Classification of the
Planets

Much of our concern this semester will be with the development of our present understanding of the Solar System. We begin with a brief overview of the modern and ancient classifications of the planets.

The Modern Solar System

The planets of the modern solar system are grouped into several different and sometimes overlapping classifications, as illustrated in the following figure:

  1. The planets inside the orbit of the earth are called the Inferior Planets: Mercury and Venus.

  2. The planets outside the orbit of the earth are called the Superior Planets: Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto.

  3. The planets inside the asteroid belt are termed the Inner Planets (or the Terrestrial Planets): Mercury, Venus, Earth, and Mars.

  4. The planets outside the asteroid belt are termed the Outer Planets: Jupiter, Saturn, Uranus, Neptune, and Pluto.

  5. The planets sharing the gaseous structure of Jupiter are termed the Gas Giant (or Jovian) Planets: Jupiter, Saturn, Uranus, and Neptune.

The 7 Planets of the Ancients

The term "planet" originally meant "wanderer": it was observed long ago that certain points of light wandered (changed their position) with respect to the background stars in the sky. In ancient times, before the invention of the telescope and before one understood the present structure of the Solar System, there were thought to be 7 such wanderers or planets: Mercury, Venus, Mars, Jupiter, Saturn, the Moon, and the Sun. This list is different in several respects from our modern list of planets:
  1. The Earth is missing, because it was not understood that the points of light wandering on the celestial sphere and the Earth on which we stood had anything in common.

  2. Uranus, Neptune, and Pluto are missing because they would only be discovered when the telescope made them easily visible.

    • Uranus is barely visible to the naked eye; it was discovered in 1781.

    • Neptune and Pluto are too faint to see at all without a telescope; they were discovered in 1846 and 1930, respectively.

  3. The Sun and the Moon were classified as planets because they wandered on the celestial sphere, just like Mars and Jupiter and the other planets.
A central theme of our initial discussion will be how the "7 planets of the Ancients" (only 5 of which are really planets) evolved into our present list of Solar System planets.

Stars Look Different from Planets

Planets (and the Sun and Moon) have some observational characteristics that distinguish them from what we would now call the stars:

Observational Differences between Planets & Stars
PLANETS
STARS
The planets move relative to stars on celestial sphere The relative positions of the stars are fixed on celestial sphere
The nearer and larger planets appear as disks in telescope The stars appear as "points" of light, even through the telescope
The brighter planets do not "twinkle" The stars appear to "twinkle"
The planets are always near the imaginary yearly path of the Sun on the celestial sphere (the ecliptic) Stars can be anywhere on the celestial sphere

These observational differences, particularly the "wandering" of the planets on the celestial sphere, attracted a lot of attention from ancient observers of the sky. The attempt to explain these differences ultimately led to the birth of modern astronomy.

Twinkling of starlight.

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