According to Vedas and Shastras there are many planets & stars but only Nine (Nav=9) Planets (Grah) and 27stars that fell into immediate circumference of the solar system and are likely to affect our lives depending upon their position in our natal chart are considered.
These nine planets form an orbit called as Navagraha Mandal.
The nine "grahas" or planets in our horoscope control our karma, our desires and their outcomes. Each of these nine planets exerts an influence in our lives, which is called "dasa" and it can be known from one's horoscope. Navagraha Shanti pooja is undertaken to reduce the pessimistic effects and improve the optimistic energies related to a person.
The nine planets have special kshetras ( place of worship ) in South India. Most of them are concentrated near the Thanjavur district. Navagraha pooja will be helpful, when the concerned planet dasa or sub-dasa is in operation or if the planet is associated with malefics or is in the 3rd, 6th, 8th, or 12th house.
Also as said earlier only 9 planets and 27 stars are considered.