Planets and Satellites

Clues as to how the planets were formed lie in the regularities of their orbital motions, their satellite systems, and their chemical compositions. Compared to their sizes, the separations of planets from each other are enormous; and, apart from a diffuse solar wind and minor debris, interplanetary space is remarkably empty. Thus, as a general rule, the planets have been well isolated dynamically and chemically since their birth, and the present configuration of the solar system provides hints of the initial conditions, in spite of the more than 4 × 109 years of subsequent evolution.

With the exception of Mercury and Pluto, the orbits of the planets are all nearly circular; they lie within a few degrees of the same plane; and they have the same direct sense of revolution as the rotation of the Sun. Since these facts were first noted, they have suggested to philosophers and scientists such as Kant and Pierre-Simon Laplace of France that the planets of the solar system must have originally formed from a flat nebular disk that revolved about the primitive Sun. The exceptions, Mercury and Pluto, are not troublesome; they both suffer strong resonant interactions with other bodies that may have considerably modified their original orbital characteristics.

In the inner planetary system where the terrestrial planets--Mercury, Venus, Earth, and Mars--reside, the distance between successive planets is relatively small in comparison with the outer planetary system where the Jovian planets--Jupiter, Saturn, Uranus, and Neptune--reside. Moreover, the terrestrial planets are small and rocky or ironlike, while the Jovian planets (also called the giant planets) are large and gaseous or icy. Neither the terrestrial nor the Jovian planets exhibit the chemical elements in their cosmic proportions, but the latter, particularly Jupiter and Saturn, approach these proportions to a much closer degree. This implies that the process of planet building, unlike the mechanism of star formation, probably involves forces other than just gravity, for gravitation is universal and does not distinguish between different elements if they are in a gaseous form. Condensation (i.e., the separation of solid phases of matter from gaseous phases if the temperature drops to sufficiently low values) suggests itself as an important process.

From this point of view, the terrestrial planets have managed only to gather into their bodies mostly materials containing elements heavier than hydrogen and helium--materials such as silicate rocks and metallic iron or nickel, which can condense as solids from a gaseous phase even at relatively high temperatures (between 1,200 and 2,000 K). In contrast, Uranus and Neptune have not only accumulated rocky and metallic compounds but also ices of water, ammonia, and methane, which can condense from nebular gas only at much lower temperatures (between 100 and 200 K). Jupiter and Saturn succeeded additionally in capturing substantial amounts of hydrogen and helium (in their envelopes). Since hydrogen and helium at plausible nebular pressures do not solidify unless the temperature is lower than even in the coldest regions of interstellar space, this suggests that in the two largest planets of the solar system gravitation did play a role in the direct acquisition of massive amounts of these gases.

Pluto, which is small and icy and orbits farthest from the Sun, is not readily classifiable in the scheme outlined above. The discrepancy is not disruptive, however, because Pluto, discovered in 1930, and its moon, Charon, discovered in 1978, are relatively minor bodies similar in composition to the comets.

The terrestrial and Jovian planets possess other systematic differences: the former generally have no rings or satellites, while the latter each have a set of rings and many satellites. Here, Earth and Mars are exceptions to the rule. Earth has of course one satellite, the Moon; Mars has two, Phobos and Deimos. Of these exceptions, the more difficult case to explain has long remained the Moon because it is an unusually large object for a satellite. Indeed, the Moon is only somewhat smaller than the largest and most massive satellites in the solar system: Jupiter's Ganymede, Saturn's Titan, and Neptune's Triton. In comparison, Phobos and Deimos are tiny objects that may well have been captured after Mars had already formed.

The satellite and ring systems of the giant planets, particularly those of Jupiter and Saturn, resemble miniature planetary systems. As an analogy, one may say that moons and rings are to the giant planets what the planets and the asteroid belt are to the Sun. The moons of the giant planets can be classified as either regular or irregular. The regular satellites have nearly circular orbits lying in the same plane as the equator of the parent planet and revolve in the same direction as its rotation. The irregular satellites violate one or more of the above rules. In addition, they generally tend to be small bodies and to lie at large distances from the central planet. The regular satellites may have formed from protoplanetary disks that encircled the planet in the same manner as a protostellar disk encircled the Sun in the nebular hypothesis. The most likely explanation for the irregular satellites is that they are captured bodies.

The thin flat rings that encircle Jupiter, Saturn, Uranus, and Neptune are composed of innumerable small solid bodies. Each piece of the ring is in a nearly perfect circular orbit about the central planet. Theory suggests that noncircular motions are damped by mutual inelastic collisions of the particulate matter to very small values. These collisions would have led to gradual agglomeration into larger bodies had the rings not lain in such close proximity to the planet (i.e., within the Roche limit). The strong tidal forces that exist inside the Roche limit of a planet are believed to be capable of tearing apart loosely bound aggregates of particulate matter and thereby preventing their agglomeration into moons. It is unclear, however, whether planetary rings are the natural debris left over from an earlier period of satellite formation in a protoplanetary disk that extended almost to the planet's surface or whether they arose from the more recent breakup and erosion (by continual collisions and by micrometeoroid bombardment) of some larger parent body. There does exist some evidence from dynamic studies of the gravitational interactions of the rings and satellites of Saturn that the rings may be appreciably younger than the solar system in general.