1. Centre for Fusion, Space and Astrophysics, Department of Physics, University of Warwick , Coventry CV4 7AL, UK
2. Centre for Exoplanets and Habitability, University of Warwick , Coventry CV4 7AL, UK
3. European Space Agency (ESA), European Space Research and Technology Centre (ESTEC) , Keplerlaan 1, NL-2201 AZ Noordwijk, the Netherlands