Mercury, the innermost planet in our solar system, is so weird in terms of its small size yet proportionally large core, isolated orbit, sulfur-rich surface, and magnetic field that scientists have typically ignored it when modelling the origin of the planets. Previous dynamical studies such as the Nice Model and Grand Tack focus on giant planet migration in the outer solar system and dynamical scattering of the asteroid belt. In these models, some planetary embryos that formed in the inner solar system may have migrated outward. However, these models have had difficulty recreating the small size of Mercury and explaining the lack of planets inside its orbit compared to exoplanet systems around Sun-like stars.

A new numerical model for early inner solar system dynamics, created by Matthew S. Clement and John E. Chambers of the Earth and Planets Laboratory/Carnegie Institution for Science in Washington D.C. and Sean N. Raymond of Laboratoire d’Astrophysique de Bordeaux in France, suggests that Mercury could be a ‘relic’ of outward migration of Earth and Venus. In successful simulations, Earth and Venus accreted mostly from dry, enstatite chondrite-like material as they swept up material from around Mercury’s current orbit and moved outward to their present-day locations. These simulations provide simple explanations for the masses of all four terrestrial planets and inferred isotopic differences between Earth and Mars and predict Venus’s composition to be similar to Earth’s. This exodus of Earth and Venus also left less material for similar planets to form within Mercury’s orbit, reproducing Mercury’s small mass and isolated orbit. Unfortunately, these models do not produce strong constraints on Mercury’s bulk composition. There is hope to refine dynamical and compositional constraints on the origin of the terrestrial planets, especially the oddball Mercury, with the European Space Agency’s and Japan Aerospace Exploration Agency’s joint BepiColombo mission primed to provide a voluminous new source of data when it arrives at Mercury in 2025. READ MORE