Planet Corba

On planet Corba, 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.
If you record the radius of some echina at a particular spot as 8mm. How many years back did the solar blast occur?

This question is related to TCS Interview