The quest for Planet X always starts out with celestial objects behaving badly. Astronomers notice that a known planet, or a bunch of comets, begin moving in ways Newton's laws of motion can't explain. They propose that it's caused by the gravity of something massive and still undiscovered lurking out in the Solar System, and they head to their telescopes to search for it.
http://www.time.com/time/health/article/0,8599,2049641,00.html?xid=newsletter-weekly
