The Planet X Hypothesis: The History and Legacy of the Theory that the Solar System has a Hidden Planet

Naturally, as technological advances and the creation of flying aircraft became realities, the sighting of UFOs increased, as did the interest in potential contact with aliens. While incidents like the one at Roswell led to conspiracies and a craze among those who insisted the government was hiding proof of extraterrestrials' existence, governments across the world were actually secretly studying UFO sightings by the mid-20th century.

Given all of that, it was hardly groundbreaking for scien...