An astronaut, Dr. Aris Thorne, is alone in orbit around a newly discovered exoplanet, designated Kepler-186f. Using the ship's most advanced optical telescope, he observes what appears to be a large, perfectly rectangular structure on the planet's surface. It seems to be slowly but consistently traversing the rugged alien landscape. This is perplexing, as no probes have ever landed there, and the planet is believed to be uninhabited. What is the most logical explanation for what Dr. Thorne is observing?
Correct: A piece of debris or dust has settled on one of the telescope's internal lenses.
The trick here lies in shifting your perspective from the observed object to the observer's instrument. A piece of dust or debris on an internal lens of the telescope would be magnified along with the distant view. As Dr. Thorne's ship orbits the exoplanet, the planet's surface would appear to 'scroll' past in the eyepiece. Since the dust particle is fixed relative to the telescope (and thus the eyepiece), it would create the illusion that the 'rectangular structure' (the dust particle) is slowly moving across the background of the alien landscape. This explains the 'perfectly rectangular' shape (a common shape for dust or debris) and its consistent, slow 'movement' without needing an external, impossible explanation on the planet itself.