It's generally well known that the innermost part of Earth is made mostly of iron (about 85%). Nickel accounts for about 10%. That last 5% however, has remained a bit of a mystery.

A Japanese research team has been searching for that missing element for decades, and now believes that the final 5% is most likely made from silicon, reports the BBC. They presented their results at the Fall Meeting of the American Geophysical Union in San Francisco.

The Earth's solid core, which lies about 3,000 km below the surface, is thought to have a radius of about 1,200 km. Since it's so deep, it's impossible to directly test it, and find out precisely what the core is made of. (For comparison, the world's deepest mines only reach a depth of about 4 km.)

Image: Soylent Green

Silicon has been a contender as the missing element in the core for a while. Researchers knew that it had to be a lighter element, and silicon has been suggested many times, because of its properties, such as how well it bonds to metals.

So instead of digging, the team from the University of Tohoku decided to create a miniature Earth — crust, mantle core, and all — in the lab. First, they created alloys of iron and nickel and mixed them with silicon, then subjected them to the huge pressures and temperatures that exist within the core — about 6,000 degrees Celsius.

These conditions matched the seismic data of the Earth's core, which is information from the seismic waves that emanate from near the centre of the Earth. This gave the team sufficient evidence that silicon was probably the missing element.

"These difficult experiments are really exciting because they can provide a window into what Earth's interior was like soon after it first formed, 4.5 billion years ago, when the core first started to separate from the rocky parts of Earth," Professor Simon Redfern, professor of mineral physics at the University of Cambridge told the BBC news: "But other workers have recently suggested that oxygen might also be important in the core."