More than 20 years ago, scientists discovered a curious fact about the Sun. The outer 5 percent of our star rotates far more slowly than its chaotic interior.

The discovery was puzzling, but American and Brazilian astronomers might have finally cracked this mystery. The solar radiation is responsible for slowing down the plasma in the top 35,000 kilometers (21,000 miles) due to an effect similar to the Poynting-Robertson photon braking.

That effect has been observed in dust particles around the Sun. The specks of dust are small enough to be strongly pushed around by the momentum of the photons, and these photons create a drag, which over time slows them down until they fall into the Sun.

The research, published in the Physical Review Letters and available on arXiv, suggests that a similar effect is making plasma particles lose angular momentum and so end up slowing down the external shell of our star. The effect is very small but it’s constant, and over time has become very noticeable.

“This is a gentle torque that is slowing it down, but over the Sun's 5-billion-year lifetime it has had a very noticeable influence on its outer 35,000 kilometers," team leader Jeff Kuhn, from the University of Hawaii Institute for Astronomy, said in a statement.

The researchers used a technique known as asteroseismology, and just like seismology helps scientists understand Earth’s interior, astronomers can look at the light of the stars and see how it is affected by the oscillation from inside. This led them to the discovery that the rotation depends on the distance from the center and from the equator.

“The Sun won't stop spinning anytime soon, but we've discovered that the same solar radiation that heats the Earth is 'braking' the Sun because of Einstein's Special Relativity, causing it to gradually slow down, starting from its surface,” Kuhn explained.

It is not certain that this effect is responsible but the team believes that is a good explanation and they suggest that it should be investigated further. If the photon-breaking effect is truly responsible for the different spin speed, it’s important not only for the Sun but for every star, as they might have similar effects.