Solar System

One of the central and age-old questions concerning the solar system is, "How did it form?" From the very outset we know that such a question has no simple answer, and rather than attempting to explain specific observations about our solar system, scientists have tried to build-up a general picture of how stars and planets might form. Therefore, scientists do not try to explain why there are nine major planets within our solar system, or why the second planet is 17.8 times less massive than the seventh one. Rather, they seek to explain, for example, the compositional differences that exist between the planets. Indeed, it has long been realized that it is the chemical and dynamical properties of the planets that place the most important constraints on any theory that attempts to explain the origin of our solar system.

The objects within our solar system demonstrate several essential dynamical characteristics. When viewed from above the Sun's north pole, all of the planets orbit the Sun along near-circular orbits in a counterclockwise manner. The Sun also rotates in a counterclockwise direction. With respect to the Sun, therefore, the planets have prograde orbits. The major planets, asteroids and short-period comets all move along orbits only slightly inclined to one another. This is why, for example, that when viewed from Earth, the asteroids and planets all appear to move in the narrow zodiacal band of constellations. All of the major planets, with three exceptions, spin on their central axes in the same direction that they orbit the Sun. That is, the planets mostly spin in a prograde motion. The planets Venus, Uranus, and Pluto are the three exceptions, having retrograde (backwards) spins.

The distances at which the planets orbit the Sun increase geometrically, and it appears that each planet is roughly 64% further from the Sun than its nearest inner neighbor. This observation is reflected in the so-called Titius-Bode rule which is a mathematical relation for planetary distances. The formula for the rule is d(AU) = (4 + 3 × 2ⁿ) / 10, where n = 0, 1, 2, 3,...,etc. represents the number of each planet, and d is the distance from the Sun, expressed in astronomical units. The formula gives the approximate distance to Mercury when n = 0, and the other planetary distances follow in sequence. It should be pointed out here that there is no known physical explanation for the Titius-Bode rule, and it may well be just a numerical coincidence. Certainly, the rule predicts woefully inaccurate distances for the planets Neptune and Pluto.

One final point on planetary distances is that the separation between successive planets increases dramatically beyond the orbit of Mars. While the inner, or terrestrial planets are typically separated by distances of about four-tenths of an AU, the outer, or Jovian planets Figure 1. Illustration by Hans & Cassidy. Courtesy of Gale Group. are typically separated by 5-10 AU. This observation alone suggests that the planetary formation process was "different" somewhere beyond the orbit of Mars.

While the asteroids and short-period comets satisfy, in a general sense, the same dynamical constraints as the major planets, we have to remember that such objects have undergone significant orbital evolution since the solar system formed. The asteroids, for example, have undergone many mutual collisions and fragmentation events, and the cometary nuclei have suffered from numerous gravitational perturbations from the planets. Long-period comets in particular have suffered considerable dynamical evolution, first to become members of the Oort cloud, and second to become comets visible in the inner solar system.

The compositional make-up of the various solar system bodies offers several important clues about the conditions under which they formed. The four interior planets—Mercury, Venus, Earth, and Mars—are classified as terrestrial and are composed of rocky material surrounding an iron-nickel metallic core. On the other hand, Jupiter, Saturn, Neptune, and Uranus are classified as the "gas giants" and are large masses of hydrogen in gaseous, liquid, and solid form surrounding Earth-size rock and metal cores. Pluto fits neither of these categories, having an icy surface of frozen methane. Pluto more greatly resembles the satellites of the gas giants, which contain large fractions of icy material. This observation suggests that the initial conditions under which ices might have formed only prevailed beyond the orbit of Jupiter.

In summary, any proposed theory for the formation of the solar system must explain both the dynamical and chemical properties of the objects in the solar system. It must also be sufficient flexibility to allow for distinctive features such as retrograde spin, and the chaotic migration of cometary orbits.