In Vedic times, graha meant malevolent spirits who hurt mothers and babies. It had nothing to do with planets. The 3000-year-old Vedic Jyotisha had nothing to do with predicting the future. It was a practical science of timekeeping — fixing solstices, calibrating the lunar calendar, scheduling sacrifices. The Rig Veda mentions no grahas at all. It does, however, preserve a shadowy figure called Svarbhanu who causes a solar eclipse before the sage Atri drives him off. 

By the Puranic era, 2000 years ago, the sky had become a divine family. Vivasvan, the sun, fathered planets including Shani (Saturn), and the old eclipse-demon was elaborated into a fuller drama: an asura tries to steal the amrita of immortality, the sun and moon expose him, and Vishnu beheads him — his head becoming Rahu, his body Ketu. Yet the Puranas could not agree on how many grahas there were. Early reliefs at Eran in Madhya Pradesh from 500 AD show seven, panels on Odisha temples from 800 AD show eight, and the complete set of nine appears in stone only after 1000 AD.