The Sunspot Cycle

Claims versus evidence: how to weigh an idea like this

An idea about nature earns its place only if it can pass three tests, in order. First it needs a believable mechanism: a real physical way for the cause to produce the effect. Second it has to survive the numbers: the proposed force must actually be strong enough to do the job. Third it must match the data in phase: the predicted timing has to keep step with what we observe, not just for a cycle or two but across the whole record.

The planetary idea has been argued at three very different levels of seriousness, and these three tests sort them out cleanly.

A cautionary tale: Tice's "Jovial cycle"

In the 1870s an American weather forecaster named John H. Tice claimed that Jupiter, on reaching certain points in its orbit, set off an electrical disturbance across the solar system that triggered sunspots, storms, and even earthquakes on a roughly twelve-year "Jovial cycle." It failed every test.

There was no real mechanism, only an assumed one; the system-wide electrical forces he imagined do not exist; and when his predictions missed, he explained them away by appealing to a planet, Vulcan, that turned out not to exist. Reviewers of his own day dismissed the theory as built on a groundless assumption. Tice is the textbook example of how a tidy-looking cycle can be pure coincidence dressed up as a law.

A serious modern question

A small number of present-day scientists, publishing in peer-reviewed journals, ask a far more careful version of the question. They notice the 11.07-year Venus-Earth-Jupiter beat and Jupiter's 11.86-year orbit, and they wonder whether the planets might gently synchronize the Sun's internal clock, the way a faint but regular push can keep a child's swing going in time. This is not astrology, and it is not Tice.

It passes the first test, because a plausible mechanism has been proposed: a slight nudging of waves deep inside the Sun. But it stumbles on the second. The tidal bulge the planets raise on the Sun is less than a millimeter high, and Jupiter's tug at the depth where the Sun's magnetism is generated is about ten thousand times weaker than the churning gas motions already at work there.

To rescue the idea, its supporters have to assume the Sun hugely amplifies that tiny push, a step most solar physicists do not accept. It also struggles with the third test: critics have shown that the supposed 11.07-year signal does not actually stand out in the planetary tides, and a rigid planetary clock drifts away from the real cycle within a few decades, exactly as the chart above shows.

What the evidence actually supports

The mainstream explanation needs no planets at all.

The Sun generates its own magnetism through a dynamo: its churning, electrically charged gas, wound up by the Sun's own rotation, builds and then sheds magnetic field on its own schedule. The mainstream Babcock-Leighton dynamo model reproduces both the 11-year Schwabe cycle and the 22-year Hale flip directly from the physics of the Sun, and it remains the model that best fits the observations.

So the honest summary is this: the planets almost certainly do not cause the sunspot cycle. Whether they very slightly influence its timing is a genuine open question that a few researchers continue to test, and the burden of proof, rightly, still rests with them. That is the difference between a coincidence, a hypothesis, and an established explanation, and it is worth keeping in mind whenever any cycle is said to drive another.