Mercury-like planet might have crashed into Earth to give it its magnetic field. NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

Earth might have swallowed up a massive Mercury-like planet early on in its formation, a collision that provided it its layers and magnetic field.

That is according to two Oxford University geochemists, who say the current theories about the Earth formation – that it came from small rocky bodies – are not a perfect fit and do not explain the energy source that drives the planet's magnetic field.

Publishing their findings in the journal Nature, the authors said the main problem is you need radioactive elements that give off heat as they decay to be in the planet's iron core. However, these elements like oxygen, so would be expected to rise towards the surface, and do not like iron, the LA Times reports.

Current models cannot explain how enough radioactive material is kept in Earth's centre to power our magnetic field.

Instead, Bernard Wood and Anke Wohlers say if you had a source of reduced sulphides (sulphur compounds without oxygen), the radioactive elements would stick with the metal better. Findings showed that this would solve the problem – and that it would explain a conundrum about the ratio of two rare-Earth elements in Earth's layers.

Explaining how this could have come to be, researchers say the sulphides probably came from a body like Mercury – which is rich in sulphur and poor in oxygen.

They said that with the "addition of a reduced Mercury-like body rich in sulphur to the early Earth" would generate the elements in the mantle.

By consuming such a body, the sulphides would have allowed the radioactive material to stay in the core. "We think that that is quite conceivable," Wood is quoted as saying. "It's kind of exciting to think that this reduced body could actually be the thing which caused the moon."

Speaking to Smithsonian magazine, Wood said the Mercury-like object was probably less than half the size of Earth: "We would need a body 20 to 40% the size of Earth."

However, Richard Carlson from the Carnegie Institution for Science said the study provides only "limited (and controversial) experimental evidence" and that "a more stringent test" to see if uranium got into the core in this way is required.