How did William Thomson estimate the age of the Earth?

William Thomson, later known as Lord Kelvin, estimated the age of the Earth by applying his principles of thermodynamics to the study of heat flow.1

He operated under the assumption that the Earth was originally a molten body that had been cooling over time through thermal conduction and that the planet’s substance was inert, meaning it was incapable of generating new heat. Based on these calculations, he initially proposed in 1862 that the Earth was approximately 100 million years old, a figure he later revised downward to between 20 and 40 million years by 1899.12

Thomson’s estimate was ultimately proven incorrect following the discovery of radioactivity. Scientists realized that the decay of radioactive isotopes within the Earth provides a continuous internal source of heat, meaning the planet was not simply cooling from an initial state as Thomson had assumed.12