  The existence of an outer planet beyond Pluto has been a matter of debate for
decades and the recent discovery of 2012 VP113 has just revived the interest
for this controversial topic. This Sedna-like object has the most distant
perihelion of any known minor planet and the value of its argument of
perihelion is close to 0 degrees. This property appears to be shared by almost
all known asteroids with semimajor axis greater than 150 au and perihelion
greater than 30 au (the extreme trans-Neptunian objects or ETNOs), and this
fact has been interpreted as evidence for the existence of a super-Earth at 250
au. In this scenario, a population of stable asteroids may be shepherded by a
distant, undiscovered planet larger than the Earth that keeps the value of
their argument of perihelion librating around 0 degrees as a result of the
Kozai mechanism. Here, we study the visibility of these ETNOs and confirm that
the observed excess of objects reaching perihelion near the ascending node
cannot be explained in terms of any observational biases. This excess must be a
true feature of this population and its possible origin is explored in the
framework of the Kozai effect. The analysis of several possible scenarios
strongly suggest that at least two trans-Plutonian planets must exist.
