According to a paper published in the May 2019 issue of Solar Physics,2 three researchers in Germany correlated the planetary alignments – which occur every 11.07 years (sound familiar?) – with the times of solar minimum going back 90 solar cycles – covering more than 1000 years – and came up with a 100% match. Their hypothesis is that the relatively tiny pull of the three planets' combined gravity has a tidal effect on the Sun that sets off something called a Tayler instability, which in turn, leads to the reversal of the Sun's magnetic field that is typically associated with the birth of a new solar cycle.
While their research is continuing, the researchers feel this association will make it much easier to precisely predict the start of each new cycle. They do not, however, offer an explanation of why some solar cycles are much stronger or weaker than others. (Tnx to the HAMSci reflector)
1. For you young whippersnappers out there, that was from the opening line of the song, "Aquarius / Let the Sun Shine In," by the 5th Dimension, from the 1960s musical "Hair." Obviously, we're still waiting for that age of peace and love.
2. "A Model of a Tidally Synchronized Solar Dynamo," Stefani F., Giesecke A., Weier T., Sol Phys (2019) 294:60. <https://doi.org/10.1007/s11207-019-1447-1>