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Planetary Systems

Formation and Evolution of the Solar System

Disk Formation 1 Disk Formation 2 Disk Formation 3 The solar system, it is thought, began as a subcondensation in an interstellar cloud of gas and dust, from which probably hundreds of other stars also formed. To begin with, this presolar cloud was spheroidal, slowly rotating, and quite large, with a diameter of perhaps one

Figure 07-02a Disk Formation 1 [view large image]

Figure 07-02b Disk Formation 2 [view large image]

Figure 07-02c Disk Formation 3 [view large image]

or two light-years (Figure 07-02a). As it con- densed, the gas in the cloud's equatorial plane moves inward more slowly because its rotation starts to balance the gravity, causing it to
become increasingly flattened (Figure 07-02b). Over time, all the material in the cloud falls into the equatorial plane, where the gas becomes rotationally supported - its motion holds it up against gravity (Figure 07-02c). In the middle of the disk, where the density was greatest, the protosun began its final condensation. By the time the Sun had initiated nuclear fusion reactions in its core, the pancake-shaped protoplanetary disk had started to form agglomerations at various distances from the center. This mechanism for disk formation is common for a variety of astronomical objects such as spiral galaxies, quasars, and black holes. Sometimes the system displays a pair of jets perpendicular to the rotational plane. It seems to be produced by charged particles moving along twisted magnetic field lines.

The gas and dust in the protoplanetary disk formed small bodies between 1-10 km in diameter. These bodies are known as planetesimals. Initially they formed small fragments of solar dust up to about 1 cm in diameter by the processes of cohesion (sticking together by weak electrostatic forces) and by gravitational instability. Larger bodies formed later by collisions at low speed which caused the material to stick together by gravitational attraction. Support for this view of the process of accretion comes from a region on the edge of the solar system known as the Kuiper Belt, where it is thought that the accretive 'mopping up' had failed to complete and the raw materials are still around as comets. The final stage of accretion has been described as 'runaway accretion'. Planetesimals were swept up into well defined zones around the sun close to the present orbits of the terrestrial planets. The process led eventually to a small number of large planetary bodies. Evidence for this impacting process can be seen in the early impact craters found on planetary surfaces.

Two key factors determine what kind of planet a protoplanet will become: its mass and its distance from the central star. Planets of low mass cannot retain hydrogen and helium, the lightest and most abundant gases, especially if their temperature rises to the point at which the lightest molecules escape. When the planets were in their early accretive phase, the mass that agglomerated before the Sun began to shine helped determine how well the planet could retain its hydrogen and helium. The other crucial factor, the distance of the planet from the Sun, also influenced the escape of hydrogen and helium from the planet's gravity, because inner planets become hotter and so have more difficulty in retaining the lightest gases with a given amount of gravitational force. These considerations explain well the overall structure of the solar system. The four small, inner planets were unable to hold on to any free hydrogen and helium with which they may have started out. However, the four gas giants, lying much further out from the Sun and therefore having much lower temperatures, not only retained their light gases but, through their powerful gravitational pulls, continued to draw in more material after the Sun had turned on.

Planetary Formation Figure 07-02d is an artist's conception of the formation of a planetary system. The first three lower inset boxes zoom in from the spiral arm of the Milky Way to a star-forming region such as Orion, and then to a newly-forming star with its gas disk. The upper picture shows that the disk has become thin and is beginning to break into rings of gas and dust. The dust rings will condense into rocky "planetesimals" that will eventually merge to become planets, as shown in the inset at the lower right. Jets of gas flow out from the newborn star in the polar directions.

Figure 07-02d Formation of Planetary System

[view large image]

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