Earth’s inner core may have started spinning other way

Far below our feet, a giant may have started moving against us.

Earth's inner core, a hot iron ball the size of Pluto, has stopped spinning in the same direction as the rest of the planet and might even be rotating the other way, according to a research.

Roughly 5,000 kilometres (3,100 miles) below the surface we live on, this "planet within the planet" can spin independently because it floats in the liquid metal outer core.

Exactly how the inner core rotates has been a matter of debate between scientists.

What little we know about the inner core comes from measuring the tiny differences in seismic waves -- created by earthquakes or sometimes nuclear explosions -- as they pass through the middle of the Earth.

Seeking to track the inner core's movements, the new research, published in the journal Nature Geoscience recently, analysed seismic waves from repeating earthquakes over the last six decades.

The study's authors, Xiaodong Song and Yi Yang of China's Peking University, said they found that the inner core's rotation "came to near halt around 2009 and then turned in an opposite direction".

"We believe the inner core rotates, relative to the Earth's surface, back and forth, like a swing," they told AFP.

"One cycle of the swing is about seven decades", meaning it changes direction roughly every 35 years, they added.

They said it previously changed direction in the early 1970s and predicted the next about-face would be in the mid-2040s.

The researchers said this rotation roughly lines up with changes in what is called the "length of day" -- small variations in the exact time it takes Earth to rotate on its axis.