Planets in Sanskrit are called 'Grahas', which means that which seizes the consciousness.
For the purpose of astrology, the ancients gave the same status, that of a 'Graha' to the Sun, the Moon, and anything in the cosmic space within the Solar System. Even Rahu and Ketu (the two points at which the orbit of the Moon crosses the ecliptic, the north and south node of the Moon known as Rahu and Ketu in Jyotisha). The human psyche and its relationship with the collective consciousness was seen to be projected outwardly within the inanimate world of objects. Simple put, the outer-world and the inner-world were seen as a reflection of each other. Jyotisha or Astrology makes this grand metaphysical claim practicable, self-evident, observable. A sophisticated system giving practical explanations of the otherwise abstract metaphysical phenomenon.
When the world is seen as a psychic phenomenon, everything seems to be possessed or seized, and to the ancients, the world was a place of action rather than a place of things. The ancients were deeply and unknowingly/spontaneously engaged with the imagistic contents of the unconscious mind, inter-acting and deriving meaning from it.
In the ancient astrological texts such as Brihat Parashara Hora Shashtra, Grahas are given a physical form, that of a humanoid. Every planet has a peculiar personality and symbols associated with it, in fact, every character trait is a symbol in itself. For instance, Saturn or Shani is described as tall, dark, dirty, lame, thin, with coarse dry skin and hair, etc. These traits are symbols that reveal the ways and effects of Saturn when it seizes/possesses the Self-Psyche complex.