Solar system conspiracy theorists have long harbored suspicions that a hidden extra planet or dwarf star lies beyond the orbit of Pluto. As a planet, it’s been called Tyche or simply Planet X. As a star, Nemesis.
But an exhaustive search has found no hints of this long-rumored object. NASA's Wide-Field Infrared Survey Explorer has scanned the entire sky in infrared light—twice—and ruled out any hidden bodies larger than Saturn out to 10,000 times the distance from the Earth to the sun.
Furthermore, it found no evidence for anything bigger than Jupiter out to 26,000 times the Earth-sun distance. Pluto, for comparison, lies only 40 times farther from the sun than Earth. [Kevin L. Luhman, A Search For a Distant Companion to The Sun with the Wide-Field Infrared Survey Explorer, in the Astrophysical Journal]
The finding deals a blow to theories that a Planet X- or Nemesis-type object may have caused periodic mass extinctions on Earth. Some theorists have mused that a hidden planet might have swept through bands of comets in the solar system, sending them crashing into Earth. Now it seems that for such mass species die-offs, the fault lies not in the stars.
[The above text is a transcript of this podcast.]