  The Solar System's orbital structure is thought to have been sculpted by an
episode of dynamical instability among the giant planets. However, the
instability trigger and timing have not been clearly established.
Hydrodynamical modeling has shown that while the Sun's gaseous protoplanetary
disk was present the giant planets migrated into a compact orbital
configuration in a chain of resonances. Here we use dynamical simulations to
show that the giant planets' instability was likely triggered by the dispersal
of the gaseous disk. As the disk evaporated from the inside-out, its inner edge
swept successively across and dynamically perturbed each planet's orbit in
turn. The associated orbital shift caused a dynamical compression of the
exterior part of the system, ultimately triggering instability. The final
orbits of our simulated systems match those of the Solar System for a viable
range of astrophysical parameters. The giant planet instability therefore took
place as the gaseous disk dissipated, constrained by astronomical observations
to be a few to ten million years after the birth of the Solar System.
Terrestrial planet formation would not complete until after such an early giant
planet instability; the growing terrestrial planets may even have been sculpted
by its perturbations, explaining the small mass of Mars relative to Earth.
