  The make-up of the outer planets, and many of their moons, are dominated by
matter from the H-C-N-O chemical space, commonly assumed to originate from
mixtures of hydrogen and the planetary ices H$_2$O, CH$_4$, and NH$_3$. In
their interiors, these ices experience extreme pressure conditions, around 5
Mbar at the Neptune mantle-core boundary, and it is expected that they undergo
phase transitions, decompose, and form entirely new compounds. In turn, this
determines planets' interior structure, thermal history, magnetic field
generation, etc. Despite its importance, the H-C-N-O space has not been
surveyed systematically. Asked simply: at high-pressure conditions, what
compounds emerge within this space, and what governs their stability? Here, we
report on results from an unbiased crystal structure search amongst H-C-N-O
compounds at 5 Mbar to answer this question.
