#78 An Overview of the Solar System

Orbits

The solar system consists of the Sun; the eight official planets, at least three "dwarf planets", more than 130 satellites of the planets, a large number of small bodies (the comets and asteroids), and the interplanetary medium. (There are probably also many more planetary satellites that have not yet been discovered.)

The inner solar system contains the Sun, Mercury, Venus, Earth and Mars

The main asteroid belt lies between the orbits of Mars and Jupiter. The planets of the outer solar system are Jupiter, Saturn, Uranus, and Neptune (Pluto is now classified as a dwarf planet)

The first thing to notice is that the solar system is mostly empty space. The planets are very small compared to the space between them. Even the dots on the diagrams above are too big to be in proper scale with respect to the sizes of the orbits.

The orbits of the planets are ellipses with the Sun at one focus, though all except Mercury 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. The above diagrams show the relative sizes of the orbits of the eight planets (plus Pluto) from a perspective somewhat above the ecliptic (hence their non-circular appearance). They all orbit in the same direction (counter-clockwise looking down from above the Sun's north pole); all but Venus, Uranus and Pluto also rotate in that same sense.

Sizes

The above composite shows the eight planets and Pluto with approximately correct relative sizes (see another similar composite and 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 everything is reduced in size by a factor of a billion. Then the model Earth would be about 1.3 cm in diameter (the size of a grape). The Moon would be about 30 cm (about a foot) from the Earth. The Sun would be 1.5 meters in diameter (about the height of a man) and 150 meters (about a city block) from the Earth. Jupiter would be 15 cm in diameter (the size of a large grapefruit) and 5 blocks away from the Sun. Saturn (the size of an orange) would be 10 blocks away; Uranus and Neptune (lemons) 20 and 30 blocks away. A human on this scale would be the size of an atom but 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; 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; and the many small icy bodies beyond Neptune in the Kuiper Belt. 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.

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 many small moons that are probably started out as asteroids and were only later captured by a planet;
• comets sometimes fizzle out and become indistinguishable from asteroids;
• the Kuiper Belt objects (including Pluto) 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; the actual situation is too complicated for simple categorization. In the pages that follow, I will use the conventional categorizations.

The eight bodies officially categorized 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.
• by size:
  • small planets: Mercury, Venus, Earth, Mars.
    • 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.
  • 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.
  • 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 Neptune.
    • farther from the Sun than Earth.
    • The superior planets always appear full or nearly so.
• by history:
  • classical planets: Mercury, Venus, Mars, Jupiter, and Saturn.
    • known since prehistorical times
    • visible to the unaided eye
    • in ancient times this term also refered to the Sun and the Moon; the order was usually specificied as: Saturn, Jupiter, Mars, Sun, Venus, Mercury and Moon, based on the time for them to go "all the way round" the sphere of the "fixed" stars).
  • modern planets: Uranus, Neptune.
    • discovered in modern times
    • visible only with optical aid
  • Earth.
  • The IAU has recently decided that "classical" should refer to all eight planets (Mercury thru Neptune, including Earth but not Pluto). This is contrary to historical usage but makes some sense from a 21st century perspective.