The analysis and interpretation of exoplanet spectra from time-series observations remains a significant challenge to our current understanding of exoplanet atmospheres , due to the complexities in understanding instrumental systematics .
Previous observations of the hot Jupiter WASP-31b using transmission spectroscopy at low-resolution have presented conflicting results . Hubble Space Telescope ( HST ) observations detected a strong potassium feature at high significance ( 4.2 \sigma ) , which subsequent ground-based spectro-photometry with the Very Large Telescope ( VLT ) failed to reproduce .
Here , we present high-resolution observations ( R > 80,000 ) of WASP-31b with the UVES spectrograph , in an effort to resolve this discrepancy .
We perform a comprehensive search for potassium using differential transit light curves , and integration over the planet ’ s radial velocity .
Our observations do not detect K absorption at the level previously reported with HST , consistent with the VLT observations .
We measure a differential light curve depth \Delta F = 0.00031 \pm 0.00036 using 40 Å bins centred on the planet ’ s K feature , and set an upper limit on the core line depth of \Delta F \leq 0.007 ( 3 \sigma ) at a few times the resolution limit ( \approx 0.24 Å ) .
These results demonstrate that there are still significant limitations to our understanding of instrumental systematics even with our most stable space-based instrumentation , and that care must be taken when extracting narrow band signatures from low-resolution data .
Confirming exoplanet features using alternative instruments and methodologies should be a priority , and confronting the limitations of systematics is essential to our future understanding of exoplanet atmospheres .