Angelique Kahle   (Germany)

kahle @ mpia.de

Atmosphere characterization of sub-Neptunes with the Hubble Space Telescope

In recent decades, more than 5000 extrasolar planets have been discovered orbiting the stars of the night sky. Among these typically short orbital period planets, sub-Neptunes are the most abundant class, with roughly between 2 and 3.5 earth radii. Interestingly, these common objects have no analog in the solar system, making them attractive targets for exoplanet research.

A tool for studying the composition and conditions on exoplanets is transmission spectroscopy, which analyzes the spectrum of the starlight that is filtered through the planet's atmosphere. This technique requires observations with high sensitivity, as the transiting planets cover only a small fraction of their host star. While currently operating facilities are well suited for observing giant planets, observing the smaller sub-Neptunes is more challenging and has therefore only been done for very few of them.

Many of the published sub-Neptune spectra are either featureless or show muted spectral features, possibly due to high mean molecular weight atmospheres or alternatively high altitude hazes or clouds. The Sub-neptune Planetary Atmosphere Characterization Experiment (SPACE) with the Hubble Space Telescope will systematically investigate the reason behind these findings. For this, we observe and analyze the transmission spectra of 8 sub-Neptunes that span a diverse sample in radius (2.2-3.4 earth radii) and temperature (320K-1300K), together with UV spectra of their host stars. With these new observations, we will establish demographic trends among cloudy and cloud-free sub-Neptunes which can be used to distinguish between the possible feature attenuation scenarios. The detected spectral features will inform us about the chemical processes and dominant molecular species in the atmospheres, providing a starting point for exploring the wide range of atmospheric compositions predicted by formation models.

Supervisor:    Laura Kreidberg  (MPIA)