My summary of the full New Scientist article:
[link to space.newscientist.com] Lykawa is NOT alone in the astronomical community in his belief that something out there is causing unexplained features in the Kuiper Belt, the belt of icy bodies of which Pluto is one of the largest members. He points to the so-called 'Kuiper cliff', the region where the number of KBOs falls off very sharply. Using Saturn's rings as an analogy, this suggests that a large object orbiting further out is causing this sharp edge to the belt. Then he points to the fact that they form at least three very distinct orbits - suggesting the same thing. Lykawa did a computer simulation follow-up study to some work by scientists at Queen May College in London and the National University of La Plata in Argentina, which hypothesized that a Planet X was responsible but which failed to explain all of the Kuiper belt's features. Lykawa's new work ruled out this hypothesis, as well as many other Planet X proposals. So he started to examine the possibility of a more distant unseen planet having created the peculiar, extremely eccentric orbit of Sedna, which has never been satisfactorily explained. He now thinks that there is a Planet X that was ejected by a young Neptune into an elongated orbit that stirred up the Kuiper belt, sweeping part of it clean of debris and creating the Kuiper cliff. According to Kuiper, Neptune's outward migration over millions of years pushed Planet X into a distant orbit that gave Sedna its peculiar orbit. His Planet X would take anywhere between 1000 and 2500 years to complete one orbit, compared with Pluto's 248 years. It would never get nearer to the sun than 80 AU and its orbital inclination to the Ecliptic could be as much as 40 degrees.

Other astronomers think Lykawa's proposals for explaining the structure of the Kuiper belt are plausible. Astronomers with rival theories are more critical and have explained the sharp-edge features of the belt without resorting to hypothesizing a Planet X (www.arxiv.org/abs/0712.0553), although they admit their theory has its own problems. However, they judge that its successes outweigh its difficulties.

Caltech astronomer Brown thinks Planet X could easily have been missed in his hunt for large KBOs, which is far from complete. Future comprehensive surveys like Pan-STARRS will prove whether a large, Mar-like object beyond Pluto exists out.
(end of summary)

So the message of this tale is this. Astronomers do NOT rule out the possibility of a large planet being found beyond Pluto. Any debunker of Planet X who tells you it should already have been discovered is totally uninformed about what is going on in the on-going search by astronomers for more KBOs.