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Why did early estimates of Earth’s age by Lord Kelvin turn out to be too low?

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Lord Kelvin’s estimates of Earth’s age were significantly too low because they were based on the flawed assumption that the planet is an inert body that has been cooling steadily since its formation.1

Applying principles of thermodynamics, Kelvin calculated the time required for a molten Earth to cool to its current temperature, concluding in 1899 that the planet was between 20 and 40 million years old. This calculation failed to account for the internal production of heat, which was unknown at the time.12

The discovery of radioactivity by Henri Becquerel and the subsequent work of Marie and Pierre Curie revealed that the decay of radioactive isotopes—such as uranium and thorium—provides a continuous, dynamic source of heat within the Earth’s interior. Because the Earth is not simply cooling from an initial state but is instead actively generating heat, Kelvin’s thermodynamic model could not accurately determine the planet's true age.12