[fastaguy88]

Just because one can imagine an impossible experiment does not prove that a “science” is not falsifiable. There are lots of predictions that climate science makes that are falsifiable. And one can imagine that at some point in the future, we will be able to do experiments on appropriately paired sets of planets.

The phase “unfalsifiable” is often used to suggest that something is not scientific. My recollection is that Popper thought that evolution was not a scientific theory for the same reason. But unfalsifiable depends a lot on the kinds of experiments that are possible, or might become possible in the future.