Navagrahas are nine deities that influence human life on Earth according to Hinduism and Jyotisha Shastra. The nine grahas viz. Sun (Adhithya), moon (Soma), Mars (Angaraka), Mercury (budha), Jupiter (Brahaspathi), Venus (Shukra), Shani (Saturn), Rahu (north node of the moon), and Kethu (South node of the moon). The last two of them are called shadow planets. The mention of the Navagraha is in RigVeda/Yajurveda/Atharvanaveda. There are a bunch of Vedic mantras together called the "Navagraha Suktam" which is a hymn dedicated to the Navagrahas. The term Nava- Graha is derived from Sanskrit language (“nava” means nine and “graha” is derived form of the root verb “grah” which means to grab, seize, hold, receive or exert an influence over something or someone. (Examples, anugraha, paani grahana, grahaka etc.)
Navagraha’s should not be mistaken as the Nine Planets as referred to in the solar system.The English word planet is used in its current meaning, only in the early 1600s. It finds its origins in the Latin word "planeta", and Greek "planētēs", both of which mean, to move, or to wander. Once the west realized that in the solar system, the Sun is relatively fixed and the other celestial objects move around it, they started using the word planet, which conveys just that, celestial objects in motion. The nine planets mentioned in the solar system includes Earth, Uranus, Pluto and Neptune. It is important to note that the Earth, Uranus, Neptune & Pluto are not included in the Navagrahas.
The very word 'graha' on the other hand, is used with the earth as the plane of reference. Not because of a lack of understanding that objects in the solar system move around the Sun; in fact the very arrangement of the Nava-Grahas with the Sun at the center demonstrates this understanding. Our interest, rather, was in studying how these external celestial entities exert their forces upon us here on earth. The study is done, with the earth as the plane of reference. That is why we see that Earth is missing in this arrangement of nava-grahas.
It is with this understanding, that the nava-grahas, or the nine celestial influences were identified. They are listed out in the order of the strongest influencers first. So, we see the Sun and the Moon as the first two grahas. The seven days of the week in the Hindu Calendar also correspond with the Navagraha, and are named accordingly in various languages of the Indian subcontinent. Most temples in India have a designated place dedicated for Navagraha worship.
The 'nava-grahas', are verily named, because of their influence on us. The concept of nava-grahas includes Sun, moon, and ecliptic phenomena. Elaborate systems of knowledge were created around identifying these influences, be it favorable or unfavorable. Practices and rituals were designed to either pacify these effects or to move beyond them completely taking life into one's own hands. This is the basis of the entire field of Jyotisha Shastra.