"...calculated that it would require 648,000 comets, each 200 kilometres wide..."So apparently it would require an obscenely great number of very large things to be passing the star.
While I would suggest that it is possible that this is the case, A cloud of hydrogen from an exploded star could be passing this star providing increased consistent dimming over time as hydrogen builds between us and the star at a constant rate. But how does this explain the large object variation in the dimming? It's explained by planets passing through the hydrogen flow. These planets and star collect hydrogen leaving shadows of reduced hydrogen in the hydrogen flow.
So a colossal hydrogen flow headed our way equals extinction level event. Our own planet will suffocate as hydrogen turns to water with air, meanwhile the sun will transmit heat more efficiently through the hydrogen choking the solar system. On Venus the hydrogen will cook with the CO2 atmosphere creating long chain hydrocarbons and the beginnings of life.
We now need to know:
- Is it a hydrogen cloud?
- How long do we have until it gets here?
- How fast is it travelling?
- How big an area does it occupy?