Susan Fraser, of Wellington, asks :-
Could it be true that the solar system exerts a modulating influence on solar cycles?
Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.
I don't think there is evidence that the tidal pull of planets has any effect on solar activity. If there was then it would be much more discussed in the mainstream scientific literature.
The tidal effects of planets are minute. Tidal effects fall away as the distance of the object cubed. That is, if the object is twice as far away then its tidal effect is 1/8th (= 1/2 x 1/2 x 1/2) of what it would be at a unit distance. Take it three times the original distance further away and the tidal effect is 1/27th, and so on. One gets an idea of this from the fact that the sun and moon make roughly similar tides on our oceans even though the sun is 27 million times the mass of the moon.
The causes of the sun's activity are complex and not fully understood. The biggest force acting on the sun's material is the heat flow from within the sun. It makes convection currents, etc. The second force is the magnetism of the sun. Because the gases of the sun are ionized (charged) they are strongly affected by any magnetism. The sun has a strong magnetic field that gets wound up by the rotating gases. It is the magnetic field popping through the surface that makes sunspots.
Much is being learned about the sun's interior by new technologies measuring low- frequency sound waves going through the sun. It's similar to earthquake waves going through the earth. To get an idea of all this research start with http://umbra.nascom.nasa.gov/index.html/
An interesting suggestion coming from this work is that the sun may soon enter a 'Maunder Minimum'. That is when it shows no sunspots for decades. It did this from about 1645 till 1715. Unfortunately there are only about 20 years of observations to base this suggestion on.
I think the idea that the planets have an effect on the sun's activity is in the same category as Ken Ring's attempts to predict long-range weather by atmospheric tides on the Earth. Sure, there are atmospheric tides. But they are a minute force compared to the convection effects caused by the temperature difference between the equator and the poles. Earth's rotation adds another force to the moving air, causing it to rotate as it moves in latitude. That makes the anticyclones and depressions. Any tidal forces are overwhelmed by convection and other effects.