Cristiano Longarini, of the University of Cambridge, presents this Institute for Advanced Study astrophysics seminar on how planets might form through gravitational instability in protoplanetary discs. He revisits a mechanism once dismissed because it tends to produce stellar companions rather than planets, arguing that new attention to dust dynamics revives its plausibility. Longarini explains how gravitational instability generates large scale spiral structures in young discs that trap dust efficiently, and how that trapped dust can become concentrated enough to collapse directly into Earth mass cores. He walks through the analytic characterization of this process and the numerical simulations built to test it, showing planetary cores forming at large orbital radii earlier in disc evolution than standard core accretion models allow. The talk addresses why substructures observed in real protoplanetary discs suggest planet formation must overlap with the disc's gravitationally unstable youth, giving this historically sidelined theory a renewed physical mechanism.