There's no evidence for the existence of Planet X, despite a NASA space telescope's best efforts to track it down.
The hypothetical world that may or may not be orbiting the sun beyond the orbit of Pluto has inspired many a doomsday theory. In the run-up to the much anticipated "Mayan Doomsday" of Dec. 21, 2012, the marauding Planet X was scheduled to make a inner-solar system dash, sparking gravitational mayhem, triggering civilization-ending solar flares. Some doomsayers held onto the crackpot notion that Planet X could be the fictional planet "Nibiru" that is inhabited by the Annunaki, an alien race hellbent on re-claiming Earth as their own.
15 months later, we all know how that alien invasion went - apparently we won.
Top Doomsday Predictions Gone Bust
All this doomsday nonsense to one side, the hunt for "Planet X" actually has roots in real science. In the mid- to late-19th Century, astronomers were tracking the gravitational perturbations of the gas giant planets in an effort to track down an undiscovered world in the outermost reaches of the solar system - this hypothetical massive planet was dubbed "Planet X." However, this fascinating trail of discovery ended at the discovery of tiny Pluto in 1930. Lacking the gravitational oomph to explain the gravitational perturbations, it turned out that Pluto wasn't the Planet X astronomers thought it would be. After the realization that the gravitational perturbations observed were more likely observational error, Planet X became a story of legend.
The idea that the sun may have a stellar partner has also been investigated - perhaps there's a brown dwarf (a failed star) going unnoticed out there. Nicknamed "Nemesis," this binary partner could be evading detection.
A few oddities in the outer solar system have given astronomers pause to think that something massive might be lurking out there, however, whether it be a massive planet or sub-standard star. One strong piece of evidence laid in the discovery of the "Kuiper Cliff," a sudden drop-off of Kuiper Belt objects in the region just beyond Pluto. Could the Cliff be caused by a previously overlooked world? Also, geological record has suggested there's a regularity to mass extinctions on Earth linked to comet impacts - could a distant orbiting body be perturbing comets, sending them our way on a cyclical basis?
ANALYSIS: Is ‘Planet X 2.0′ Lurking Beyond Pluto's Orbit?
"The outer solar system probably does not contain a large gas giant planet, or a small, companion star," said Kevin Luhman of the Center for Exoplanets and Habitable Worlds at Penn State University, University Park, Pa.
Luhman and his team have analyzed data from NASA's Wide-Field Infrared Survey Explorer (WISE), a space telescope that carried out a detailed infrared survey of the entire sky from 2010 to 2011. If something big is lurking out there, WISE would easily have spotted it. Alas, WISE has turned up no Planet X candidate. Previous observations by WISE have also ruled out the Planet X-comet perturbation theory.
According to a NASA news release, "no object the size of Saturn or larger exists out to a distance of 10,000 astronomical units (AU), and no object larger than Jupiter exists out to 26,000 AU. One astronomical unit equals 93 million miles. Earth is 1 AU, and Pluto about 40 AU, from the sun."
ANALYSIS: Does a Massive Planet Lurk in the Outer Solar System?
However, the modern search for a Planet X was never WISE's prime mission. In a second study, the discovery of 3,525 stars and brown dwarfs within 500 light-years of the sun are detailed. In cosmic distances, these objects are right on our galactic doorstep. Both studies have been published in The Astrophysical Journal.
"Neighboring star systems that have been hiding in plain sight just jump out in the WISE data," said WISE principal investigator Ned Wright of the University of California, Los Angeles.
During its prime mission, WISE was able to capture two full scans of the infrared sky approximately 6 months apart. By comparing the positions of objects in the two scans, astronomers are able to deduce how much the objects have moved. The greater the positional shift, the closer the object is to Earth. This is known as the parallax effect and provides astronomers with a valuable tool to detect how close a celestial object is to Earth.
When WISE's cryogenic helium ran dry, its primary mission came to an end, but late last year, the space telescope was rebooted to continue to search for the infrared signals of near-Earth objects and renamed NEOWISE.