I have said in a previous article that the Enneagram Planet tends to be the one which is "least integrated" in the astrological birth chart. What does this mean in plain English? In order to answer that question, one does need to understand some very basic ideas of astrology.
Have you ever known someone who did NOT resemble their astrological birth sign description at all? For example, maybe they were a Libra....supposedly a very fair minded, rational and relationship oriented sign. Yet the individual in question may have been brash, headstrong, opinionated and competetive, like an Aries. Upon further investigation, the birth chart shows the Sun in Libra...and the Moon, Mars and Jupiter in Aries. No wonder they act like an Aries; it is the dominant expression of their energies when the whole chart is considered.
The birth chart shows the true complexity of an individual, composed of many different drives and needs--more like a "committee" than a singular type of person. In fact one glance at a birth chart will show if a person is highly complex, with planets scattered all over the chart, or more easily understood with a few clusters of planets showing definate areas of focus and clear personal preferences.
In the symbolism of astrology, the geometric positions of the planets relative to one another, called "aspects", indicates whether the two parts of self (symbolized by the planets) are in easily harmonized styles of operation, or in contrasting, or conflicting styles of expression. The sextile and trine angles usually connect planets in the same or harmonious element (air with fire, water with earth), whereas the squares and oppositions indicate contrasting elements or a position of polar opposition in the chart.
If a planet is in square or opposition to many other planets in the chart, --or even completely unaspected or isolated--it shows that it is "odd man out", that is, different and in contrast to the rest of the birth chart. This means it will be more challenging for the individual to live out the energies of that planet in a graceful and balanced way. For example, if you are right handed and are suddenly called upon to use your left hand to do something, you feel awkward, clumsy, less sure of yourself. You tend to over do, or under-perform, in that area. In a similar manner, the "least integrated planet" begins to act like the negative Enneagram personality type it pertains to.
There are other ways for a planet to become "odd man out". The birth chart is divided into twelve houses, each representing a different area of life experience. A planet tucked away in the twelfth house, (that is the personal unconscious), or in the seventh house of relationships (thus likely to be projected on to other people) can also provide clues about which is the Enneagram Planet.
The goods news is, once the Enneagram Planet is identified, it is clear that one can learn to accept, integrate and express those energies more gracefully and completely.