Radioactive decay of uranium, thorium and potassium is responsible for about 50% of the heat coming from the Earth's internal area, a study published in Nature Geoscience has found.

The Earth has cooled since its formation, yet the decay of radiogenic isotopes ... in the planet's interior provides a continuing heat source, the international team of American, Japanese, and Dutch researchers behind the project wrote, reported. Further, the current total heat flux from the Earth to space is approximately 44.2 terawatts but the relative contributions from residual primordial heat and radiogenic decay remain uncertain.

In other words, the Earth, which scientists estimate is more than 4.5 billion years old, is still in the process of cooling.

In all, the roughly 44 trillion watts of heat continually flow from the Earth's interior into space. The 44 terawatts works out to 44,000 billion watts, reported. That heat is hot enough to melt iron ore in the outer core, among other physical achievements.

Scientists in the Japan-based KamLAND (Kamioka Liquid-scintillator Antineutrino Detector) collaboration first showed that there was a way to measure the contribution directly. 

Previous estimates of what scientists call radiogenic heat are roughly the same as the new figure, but they were based on the inferences of Earth's chemical composition derived from analyses of meteorites, reported.