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