Solar System

Astronomers almost universally believe that the best descriptive model for the formation of the solar system is the solar nebula hypothesis. The essential idea behind the solar nebula model is that the Sun and planets formed through the collapse of a rotating cloud of interstellar gas and dust. In this way, planet formation is thought to be a natural consequence of star formation.

The solar nebula hypothesis is not a new scientific proposal. Indeed, the German philosopher Immanuel Kant first discussed the idea in 1755. Later, the French mathematician Pierre-Simon Marquis de Laplace developed the model in his text, The System of the World, published in 1796. The model is still under development today.

The key idea behind the solar nebula hypothesis is that once a rotating interstellar gas cloud has commenced gravitational collapse, then the conservation of angular momentum will force the cloud to develop a massive, central condensation that is surrounded by a less massive flattened Figure 2. Schematic of present-day solar system. Illustration by Hans & Cassidy. Courtesy of Gale Group. ring, or disk of material. The nebula hypothesis asserts that the Sun forms from the central condensation, and that the planets accumulate from the material in the disk. The solar nebula model naturally explains why the Sun is the most massive object in the solar system, and why the planets rotate about the Sun in the same sense, along nearly circular orbits and in essentially the same plane.

During the gravitational collapse of an interstellar cloud, the central regions become heated through the release of gravitational energy. This means that the young solar nebular is hot, and that the gas and (vaporized) dust in the central regions is well-mixed. By constructing models to follow the gradual cooling of the solar nebula, scientists have been able to establish a chemical condensation sequence. Near to the central proto-sun, the nebular temperature will be very high, and consequently no solid matter can exist. Everything is in a gaseous form. As one moves further away from the central proto-sun, however, the temperature of the nebula falls off. At distances beyond 0.2 AU from the proto-sun, the temperature drops below 2,000 K (3,100°F; 1,700°C). At this temperature metals and oxides can begin to form. Still further out (at about 0.5 AU), the temperature will drop below 1,000K (1,300°F; 730°C), and silicate rocks can begin to form. Beyond about 5 AU from the proto-sun, the temperature of the nebula will be below 200 K (-100°F; -73°C), and ices can start to condense. The temperature and distance controlled sequence of chemical condensation in the solar nebula correctly predicts the basic chemical make-up of the planets.