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We show that the merger and tidal disruption of a C/O white dwarf (WD) by aneutron star (NS) binary companion provides a natural formation scenario forthe PSR B1257+12 planetary system. Starting with initial conditions for thedebris disk produced of the disrupted WD, we model its long term viscousevolution, including for the first time the effects of mass and angularmomentum loss during the early radiatively inefficient accretion flow (RIAF)phase and accounting for the unusual C/O composition on the disk opacity. Forplausible values of the disk viscosity $\alpha \sim 10^{-3}-10^{-2}$ and theRIAF mass loss efficiency, we find that the disk mass remaining near the planetformation radius at the time of solid condensation is sufficient to explain thepulsar planets. Rapid rocky planet formation via gravitational instability ofthe solid carbon-dominated disk is facilitated by the suppression of verticalshear instabilities due to the high solid-to-gas ratio. Additional evidencesupporting a WD-NS merger scenario includes (1) the low observed occurrencerate of pulsar planets ($\lesssim 1\%$ of NS birth), comparable to the expectedWD-NS merger rate; (2) accretion by the NS during the RIAF phase is sufficientto spin PSR B1257+12 up to its observed 6 ms period; (3) similar models of `lowangular momentum' disks, such as those produced from supernova fallback, findinsufficient mass reaching the planet formation radius. The unusually highspace velocity of PSR B1257+12 of $\gtrsim 326\,{\rm km\,s}^{-1}$ suggests apossible connection to the Calcium-rich transients, dim supernovae which occurin the outskirts of their host galaxies and were proposed to result frommergers of WD-NS binaries receiving SN kicks. The C/O disk composition impliedby our model likely results in carbon-rich planets with diamond interiors.

Merger of a White Dwarf-Neutron Star Binary to $10^{29}$ Carat Diamonds: Origin of the Pulsar Planets