If Planet X is out there in the icy reaches of the Solar System, we should see its affect on the orbital motion of the inner planets
"Does the Solar System contain undiscovered massive planets or a distant stellar companion of the Sun?" asks Lorenzo Iorio at the Istituto Nazionale di Fisica Nucleare in Pisa, Italy.
If it does, then the orbital period of such a body would be so long that any gravitational influence on the known planets can be thought of as a constant, tiny perturbation.
Iorio has looked at a the perihelion precessions of Venus, Earth and Mars in the last century and asked whether there is any indication of such a force in action.
Apparently not, he says, but that doesn't rule out its presence. Instead, it has allowed Iorio to place limits on how far away such a planet might be.
His conclusion is that if Planet X is out there and about the size of Earth, it must more than 130 AU away. If it is the size of Jupiter, it cannot be closer than 886 AU and if it were a brown dwarf with a mass around 80 times that of Jupiter, it would have to be more than 3800 AU away.
Let's get looking!