  The radio tracking apparatus of the New Horizons spacecraft, currently
traveling to the Pluto system where its arrival is scheduled for July 2015,
should be able to reach an accuracy of 10 m (range) and 0.1 mm s^-1
(range-rate) over distances up to 50 au. This should allow to effectively
constrain the location of a putative trans-Plutonian massive object, dubbed
Planet X (PX) hereafter, whose existence has recently been postulated for a
variety of reasons connected with, e.g., the architecture of the Kuiper belt
and the cometary flux from the Oort cloud. Traditional scenarios involve a
rock-ice planetoid with mX = 0.7mE at some 100 - 200 au, or a Jovian body with
mX = 5mJ at about 10,000 - 20,000 au; as a result of our preliminary
sensitivity analysis, they should be detectable by New Horizons since they
would impact its range at a km level or so over a time span six years long.
Conversely, range residuals statistically compatible with zero having an
amplitude of 10 m would imply that PX, if it exists, could not be located at
less than about 4,500 au (mX = 0.7mE) or 60,000 au (mX = 5mJ), thus making a
direct detection quite demanding with the present-day technologies. As a
consequence, it would be appropriate to rename such a remote body as Telisto.
Also fundamental physics would benefit from this analysis since certain subtle
effects predicted by MOND for the deep Newtonian regions of our Solar System
are just equivalent to those of a distant pointlike mass.
