The word "planet" derives from the Greek πλανητησ, "wanderer". The classical planets were those celestial bodies which moved in regular, permanent cycles across the celestial sphere: The Sun and Moon, and the five "true" planets visible to the naked eye, Mercury, Venus, Mars, Jupiter, and Saturn. This grouping of seven parallels many other esoterically prominent groups of seven, such as the classic metals, the chakras, and the days of the week.
Astronomers have discovered three new planets since classic times. Uranus is barely visible to the naked eye under ideal conditions, but appears to have escaped notice until its discovery (through gravitational analysis) in the 18th century. Neptune is too faint to be seen without good binoculars, and tiny, distant Pluto is a difficult target even with a small telescope.
The Minor Planets
In addition to these, numerous smaller bodies occupy planet-like orbits around the Sun, including the main belt asteroids between Mars and Jupiter, a few asteroids outside the main belt, and the icy denizens of the remote Kuiper Belt and still more remote Ooort Cloud. There is considerable controversy among astronomers as to what constitutes a "planet"; for example, Pluto itself would probably be classified as a large "Kuiper Belt object" rather than a planet if it were discovered today.
The Planets in Astrology
In Western astrology, the planets generally represent emotional states or modes of being, with their relations to one another, to the local horizon, and to the zodiacal signs contributing to the interpretation of a given chart. For example, Venus is said to rule matters of love, relationships, and vanity, while Mercury presides over intellect, communication, and trickery. These attributions generally mirror those of the Roman god for whom the planet was named.
Astrologers are divided on how to incorporate the trans-Saturnian and "minor" planets into the system of astrology. There are numerous schools of thought regarding the symbolic meaning of the three outer planets, but broad consensus on their general character. This cannot be said of the "minor" planets, which some astrologers ignore entirely, while others use them in wildly differing ways.