Figure 1: The unpolarized stellar light scattered by a planet is linearly polarized perpendicular to the scattering plane. Partially polarized light from an unresolved star-planet system reaches a polarimeter which thereby detects directly the scattered light.

Figure 2: The geometrical albedo of the hot Jupiter HD189733b measured using the polarized light technique (Berdyugina et al. 2011). The planet is found to be as blue as Neptune in our Solar System.

Hot Molecules in Exoplanets and Inner Disks

Understanding the nature and distribution of habitable environments in the Universe is one of the fundamental goals of modern astrophysics. For the life we know, liquid water on the planetary surface is a prerequisite. However, a direct detection of liquid water on exoplanets, and especially on a potentially habitable Earth-size planet, is not yet possible. The existence of water almost certainly implies the presence of atmospheric water vapour which must evaporate under stellar irradiation from a cloud deck or from the surface, together with other related molecules. Therefore, devising sensitive methods to detect hot molecules on exoplanets is of high importance.

This project develops several exploratory theoretical and observational aspects of precision spectropolarimetry for detecting water vapour and other volatiles on exoplanets and in the inner part of protoplanetary disks. These are new tools for making progress in our understanding which fraction of planets acquires water and how planet formation influences their habitability. As a "double differential" technique, spectropolarimetry has enormous advantages for dynamic range problems, like detection of weak line signals against a large stellar background and exploration at scales beyond the angular resolution of telescopes, which are crucial for both exoplanets and inner disks. Direct detection of polarized spectral lines (Fig. 1) enables recovering precise orbits of exoplanets (including non-transiting systems) and evaluating their masses as well as potentially their magnetic fields. First applied to hot Jupiters the developed tools will create a firm foundation for future exploration of Earth-like planets with larger telescopes. The same technique applied to planetesimals in the inner disks of young stars yields their orbits, temperature, and chemical composition. These will provide constraints on the formation of a planetary atmosphere in the vicinity of the star and its habitable zone.

Earlier, in 2008, our team was the first to detect reflected light from an extrasolar planet with the help of polarimetry (Berdyugina et al. 2008). In 2011, we have reported the first exoplanet geometrical albedo measurement for the hot Jupiter HD189733b (Berdyugina et al. 2011, see Fig. 2). We have found that the planet is of blue color similar to Neptune. This is in contrast to earlier, theory induced expectations for hot Jupiters to be similar to the Solar system Jupiter's color. Our result was confirmed in 2013 by Evans et al. who have used Hubble Space Telescope to measure the amount of the reflected light during the secondary eclipse.

Our polarimetric technique is now further developed for detecting molecules in exoplanetary atmospheres and planetesimals, and in the future for studying extraterrestrial life.

The key questions of this project are

What is the range of atmospheric conditions on planets with water? Detecting water vapor in known exoplanets provides valuable constraints on the frequency of potentially habitable planets.

What is the amount of hot volatiles in planetesimals residing in the inner part of protoplanetary disks (in the vicinity of the star and its habitable zone) at different stages of planet formation? This constrains the time when potentially habitable planets acquire water and atmospheres.

How do planetary magnetic fields originate and evolve and how efficiently do they shield the atmospheres from dehydration by the stellar wind? Detecting magnetic fields in planetesimals and exoplanets will provide a new dimension in our understanding of habitability.

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