What assumption about Earth’s internal heat caused William Thomson’s age calculation to fail?

William Thomson, later known as Lord Kelvin, calculated the age of the Earth based on the erroneous assumption that the planet was an inert body incapable of producing its own internal heat.1

By applying his principles of thermodynamics to heat flow, Thomson concluded that the Earth was cooling from an initially molten state and estimated its age to be between 20 and 100 million years. This calculation failed because it did not account for the continuous generation of heat within the Earth's interior through the decay of radioactive isotopes, such as uranium and thorium. Once the role of radioactivity was understood, it became clear that the Earth was not simply a cooling, inert mass, but a dynamic system with a sustained internal heat source.12