On The Compatibility of Ground-based and Space-based Data: WASP-96 b, An Example

Abstract

The study of exoplanetary atmosphere relies on detecting minute changes in the transit depth at different wavelengths. To date, a number of ground and space based instruments have been used to obtain transmission spectra of exoplanets in different waveband. One common practice is to combine observations from different instruments in order to achieve a broader wavelength coverage. We present here two inconsistent observations on WASP-96 b, one by Hubble Space Telescope (HST) and the other by Very Large Telescope (VLT). We present two key findings in our investigation: 1.) a strong water signature is detected via the HST WFC3 observations. 2.) A notable offset in transit depth (>1100 ppm) can be seen when the ground-based and space-based observations are combined together. The discrepancy raises the question of whether observations from different instruments could indeed be combined together. We attempt to align the observations by including an additional parameter in our retrieval studies but are unable to definitively ascertain that the aligned observations are indeed compatible. The case of WASP-96 b signals that compatibility of instruments should not be assumed. While wavelength overlaps between instruments can help, it should be noted that combining datasets remains a risky business. The difficulty in combining observations also strengthens the need for next generation instruments which will possess broader spectral coverage.

Authors 8

1. Kai Hou Yip corresponding

   University College London

   Affiliation as printed

   University College London, Physics and Astronomy, London, United Kingdom of Great Britain and Northern Ireland (zcapkhy@ucl.ac.uk)

2. Quentin Changeat

   University College London

   Affiliation as printed

   University College London, Physics and Astronomy, London, United Kingdom of Great Britain and Northern Ireland (zcapkhy@ucl.ac.uk)

3. Billy Edwards corresponding

   University College London

   Affiliation as printed

   University College London, Physics and Astronomy, London, United Kingdom of Great Britain and Northern Ireland (zcapkhy@ucl.ac.uk)

4. Mario Morvan corresponding

   University College London

   Affiliation as printed

   University College London, Physics and Astronomy, London, United Kingdom of Great Britain and Northern Ireland (zcapkhy@ucl.ac.uk)

5. Katy L. Chubb Aachen SRON Netherlands Institute for Space Research

   University College London · Space Research Organisation Netherlands

   Affiliation as printed

   SRON Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA, Utrecht, Netherlands

   University College London, Physics and Astronomy, London, United Kingdom of Great Britain and Northern Ireland (zcapkhy@ucl.ac.uk)

6. Angelos Tsiaras

   University College London

   Affiliation as printed

   University College London, Physics and Astronomy, London, United Kingdom of Great Britain and Northern Ireland (zcapkhy@ucl.ac.uk)

7. Ingo P. Waldmann

   University College London

   Affiliation as printed

   University College London, Physics and Astronomy, London, United Kingdom of Great Britain and Northern Ireland (zcapkhy@ucl.ac.uk)

8. G. Tinetti corresponding

   University College London

   Affiliation as printed

   University College London, Physics and Astronomy, London, United Kingdom of Great Britain and Northern Ireland (zcapkhy@ucl.ac.uk)