The Earth is almost 4.5 billion years old, but it's young at heart – literally. Researchers from the University of Rochester have now dated the solid inner core of the planet to just 565 million years, making it a relative toddler compared to the rest of Earth.

Since Earth started life as a growing clump of rock, it's easy to assume that the core is the oldest part of the planet, but that's not quite the case. Today, it's divided into two regions: a solid ball of iron in the inner core, which is surrounded by a swirling pool of liquid iron. When exactly that inner core solidified has long been up for debate, with conventional thinking placing it somewhere between half a billion and 2.5 billion years.

But now the Rochester researchers have narrowed it down to the lower end of that scale. The key to the discovery is the magnetic field that surrounds and protects Earth, and measuring how that's changed over time.

To do so, the team collected samples of ancient crystals from the Sept-Îles Complex in Quebec. Inside these crystals are tiny magnetic needles that preserve a record of the magnetic field at the time they were first locked away in the mineral. The researchers found that about 565 million years ago, the Earth's magnetic field was the weakest it's ever been – about a tenth of its current strength – and was on the verge of collapse.

Since life is still here today and we're able to go outside without being bombarded with deadly cosmic radiation, the magnetic field obviously bounced back from that low point. But how? The Rochester team says that a newly-formed solid inner core could be responsible.

The Earth's magnetic field is generated by the flowing fluid iron, in a process called a geodynamo. The team says Earth has probably had a weak geodynamo for billions of years, created by a core that was mostly molten iron. But this process slowed down over time, until that turning point 565 million years ago when it was stabilized, possibly by the arrival of the solid inner core we know today.

"This is a critical point in the evolution of the planet," says John Tarduno, corresponding author of the study. "The field did not collapse because the inner core started to grow and provided a new energy source for the formation of the geodynamo."

The team says the idea is backed up by other data sets and simulations, but at will need more work to confirm it. The study doesn't just help us understand our own planet a bit better – it could aid in the search for exoplanets that are capable of supporting life.

"The same factors that drive dynamos on Earth might affect the magnetic shielding on exoplanets," says Tarduno. "It could be the case that some planets don't have long-lived dynamos and those planets would not have the magnetic shielding we have, meaning that their atmosphere and water might be removed."

The research was published in the journal Nature Geosciences.

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