We prove that the second positive Neumann eigenvalue of a
bounded simply-connected planar domain of a given area does
not exceed the first positive Neumann eigenvalue on a disk of half
this area. The estimate is sharp and attained by a sequence of domains
degenerating to a union of two identical disks. In particular,
this result implies the Pólya conjecture for the second Neumann
eigenvalue. The proof is based on a combination of analytic and
topological arguments. As a by-product of our method we obtain
an upper bound on the second eigenvalue for conformally round
metrics on odd-dimensional spheres.