Sketch of a possible symbol proposal as one could be done if Planet Nine will in fact be detected. It has the overall shape of the digit 9 in a way which resembles also the mirrored shape of Arabic and Devanagari forms of this digit. The small partial ellipse within the head resembles the orbits of the trans-Neptunian objects, on which the claim of the existence of Planet Nine was based at first.
Own work , own idea
Karl432