The planet Tyche is a theorised, hypothetical object orbiting the Sun in the far reaches of the Solar System. First proposed in 1999, the name was chosen for a reason. Tyche was the Greek goddess of fortune, or plain old luck and is a sort of "good" counterpart of another Greek goddess, Nemesis. This, as you may have seen already from this site, is the name given to a hypothetical star orbiting the Sun out beyond the Oort Cloud and thus the planet's name was chosen to reflect the connection.
Let's just consider the Oort Cloud for a moment...
Now, I'm fully aware that the Oort Cloud has its own page on this site here, but certain features of it have to be reiterated to explain why planet Tyche was theorised in the first place.
First off, it's thought to be where long-period comets come from. Lying maybe as much as one light year from the Sun, it is assumed to be spherical in shape, because these comets appear randomly from all points of the sky.
But then, along came three University of Louisiana astrophysicists, John Matese, Patrick Whitman and Daniel Whitmire, who, in 1999, challenged this view.
They didn't dispute the fact that the Oort Cloud was spherical; their assertion was that comets didn't appear from random points at all, their orbits were all contained within a band that was inclined to the plane of the Solar System.
The only way to explain this, they suggested, was the existence of an object, larger than Jupiter, situated way out there, nudging the comets into these common orbits. Thus the Tyche theory was born.
It was proposed that a planet of around four Jupiter masses was orbiting at a distance of nearly one quarter of a light year from the Sun, every 1.8 million years.
The orbit would be in a different plane from that of the Solar System, thus accounting for the scientists suggested comet orbit anomaly.
They further speculated that, despite its larger mass, Tyche's size would be comparable to that of Jupiter and would have a comparatively warm surface temperature - minus 17 degrees Celsius.
The NASA space telescope Wide-field Infrared Survey Explorer (WISE) has conducted a number of all-sky surveys since 2012, which included all locations where the scientists predicted that Tyche might be found.
The latest conclusion, in March 2014, is that there is no evidence of any object of Jupiter's size or above to be found anywhere, right out to 0.41 light years.
Not very promising, then, is it?