Since the early 1990s, space scientists have known that the sun’s surface rotates more slowly than its interior. But they haven’t been able to pinpoint why.

Now, astrophysicists at the University of Hawaii say they have a hunch. They’ve identified that the light from the sun acts like a “centrifugal sprinkler system” to blast photons into outer space, prompting an exchange of momentum that causes the rotation of the outside of the sun’s surface, where plasma is thinnest, to lag behind its core.

The sun and other stars shed photons in every direction. These photons have no mass, but are pure energy. Because of this, the particles carry a miniscule amount of angular momentum as they leave the sun.

Jeff Kuhn, a U. of Hawaii astrophysicist who co-authored a new study published in Physical Review Letters, says that when photons are ejected from the sun into outer space, they take some of the sun’s rotation with them. The slowdown is almost imperceptible, but by using helioseismology — the study of how sound and gravity waves move through the sun — the astronomers discovered this solar feature.

“The sprinkler has the nozzles directed off at an angle, and the water carries the momentum just like the photons,” Kuhn says.

The slowdown will eventually affect the entire sun, but for right now, it’s only visible in the surface layer

The team obtained their data from NASA’s Solar Dynamics Observatory over a period of three years, beginning in 2010. Observing the entire sun would have produced images with a limited resolution, so the researchers zoomed in on the edge of the sun, where they could see the surface activity in more detail.

Their model of the sun’s movement revolves around the star’s turbulent plasma. Plasma is a high energy state of matter, in which electrons and protons float freely. The sun’s plasma gets thicker as it moves inward towards the “tachocline,” an area deep within the star that separates the solid-like interior from the turbulent outer zones.

As the photons interact with the different thicknesses of material deep beneath the sun’s surface, they exchange momentum. This effect is more noticeable it the outer layers, where the plasma is thinnest.

“A similar effect happens with micrometeoroids in the inner solar system, but in reverse, that makes them spiral into the sun so there aren’t any in the inner solar system,” said William Dean Pesnell, a project scientist for NASA’s Solar Dynamics Observatory who wasn’t involved in the study.

Only five percent of the sun’s total volume — the 43-mile thick outer layer of the sun’s photosphere — has any sort of noticeable reduction in speed. The researchers posit that all stars may experience these slowdowns, but the effect becomes more apparent in larger and brighter stars.

The jury is still out on whether the theory from Kuhn and his colleagues holds up. Pesnell said scientists would have to study the sun for another million years to know for sure.

But researchers estimate the rate of the sun’s slowdown is, to put it simply, slow. The sun will expand and consume our planet long before we would need to worry about the sun’s rotation stopping for good, they say.