Blanc SIMI 5 - Blanc - SIMI 5 - Physique subatomique et théories associées, astrophysique, astronomie et planétologie

Atmosphere et Evaporation of Exoplanets – Exo-Atmos

Atmosphere and Evaporation of exoplanets

The present “Exo-Atmos” programme is aimed at constraining both the extended upper atmosphere of evaporating planets, and the deeper atmosphere of a large variety of exoplanets. These objectives will be reached through: 1) Observation with the best telescopes presently available: Very Large Telescope (VLT) and the Hubble Space Telescope (HST); 2) Enlargement of the sample of planets for which atmosphere are detected and analysed; 3) Development of numerical simulations.

Observational and theoretical exploration of exoplanets' atmosphere.

The aim of the project is to reach a better understanding of the exoplanets' atmospheres. We wish to improve our knowledge on the exoplanets atmosphere diversity, and better understand the origin and the key factor of this diversity. Additionally, acknowledging that exoplanet atmospheres evolved on long time scale under the effect of evaporation, we need to understand the mechanisms triggering the atmospheric escape and its impact on the planets structure. <br />The challenges and the difficulties to be overcome are the following: <br />- Presently, we have a very few atmospheric detections and they bring only a small number of observational constraints<br />- The few results available are surprising (discovery of evaporation was not foreseen), and therefore in the theoretical side, they are difficult to interpret.<br />However, this difficulties can be circumvent. We can contribute to this new step in the exoplanet science using two paths:<br />1) obtaining new observational constraints <br />2) new theoretical results raising from massive computing simulations.

Yet, we have obtained a large amount of time on the HST and the VLT telescopes to observe the atmospheres and evaporation of extrasolar planets. For instance, with 3 HST programs in 2012 and one program obtained in 2013, we observed a total of 10 planets (9 exoplanets + Venus as a benchmark). This allows us to scrutinize the atmosphere of an unprecedented large sample of exoplanets, and will open the field of comparative exoplanetology.
We are developing numerical simulations for the analysis and interpretation of HST spectroscopic observations, and modeling of the gas escape. These simulations are designed to better constrain the structure, dynamics and composition of the atmospheres of planets observed by transit spectroscopy. In particular, a significant part of the CPU time is dedicated to issues related to the evaporation of planets orbiting close to their star, an area in which our team has played a major role. Combined with the results of these simulations, the HST observations allow us to better constrain the escape rate and the evaporation mechanisms.

The analysis of HST observations to characterize the atmosphere of the greatest number of exoplanets is underway. This work resulted in a number of publications in peer review journals: Ehrenreich et al. 2013; Bourrier et al. 2013; Sing et al. 2013; Vidal-Madjar et al. 2013.
Of particular interest is the remarkable result of the discovery of neutral magnesium, which constrains the electron density in the upper atmosphere, the thermosphere and the base of the exosphere (Vidal-Madjar et al. 2013).
Modeling of atmospheric escape has also made significant progress (Bourrier et al 2013. Bourrier & Lecavelier 2014). Using the new models that we have developed, the fit of the Lyman-alpha observations obtained with the Hubble telescope now allows us to constrain not only the atmospheric escape rate, but also the speed of the planetary wind at the 'exobase. This is an important constraint for all models that seek to understand the mechanisms at work in the atmospheric escape of hot Jupiters.
using spectroscopic observations with the ESO/HARPS spectrograph, we discovered a new system with evaporating exocomets transiting the star HD172555 (Kiefer et al. 2013).
In addition, a statistical analysis of several hundred transits of evaporating exocomets around the star Beta Pictoris has highlighted the existence of two families of comets in this young exoplanetary system (Kiefer, Lecavelier et al. Nature, 2014).
Finally, we explore the performance of multi-object spectrographs (such as the new instrument KMOS/VLT) for transmission spectroscopy of atmospheres.

The project is progressing nominally, with very satisfactory results for all of the partners.
The arrival of two post-docs in the two Partner teams in mid-2014 has further boosted the project.

16 articles were published in articles in refereed with the work under the project.


The discovery of extrasolar planets is a revolution in modern astrophysics which impacts not only our knowledge of planet formation and evolution, but also our understanding of the place of the Earth in the Universe. In this domain major advances have been made by using spectroscopic observations of transiting planets. Using this technique, we discovered an unexpected phenomenon : the evaporation of hot-Jupiters, and we made a detailed study of the atmosphere of the exoplanets HD209458b and HD189733b. Observations of transiting planets are now widely recognized as a powerful method to scrutinize the atmosphere of these exoplanets.
The present “Exo-Atmos” programme is aimed at constraining both the extended upper atmosphere of evaporating planets, and the deeper atmosphere of a large variety of exoplanets, using transit spectroscopy observations. These objectives will be reached by two means: 1) We will observe with the best telescopes presently available: Very Large Telescope (VLT) and the Hubble Space Telescope (HST). 2) We will also enlarge the sample of planets for which atmosphere are detected and analysed.
Indeed, we have obtained a large amount of time on the HST and the VLT telescopes to observe the atmospheres and evaporation of extrasolar planets. For instance, with 3 HST programs in 2012 we will observe a total of 10 planets (9 exoplanets + Venus as a benchmark). This will allow us to scrutinize the atmosphere of an unprecedented large sample of exoplanets, and will open the field of comparative exoplanetology.
However, to obtain the best scientific return of these programs, a substantial financial support is required. Here we propose an ANR program to support this work.
In addition to financial support for missions and equipment, we ask for 2 post-doctoral fellowships of 2 years, one for each of the two partners of the project.
Finally, we ask for financial support for the acquisition of a powerful computer that will be dedicated to numerical simulations for the analysis and interpretation of HST spectroscopic observations, and modeling of the gas escape. These simulations will be designed to better constrain the structure, dynamics and composition of the atmospheres of planets observed by transit spectroscopy. In particular, a significant part of the CPU time will be dedicated to issues related to the evaporation of planets orbiting close to their star, an area in which our team has played a major role. Combined with the results of these simulations, the HST observations will allow to better constrain the escape rate and the evaporation mechanisms.
At the end of this 3 year program "Exo-Atmos," we aim to better understand the atmosphere and evaporation of exoplanets.

Project coordination

Alain LECAVELIER (Institut d'astrophysique de Paris) – lecaveli@iap.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

IAP Institut d'astrophysique de Paris
IPAG Institut de Planétologie et d'Astrophysique de Grenoble

Help of the ANR 454,246 euros
Beginning and duration of the scientific project: December 2012 - 36 Months

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