Earth’s magnetic field, which acts as a protective blanket against harmful solar radiation and makes it possible for life to exist on the planet, might be older than previously thought. According to a new study, published Friday in the journal Science, Earth might have developed a magnetic field at least four billion years ago -- making it nearly 500 million years older than previous estimates.

While it is widely accepted that the planet’s magnetic field is generated by molten iron swirling around in the planet’s outer core -- resulting in a self-sustaining geomagnetic dynamo -- determining its exact age has proved to be a daunting task, as only rocks and crystals that have remained untouched since their formation can hold an accurate record of the ancient magnetic field.

Magnetite -- magnetic variant of rust, or iron oxide -- is one such mineral. Microscopic grains inside magnetite are capable of storing information about the planet’s magnetic field, including the direction and intensity, from the time the minerals cooled and solidified. With proper instruments, these magnetite grains can be read like a tape recorder.

For this particular study, a team of scientists examined four-billion-year-old zircon crystals -- capable of surviving geological upheavals -- from the Jack Hills region of Western Australia. When the faint magnetic signals in zircon samples containing magnetite were observed using a high-resolution magnetometer, the researchers found that not only did Earth’s magnetic field exist four billion years ago, it also exhibited fluctuations in strength similar to those seen now.

“We know the zircons have not been moved relative to each other from the time they were deposited,” lead author John Tarduno, a geophysicist at the University of Rochester, New York, said in a statement. “As a result, if the magnetic information in the zircons had been erased and re-recorded, the magnetic directions would have all been identical.”

In addition to determining the magnetic field’s date of birth, the observations also provide evidence that the dynamo in the planet’s core was present even in its infancy. This, in turn, hints at the existence of the plate tectonics at the time, which would have been needed to release the built-up heat.

“It’s surprising, because some of the models of the ancient Earth suggest that a magnetic field or plate tectonics could not have occurred that early,” Tarduno told Live Science. “Those models need to be rethought to include potential ways of cooling Earth’s interior early on.”