Multiwavelength Exoplanet Atmophseric Characterization

Transiting planets offer unique opportunities for the study of exoplanet atmospheres, as we can observe them passing in front and behind their host star. During an occultation, the host star blocks the flux emerging from the planetary dayside. By comparing the flux in- and out-of occultation, one can measure the planet-to-star brightness ratio. Observations in different pass-bands yield a measure of the planetary emission spectrum, which encodes information on planetary atmospheric structure, composition, and energy balance.

Transit observations allow one to study the composition of planetary atmospheres in greater detail, as the stellar light is filtered through the planetary atmosphere on its way to the observer. Then, spectrally resolved transit observations allow one to probe the planetary transmission spectrum and infer the elemental and molecular signatures contained in it.

Our team is involved in a number of projects using the above techniques to study bright transiting systems with close-in planets. We are using multi-wavelength observations from ground- and space-based facilities (ESO/VLT, Gemini, Spitzer, and HST) to study the atmospheres of key exoplanets in transmission and emission, and detect the absorption and scattering signatures of chemical elements, molecules and haze components.

We use ultraviolet observations obtained with HST to characterize the physical and chemical properties of the upper atmosphere of close-in planets to constrain atmospheric escape and evolution models. We observe and study both gaseous and rocky close-in planets through transmission spectroscopy.

We also study the star–planet interaction phenomenon by measuring stellar activity at different wavelengths looking for magnetic and physical interactions (e.g., planet-to-star mass transfer).


The CHaracterising ExOPlanet Satellite (CHEOPS) is an S-class ESA satellite mission dedicated to the study of extrasolar planets. CHEOPS is currently foreseen for launch in 2017 and will observe planetary systems at an unprecedented photometric precision.

The main science goals of CHEOPS are to find transits of small planets, known to exist from radial-velocity surveys, measure precise radii for a large sample of planets to study the nature of Neptune- to Earth-sized planets, and obtain precise observations of transiting giant planets to study their atmospheric properties. Detailed information can be found at this web site:

Our institute is heavily involved in the CHEOPS mission, a group from IWF is building one of the two on-board computers. Our team is involved in the preparation of the CHEOPS mission at the core science team level (performing observation feasibility modeling, understanding the host star properties and photometric behavior, and CHEOPS observing program definition).

Within the context of the CHEOPS mission, our group is currently running a theoretical and observational project aiming at predicting the radius (mass) of hydrogen-dominated sub-Neptune planets (< 12 Earth masses) on the basis of the measured mass (radius). With this project we aim also at predicting the planets' atmospheric evolution and presence of high-altitude clouds or hazes (e.g., cloud/haze altitude).

Atmospheric Modeling and Retrieval

Our team is part of the core developers of the Bayesian Atmospheric Radiative Transfer (BART) project, which consists of a radiative-transfer solver (Transit) coupled to a thermochemical equilibrium abundance code (TEA, project leaded by J. Blecic), and a Bayesian MCMC sampler (MC3). The BART code can compute forward-modeling exoplanet emission and transmission spectra including line-by-line, cross-section, alkali, Rayleigh, and cloud opacities. BART also works in a retrieval configuration to constrain the temperature and composition of exoplanet atmospheres upon comparing the theoretical spectra with observed eclipse or transit data.

BART is an open-source, open-development code available at under a Reproducible Research license. Each of the main components of BART —Transit, TEA, and MC3— work independently in a modular way, and thus, can be applied to a multitude scientific projects beyond their designed purpose. The following video shows an example of emission spectra computed with Transit:

Space Research Institute, Austrian Academy of Sciences. Schmiedlstrasse 6, 8042 Graz, Austria.
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