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Atmospheric Characterisation of Exoplanets

Final Report Summary - ACE (Atmospheric Characterisation of Exoplanets)

The project consists in taking up the challenge of the atmospheric characterisation of Earth-like exoplanets, in search for life. The project objectives aim at reaching intermediate milestones toward this long-term goal, that could be achieved around 2020.

The first objective is to detect new worlds suited for atmospheric characterisation. The aim is to propose observations with international facilities in order to characterise the atmospheres of new exoplanets detected at the University of Geneva. Alternatively, these proposed observations should lead to the possibility to perform the atmospheric characterisation (e.g. detecting the transit of an exoplanet detected with Doppler velocimetry). The second objective is to study the atmospheres of exoplanets, from hot jupiters to telluric planets. The aim is to exploit existing observations of exoplanetary atmospheres, to infer the atmospheric properties of known exoplanets. Finally, the long-term objective is to prepare for the detection and characterisation of Earth-like planets: The aim is to participate to the science preparation of space missions that will find these exoplanets, namely CHEOPS and PLATO.

The development of activities around the CHEOPS mission, which got selected by ESA in November 2012 (just 3 months after the start of the project), led to a substantial shortening of the initial project, because the researcher was offered a position of CHEOPS Mission Scientist at the University of Geneva, where the Science Operations Centre of CHEOPS is being developed. Therefore, the overview of the scientific activities described below covers a four-month period, between Sept. 2012 and Dec. 2012, when the project was suspended (it was not resumed afterwards).

Contemporaneously to the researcher arrival at the host institution, the exoplanet team was finalising the publication about the Earth-mass exoplanet around α Centauri B (Dumusque et al. 2013). This exoplanet, the lightest and closest ever detected, was discovered through radial velocity measurements. The signal semi-amplitude (51 cm/s) makes it one of the most challenging planet to detect. The researcher proposed to search for a transit in order (i) to confirm the existence of the planet and (ii) to allow for its atmospheric characterisation via transmission spectroscopy. The transit search is extremely challenging due to the brightness of the star (one of the brightest star in the sky, blinding most cameras) and the shallowness of the expected transit (100 parts per million). The researcher submitted as principal investigator a Director’s Discretionary Time request to the Hubble Space Telescope (HST) and an observing proposal to the European Southern Observatory (ESO), aimed at using the Very Large Telescope set in the Atacama desert of Chile. Both proposals were successfully awarded time (15 HST orbits and 2 nights at VLT). Moreover, a competiting US-based proposal on α Centauri B was also accepted by the Space Telescope Science Institute (STScI), who advised that the researcher be nominated PI of the merged Swiss and US teams. The observations were scheduled for Summer 2013.

During the project, the researcher finalised the reduction, analysis, interpretation and publication of HST data from a programme executed before the start of the reporting period (“Search for a photodissociated evaporating ocean on the super-Earth 55 Cancri e”, PI: Ehrenreich). The article on the HST far-ultraviolet observations of the 55 Cancri system (Ehrenreich et al. 2012, A&A 547, A18) reports on the non-detection of an extended atmosphere around the hot super-Earth 55 Cancri e, providing constraints on the composition of a potential gas envelope to the planet, and the surprising detection of such an extended atmosphere on the warm gas giant 55 Cancri b, which was not known as a transiting planet. It seems, however, that the extended atmosphere of this planet is partially transiting the star, which had never been seen before. This result shows that moderately irradiated exoplanets can also bear evaporating atmospheres.

The research also participated as a co-investigator in two other observational studies based on archival HST observations of the giant exoplanets HD 209458b and HD 189733b, and one theoretical study about the intrinsic emission signatures of HD 209458b. The researcher participated to a study (Bourrier et al. 2013, A&A 551, A63) about the detection of atmospheric escape from the hot gas giant HD 198733b, based on HST far-ultraviolet observations. This paper presents a detailed data analysis that firmed up the previously known transit signal in the Lyman-α line of neutral hydrogen and hinted at the existence of a bow shock ahead of the planetary transit. This bow shock, resulting from the interaction between the planet and stellar winds, could produce early absorption signals in the lines of ionised elements (silicon and nitrogen), which are tentatively detected.
The researcher participated to a study reporting on the detection of magnesium in the atmosphere of the planet HD 209458b (Vidal-Madjar et al. 2013, A&A 560, A54). We established the signature of magnesium as a probe to study the transition region between the thermosphere (probed with the sodium lines) and the exobase (probed with the H i Lyα line), which will be useful for future studies of a whole sample of exoplanets.
Finally, the researcher participated to a theoretical work led by planetary scientists in Grenoble (Menager et al. 2013, Icarus 226, 1709), which aimed at predicting the planetary Lyα emission signals from auroræ arising in magnetised hot Jupiters. The result was extrapolated from an electron transport code used for giant planets in the Solar System. The researcher advised the team on how to transfer this code to the conditions of hot giant exoplanets. The resulting signals will be challenging to detect.