Chapter 1 - Sections 1.1-1.8 - Exercises - Problems by Topic - Page 40: 78d

There are seven significant figures in $1.563300 \times 10^{11}$.

All non-zero numbers are significant (1.5633 in this case), as well as any trailing or final zeroes after the decimal point (so the two zeroes after 1.5633 in this case). $10^{11}$ is just to express how many more zeroes you must add, so they are not significant.