Principal Investigator: Catherine Huitson, University of Colorado at Boulder (CASA)

Program Summary:

Recent exoplanet transit observations have detected evidence of high-altitude clouds in the majority of targets, although the species remain unknown. Our survey is the first program dedicated to not only detecting clouds, but also discovering the cloud condensate species. Knowledge of cloud species is essential in order to dis-entangle cloud signatures from atomic and molecular signatures in exoplanet transit spectra (a transmission spectrum measured when the planet passes in front of the star). The atomic and molecular measurements are the only way to detect atmospheric compositions of these planets, which are important for understanding formation history and searching for biosignatures in future observations.

So far, there is no correlation between presence of clouds and planet temperature, with even >2000 K atmospheres showing evidence of substantial cloud cover. Given the diversity of atmospheres shown to exhibit clouds, we propose to sample a representative cross-section of the hot gas giant population to try to link cloud properties with easily-measurable bulk properties such as planet temperature and surface gravity (Figure 1). This will unable us to better understand cloud formation pathways in the exoplanet population.

Figure 9: Mass-radius diagram of known extrasolar planets focusing on larger objects, which are accessible to transmission spectroscopy. Our target selection will explore a representative sub-sample of the gas giant population.

Figure 10: Example transmission spectra in the blue optical produced by different types of clouds, with different condensate grain sizes. Also shown are the GMOS wavelength coverage and predicted co-added precisions for the data.

We plan to use 95 hours of Gemini/GMOS time over 2 years to observe blue optical transmission spectra of our targets multiple times to reach high-precisions. In the blue optical, planet atmospheres display no atomic or molecular features and cloud scattering signatures dominate the transmission spectra. Modeling of the scattering signatures will be used to identify cloud condensate grain sizes and hence cloud species (Figure 2). The results can then be used to understand the contribution of clouds to the longer-wavelength transmission spectra, where atomic and molecular features are observed. This will enable reliable retrieval of abundance and composition measurements.

Co-Investigators:

• Jean-Michel Desert, University of Colorado at Boulder (CASA)
• Jacob Bean, University of Chicago
• Jonathan Fortney, University of California at Santa Cruz
• Kevin Stevenson, University of Chicago
• Marcel Bergmann, NOAO/Gemini