Scientists say they have discovered the Sun’s core is rotating four times faster than its surface, a surprising discovery that may be a relic of its formation.

The finding was led by scientists from the University of the Côte d'Azur in France, and published in the journal Astronomy and Astrophysics. The core makes up about a quarter of the Sun’s radius of 696,000 kilometers (432,000 miles).

It’s thought this phenomenon may be related to the period after the formation of the Sun 4.6 billion years ago, when solar wind slowed the rotation of the outer portion. The core, however, could have been unaffected and retained its original pace. This idea has been touted for 20 years, but it has never been discovered before.

“The most likely explanation is that this core rotation is left over from the period when the Sun formed,” said Roger Ulrich from the University of California, Los Angeles, a co-author of the study, in a statement. “It’s a surprise, and exciting to think we might have uncovered a relic of what the Sun was like when it first formed.”

The discovery was made using an instrument called GOLF (Global Oscillations at Low Frequency) on NASA and ESA’s SOHO (the Solar and Heliospheric Observatory) spacecraft. This observed a type of seismic wave in the Sun, called g-modes, and saw how they interacted with the core.

“This is certainly the biggest result of SOHO in the last decade, and one of SOHO’s all-time top discoveries,” said Bernhard Fleck, ESA’s SOHO project scientist based at NASA’s Goddard Space Flight Center in Greenbelt, Maryland in a separate statement.

Using 16.5 years of data collected by GOLF, the scientists were able to see how long it took acoustic waves to travel through the Sun and back to the surface. This journey takes about four hours and seven minutes.

But they were also able to measure the impact of gravity waves in the Sun’s core – basically fluid waves like you see on an ocean. The imprint of these showed the core was rotating once per week, compared to the surface, which takes 25 days at the equator and 35 days at the poles.

Now, scientists can start studying the Sun in a whole new way, and see how this differing rotation affects the surface. It’s thought that the varying rotation may even play a part in the creation of vast sunspots on the surface.