A New Ninth Planet?

For most of human history, our solar system had 6 planets (counting the Earth), the ones that could be observed by eye, wandering across the night sky. In 1781, William Herschel's telescope revealed a seventh -- Uranus -- but it was the eighth planet that became the great triumph of nineteenth century Newtonian celestial mechanics. In the 1840s, unexplained perturbations in the orbit of Uranus led French mathematical astronomer Urbain Jean Joseph Le Verrier and English astronomer John Couch Adam...