The planet deities are also referred to as nava-graha and are suppose to have a significant impact on the lives of individual and therefore a Hindu adorns these planets as deities, so that they may bring peace & harmony in their life and to avert any mishap.
The nine planets include 7 visible planets (Sun, Moon, Mercury, Jupiter, Venus, Mars and Saturn) and two lunar nodes Raahu & Ketu).

Astrology is a subject, which studies the position of these planets & accordingly their impact on a person's life in co-relation with the date and time of birth & the corresponding positions of these planets at that time