A graph G is a k-dot product graph if there exists a vector labelling u : V (G) -> R(k) such that u(i)(T)u(j) >= 1 if and only if ij is an element of E(G). Fiduccia, Scheinerman, Trenk and Zito [Discrete Math., 1998] asked whether every planar graph is a 3-dot product graph. We show that the answer is "no". On the other hand, every planar graph is a 4-dot product graph. We also answer the corresponding questions for planar graphs of prescribed girth and for outerplanar graphs.