A Preparatory Program to Identify the Single Best Transiting Exoplanet for JWST Early Release Science

JWST will revolutionize transiting exoplanet atmospheric science due to its capability for continuous, long-duration observations and, compared to existing space-based facilities, its larger collecting area, spectral coverage, and resolution. However, it is unclear precisely how well JWST will perform and which of its myriad instruments and observing modes will be best suited for transiting exoplanet studies. The Early Release Science (ERS) program was devised to provide early and open access to a broad suite of JWST science observations subject to key data analysis challenges so that the community can quickly build experience and develop a list of best observing practices prior to the Cycle 2 proposal deadline. In a recent paper, we identified 12 transiting exoplanets (dubbed "community targets") that may be suitable for time-series observations within the ERS program; however, a critical unknown for the most favorable targets is the presence of obscuring clouds. To properly assess each observing mode, it is vital that the selected community target has measurable and identifiable spectroscopic features.

We propose HST/WFC3 observations of four exoplanets to identify the single best target by first measuring the size of their 1.4-micron water vapor features. Next, we will perform follow-up Spitzer observations of the top two targets to determine the slopes in their infrared transmission spectra. Together, these measurements will provide the most robust determination of clouds/hazes with the minimum amount of telescope time. Cycle 24 is our final opportunity to identify suitable community targets with cloud-free atmospheres prior to the ERS proposal deadline in mid-2017.