Why were William Thomson’s estimates of Earth’s age incorrect?

William Thomson’s estimates of Earth’s age were incorrect because they were based on the assumption that the Earth is an inert body incapable of producing its own heat.1

Applying his principles of thermodynamics to heat flow, Thomson calculated the age of the Earth by estimating how long it would take for a molten planet to cool to its current temperature. Based on this, he famously concluded in 1899 that the Earth was between 20 and 40 million years old.12

This estimate was proven wrong following the discovery of radioactivity in the late 1890s. Scientists, including Marie and Pierre Curie and Ernest Rutherford, recognized that the decay of radioactive isotopes—such as uranium and thorium—provides a continuous, dynamic source of internal heat. Because Thomson did not account for this internal heat production, his model failed to recognize that the Earth was not simply cooling from an initial state, but was being internally reheated over time.12