Ever since the discovery of the ninth planet, Pluto, astronomers have speculated about whether a still more distant, tenth planet may exist. This thinking was initially based on two lines of reasoning. First, astronomers had been successful at discovering Uranus, and then Neptune, and then Pluto. This led them to believe that additional planets might await discovery farther out, if they only searched hard enough for them. Second, the search for Pluto, like the search for Neptune before it, had been based on the apparent tug of a giant, unseen world affecting the orbit of its predecessor. Yet after Pluto's discovery, it became clear that the newly found planet was very small, smaller than the United States. Such a small world could not have tugged significantly on Neptune.

Not long after the discovery of Pluto, however, it was discovered that the discrepancies in the orbit of...