Below are the only known references to the Law of Planetary Affinity in the writings of Alice A. Bailey (AAB) and Djwhal Khul (DK). Links on these pages of the Laws of Alice A. Bailey are to the online books at the Lucis Trust website.
For a discussion of what AAB means by a “law” please see Alice A. Bailey – What is a Law?
I. The Subsidiary laws. [ of the Law of Attraction ]
2. Law of Progress.
3. Law of Sex.
4. Law of Magnetism.
5. Law of Radiation.
6. Law of the Lotus.
7. Law of Colour.
9. Law of Planetary Affinity.
10. Law of Solar Unity.
11. Law of the Schools.
9. The Law of Planetary Affinity. This term is used in the occult teaching specifically in connection with the interaction of the planets with each other and their eventual marriage. As we know, the planetary schemes (the seven sacred planets) will eventually synthesise, or absorb the life of the planets which are not termed sacred and the numerous planetoids, as far as the four kingdoms  of nature are concerned. The absorption of the Spirit aspect proceeds under the Law of Synthesis. The minor four planetary schemes become first the two, and then the one. This one, with the major three, forms a second and higher quaternary which again repeats the process, producing from the four, the two, and from the two, the one. This final one is eventually merged in the Sun, producing in this prolonged process, and over a vast period of time, the appearance of the “seven Suns who run together, and thus blaze forth, producing one flaming ball of fire.”
You are welcome to post your comments and understanding of this law.