Solar Events

New Planets

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 this article, we 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.
  • by history:

    • classical planets: Mercury, Venus, Mars, Jupiter, and Saturn.

      • known since prehistorical times
      • visible to the unaided eye
    • modern planets: Uranus, Neptune, Pluto.

      • discovered in modern times
      • visible only with telescopes
    • Earth.