar to the solar nebula in the protoplanet hypothesis, form around other stars in our Galaxy as a natural part of the processes responsible for the birth of stars. Hence other Solar Systems probably exist somewhere out in the distant reaches of space.

This completes our discussion of the Solar System and from this point we want to branch out to a study of the stars. Of all the stars we know most about the Sun. However, our knowledge of it is still quite incomplete. We know of the existence of stars from the radiation they send out that reaches across the vastness of space to link our lives with theirs. Therefore let us begin in the next chapter with a study of the radiation emitted by the Sun and stars.


Copyright 1995 J. C. Evans
Physics & Astronomy Department, George Mason University
Maintained by J. C. Evans; jevans@gmu.edu