Gamma-Ray Bursts and the Origin of Chondrules and Planets

The effect of a nearby $\gamma$-ray burst (GRB) on the preplanetary solar nebula is considered. The intense irradiation by x-rays and $\gamma$-rays would have caused dust balls to become molten in a matter of seconds, cooling more slowly to form chondrules. The role of iron is crucial in this process because it was the major absorber of x-rays between 7 keV and 30 keV. In this scenario, chondrules formed at the same time across the side of the nebula toward the GRB source. At least 27 Earth masses (M$\oplus$) could have been produced in the nebula with well mixed gas and dust of solar composition, increasing to 100 M$\oplus$ with only moderate depletion of nebular gases. The chondrules combined to form meteorites and possibly the terrestrial planets, the cores of the giant planets and chondrules in comets. Assuming GRBs are linked to massive stars like supernovae, the probability of a GRB within 100 pc which could form chondrules is about 10$^{-3}$ and the same small probability may apply to other planetary systems being akin to our solar system. A GRB in a nearby galaxy will reveal protoplanetary disks by delayed transient infrared emission from the chondrule formation process. We suggest that a GRB was first detected about 4.5 Gyrs ago and its signature was written in stone and preserved by the chondrules in meteorites.