Solar System

V

Movements of the Planets and Their Satellites

If one could look down on the solar system from far above the North Pole of Earth, the planets would appear to move around the Sun in a counterclockwise direction. All of the planets except Venus and Uranus, and the dwarf planet Pluto, rotate on their axes in this same direction. The entire system is remarkably flat—only Mercury among the major planets has an obviously inclined orbit. However, the dwarf planets Pluto and Eris have orbits that are strongly tilted out of the main plane of the solar system, Pluto at 17.2° and Eris at 44°. Both objects also have highly elliptical orbits. Pluto’s orbit sometimes takes it closer than Neptune to the Sun. At its nearest point to the Sun, Eris passes inside the orbit of Pluto, though well beyond the orbit of Neptune.

The satellite systems mimic the behavior of their parent planets and move in a counterclockwise direction, but many exceptions are found. Jupiter, Saturn, Uranus, and Neptune each have a number of satellites that move around the planet in a retrograde orbit (clockwise instead of counterclockwise), and several satellite orbits are highly elliptical. Uranus has some satellites that follow its clockwise direction and others that move in counterclockwise orbits. Jupiter, moreover, has trapped two clusters of planetesimals or small rocky bodies (the so-called Trojan asteroids) leading and following the planet by 60° in its orbit around the Sun. Neptune also has groups of planetesimals that share its orbit. (Some satellites of Saturn have done the same with smaller bodies that occupy different parts of the same orbits as the satellites.) The long-period comets exhibit a roughly spherical distribution of orbits around the Sun, while most of the short-period comets appear to originate from the disklike distribution of Kuiper Belt Objects.

Within this maze of motions, some remarkable patterns exist: Mercury rotates on its axis three times for every two revolutions about the Sun; no asteroids exist with periods 1/2, 1/3, … 1/n (where n is an integer) the period of Jupiter; the three inner Galilean satellites of Jupiter have periods in the ratio 4:2:1. Some Kuiper Belt Objects (including Pluto) orbit the Sun in a 2:3 ratio to Neptune’s orbit. These and other examples demonstrate the subtle balance of forces that is established in a gravitational system composed of many bodies.

VI

Theories of Origin

Despite their differences, the members of the solar system probably form a common family. They seem to have originated at the same time; few indications exist of bodies joining the solar system, captured later from other stars or interstellar space.

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Early attempts to explain the origin of this system include the nebular hypothesis of the German philosopher Immanuel Kant and the French astronomer and mathematician Pierre Simon de Laplace, according to which a cloud of gas broke into rings that condensed to form planets. Doubts about the stability of such rings led some scientists to consider various catastrophic hypotheses, such as a close encounter of the Sun with another star. Such encounters are extremely rare, and the hot, tidally disrupted gases would dissipate rather than condense to form planets.

Current theories connect the formation of the solar system with the formation of the Sun itself, about 4.6 billion years ago. The fragmentation and gravitational collapse of an interstellar cloud of gas and dust, triggered perhaps by nearby supernova explosions, may have led to the formation of a primordial solar nebula. The Sun would then form in the densest, central region. It is so hot close to the Sun that even silicates, which are relatively dense, have difficulty forming there. This phenomenon may account for the presence near the Sun of a planet such as Mercury, having a relatively small silicate crust and a larger than usual, dense iron core. (It is easier for iron dust and vapor to coalesce near the central region of a solar nebula than it is for lighter silicates to do so.) At larger distances from the center of the solar nebula, gases condense into solids such as are found today from Jupiter outward. Evidence of a possible preformation supernova explosion appears as traces of anomalous isotopes in tiny inclusions in some meteorites. This association of planet formation with star formation suggests that billions of other stars in our galaxy may also have planets. The high frequency of binary and multiple stars, as well as the large satellite systems around Jupiter and Saturn, attest to the tendency of collapsing gas clouds to fragment into multibody systems.

The formation of our solar system may have been an even more complex process than once thought. Recent studies of the chemistry of comets based on NASA’s Deep Impact and Stardust missions indicate that such primitive objects contain a surprising mix of materials that formed in both the hot inner regions and the cold outer regions of the early solar system. Some computer models show that the giant planets may have formed closer to the Sun, then moved outward over time, changing the orbits of other planets. Other models suggest inward migration of Jupiter and Saturn, imitating the orbital histories of some giant extrasolar planets that have been found orbiting very close to their parent stars. Our early solar system likely contained additional planets that were either destroyed in collisions with other planets or were thrown out of the solar system completely. The study of solar systems around other stars promises to provide important additional insights.

See separate articles for most of the celestial bodies mentioned in this article. See also Exobiology.

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