2017MNRAS.469.3994B

Chemical compositions of giant planets provide a means to constrain how and where they form. Traditionally, super-stellar elemental abundances in giant planets were thought to be possible due to accretion of metal-rich solids. Such enrichments are accompanied by oxygen-rich compositions (i.e. C/O below the disc's value, assumed to be solar, C/O = 0.54). Without solid accretion, the planets are expected to have sub-solar metallicity, but high C/O ratios. This arises because the solids are dominated by oxygen-rich species, e.g. H₂O and CO₂, which freeze out in the disc earlier than CO, leaving the gas metal poor but carbon rich. Here we demonstrate that super-solar metallicities can be achieved by gas accretion alone when growth and radial drift of pebbles are considered in protoplanetary discs. Through this mechanism, planets may simultaneously acquire super-solar metallicities and super-solar C/O ratios. This happens because the pebbles transport volatile species inwards as they migrate through the disc, enriching the gas at snow lines where the volatiles sublimate. Furthermore, the planet's composition can be used to constrain where it formed. Since high C/H and C/O ratios cannot be created by accreting solids, it may be possible to distinguish between formation via pebble accretion and planetesimal accretion by the level of solid enrichment. Finally, we expect that Jupiter's C/O ratio should be near or above solar if its enhanced carbon abundance came through accreting metal-rich gas. Thus, Juno's measurement of Jupiter's C/O ratio should determine whether Jupiter accreted its metals from carbon-rich gas or oxygen-rich solids.