On planet Zorba, a solar blast has melted the ice caps on its equator. 8 years…

On planet Zorba, a solar blast has melted the ice caps on its equator. 8 years after the ice melts, tiny plantoids called echina start growing on the rocks. Echina grows in the form of a circle, and the relationship between the diameter of this circle and the age of echina is given by the formula d = 4·√(t − 8) for t ≥ 8, where d represents the diameter in mm and t the number of years since the solar blast. Jagan recorded the radius of some echina at a particular spot as 8 mm. How many years back did the solar blast occur?