  The planet formation process and subsequent planet migration may lead to
configurations resulting in strong dynamical interactions among the various
planets. Well-studied possible outcomes include collisions between planets,
scattering events that eject one or more of the planets, and a collision of one
or more of the planets with the parent star. In this work we consider one other
possibility that has seemingly been overlooked in the various scattering
calculations presented in the literature: the tidal capture of two planets
which leads to the formation of a binary planet (or binary brown dwarf) in
orbit about the parent star. We carry out extensive numerical simulations of
such dynamical and tidal interactions to explore the parameter space for the
formation of such binary planets. We show that tidal formation of binary
planets is possible for typical planet masses and distances from the host star.
The detection (or lack thereof) of planet-planet binaries can thus be used to
constrain the properties of planetary systems, including their mutual spacing
during formation, and the fraction of close planets in very eccentric orbits
which are believed to form by a closely related process.
