Chapter 3 The Origin of Earth and the Solar System
Chapter 3 The Origin of Earth and the Solar System
Karla Panchuk
A solar system consists of a collection of objects orbiting one or more central stars. All solar systems start out the same way. They begin in a cloud of gas and dust called a nebula. Nebulae are some of the most beautiful objects that have been photographed in space, with vibrant colors from the gases and dust they contain, and brilliant twinkling from the many stars that have formed within them (Figure 3.3.1). The gas consists largely of hydrogen and helium, and the dust consists of tiny mineral grains, ice crystals, and organic particles.
Step 1: Collapse a nebula
A solar system begins to form when a small patch within a nebula (small by the standards of the universe, that is) begins to collapse upon itself. Exactly how this starts isn’t clear, although it might be triggered by the violent behavior of nearby stars as they progress through their life cycles. Energy and matter released by these stars might compress the gas and dust in nearby neighborhoods within the nebula.
Once it is triggered, the collapse of gas and dust within that patch continues for two reasons. One of those reasons is that gravitational force pulls gas molecules and dust particles together. But early in the process, those particles are very small, so the gravitational force between them isn’t strong. So how do they come together? The answer is that dust first accumulates in loose clumps for the same reason dust bunnies form under your bed: static electricity. Given the role of dust bunnies in the early history of the solar system, one might speculate that an accumulation of dust bunnies poses a substantial risk to one’s home (Figure 3.3.2). In practice, however, this is rarely the case.
Step 2: Make a disk and put a star at its center
As the small patch within a nebula condenses, a star begins to form from material drawn into the center of the patch, and the remaining dust and gas settle into a disk that rotates around the star. The disk is where planets eventually form, so it’s called a protoplanetary disk. In Figure 3.3.3 the image in the upper left shows an artist’s impression of a protoplanetary disk, and the image in the upper right shows an actual protoplanetary disk surrounding the star HL Tauri. Notice the dark rings in the protoplanetary disk. These are gaps where planets are beginning to form. The rings are there because incipient planets are beginning to collect the dust and gas in their orbits. There is an analogy for this in our own solar system, because the dark rings are akin to the gaps in the rings of Saturn (Figure 3.3.3, lower left), where moons can be found (Figure 3.3.3, lower right).
Step 3: Build some planets
In general, planets can be classified into three categories based on what they are made of (Figure 3.3.4). Terrestrial planets are those planets like Earth, Mercury, Venus, and Mars that have a core of metal surrounded by a mantle of rock. Jovian planets (also called gas giants) a