It turns out that the only dimensions $n$ for which a sphere $S^n$ admits a unique differentiable structure are exactly $n=1, 2, 3, 5, 6, 12, 56, 61$.
In all other dimensions, except $n=4$, we get the existence of exotic spheres.
What happens at dimension $4$ is still largely unknown and it goes by the name of smooth Poincare conjecture, the only open case of generalized Poincare conjecture.