(Proposal ID) S17A-131

(PI) de Mooij, Ernst

(Proposal Title)
Comparative Exoplanetology: Probing the Atmospheres of Hot Saturns

(Abstract)
Characterizing exoplanet atmospheres presents a formidable challenge, primarily
due to the extreme brightness contrast between the planet and the host star. 
However, transmission spectroscopy during transits, when stellar light passes 
through a planet's atmosphere, offers a rare opportunity to identify the 
presence of atomic and molecular species and other atmospheric features. 
Here we propose to extend the technique to characterize atmospheres of two 
Saturn-mass planets using HDS, specifically targeting the Sodium and Potassium 
lines at optical wavelengths. Both planets have a very low surface gravity, 
resulting in very extended atmospheres. Low-resolution space-based observations
hint at the presence of alkali metals. These high-resolution observations will 
allow us to not only detect the species, but also to use the line-broadening 
and positions to determine the temperature-pressure profile as well as the wind
speeds in their upper atmospheres. In addition, we will investigate the 
alignment between the planet's orbit and stellar rotation using the Rossiter-
McLaughlin effect. Both planets have relatively large orbital separations and 
small mass ratios, resulting in long timescales for the alignment of the 
convective stellar envelope, which has been invoked to explain high obliquities.