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The analysis and interpretation of exoplanet spectra from time-seriesobservations remains a significant challenge to our current understanding ofexoplanet atmospheres, due to the complexities in understanding instrumentalsystematics. Previous observations of the hot Jupiter WASP-31b usingtransmission spectroscopy at low-resolution have presented conflicting results.Hubble Space Telescope (HST) observations detected a strong potassium featureat high significance (4.2\sigma), which subsequent ground-basedspectro-photometry with the Very Large Telescope (VLT) failed to reproduce.Here, we present high-resolution observations (R>80,000) of WASP-31b with theUVES spectrograph, in an effort to resolve this discrepancy. We perform acomprehensive search for potassium using differential transit light curves, andintegration over the planet's radial velocity. Our observations do not detect Kabsorption at the level previously reported with HST, consistent with the VLTobservations. We measure a differential light curve depth $\Delta F = 0.00031\pm 0.00036$ using 40\AA\ bins centred on the planet's K feature, and set anupper limit on the core line depth of $\Delta F \leq 0.007$ (3\sigma) at a fewtimes the resolution limit ($\approx0.24\AA$). These results demonstrate thatthere are still significant limitations to our understanding of instrumentalsystematics even with our most stable space-based instrumentation, and thatcare must be taken when extracting narrow band signatures from low-resolutiondata. Confirming exoplanet features using alternative instruments andmethodologies should be a priority, and confronting the limitations ofsystematics is essential to our future understanding of exoplanet atmospheres.

Revisiting the potassium feature of WASP-31b at high-resolution