We analyze the single particle states at the edges of disordered graphene
quantum dots. We show that generic graphene quantum dots support a number of
edge states proportional to circumference of the dot over the lattice constant.
Our analytical theory agrees well with numerical simulations. Perturbations
breaking electron-hole symmetry like next-nearest neighbor hopping or edge
impurities shift the edge states away from zero energy but do not change their
total amount. We discuss the possibility of detecting the edge states in an
antidot array and provide an upper bound on the magnetic moment of a graphene
dot.Comment: Added figure 6, extended discussion (version as accepted by Physical
Review B