A solar C/O and sub-solar metallicity in a hot Jupiter atmosphere

Measurements of the atmospheric carbon (C) and oxygen (O) relative to hydrogen (H) in hot Jupiters (relative to their host stars) provide insight into their formation location and subsequent orbital migration(1,2). Hot Jupiters that form beyond the major volatile (H2O/CO/CO2) ice lines and subsequently migrate post disk-dissipation are predicted have atmospheric carbon-to-oxygen ratios (C/O) near 1 and subsolar metallicities(2), whereas planets that migrate through the disk before dissipation are predicted to be heavily polluted by infalling O-rich icy planetesimals, resulting in C/O < 0.5 and super-solar metallicities(1,2). Previous observations of hot Jupiters have been able to provide bounded constraints on either H2O (refs. (3-5)) or CO (refs. (6,7)), but not both for the same planet, leaving uncertain(4) the true elemental C and O inventory and subsequent C/O and metallicity determinations. Here we report spectroscopic observations of a typical transiting hot Jupiter, WASP-77Ab. From these, we determine the atmospheric gas volume mixing ratio constraints on both H2O and CO (9.5 x 10(-5)-1.5 x 10(-4) and 1.2 x 10(-4)-2.6 x 10(-4), respectively). From these bounded constraints, we are able to derive the atmospheric C/H (0.35(-0.10)(+0.17) x solar) and O/H (0.32(-0.08)(+0.12) x solar) abundances and the corresponding atmospheric carbon-to-oxygen ratio (C/O = 0.59 +/- 0.08; the solar value is 0.55). The sub-solar (C+O)/H (0.33(-0.09)(+0.13)x solar) is suggestive of a metal-depleted atmosphere relative to what is expected for Jovian-like planets(1) while the near solar value of C/O rules out the disk-free migration/C-rich(2) atmosphere scenario.