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The Sun, which contains over 99% of the Solar System's mass, completes one rotation in about 24 days. In contrast, the largest planets, Jupiter and Saturn, rotate once in about 10 hours. When combined with their orbital motion, it turns out that Jupiter accounts for some 60% of the Solar System's angular momentum, with another 25% accounted for by Saturn. This compares with about 2% for the sluggardly Sun.

Any theory of cosmogony that attempts to account for the formation of the Solar System must take into account the angular momentum of the Solar System objects, as well as the facts that all of the planets travel in the same direction and more or less in the same plane. The obvious conclusion is that they all formed in the same manner and at about the same time.

Scientists have usually considered two main possibilities: the planets were either created by material derived from the Sun or a nearby companion star, or they formed from a cloud of diffuse matter that surrounded the Sun. However, theorists have struggled for centuries to match the hypotheses to the known facts, in order to choose between them.

One of the earliest, and most successful, attempts to explain how the Solar System came about was the nebular hypothesis – the idea that the Sun and planets formed from a vast, slowly rotating disk of gas and dust. A modified version of this hypothesis is the generally accepted explanation today.

Some of the key evidence comes from modern observations of distant star systems. Today, spaceborne telescopes can peer into the hearts of giant molecular clouds, such as the Orion Nebula, and search for young, Sun‐like stars that replicate the conditions that prevailed in our Solar System some 4.6 billion years ago.

These observations show that so‐called protoplanetary disks, or proplyds, exist around most very young stars – those less than 10 million years old (Figure 1.16). Many of the disks are larger than our Solar System. Observations of slightly older stars show how these disks evolve as time goes by, with the formation of swarms of rocky and icy debris and gaps in the clouds created by fledgling planets.

As currently envisaged, the Solar System began with the collapse of a cloud of interstellar gas. The trigger for this collapse may have been the passage of an externally generated shock wave from a supernova explosion, density waves passing through the galaxy, or a major reduction in the cloud's magnetic field or temperature.

The first of these explanations is the prime candidate, since many stars form in clusters within clouds containing thousands of solar masses of material. When the giant stars of the cluster run through their short life spans, they are likely to produce a series of supernovas, preceded by powerful stellar winds.

Evidence from meteorites and dynamical modeling of supernova shock wave propagation into giant molecular clouds indicate that a supernova explosion compressed part of a cloud, causing this region to collapse. The shock wave would also have injected material from the exploding star into the solar nebula. Scientists have detected evidence of this material in the form of the decay products from radioactive isotopes, particularly iron‐60. These are found in primitive meteorites and can only form in the giant stars that end their lives as supernovas.

Over millions of years, the original cloud may be broken up into smaller fragments, each mixed with heavier elements from the dying stars, as well as the ubiquitous hydrogen and helium gas. Once a fragment reaches a critical density, it is able to overcome the forces associated with gas pressure and begins to collapse under its own gravity.

The contracting cloud begins to rotate, slowly at first, then faster and faster – rather like when an ice skater pulls in his arms. Since material falling from above and below the plane of rotation collides at the mid‐plane of the collapsing cloud, its motion is cancelled out. The cloud begins to flatten into a disk, with a bulge at the center where the protostar is forming. The disk was probably thicker at a greater distance from the protostar, where gas pressure was lower.

Figure 1.12 The axial inclinations (obliquities) of the planets and Pluto compared to their orbital planes. Most of the planets have axial tilts of less than 90°, so they rotate in a prograde direction, from west to east. Venus, Uranus, and Pluto have obliquities greater than 90°, so they are said to rotate in a retrograde (backwards) direction.

(Peter Bond)

Such a nebula would almost certainly rotate slowly in the early stages, but as it contracts, conservation of angular momentum causes the cloud to spin faster. If this process continues, the core forming at the center of the nebula will spin up so fast that it flies apart before it has a chance to form a star. Somehow, that angular momentum must be removed before a star can form.

Studies of other young stars and their surrounding disks provide evidence that, as the interstellar gas collapses, it also winds up the magnetic field which permeates the nebula. Gas which is rotating too fast to collapse is expelled and dispersed along the magnetic field.

This process naturally forms a spiral‐shaped magnetic field that helps to generate polar jets and outflows associated with very young stars. At the same time, the jets remove angular momentum, allowing other material to accrete and collapse. Gravitational instability, turbulence, and tidal forces within the “lumpy” disk may also play a part, helping to transfer much of the angular momentum to the outer regions of the forming disk.

The protoplanetary disk is heated by the infall of material. The inner regions, where the cloud is most massive, become hot enough to vaporize dust and ionize gas. As contraction continues and the cloud becomes increasingly dense, the temperature at its core reaches the point where nuclear fusion commences. The emerging protostar begins to emit copious amounts of ultraviolet radiation. Radiation pressure drives away much of the nearby dust, causing the star to decouple from its nebula.

The young star may remain in this T Tauri stage for perhaps 10 million years, after which most of the residual nebula has evaporated or been driven into interstellar space. All that remains of the original cloud is a rarefied disk of dust grains, mainly silicates and ice crystals.

Meanwhile, the seeds of the planets have begun to appear. More refractory elements condense in the warm, inner regions of the nebula, while icy grains condense in the cold outer regions. Individual grains collide and stick together, growing into centimeter‐sized particles. These swirl around at different rates within the flared disk, partly due to turbulence and partly as the result of differences in the drag exerted by the gas. After a few million years, these dusty or icy golf balls accrete into kilometer‐sized planetesimals and gravity becomes the dominant force.

The Solar System now resembles a shooting gallery, with objects moving at high speed in chaotic fashion and enduring frequent collisions with each other. Some of these impacts are destructive, causing the objects to shatter and generate large amounts of dust or meteorite debris. Other collisions are constructive, resulting in a snowballing process. Over time, the energy loss resulting from collisions means that construction eventually dominates.

Eventually, the system contains a relatively small number of large bodies or protoplanets. Millions of years pass as they continue to mop up material from the remnants of the solar nebula and to collide with each other, finally resulting in a population of widely separated worlds occupying stable orbits and traveling in the same direction around the young central star.

It is likely that the largest planets in the Solar System, Jupiter and Saturn, formed first. They presumably accumulated their huge gaseous envelopes of hydrogen and helium before the solar nebula dispersed.