The C/O ratio is predicted to regulate the atmospheric chemistry in hot Jupiters .
Recent observations suggest that some exo-planets , e.g .
Wasp 12-b , have atmospheric C/O ratios substantially different from the solar value of 0.54 .
In this paper we present a mechanism that can produce such atmospheric deviations from the stellar C/O ratio .
In protoplanetary disks , different snowlines of oxygen- and carbon-rich ices , especially water and carbon monoxide , will result in systematic variations in the C/O ratio both in the gas and in the condensed phase .
In particular , between the H _ { 2 } O and CO snowlines most oxygen is present in icy grains – the building blocks of planetary cores in the core accretion model – while most carbon remains in the gas-phase .
This region is coincidental with the giant-planet forming zone for a range of observed protoplanetary disks .
Based on standard core accretion models of planet formation , gas giants that sweep up most of their atmospheres from disk gas outside of the water snowline will have C/O \sim 1 , while atmospheres significantly contaminated by evaporating planetesimals will have stellar or sub-stellar C/O when formed at the same disk radius .
The overall metallicity will also depend on the atmosphere formation mechanism , and exo-planetary atmospheric compositions may therefore provide constraints on where and how a specific planet formed .