THE 10-Earth-mass planet1 in a nearly circular 0.7-AU orbit around PSR1829-10 is unlikely to have survived the supernova, or especially the pre-supernova evolution of the star that became the pulsar. Here we describe how the planet might have been created inside the young supernova remnant1-3. The principal difficulty lies not in providing enough mass or conducive thermo-dynamic conditions for planet formation, but in explaining the large angular momentum (~3 x 1048 erg s) and small eccentricity (<0.1) of the orbit. We propose that the planet formed from a rotationally supported disk of ~0.02 solar mass of heavy elements that fell back from the supernova explosion to an initial radius of about 1,000 km. Viscous evolution of the disk then concentrated most of its angular momentum into a small amount of material at the disk's outer extremity: 10 Earth masses at 1013 cm. Here, dust grains that had condensed and precipitated towards the mid-plane grew through cohesive collisions and gravitational instabilities into 100-km planetesimals, which coagulated into the planet on a million-year timescale. We find the presence of a second planet, more massive and more distant, unlikely, although residual planetesimals may provide the fuel for γ-ray bursts.