Every 11 years or so the Sun has a peak in its activity; the number of solar spots increase, its luminosity grows, and its magnetic field gets all twisty. This solar cycle is not unusual as other stars are expected to undergo periodic oscillations of their magnetic fields, but models have so far struggled to explain exactly what is going on.

Researchers from the Helmholtz-Zentrum Dresden-Rossendorf, a German Institute, have found evidence that the peculiarity of the regular solar cycle is to be found not within the Sun, but in its planets. Venus, Earth, and Jupiter have a tidal pull on our star that affects the hot plasma on its surface, much like the gravitational pull of the Moon affects tides on Earth. The team considered their regular alignment, every 11.07 years, to be the one keeping the tempo for the solar cycle. The model is published in the journal Solar Physics.

The coincidence in the periods of alignment and the solar cycle has been known for a long time, but the tidal forces exerted by the planets on the surface remained tiny compared to the phenomenal forces in the interior of the Sun. But researchers discovered that the magnetic field in plasma can be dramatically disrupted by tiny oscillations, in certain conditions. The team believes that these conditions might exist in the Sun. 

The physical effect, known as the Tayler instability, can be seen in the team’s simulation. A small force can suddenly change the solar plasma's helicity, flowing in either a clockwise or anticlockwise corkscrew motion.

“When I first read about ideas linking the solar dynamo to planets, I was very skeptical,” lead author Frank Stefani said in a statement. “But when we discovered the current-driven Tayler instability undergoing helicity oscillations in our computer simulations, I asked myself: What would happen if the plasma was impacted on by a small, tidal-like perturbation? The result was phenomenal. The oscillation was really excited and became synchronized with the timing of the external perturbation.”

The team corroborated the findings with solar observations from the last 1,000 years, with the alignment of the three planets. The Sun is also subjected to longer cycles and changes in the intensity of each cycle and the team put forward the idea that the planets might play a role in that too.  

“There is an astonishingly high level of concordance: what we see is complete parallelism with the planets over the course of 90 cycles,” added Stefani. “Everything points to a clocked process."

The prospect is certainly intriguing and researchers are now setting up a liquid metal experiment to simulate the type of oscillations that could generate those Tayler instabilities in the lab. They hope to strengthen their hypothesis with such a setup.  

The Sun holds over 99 percent of the mass of the Solar System but despite its size can still be influenced by planets. For example, Jupiter is big enough (2.5 times the mass of all the other planets combined) and far away enough, that the center of mass of the Jupiter-Sun system is actually outside the Sun, which means Jupiter doesn’t orbit the Sun. It actually orbits a point about 50,000 kilometers (30,000 miles) from the surface of the Sun.