  We advocate the idea that proton decay may probe physics at the Planck scale
instead of the GUT scale. This is possible because supersymmetric theories have
dimension-5 operators that can induce proton decay at dangerous rates, even
with R-parity conservation. These operators are expected to be suppressed by
the same physics that explains the fermion masses and mixings. We present a
thorough analysis of nucleon partial lifetimes in models with a string-inspired
anomalous U(1)_X family symmetry which is responsible for the fermionic mass
spectrum as well as forbidding R-parity violating interactions. Protons and
neutrons can decay via R-parity conserving non-renormalizable superpotential
terms that are suppressed by the Planck scale and powers of the Cabibbo angle.
Many of the models naturally lead to nucleon decay near present limits without
any reference to grand unification.
