That’s not enough to support the claim Mars migrated inward, so the physics of such a maneuver had to be considered. If Mars formed in the asteroid belt, it would have encountered many objects smaller than itself. Simulations indicate that Mars would have grown to its current size in five to 10 million years after the solar system formed. Interacting with the asteroids in the belt would have been a net loss of energy for Mars, and the simulations predict it would have slipped into a tighter orbit around the sun after about 100 million years. The rest of the planet’s history would have unfolded as we currently understand it — the sun grows more intense, Mars loses its atmosphere, and most of the surface water dries up.

The next step is to run more simulations to see if any of them can provide a better match for the properties of Mars we currently observe. If scientists settle on Martian migration as a fact, it could tell us a lot about how all rocky planets form.