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The Formation of the Solar System
From: Cambridge University Press
| By:
Iain Nicolson |
EDITOR'S INTRODUCTION |
Where did we come from? At least part of the answer is offered by astronomers exploring the origin of the Earth as a planet. In this excerpt from his book Unfolding Our Universe, Iain Nicolson explains the latest thinking about how the Earth and the other planets of the solar system were formed in orbits around the Sun. |
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| Saturn, viewed by the Hubble Space Telescope. | |
 ntil comparatively recently, research on the formation of planets concentrated almost exclusively on the origin of our own Solar System. After all, it was the only planetary system that we knew for certain to exist and the only one whose properties we could study in detail. It has become apparent that planetary systems are commonplace and that their formation may be a natural by-product of the way in which stars like the Sun are born. Nevertheless, it is worth looking in detail at the Solar System in particular. |
With the exception of Mercury and Pluto, all of the planets follow orbits that lie within a couple of degrees of a common plane (the plane of the ecliptic), which lies close to, but does not exactly coincide with, the plane of the Sun's equator. All of the planets revolve round the Sun in the same direction — the direction in which the Sun spins on its axis — and most of them rotate around their own axes in this same direction. |
There are, however, distinct chemical and physical differences between them, which are linked to distance from the Sun and to the masses of the planets. The four innermost planets, the terrestrials, are relatively small dense bodies composed of rocks and metals. Jupiter and Saturn consist mainly of hydrogen and helium, but Uranus and Neptune contain a higher proportion of icy materials. The asteroids of the inner Solar System are rocky or metallic bodies, but the outer fringes of the system are populated by icy or ice-rich bodies. |
Changing views
The first serious theory of the origin of the Solar System was the "nebular hypothesis," proposed in slightly different forms during the eighteenth century by Immanuel Kant and Pierre-Simon de Laplace. The basic idea was that the Solar System formed when a large cloud of gas contracted under the action of its self-gravitation. As it contracted, it began to spin ever more rapidly. The rapid spin caused it to flatten into a disc shape. The central part of the cloud became the Sun, and the planets formed out of the surrounding disc. |
Theories of this kind had a number of points in their favor (e.g., they explained why all the planets revolve in a similar plane in the same direction as the Sun rotates). To nineteenth- and early-twentieth-century theorists, however, they appeared to have one fatal flaw — the distribution of angular momentum in the Solar System. |
Angular momentum is a measure of rotational motion. A rotating body possesses a quantity of angular momentum that depends on its mass, on the distribution of mass in its globe, and on its angular rate of rotation. An orbiting body possesses angular momentum that depends on its mass, on the radius of its orbit, and on its speed. |
It was argued that if the Sun and planets had formed from the same cloud of material, then because 99.8 percent of the mass of the Solar System resides in the Sun, the Sun should also contain a similar proportion of the total angular momentum of the system. In fact, the Sun, which rotates in a leisurely fashion, contains less than 1 per cent of the system's angular momentum; more than 99 per cent is contained in the orbital motions of the planets. |
This apparent anomaly forced astronomers to consider alternative possibilities. Of particular importance was the tidal, or encounter, theory that was proposed during the first decade of the twentieth century by T.C. Chamberlain and F.R. Moulton and developed notably by Sir James Jeans. According to this theory, the planets condensed out of a filament of gas dragged from the Sun during a close encounter with a passing star. This theory had the merit that the angular momentum of the planets was derived from the relative motion of the Sun and the passing star and was independent of the angular momentum of the Sun itself. |
It was later shown, however, that a planetary system such as ours could not have formed from a filament of high-temperature gas that had been dragged from the Sun in this way. Furthermore, because close encounters between stars are rare events, theories of these kinds implied that planetary systems must be very rare in the Galaxy. |
Now that there is good observational evidence to show that many young stars are surrounded by flattened discs of gas and dust, and there are sound theoretical mechanisms to explain how newly forming stars can shed angular momentum, the original objection to nebular-type hypotheses has been discarded. |
Evolution of the solar nebula
The collapsing cloud that gave birth to the Sun and the Solar System consisted of a mixture of gas and interstellar dust grains. As it contracted and flattened into a disc shape, kinetic energy released by infalling matter accreting onto the protosun would have raised the temperature of the innermost part of the disc sufficiently to vaporize all of the original grains, but farther out, where the temperature was lower, an increasing proportion of the original grains would have survived. |
As the newly forming Sun contracted and the surrounding disc began to cool, solid particles started to condense out of the vapor state. In the inner part of the disc only refractory substances (i.e., materials that remain solid at high temperatures) such as iron and various silicates (i.e., rock-forming materials) could have remained as solid particles or could subsequently have condensed into solid grains. Farther out, a wide variety of grains, including volatile materials (materials that evaporate at relatively low temperatures) such as ices of water, ammonia, or methane, would have survived or subsequently condensed. |
The vertical component of the flattened cloud's gravitational field may have caused the solid grains to settle quite rapidly (i.e., within a few thousand years) into a thin sheet in the plane of the disc. The rate of descent, however, would have depended on the size and density of the grains and on the frictional resistance provided by the nebula. If the grains were "sticky," then they would have collided and coagulated quickly into larger grains, which would have taken considerably longer to settle to the disc plane. |
Protoplanets and planet building
According to the gas instability, or "giant gaseous protoplanets" theory, originally proposed in 1951 by Gerard Kuiper, the planets formed by the direct gravitational collapse of clumps of gas within the solar nebula — a process analogous to the way in which stars themselves form within molecular clouds. Although a process of this kind provides a plausible mechanism for the birth of giant planets, it appears less satisfactory for the terrestrial worlds. In any case, for this approach to work, the mass of the solar nebula would have had to be substantially higher than most estimates suggest. |
There is wide general support for the alternative view that the terrestrial planets, and the cores of the giant planets, formed by accretion — the progressive aggregation of small bodies into larger ones. The process is thought to have begun in one of two ways. One possibility, originally discussed in the 1970s by Goldreich and Ward, is that when the dust component of the solar nebula settled into a thin sheet, the sheet broke up into clumps that collapsed directly under their own gravity to form solid bodies called planetesimals. Another view is that the planetesimals may have been assembled by the gradual sticking together of dusty grains as a result of random collisions. Either way, the end result was the production of a population of some ten billion planetesimals, each about 5-10 km in size. |
Mutual gravitational perturbations then diverted the planetesimals into intersecting orbits, and the resulting collisions led to the growth of some and the break-up of others. Simulations suggest that when one planetesimal grows to be more massive than its neighbors, it quickly mops up many of the others in a process of runaway growth that leads to the formation of bodies comparable in size to the Moon or the planet Mercury. |
Subsequent collisions between these bodies (i.e. giant impacts) resulted in the formation of the terrestrial planets and the cores of the giant planets. Modeling suggests that the time taken for the proto-Earth and proto-Venus to grow to about half of their final masses was about 10 million years, and to achieve their final masses, about 100 million years. |
The last giant impacts probably exerted great influence not only on the surface features and the internal structure of the planets but also on their rotation rates and on the orientations of their axes. For example, the slow retrograde rotation of Venus and the curious axial tilt of Uranus may have been induced by the last major impact in each case. |
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| Jupiter, viewed by the Hubble Space Telescope. | |
Because the powerful stellar wind that flowed away from the newborn Sun would have swept away most of the remnants of the solar nebula within a few million years, the cores of the giant planets would have had to have become sufficiently massive to enable them to accrete massive envelopes of gas from the solar nebula within this timescale. Because the average density of material in the disc declined with distance from the Sun, it is unlikely that the cores of the giants could have formed quickly enough had they been assembled solely from rocks and metals. |
At the distances at which the giant planets formed (beyond 5 AU), however, the temperature was low enough for water ice to condense into solid particles, and this increased the amount of solid material available. Taking this into account, it is likely that runaway accretion would have been able to assemble the massive cores that enabled Jupiter and Saturn to attract their huge gaseous envelopes before the enhanced solar wind stripped away the nebula. Slower growth of the cores of Uranus and Neptune may have restricted their ability to acquire envelopes as deep and as massive as those of their larger siblings. |
Satellites and surviving planetesimals
Some of the planetary satellites (e.g., the major moons of Jupiter) may have formed from rotating discs of material surrounding their parent planets, whereas others (e.g., the two tiny moons of Mars and the outer satellites of the giants) appear to be planetesimals subsequently captured by the planets. Earth's Moon seems likely to have been formed from material blasted into space by a collision between the proto-Earth and a body comparable in mass with the planet Mars (about one tenth of the Earth's mass). |
Such a collision, if it occurred after the Earth had differentiated, would have removed mainly mantle and crustal material, the volatile components of which would largely have evaporated, thereby producing a Moon with a mean density comparable to Earth's crustal rocks, a very modest core, and a substantially reduced proportion of volatiles compared with that of the Earth. |
The gravitational influence of Jupiter is believed to have stirred up the remaining planetesimals to such an extent that there was no possibility of their accumulating to form a planet between Mars and Jupiter. Remnant rocky and iron-rich planetesimals modified by growth and fragmentation, provided the bodies that populate the main asteroid belt today. |
The perturbing effect of Jupiter, and to a lesser extent other planets, was probably responsible for catapulting many of the icy planetesimals outward into what is now the Kuiper belt and Oort cloud. Those that were perturbed inward would have collided with the terrestrial planets, thereby supplying the Earth and the other terrestrial worlds with water and other volatile materials. |
During the first few hundred million years after the formation of the planets and their satellites, a heavy bombardment of impacting planetesimals continued. The planetesimals excavated craters, such as those that remain in evidence on the Moon today, and triggered global changes to planetary crusts and atmospheres. Plate tectonics on Earth may have been initiated by giant impacts fracturing the lithosphere. The atmospheres of the terrestrials were produced by outgassing of gaseous materials from their interiors as a result, for example, of volcanism, whereas the atmospheres of the giants are essentially the original envelopes that they acquired at the time of their formation. |
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