The astronomer who discovered the dwarf planet Eris suggests there might be another object far beyond the Kupier belt. If this Planet X exists, it would be about 10 times the mass of Earth and 2-3 times the size of Earth, putting it in the ice giant category, and have an orbit with a semi-major axis of 700 AU. You can read more about this object on NASA's page. If this object exists, what would we classify it as?

These objects would be classified as extreme trans Neptunian object (ETNO).

Explanation:

ETNO’s are the objects lying beyond the planet Neptune and orbiting the Sun. They follow a highly eccentric path which is tilted. ETNO has been grouped into three major according to their respective perihelia.  

Within this region (beyond Neptune’s orbit), a hypothetical planet has been discovered. It was discovered following its gravitational effect on the other objects of Kuiper Belt (region beyond the orbit of the Neptune- the last planet of our Solar system)

The Planet is assumed to be around 2 times the Earth’s size and around 10 times heavier than Earth.  

b) phosphorus pentoxide