“This scenario implies that the giant planet instability is not the source of the Late Heavy Bombardment and that terrestrial planet formation finished with the giant planets in their modern configuration.” -Nathan A. Caib & John E. Chambers
It’s a common but nonetheless spectacular occurrence when our leading scientific theories — the ones with the best explanations for the observed phenomena — run into conflict with what we see. Sometimes, it’s the power of simulations that show us the way.
In 2005, scientists put forth the “Nice Model” to explain the configuration of the Solar System’s planets. It was thought that the outer planets, Jupiter in particular, migrated through the inner Solar System, and were then pulled back out by the presence of the outer giants, causing the late heavy bombardment of the terrestrial planets as it crossed the asteroid belt. But not only are extra gas giants that have since been ejected required to explain the outer worlds, but the migration would have ejected the inner, terrestrial worlds, indicating that the rocky planets finished forming after the gas giants were already in place.