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Introduction to the Solar System

Our Solar System is a collection of planets, asteroids, satellites, comets, meteoroids, dust, and gas orbiting our sun, a G2 V star. All the macroscopic bodies orbit the Sun in accordance with Kepler's laws, while the microscopic particles are affected by the solar wind and the magnetic fields of the Sun and planets. The Sun contains about 99.9 percent of the mass of the system. The jovian planet Jupiter contains about 0.1 percent of the mass while Saturn about 0.03 percent. The largest of the terrestrial planets, our Earth, has 1/328700 the mass of the sun.

In the Beginning...

The Solar System is believed to be about 5 billion years old and, presumably, formed at the same time as the Sun. To understand the formation of the Solar System, we must first understand the formation of the Sun. Computer models of star formation suggest that the angular momentum of a collapsing cloud of gas is too high to form a rotationally stable star. Stars may need associated bodies, such as companion stars (multiple star systems) or planets, into which to distribute the angular momentum of the system. Jupiter contains most of the angular momentum of the Solar System and Jupiter and Saturn combined possess nearly 90% of it.

The Solar nebula collapsed until the sun was several times larger than it is now surrounded by a disk of dust and gas. Solar magnetic fields may have transferred the angular momentum of the sun into the disk. The disk itself then fragmented into the planets at spacings corresponding to the mysterious Titius-Bode Law.

Those closest to the sun were too warm and small to hold most of the original gases; most of the material escaped back into the Solar nebula leaving only a residue upon which, in the case of the Earth, we live. These planets, Mercury, Venus, Earth, and Mars are called the terrestrial planets. The rest of the planets, forming further from the Sun, were cool and massive enough to keep most of their hydrogen. These massive, giant planets are called the jovian planets.

Much of the nebula was blown out of the solar system by the pressure of the Sun's radiation. Many fragments still exist: meteoroids, asteroids, comets, and dust particles.

The Sun

At the heart of the Solar System is the Sun. Luckily for us, the Sun is quite an ordinary star, with no exciting changes in brightness. Since the Sun is the primary source for almost all the energy on Earth, a stable environment makes the Earth's ecosystem, and all the life within it, possible. The surface of the Sun has a temperature of about 5500 Kelvin. The outer atmosphere of the Sun, the Corona, has a temperature of over a million degrees. The corona is visible during the spectacular event of a total solar eclipse.


The planet closest to the sun, Mercury is a rocky planet too close to the sun to maintain any detectable atmosphere. Images taken of Mercury by space vehicles reveal a landscape very similar to that of the Earth's moon.


In many respects the twin of the Earth in size and, probably, initial circumstances, Venus is shrouded by a dense atmosphere dominated by carbon dioxide. The pressure at the surface is about 30 times that of Earth and the temperature is over 700 degrees K.


All of what we know of the Universe is deduced from our experiences on Earth. How unique are the characteristics of the home planet? Of the four terrestrial planets, it seems to have been the only one to maintain a stable atmosphere, accommodating to life, over the billions of years it took to evolve.


Just over 10% the mass of the Earth, Mars' respectful distance from the Sun permitted a tenuous atmosphere to form. Long thought to be the planet with the best chance for life similar to that on Earth, the planet has revealed striking evidence for ancient river valleys to space probes. Are there fossils on Mars?

The Asteroids

Marking the boundary between the terrestrial planets and the massive gas giants is the asteroid belt. A jumble of irregularly shaped boulders, one cannot help but wonder if they are debris left behind from a failed, or even exploded, planet. Does the Titius-Bode Law predict a planet here? If one is to make huge space stations or enormous space ships, the asteroid belt may be one of the easiest places to find the raw materials needed to feed the smelters. The cost, in energy, of moving materials from the surface of the Earth is enormous. This fact has led to speculation that the asteroid belt might be the place to look for alien visitors to our solar system. Presumably, our little green friends would be busy mining the asteroid belt to feed their need for raw materials to expand their interstellar star fleet. Does this mean that if we find no interlopers in the asteroid belt that interstellar travel may be impossible?


The dominant planet of the Solar System, Jupiter is the first of the jovian planets. A hydrogen-rich planet whose multi-colored bands are constantly changing in color and morphology. Unlike the terrestrial planets, the gas giants have no rocky surface but an ever increasing density of gas.


The favorite of amateur astronomers, Saturn is surrounded by a thin, bright system of wide rings. The planet, a faded version of Jupiter in appearance, seems to float inside this brilliant band of light. The Saturnian rings, composed of ice particles in orbit around Saturn, are unique in planetary rings in their widths and narrow gaps. Saturn has more satellites than Jupiter (18 vs. 16) but only one, Titan, is as large as the four Galilean satellites.


Too far from Earth to be seen with an unaided eye, Uranus is the first of the discovered planets (William Herschel, 1781). Uranus is the only planet whose rotational axis lies almost in the plane of its orbit (inclination = 98 degrees from the ecliptic normal); technically, its rotation is retrograde. The atmosphere is nearly featureless; even the space probe Voyager 1 was unable to use features to measure the rotation rate of the atmosphere.


Perturbations in the orbital motions of Uranus led J.C. Adams and, independently, U.J.J. Leverrier to predict the location of the last of the jovian planets. Even further from Earth than Uranus, it seems a faint blue disk in terrestrial telescopes. Voyager 1 showed it to be a beautiful blue planet with storm features reminiscent of those of Jupiter. Neptune seems to be the planet that defines the edge of the Solar System for the clouds of comets (Kuiper Belt and the inner Oort cloud) that surround the Solar System.


Pluto was discovered by Clyde Tombaugh in 1938. Analogously to the prediction of Neptune, Pluto had been predicted from the perturbations in Neptune's orbit. Its discovery was fortuitous, however, since it is too small to have caused them. Also it is much closer than predicted by the Titius-Bode law. Its orbit is so eccentric that it crosses inside that of Neptune (where it is now) leading some to speculate that is is an escaped satellite of that planet. In the last few years, astronomers have come to think of Pluto as just an enormous comet nucleus, the largest example of a Kuiper Belt object. Pluto's relatively large satellite, Charon, was recently discovered; it is the largest satellite relative to the size of the parent planet.

Planetary Satellites

All of the jovian planets are accompanied by satellites. Of the terrestrial planets, only Earth has a significant companion: the Moon. Only Pluto has a satellite which has a mass a larger fraction of the mass of the primary than the Earth. The two moons of Mars are so small that they would appear to Martians as artificial satellites seem to terrestrial observers. The largest satellite of the Solar system is Neptune's Triton, nearly four times the mass of our Moon. Triton is the only major satellite in a retrograde orbit. Saturn's Titan, and the four Galilean satellites of Jupiter are the other major satellites. Most of the rest, about 40 others have been discovered so far, are small and, perhaps captured rather than coeval.

Comets and Meteors

The largest components of the solar system are the comets, whose tails sometimes reach between the orbits of planets. The tails are tenuous gases evaporated from the icy cores and dust liberated as the "dirty" ices melt. The comets are thought to congregate in the Kepler cloud, outside the orbit of Neptune, and the Oort cloud, which may extend well into interstellar space. Comets that stray into the into the inner (planetary part) of the solar system are often gravitationally "captured" by a jovian planet causing them to have smaller orbits and highly shortened periods. The resulting repeated passages by the Sun spells their doom as they rapidly evaporate. The smallest bodies in the Solar System are the meteoroids, floating along between planets until they crash into the atmosphere of a planet, leaving a short, bright trail, a meteor. Some are left over from the formation of the Solar System, some are the debris left behind by comets.

...And in the End

Current theories predict that in about five billion years, the sun will become a red giant star, swelling to many times its current size. This will destroy the terrestrial planets (certainly the Earth's atmosphere and water) and probably massively change the characteristics of the jovian planets. Shortly thereafter it will shrink to a white dwarf, a faint star feebly illuminating the remnants of the Solar System for many more billions of years.