"
Two centuries of observations have indicated an anomaly that suggests the 
existence of Tyche, Matese said. "The probability that it could be caused by 
a statistical fluke has remained very small," he added.

The pull of Tyche might also explain why the dwarf planet Sedna has such an 
unusually elongated orbit, the researchers added.

If Tyche existed, it would probably be very cold, roughly minus 100 degrees 
F (-73 degrees C), they said, which could explain why it has escaped 
detection for so long - its coldness means that it would not radiate any 
heat scientists could easily spot, and its distance from any star means it 
would not reflect much light.

"

http://www.space.com/scienceastronomy/secret-giant-planet-sun-companion-1011201.html