“Their (the Mi-Go’s) main immediate abode is a still undiscovered and almost lightless planet at the very edge of our solar system – beyond Neptune, and the ninth in distance from the sun… And it will soon be the scene of a strange focusing of thought upon our world in an effort to facilitate mental rapport. I would not be surprised if astronomers become sufficiently sensitive to these thought currents to discover Yuggoth when the Outer Ones wish them to do so.”
The Whisperer in the Darkness was published in Weird Tales in 1931. Pluto was discovered on February 18, 1930, about the same time that Lovecraft was writing the story.
Lovecraft on Pluto in the form of a meme:
Steal this meme.