Planet X is a theoretical and experimental enigma that has spanned over a century and a half. Planet X was theorized by a number of astronomers in the mid-1800s shortly following the discovery of Neptune as it was found that the gravitational effects of the discovered planets were not enough to explain the trajectories of Neptune and Uranus. Something was missing from our model for the solar system. Thus the theory of Planet X, a new planet that would account for these perturbations in the outer planets, was hatched.
Surveys searching for Planet X continued well into the 19th century. Along the way, Pluto was discovered and named the ninth planet. However, the mass of Pluto and thus its gravitational effects were too minuscule to account for the discrepancies in the outer planets’ orbits. There was still something else to be found in our solar system.
Some started to give up hope on finding a Planet X and abandoned the theory of an undiscovered planet for the theory of an undiscovered star, in relatively close orbit to the sun. They called this theoretical star Nemesis, or the death star. This theory was also very attractive because its gravitational effects could also explain another problem in science: the 65 million year periodicity in mass extinctions (thus its dramatic name). Theoretically, if Nemesis passed through the Oort cloud every 65 million years, it can hurtle enough chunks of ice and rock that some of them end up hitting the Earth, killing off species. You may think that it is unlikely that a star may be orbiting the sun because we would SEE it. However, whereas our sun emits light mostly in the visible spectrum, stars with lower mass emit in the infrared. This kind of light is invisible to the human eye, and much of infrared radiation is blocked by the atmosphere, so it stands to reason that such a star would be invisible to us. Surveys of the night sky in the infrared (including one at UC Berkeley carried out by Saul Perlmutter) failed to turn up Nemesis, so the main contender for the gravitational effects on the outer planets was still Planet X, though some still believe that the Nemesis theory is still a contending theory to explain the periodicity of mass extinctions as infrared surveys of the sky are far from complete.
In 2005, many more planets were unearthed including Xena (in reference to the X in Planet X as well as the warrior princess from the awesome television series), which was found to have an even larger mass than Pluto. Xena, or Sedna as it is more commonly know as today, was instrumental in demoting Pluto as a planet. Here is an artist’s conception of Sedna juxtaposed with an actual observation of Sedna. (Moral of the story, don’t trust artist’s conceptions)
So is the search over? Well, yes, but not because of the discovery of Sedna. With new observations from a fly-by space probe, they found that the mass of Neptune was very slightly smaller previously thought. Using the new mass of Neptune, the trajectory calculations matched the observed trajectory. It may seem from this narrative, then, that astronomy and planetary science is rather accidental in nature, which is not far from the truth. The discovery of Pluto and Sedna had more or less been accidents in that they might not have happened if we had better estimates for the mass of Neptune. However, behind these accidents was the truly wonderful scientific process. A theory (Planet X) was proposed to explain a phenomena (unusual orbits), the theory was tested (by trying to find Planet X), and disproved (by not finding Planet X). The theory was revised (with the mass of Neptune) and tested again (matching up trajectories) and finally the theory tested out.