Astronomers used computer models of planet formation to test where hot Jupiters—giant planets very close to their stars—most likely began life. By comparing simulated atmospheric abundances of carbon and oxygen to measured values for nine known hot Jupiters, the team finds that at least six of the nine are consistent with forming beyond the “water snowline.” The water snowline is the distance in a young disc where water turns from gas into ice, and it separates regions with different mixes of gas and solids.

The researchers ran a set of planet‑formation simulations with the ChemComp code. The model follows growing planetary cores that gather drifting icy pebbles and surrounding gas, and it lets planets migrate inward through the disc. The simulations included two chemical effects added to previous work: decomposition of carbon locked in refractory organics into volatile gas in the inner disc, and trapping of CO and CO2 inside amorphous water ice that are released when that ice crystallizes at about 130 K. The team also used measured stellar element abundances from the Hypatia catalogue to set the starting chemical mix for each system.

To compare to observations they focused on atmospheric metallicities expressed as C/H and O/H (carbon and oxygen relative to hydrogen). They ran simulations for each of nine hot Jupiters—examples in the sample include WASP‑77Ab, τ Boötis b and WASP‑121b—varying the planet’s starting orbital distance and the disc’s viscosity (α = 10−4, 5×10−4, 10−3). They fixed the disc mass at 0.07 times the stellar mass and stopped each run when the model planet reached the observed mass. The simulated C/H and O/H patterns depend on where a planet accreted most of its gas and solids: planets forming inside the water snowline tend to show oxygen enrichment, while those forming farther out can show lower oxygen and different carbon signatures.