Pluto's structure and composition suggest origin in the solar, not a planetary, nebula

The mean density of the Pluto-Charon system is now accurately known at 1.99±0.09 g cm^-3 (With formal errors five times smaller)¹, through observations of total occultations and transits². Even allowing Charon's density to vary between 1 and 3 g cm^-3 constrains Pluto's density between 1.84 and 2.14 g cm^-3. Pluto is thus very rock-rich, with a rock/(rock + H₂O-ice) mass ratio of ̃0.68-0.80, much greater than those of the icy satellites Ganymede, Callisto or Titan. It is so rock-rich that spontaneous unmixing of rock and ice phases in its convecting interior is just possible, and any significant differentiation during accretion or during Charon's formation, which may have involved a large-body impact^3-5, renders Pluto unstable to further melting and differentiation because the resulting multiple thermal boundary layers inhibit heat transport⁶. Continual loss of methane over the age of the Solar System^7,8 requires a near-surface methane reservoir, and implies that non-trivial differentiation has occurred⁹. Pluto is probably a differentiated object whose icy mantle is entirely in the ice-I stability field; its closest structural cousin is Europa. Of four explanations for Pluto's large rock/ice ratio¹⁰ - formation in the inner Solar System, volatile loss during accretion, volatile loss during the large-body impact that created Charon, and formation as a large, ice-poor^11,12 outer Solar System planetesimal - we show that only the last two are feasible, and that the depletion of water ice in Pluto is so severe that both explanations may be necessary.