There are days when it seems that our outer solar system has the singular goal of frustrating astronomers. In a way, we have no one to blame but Newton. After all, his equations of motion allowed researchers to realize that Uranus’ motion required something to be pushing and pulling on its orbit. William Herschel found Uranus in March 1781, and for the next several years, he and other observers carefully watched its slow motion through the stars. By 1845, it had completed an orbit that could only be explained by the presence of another large planet in the outer solar system. Mathematicians...