Nine Planets or Navagraha
The Hindus attach significance to Navagrahas (or Nine Planets including sun and moon) and they believe that these planets play a keyrole in their destiny. The nine planets are Sun (Surya) , Moon (Chandra), Mars (Chevvay) , Mercury (Bhudhan), Jupiter (Guru or Viyalan), Venus (Sukran), Saturn (sani), Rahu (Shaddow planets - Snake's head) and Ketu (Shadow plaents - Snake's tail) control our joy, sorrow, obstacles, business, career, marriage, progeny, health and longevity and death. The Indian calendars have assigned more importance to these planets and the week days are named in the names of these seven planets (Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn).
In lunar based Hindu astrology, significance is attached on the positions of the nine planets in the twelve constellations of the zodiac, the belt around the sky in which the movements of Sun, Moon and planets are confined. People believe that transition of Saturn, Jupiter, Rahu and Ketu from one zodiac sign to other sign will bring good or bad or mixed fortunes. All miseries and obstacles caused by one or many planets will be mitigtated by chanting Gayatris relevant to the specific plane(s).