Characterization

The K band spectra of GRAVITY are combined with archival spectroscopy of high contrast instruments where available. A coverage of the YJHK-band region is usually sufficient to put meaningful constraints on the most important properties of an exoplanet atmosphere, such as surface gravity, effective temperature, atmospheric composition. The GRAVITY K-band is particularly important for constraining the atmopsheric carbon-to-oxygen ratio (C/O).

The spectra are analyzed using either so-called self-consistent atmospheric codes (e.g., Exo-REM) that include relevant physical processes (cloud formation, chemistry, radiative and convective energy transport) to the best of our knowledge and to the degree to which they are reasonably employable within such a code.

The second analysis method is carrying out so-called free retrievals (e.g., using petitRADTRANS), where many of the abovementioned processes are more freely parameterized. If the data are of sufficient quality, they will then constrain the atmospheric composition, temperature and cloud structure.

Using both methods in tandem is key to identify missing physics as well as potential biases that may arise from using only self-consistent codes or only free retrievals.