We present one of the most precise emission spectra of an exoplanet observed so far .
We combine five secondary eclipses of the hot Jupiter WASP-18 b ( T _ { day } \sim 2900 K ) that we secured between 1.1 and 1.7 \mu m with the WFC3 instrument aboard the Hubble Space Telescope .
Our extracted spectrum ( S/N=50 , R \sim 40 ) does not exhibit clearly identifiable molecular features but is poorly matched by a blackbody spectrum .
We complement this data with previously published Spitzer/IRAC observations of this target and interpret the combined spectrum by computing a grid of self-consistent , 1D forward models , varying the composition and energy budget .
At these high temperatures , we find there are important contributions to the overall opacity from H ^ { - } ions , as well as the removal of major molecules by thermal dissociation ( including water ) , and thermal ionization of metals .
These effects were omitted in previous spectral retrievals for very hot gas giants , and we argue that they must be included to properly interpret the spectra of these objects .
We infer a new metallicity and C/O ratio for WASP-18 b , and find them well constrained to be solar ( [ M/H ] = -0.01 \pm 0.35 , C/O < 0.85 at 3 \sigma confidence level ) , unlike previous work but in line with expectations for giant planets .
The best fitting self-consistent temperature-pressure profiles are inverted , resulting in an emission feature at 4.5 \mu m seen in the Spitzer photometry .
These results further strengthen the evidence that the family of very hot gas giant exoplanets commonly exhibit thermal inversions .