This has been ruled out previously, as the mechanism was unknown. Scientists have now discovered that:

The tidal forces of Venus, Earth and Jupiter produce a gravitational pull that can cause changes to the plasma on the surface of the sun. These forces are at their strongest when there is a maximum alignment of the three planets—an event that takes place once every 11.07 years.

The researchers compared observations of solar activity over the last 1,000 years with the planetary alignment—and found the two are indeed linked. “There is an astonishingly high level of concordance,” Frank Stefani, lead author of the study, said in a statement. “What we see is complete parallelism with the planets over the course of 90 cycles. Everything points to a clocked process.”

They argue that a physical effect that can change the behavior of a liquid or plasma—known as the Tayler instability—could be responsible.
Newsweek