Why did William Thomson believe that the Earth was much younger than geologists and biologists claimed?

William Thomson, later known as Lord Kelvin, argued that the Earth was much younger than many geologists and biologists believed because he applied the principles of thermodynamics to the planet's cooling process.12

Thomson assumed that the Earth was an inert body incapable of producing new internal heat. Based on this premise, he calculated the time required for the Earth to cool from a molten state to its current temperature. By 1899, he had revised his estimate downward, concluding that the Earth was between 20 and 40 million years old.12

His calculations were ultimately proven incorrect following the discovery of radioactivity by Henri Becquerel and the subsequent realization that the decay of radioactive isotopes—such as uranium and thorium—provides a continuous, internal source of heat within the Earth. This meant the Earth was not simply cooling from an initial store of energy, as Thomson had assumed.12