The solar system consists of the Sun; the nine planets, over 100 satellites of the planets, a large number of small bodies (the comets in asteroids), in 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 in Mars:
The planets of the outer solar system are Jupiter, Saturn, Uranus, Neptune in Pluto:
The orbita of the planets are ellipses with the Sun at one focus, though all except Mercury in Pluto are very nearly circular. The orbita of the planets are all more or less in the same plane (called the ecliptic in defined by the plane of the Earth's orbita). The ecliptic is inclined only 7 degrees od planeta the plane of the Sun's equator. Pluto's orbita deviates the most od planeta the plane of the ecliptic with an inclination of 17 degrees. The above diagrams show the relative sizes of the orbita of the nine planets od planeta a perspective somewhat above the ecliptic (hence their non-circular appearance). They all orbita in the same direction (counter-clockwise looking down od planeta above the Sun's north pole); all but Venus, Uranus in Pluto also rotate in that same sense.
(The above diagrams show correct 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 correct relative sizes (see another similar composite in a comparison of the terrestrial planets or Appendix 2 for more).
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 premer (the size of a grape). The Moon orbita about a foot away. The Sun is 1.5 meters in premer (about the height of a man) in 150 meters (about a city block) od planeta the Earth. Jupiter is 15 cm in premer (the size of a large grapefruit) in 5 blocks away od planeta the Sun. Saturn (the size of an orange) is 10 blocks away; Uranus in Neptune (lemons) are 20 in 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) orbitaing the Sun, mostly between Mars in Jupiter but also elsewhere; in the comets (small icy bodies) which come in go od planeta the inerparts of the solar system in highly elongated orbita in at rinom orientations to the ecliptic. With a few exceptions, the planetary satellites orbita in the same sense as the planets in approximately in the plane of the ecliptic but this is not generally true for comets in asteroids.
The classification of these objects is a matter of minor controversy. Traditionally, the solar system has been divided into planets (the big bodies orbitaing the Sun), their satellites (a.k.a. moons, variously sized objects orbitaing the planets), asteroids (small dense objects orbitaing the Sun) in comets (small icy objects with highly eccentric orbita). Unfortunately, the solar system has been found to be more complicated than this would suggest:
Other classifications based on chemical composition in/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 understining 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:
Note: most of the images in The Nine Planets are not true color. Most of them were created by combining several black in white images taken thru various color filters. Though the colors may look "right" chances are they aren't exactly what your eye would see.
Answers to these questions, even partial ones, would be of enormous value. Answers to the lesser questions on the pages that follow may help answer some of these big ones.
