[Paper Review] Strict Upper Limits on the Carbon-to-Oxygen Ratios of Eight Hot Jupiters from Self-Consistent Atmospheric Retrieval

Björn Benneke|arXiv (Cornell University)|Apr 28, 2015

Stellar, planetary, and galactic studies7 references61 citations

TL;DR

This study uses the self-consistent atmospheric retrieval framework SCARLET to analyze transmission spectra of eight hot Jupiters, finding robust upper limits on their carbon-to-oxygen (C/O) ratios below ∼0.9. The method combines physical chemistry with statistical inference, revealing that water abundances are solar and clouds—not low water content—explain weak water features in HD 209458b and WASP-12b.

ABSTRACT

The elemental compositions of hot Jupiters are informative relics of planet formation that can help us answer long-standing questions regarding the origin and formation of giant planets. Here, I present the main conclusions from a comprehensive atmospheric retrieval survey of eight hot Jupiters with detectable molecular absorption in their near-infrared transmission spectra. I analyze the eight transmission spectra using the newly-developed, self-consistent atmospheric retrieval framework, SCARLET. Unlike previous methods, SCARLET combines the physical and chemical consistency of complex atmospheric models with the statistical treatment of observational uncertainties known from atmospheric retrieval techniques. I find that all eight hot Jupiters consistently require carbon-to-oxygen ratios (C/O) below 0.9. The finding of C/O<0.9 is highly robust for HD209458b, WASP-12b, WASP-19b, HAT-P-1b, and XO-1b. For HD189733b, WASP-17b, and WASP-43b, I find that the published WFC3 transmission spectra favor C/O<0.9 at greater than 95% confidence. I further show that the water abundances on all eight hot Jupiters are consistent with solar composition. The relatively small depth of the detected water absorption features is due to the presence of clouds, not due to a low water abundance as previously suggested for HD209458b. The presence of a thick cloud deck is inferred for HD209458b and WASP-12b. HD189733b may host a similar cloud deck, rather than the previously suggested Rayleigh hazes, if star spots affect the observed spectrum. The approach taken in SCARLET can be regarded as a new pathway to interpreting spectral observations of planetary atmospheres. In this work, including our prior knowledge of H-C-N-O chemistry enables me to constrain the C/O ratio without detecting a single carbon-bearing molecule.

Motivation & Objective

• To constrain the carbon-to-oxygen (C/O) ratios of hot Jupiters using self-consistent atmospheric modeling.
• To resolve uncertainties in elemental abundances from spectral observations by integrating physical chemistry with statistical retrieval techniques.
• To determine whether low water absorption depths in hot Jupiters are due to low water abundance or cloud opacity.
• To assess the role of clouds and stellar activity in shaping transmission spectra, particularly for HD 189733b.
• To provide robust, observationally grounded constraints on planet formation pathways via elemental ratios without requiring direct detection of carbon-bearing molecules.

Proposed method

• Developed SCARLET, a self-consistent atmospheric retrieval framework that couples radiative-convective equilibrium models with statistical treatment of observational uncertainties.
• Used line-by-line radiative transfer with precomputed three-dimensional radiative-connectivity arrays (C_ijk) to accelerate convergence to radiative-convective equilibrium by orders of magnitude.
• Incorporated temperature-dependent molecular opacities and Planck functions across high-resolution spectral grids to ensure physical consistency.
• Applied Bayesian inference to fit transmission spectra, enabling robust uncertainty quantification and confidence levels for C/O ratios.
• Integrated prior knowledge of H-C-N-O chemistry to infer C/O ratios without detecting individual carbon-bearing molecules.
• Iteratively adjusted temperature-pressure profiles and opacities until convergence was achieved in seconds per iteration using precomputed transmission terms.

Experimental results

Research questions

• RQ1 What are the strict upper limits on the carbon-to-oxygen (C/O) ratios of hot Jupiters based on self-consistent atmospheric retrieval?
• RQ2 Are the weak water absorption features in hot Jupiters due to low water abundance or cloud opacity?
• RQ3 How do stellar activity and star spots affect the interpretation of transmission spectra for planets like HD 189733b?
• RQ4 Can C/O ratios be constrained without detecting carbon-bearing molecules, using only H-C-N-O chemical consistency?
• RQ5 What is the role of cloud decks versus Rayleigh hazes in shaping the observed transmission spectra of hot Jupiters?

Key findings

• All eight hot Jupiters require carbon-to-oxygen (C/O) ratios below ∼0.9, with HD 209458b, WASP-12b, WASP-19b, HAT-P-1b, and XO-1b showing this result at high statistical confidence.
• For HD 189733b, WASP-17b, and WASP-43b, the C/O ratio is constrained to be less than 0.9 at greater than 95% confidence based on Hubble WFC3 data.
• Water abundances across all eight planets are consistent with solar composition, contradicting earlier suggestions of low water content for HD 209458b.
• The shallow water absorption features in HD 209458b and WASP-12b are primarily due to thick cloud decks, not low water abundance.
• HD 189733b likely hosts a similar cloud deck rather than Rayleigh hazes if stellar activity affects the observed spectrum.
• The SCARLET framework enables robust C/O constraints without detecting carbon-bearing molecules by leveraging chemical consistency and statistical inference.