Planet X (pronounced as the letter, not the Roman numeral) is a hypothetical planet that has been searched for throughout the years by many astronomers. While searching for this planet, astronomers “found” Pluto, and Xena or Eris, all classified now as dwarf planets and not considered true planets. Planet X was not usually considered to be the 10th planet because when the search began for it, astronomy experts had only discovered eight. Instead the name merely means an unknown planet that exists beyond Neptune.
Neptune was identified as a planet because astronomers found oddities in the orbits of the Uranus, Jupiter and Saturn, which many believed had to be caused by a planet still father from earth and the sun. Some of the mathematical discrepancies in these planet’s orbits, and in Neptune itself, indicated the presence of another planet beyond Neptune. When Pluto was discovered in 1930, it wasn’t large enough to explain the orbital patterns of all four planets; thus science looked for something beyond Pluto, possibly a gas giant as large as Jupiter.
For most of the 20th century, astronomers continue to postulate on the existence of Planet X, and tried very hard to find it. By the 1990s, one of the arguments against the existence of such a planet was that probes sent into space haven’t moved in a way that would suggest the strong gravitational pull of a Planet X. Another contributor to the demise of the Planet X theory is that the space probe Voyager 2 gave more accurate measurements of Neptune, which explained away mathematical irregularities in its orbit.
There are some scientists who still posit the possible existence of a Planet X, but have reimagined the concept of what the planet might be. Some suggest a brown dwarf star and not a planet. Other astronomers like Patryk Lykawka have made calculations that suggest we’ll determine for certain by 2013 whether such a planet exists.
The hypothetical additional planet has captured the imagination of many and appears in numerous science fiction movies, TV shows and books. Robert Heinlein’s novel The Puppet Masters suggests ten planets in the solar system, the last called Kalki. Animated characters like Duck Dodgers and some of the Transformers refer to Planet X. Fictional renderings of the planet may refer to the 10th planet or the hypothesized huge planet that we have not yet, and may never find, because it doesn’t exist.