Linking planet formation to exoplanet characteristics: C/O as a diagnostic of planet formation

Gas-giant exoplanets are test cases for theories of planet formation as their atmospheres are proposed to carry signatures of their formation within the protoplanetary disk. The metallicity and C/O are key diagnostics, allowing to distinguish formation location within the disk (e.g., relative to snowlines), and mechanism (e.g., core accretion versus gravitational instability). We can now probe the composition of the planet-forming regions of disks, and that in gas-giant exoplanets, to scrutinise these theories and diagnostics. So far, ALMA has revealed that the outer disk regions are typically metal-depleted and O-poor, whereas JWST is showing that the inner disk regions around Sun-like stars are mostly O-rich. Further, JWST is showing that most transiting gas-giant planets are typically metal-enriched and O-rich, consistent with formation at/within the water snowline and pollution by icy bodies. There is emerging an arguably "conventional" picture of gas-giant planet formation for transiting planets, to be confirmed, of course, with future data.