  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.
