Out of the Darkness: High-resolution Detection of CO Absorption on the Nightside of WASP-33b

We observed the ultra hot Jupiter WASP-33b with the Spectro-Polarim\`etre Infra-Rouge on the Canada Fance Hawaii Telescope. Previous observations of the dayside of WASP-33b show evidence of CO and Fe emission indicative of a thermal inversion. We observed its nightside over five Earth-nights to search for spectral signatures of CO in the planet's thermal emission. Our three pre-transit observations and two post-transit observations are sensitive to regions near the morning or evening terminators, respectively. From spectral retrievals, we detect CO molecular absorption in the planet's emission spectrum after transit at $\sim$6.6$\sigma$. This is the strongest ground-based detection of nightside thermal emission from an exoplanet, and only the third ever. CO appearing in absorption suggests that the nightside near the evening terminator does not have a temperature inversion; this makes sense if the dayside inversion is driven by absorption of stellar radiation. On the contrary, we do not detect CO from the morning terminator. This may be consistent with heat advection by an eastward jet. Phase-resolved high-resolution spectroscopy offers an economical alternative to space-based full-orbit spectroscopic phase curves for studying the vertical and horizontal atmospheric temperature profiles of short-period exoplanets.