Exoplanets orbiting close to their host star are expected to support a large ionosphere , which extends to larger pressures than witnessed in our Solar System .
These ionospheres can be investigated with ground-based transit observations of the optical signatures of alkali metals , which are the source of the ions .
However , most ground-based transit spectra do not systematically resolve the wings of the features and continuum , as needed to constrain the alkali abundances .
Here , we present new observations and analyses of optical transit spectra that cover the Na doublet in the atmosphere of the exoplanet XO-2 b .
To assess the consistency of our results , observations were obtained from two separate platforms : Gemini/GMOS and Mayall/KOSMOS .
To mitigate the systematic errors , we chose XO-2 , because it has a binary companion of the same brightness and stellar type , which provides an ideal reference star to model Earth ’ s atmospheric effects .
We find that interpretation of the data is highly sensitive to time-varying translations along the detector , which change according to wavelength and differ between the target and reference star .
It was necessary to employ a time-dependent cross-correlation to align our wavelength bins and correct for atmospheric differential refraction .
This approach allows us to resolve the wings of the Na line across 5 wavelength bins at a resolution of \sim 1.6nm and limit the abundance of Na .
We obtain consistent results from each telescope with a Na amplitude of 521 \pm 161 ppm and 403 \pm 186 ppm for GMOS and KOSMOS respectively .
The results are analyzed with a radiative transfer model that includes the effects of ionization .
The data are consistent with a clear atmosphere between \sim 1–100 mbar which establish a lower limit on Na at 0.4 ^ { +2 } _ { -0.3 } ppm ( [ Na/H ] =-0.64 ^ { +0.78 } _ { -0.6 } ) , consistent with solar .
However , we can not rule out the presence of clouds at \sim 10 mbar which allow for higher Na abundances which would be consistent with stellar metallicity measured for the host star ( [ Na/H ] =0.485 \pm 0.043 ) .