The Moon above you and the ground beneath your feet may both be remnants of an ancient planetary collision. This episode reconstructs the story of Theia, a hypothetical protoplanet named for the Greek Titan who was mother of the Moon goddess Selene, and follows the evidence for how it formed, how it was pushed onto a collision course, and what happened when it struck the early Earth roughly 4.5 billion years ago.

We explain why the original Mars-sized estimate struggled to produce enough debris to build our Moon and how newer simulations push Theia to 15, 30, or even 45 percent of Earth’s mass. We walk through the frost line and the water-bringer idea, how Theia could have grown inside the stable L4 or L5 Lagrange point sharing Earth’s orbit, and how rhythmic tugs from Jupiter and Venus may have nudged it loose. Then we look at what Apollo moon rocks revealed about oxygen isotopes, why that forced a shift from a glancing-blow model to a head-on impact and a vaporized synestia, and how two continent-sized dense blobs deep in Earth’s mantle under Africa and the Pacific may be Theia’s buried core.

• How Lagrange points act as gravitational eddies where debris can collect and grow into a planet
• Why identical isotope signatures on Earth and the Moon broke the sideswipe theory around 2016
• The 2020 finding that deep lunar mantle rocks carry a slightly different isotopic signature
• How seismic shear waves reveal the Large Low Shear Velocity Provinces and their 2023 link to Theia
• Why the impact may have delivered Earth’s water, stabilized its axial tilt, and helped drive plate tectonics