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WHat next? an Integrated PLanetary Atmosphere Simulator: from Habitable worlds to Hot jupiters

Periodic Reporting for period 2 - WHIPLASH (WHat next? an Integrated PLanetary Atmosphere Simulator: from Habitable worlds to Hot jupiters)

Reporting period: 2018-03-01 to 2019-08-31

With the upcoming launch of the James Webb Space Telescope (JWST), and the recent selection of the ARIEL mission (Atmospheric Remote-sensing Infrared Exoplanet Large survey) by the European Space Agency, the grand challenge of exoplanet research in the next decade is clearly the characterization of their atmospheres. This is the only way to unravel the origin of the wild, unexpected diversity we have uncovered. However, to be ready for the analysis and interpretation of such high-precision observations, we need new-generation tools fit to address the multiple challenges they will raise. Indeed, until now, most atmospheric characterization observations—e.g. transit/eclipse spectroscopy—are analyzed with spherically symmetric, steady state 1D models that cannot accurately represent the very anisotropic atmospheres of most transiting exoplanets. This issue is worsened by the ubiquity of clouds, whose inhomogeneous spatial distribution—patchiness—prevents any satisfactory treatment in 1D.
In this project, we will develop a new framework to constrain the physics and composition of exo-atmospheres that will allow us to overcome these difficulties when analyzing and interpreting observations. This will be done by developing and using a new 3D planetary atmosphere simulator that integrates a global climate model and a 3D radiative transfer code to generate observables. Using such an innovative approach, the Whiplash project will thus answer the following fundamental questions:
- What are the limits of current data interpretation tools
- What are the necessary conditions to sustain liquid water on terrestrial exoplanets? How can we infer observationally whether an atmosphere meeting these requirements is actually present?
- Can clouds explain the puzzling features of observed hot, gaseous exoplanets? What can these observations tell us on the dynamical and microphysical properties of clouds inside these atmospheres?
If we want theory to keep pace with the quality of future data, such a project is the necessary counterpart to the huge ongoing observational effort made by the community.
As expected during this first period of the project, we have laid the foundations for the project by finishing the development, testing, and validation of the 3D radiative transfer code needed to produce observable signatures from our global climate model. This tool, Pytmoshp3R, is now described in Caldas et al. (2019; A&A) and available through GitHub. Although some future upgrades are foreseen, all the bricks of our integrated 3D planet simulator are now operational and produced initial scientific results. In particular, we have been able to identify a completely new type of bias in the interpretation of transmission spectra of exoplanets due to the strong day to night side temperature gradient.

Simultaneously, the team has participated to the discovery of the first planetary system around a cool, nearby star---the Trappist-1 system. This system is becoming kind of a Rosetta stone for exoplanet science: the central star is among the smallest in the galaxy and is relatively close to us. Only around such stars can we expect to characterize the atmosphere of temperate Earth-like planets with the future space telescope. And it is not one, but seven such planets that we have discovered around it, opening the way to comparative exoplanetology.

Thanks to our planet simulator, we have been able to provide new constraints on the nature of the atmosphere of these planets and make important predictions on their observability. In parallel, we have developed several new tools to study and predict the type of rotation states available for such planets. All this theoretical expertise will be instrumental in proposing future observations and interpreting them when they are available.
The new planet simulator that we have developed, with the possibility to simulate realistic emission phase curves as well as transit and eclipse observations from a 3D atmosphere model is unique in Europe. It is thus well positioned to predict the observability of new, subtle features by future instruments. We expect to use it to propose observations on the James Webb space telescope when it will be availalbe and secure new observations of Trappist-1 and other systems.

On the longer term, we are also using it to prepare the scientific specifications for the ESA/ARIEL mission that has been selected and is being designed. Indeed, it is important to have the most realistic possible idea of what the atmosphere of exoplanets can look like to build the instruments that will observe it. In particular, the new biases that we identify in the interpretation of spectroscopic observations of exoplanets should soon give us ideas about better ways to implement those observations, or how to prioritize some observations over some others.

At the same time, we are currently using this model to shed new light on existing observations. Indeed, being one of the few models able to predict transit spectra in 3D, it has allowed us to identify new biases in current data interpretation techniques. This needs to be properly quantified and expanded in the next phase of the project.

Finally, as new extreme planets are discovered every day, we will keep upgrading the various physical ingredients making the model to keep it versatile and flexible.
Map of the temperature distribution on a tidally locked planet resembling Earth