Most people think of the zodiac first when they think of astrology. The zodiac refers to the twelve astrological constellations such as Aries, Taurus, Gemini and the rest. Most people know their "Sun Sign", which is the sign the Sun was in on the day of their birth.
Enneastrology does NOT start with the zodiac. Instead, the focus is on the symbols of the Planets, and the ACTION which they represent, for it is this action which generates the characteristic behaviors and compulsions of the nine Enneagram personality types. The best way to make this distinction is to remember the following:
A planet is a verb; an astrological sign is an adjective.
Thus: The planets are symbols of a specific kind of ACTION within the psyche or personality. The astrological sign of that planet will describe the style, or way that the action is carried out.
A very brief key to the actions of the planets is as follows:
Sun: creating an Identity or Soul connection
Moon: nurturing and security
Mercury: thinking and communicating
Mars: asserting and defending the ego
Venus: pleasure and expressing values
Jupiter: learning and adding to the self through experience
Saturn: self management and discipline
Uranus: creativity and autonomy
Neptune: merging with the collective ideal
Pluto: transformation, and personal power
It is my observation that the planetary energy which is "least integrated" in the birth chart, or most difficult to express, that becomes the "Enneagram Planet", and which creates the habit patterns and compulsions known as the Enneagram personality type.
Determining which planet is the Enneagram Planet for an individual is a complex art in itself, and integrating that planetary energy productively is the spiritual work of Enneastrology.