According to the Vedas, the planets are the instruments of God, by which the law of karma operates. The Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn, Rahu and Ketu are called the 9 "Grahas" (Sanskrit meaning is "to grab" or "to hold"). According to the ancient scripture Brihat Parashara Hora Shastra: "Vishnu (God) has incarnated as the Navagrahas (9 planets) to bestow on living beings the results of their Karmas or actions. He assumed the auspicious form of Grahas to destroy the strength of the demons (evil forces), to sustain the strength of the Devas (the divine beings) and to establish Dharma (cosmic order or law)."
The planets, besides their huge physical mass and gravitational force, are tremendous fields of subtle psychic or spiritual energies, and it is at that level that we have to understand their effects. This understanding is the basis of the science of astrology. Each planet corresponds to a specific vibratory frequency, color, subtle element, geometrical form, and even a personality type with it's own likes and dislikes, friendships and enmities.
Each individual person is made of a specific and unique combination of the energies of these planets that determines his characteristics, likes and dislikes, personality and behavior. It is interesting to note that Vedic astrology doesn't use the "outer" planets-Uranus, Neptune and Pluto-even though there is some historical evidence that the ancient Rishis knew about them; in the ancient scripture the Mahabharata, three planets are mentioned which correspond with the location and coordinates of the outer planets.