Original title: The effect of the atmospheric C/O ratio on interiors of hot Jupiters
Esther van Dijk, Yamila Miguel, Paul Mollière · September 2026
When scientists study the interiors of hot Jupiters—huge gas giant planets orbiting close to their stars—they usually assume the ratio of carbon to oxygen, or C/O, matches our Sun. But new telescopes like JWST can now measure this ratio directly. In this study, researchers tested how changing the C/O ratio affects models of these planets. They built computer models of atmospheres and connected them to models of planet interiors. Here is how it works: the C/O ratio controls which molecules dominate the atmosphere. These molecules change how well the air traps heat, shifting the temperature and pressure where the atmosphere meets the deeper, churning interior. That shift changes the planet's deep temperatures and alters its calculated size. For hot planets with high metal contents, this size difference easily beats real observation errors of 3 per cent. Looking at WASP-19 b, a higher C/O ratio meant a hotter deep interior.
Imagine baking a cake where the recipe says to use a certain amount of sugar. You follow it, but the cake comes out too small. It turns out the recipe assumed a specific type of sugar, but you used a different one that changes how the cake rises. In this paper, the carbon-to-oxygen ratio is like that sugar type—it changes how the atmosphere traps heat, which then changes the planet's calculated size.
JWST can now measure the carbon-to-oxygen ratio in exoplanet atmospheres. This paper shows that ignoring that ratio when modeling a hot Jupiter's interior leads to radius errors larger than our measurement uncertainty. For planets like WASP-19b, getting the C/O ratio wrong means inferring the wrong internal temperature, which affects everything we think we know about how these planets formed and evolved.
The abstract does not specify the range of C/O ratios tested, the exact number of models in the grid, or whether the interior model includes non-ideal effects like magnetic fields or rotation. It also notes the result applies to planets with equilibrium temperatures above 1500 K and super-solar metallicities, so cooler or lower-metallicity planets may not show the same sensitivity.
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The effect of the atmospheric C/O ratio on interiors of hot Jupiters — Esther van Dijk, Yamila Miguel, Paul Mollière
Abstract licence: CC0
Explanation generated by Synthesis from the abstract — it is not the original paper. This is a preprint, so it hasn't necessarily been peer reviewed.