Presented By: Earth and Environmental Sciences
Smith Lecture - Qian Yuan
The "missing" planet Theia and Earth's habitability

As humans explore the solar system, Earth remains the only planet with a surface ocean, plate tectonics, and life. Earth's remarkable journey is believed to have begun 4.5 billion years ago with the Moon-forming giant impact between Earth and a hypothesized planet, Theia. However, direct evidence for Theia remains elusive, obscuring its role in shaping Earth's unique evolution. In this talk, I will present my recent study, which combines impact and geodynamic simulations with seismic and geochemical insights to propose that parts of Theia’s mantle have persisted within Earth for 4.5 billion years. These remnants naturally explain Earth’s largest seismically imaged anomalies—two large low-shear-velocity provinces (LLSVPs) in the deepest mantle. I will also discuss how strong mantle plumes from LLSVPs may have initiated Earth's first subduction around 200 million years after the Moon's formation. These findings shed light on how giant impacts shaped Earth's evolution and have broader implications for terrestrial planet diversity and the search for Earth-like exoplanets.