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Basics



The solar system consists of the Sun; the nine planets, over 100 satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium. (There are also many more planetary satellites that have been discovered but not yet been officially named.) The inner solar system contains the Sun, Mercury, Venus, Earth and Mars:

The planets of the outer solar system are Jupiter, Saturn, Uranus, Neptune and Pluto:

The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury and Pluto are very nearly circular. The orbits of the planets are all more or less in the same plane (called the ecliptic and defined by the plane of the Earth's orbit). The ecliptic is inclined only 7 degrees from the plane of the Sun's equator. Pluto's orbit deviates the most from the plane of the ecliptic with an inclination of 17 degrees. The above diagrams show the relative sizes of the orbits of the nine planets from a perspective somewhat above the ecliptic (hence their non-circular appearance). They all orbit in the same di"rect"ion (counter-clockwise looking down from above the Sun's north pole); all but Venus, Uranus and Pluto also rotate in that same sense.
(The above diagrams show cor"rect" positions for October 1996 as generated by the excellent Macintosh program Starry Night; there are also many other planetarium programs available.)


The above composite shows the nine planets with approximately cor"rect" relative sizes (see another similar composite and a comparison of the terrestrial planets).

One way to help visualize the relative sizes in the solar system is to imagine a model in which it is reduced in size by a factor of a billion (1e9). Then the Earth is about 1.3 cm in diameter (the size of a grape). The Moon orbits about a foot away. The Sun is 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter is 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) is 10 blocks away; Uranus and Neptune (lemons) are 20 and 30 blocks away. A human on this scale is the size of an atom; the nearest star would be over 40000 km away.

Not shown in the above illustrations are the numerous smaller bodies that inhabit the solar system: the satellites of the planets; the large number of asteroids (small rocky bodies) orbiting the Sun, mostly between Mars and Jupiter but also elsewhere; and the comets (small icy bodies) which come and go from the inner parts of the solar system in highly elongated orbits and at random orientations to the ecliptic. With a few exceptions, the planetary satellites orbit in the same sense as the planets and approximately in the plane of the ecliptic but this is not generally true for comets and asteroids.

Classification



The classification of these objects is a matter of minor controversy. Traditionally, the solar system has been divided into planets (the big bodies orbiting the Sun), their satellites (a.k.a. moons, variously sized objects orbiting the planets), asteroids (small dense objects orbiting the Sun) and comets (small icy objects with highly eccentric orbits). Unfortunately, the solar system has been found to be more complicated than this would suggest:
  • there are several moons larger than Pluto and two larger than Mercury;
  • there are several small moons that are probably captured asteroids;
  • comets sometimes fizzle out and become indistinguishable from asteroids;
  • the Kuiper Belt objects and others like Chiron don't fit this scheme well;
  • The Earth/Moon and Pluto/Charon systems are sometimes considered "double planets".


Other classifications based on chemical composition and/or point of origin can be proposed which attempt to be more physically valid. But they usually end up with either too many classes or too many exceptions. The bottom line is that many of the bodies are unique; our present understanding is insufficient to establish clear categories. In the pages that follow, I will use the conventional categorizations.

The nine bodies conventionally referred to as planets are often further classified in several ways:
  • by composition:
    • terrestrial or rocky planets: Mercury, Venus, Earth, and Mars:
      • The terrestrial planets are composed primarily of rock and metal and have relatively high densities, slow rotation, solid surfaces, no rings and few satellites.
    • jovian or gas planets: Jupiter, Saturn, Uranus, and Neptune:
      • The gas planets are composed primarily of hydrogen and helium and generally have low densities, rapid rotation, deep atmospheres, rings and lots of satellites.
    • Pluto.
  • by size:
    • small planets: Mercury, Venus, Earth, Mars and Pluto.
      • The small planets have diameters less than 13000 km.
    • giant planets: Jupiter, Saturn, Uranus and Neptune.
      • The giant planets have diameters greater than 48000 km.
    • Mercury and Pluto are sometimes referred to as lesser planets (not to be confused with minor planets which is the official term for asteroids).
    • The giant planets are sometimes also referred to as gas giants.
  • by position relative to the Sun:
    • inner planets: Mercury, Venus, Earth and Mars.
    • outer planets: Jupiter, Saturn, Uranus, Neptune and Pluto.
    • The asteroid belt between Mars and Jupiter forms the boundary between the inner solar system and the outer solar system.
  • by position relative to Earth:
    • inferior planets: Mercury and Venus.
      • closer to the Sun than Earth.
      • The inferior planets show phases like the Moon's when viewed from Earth.
    • Earth.
    • superior planets: Mars thru Pluto.
      • farther from the Sun than Earth.
      • The superior planets always appear full or nearly so.

Pictures



Note: most of the images here are not true color. Most of them were created by combining several black and white images taken thru various color filters. Though the colors may look "right" chances are they aren't exactly what your eye would see.

More General Overview

 
Content copyright Bill Arnett - from The Nine Planets
 
   
Content Copyright © 2002-2005 John R.