Objectives The Science Interpretation WP aims at addressing the following fundamental questions:

• What is the origin of the observed exoplanet diversity?
• How and where did exoplanets form?
• What are the physical processes at work responsible for exoplanet evolution?  
  
  More specifically, transit, eclipse and phase curve observations, as analysed in WP2 and WP3, will allow us to tackle a host of science questions, including:
• What are exoplanets made of?  

• Do they have an atmosphere?
• Does the chemical/elemental composition cast light on their formation process? 

• How are they affected by starlight, stellar winds and other time-dependent processes?
• What is the energy budget?
• How do weather conditions vary with time? 

• Do any of the planets observed have habitable conditions? 

  
  This effort will deliver essential preparatory science for present and future European ground- and space-based missions to characterise extrasolar planets.

Description of work

WP5 will include the analysis and interpretation of the transit, eclipse and phase-curve spectra/photometric data acquired and analysed in WP2 and WP3. WP5 is aimed at providing a comprehensive view of the nature of exoplanet atmospheres, through an interdisciplinary approach which includes the integration of state of the art models of the star-planet interaction, atmospheric chemistry and dynamics and planet formation (Fig. WP5.1). The integrated approach proposed here is essential to provide a realistic description of physical/chemical processes which are highly correlated, as indicated in the couple of examples given below. 

Fig. WP5.1: Key physical processes influencing the composition and structure of a planetary atmosphere. While the analysis of a single planet cannot establish the relative impact of all these processes on the atmosphere, by expanding observations to a large number of very diverse exoplanets, we can use the information obtained to disentangle the various effects. The tasks covered in WP5 will make use of existing state of the art models and develop new ones to simulate all the relevant physical processes sketched here.  

The Science Interpretation WP can be broken down into two main tasks and a number of subtasks, whose completion is necessary to answering the strategic objectives spelled out above. Briefly these are:

Task 5.1 – Chemical composition of exoplanet atmospheres (coordinated by G. Tinetti (UCL), core work done by postdoc funded through this grant. Collaborators: Jéremy Leconte (Un. of Bordeaux), teams at CEA and UNIVIE, Ingo Waldmann (UCL) Michiel Min (SRON).

Giant planets are mostly made of hydrogen and helium and are expected to be always in gaseous form, so that the relevant questions concern the amounts of all elements other than hydrogen and helium, i.e. heavy elements such as C, O, N, Si, Ti, V etc., that are present. A fundamental question is by how much are these atmospheres enriched in heavy elements compared to their parent star. Such information is critical to:

1. understand the early stage of planetary and atmospheric formation during the nebular phase and the immediately following few million years. For instance, Rocchetto et al (ApJ, 833 120, 2016) have demonstrated that JWST observations will allow to retrieve confidently absolute molecular abundances and infer the C/O ratio. The C/O ratio and other elemental ratios can cast light on the provenance of the planet in the disk and on migration processes (e.g. Öberg et al., ApJL 743, 2011; Turrini et al., Exp. Astron. 40, 501, 2014; Mollière et al., 813 47, 2015). The team at CEA and SRON have great experience on planetary disks modelling and planet formation.
2. test the effectiveness of the physical processes directly responsible for their evolution. The elemental abundances, together with the atmospheric temperature and, to a lesser degree the physical processes in the atmosphere (e.g. mixing, photolysis) are the drivers of the chemical composition (i.e. the trace gases abundances which vary as a function of height and longitude), see e.g. Venot et al. (A&A 546, A43, 2012; A&A 577, A33, 2015).

When it comes to rocky-type planets, the spectrum of possibilities is indeed much larger compared to the gaseous objects: it is very difficult to guess a priori what the main atmospheric component should be, if any (e.g. Forget & Leconte, Phil. Trans. Royal Society 372 2014). The response needs to come from observations (e.g. Tsiaras et al, ApJ 820 99, 2016).

Having lower masses, their atmospheres may have evolved quite dramatically from their initial composition: lighter molecules, like hydrogen, can escape more readily. This certainly happened to the terrestrial planets in our Solar System: in Venus' and Mars' atmospheres the D/H ratio is between 5 and 200 times the Solar ratio, suggesting water on the surface was lost through time (Owen et al., 1988; Encrenaz, 2009). Impacts with other bodies, such as asteroids or comets (e.g. responsible for the delivery of Nitrogen and water on Earth), or volcanic activity (responsible e.g. for CO₂ on Venus and Mars) might also significantly alter the composition of the primordial atmosphere, not to mention life, which on Earth is responsible e.g. for the production of molecular oxygen, accounting for 21% of the overall atmospheric volume (Lovelock, 1975; Rye & Holland, 1998). Again, their atmospheric composition will be a tracer of their history, as well as addressing the much debated – and highly visible – question of habitability.

WP2 and WP3 will be able to detect, or at least constrain, the main atmospheric component and key trace-gases such as H₂O, CO₂ etc. in those atmospheres, by combining transit (an excellent probe of scale height) and eclipse (an excellent probe of the temperature) measurements. WP5.1 will interpret those results as illustrated in Fig. WP5.1. Input from WP5.2 and WP4 will be needed to account for the interaction with host star.

Fig. WP5.1: The chart on the right illustrates the theoretical procedure to be used for each of the exoplanet studied. Knowing the star-planet distance and the type of the stellar companion, we can infer the planetary equilibrium temperature. When we consider gaseous planets, which we know being composed mainly by molecular hydrogen, this information can be used as a first guess to predict the equilibrium thermochemical composition of the planetary atmosphere (Lewis, 1995, Lodders et al. 2002). If the observed chemical abundances do not match the theoretical predictions, then either other physical processes are at work or the formation and migration history played a critical role. As shown in the chart, we can test each of these hypotheses by carefully selecting the object sample and the observables.

Subtask 5.1.1 – Impact of atmospheric dynamics (Sacha Brun, Pascal Tremblin, Emeline Bolmont (CEA), Jérémy Leconte (Un. Bordeaux). Core work done by postdoc funded through this grant)

Chemistry and dynamics are often entangled. Agúndez et al. (A&A 548 A73, 2015) showed that for hot-Jupiters, for instance, the molecules CO, H₂O, and N₂ and H₂ show a uniform abundance with height and longitude, even including the contributions of horizontal or vertical mixing. For these molecules it is therefore of no relevance whether horizontal or vertical quenching dominates. The vertical abundance profile of the other major molecules CH₄, NH₃, CO₂, and HCN shows, conversely, important differences when calculated with the horizontal and vertical mixing. Phase-curves spectroscopic measurements of the dayside and terminator regions would provide a key observational test to constrain the range of models of the thermochemical, photochemical and transport processes shaping the composition and vertical structure of these atmospheres.

Longitudinal variations in the thermal properties of the planet cause a variation in the brightness of the planet with orbital phase. This orbital modulation has been observed in the IR in transiting (e.g. Knutson et al., Nature 447, 183, 2007) and non-transiting systems (Crossfield, et al., ApJ, 723 1436, 2010). In Stevenson et al. (Science 346 838, 2014) full orbit spectra have been obtained with Hubble/WFC3. One of the great difficulties in studying extrasolar planets is that we cannot directly resolve the surfaces of these bodies, as we do for planets in our solar system. The use of occultations or eclipses to spatially resolve astronomical bodies, has been used successfully for stars in the past. Majeau et al. (ApJ 747 L20, 2012) derived the two-dimensional map of the hot-Jupiter HD189733b at 8 μm with Spitzer-IRAC. JWST and dedicated space missions such as ARIEL will provide phase curves and 2D-IR maps recorded simultaneously at multiple wave-lengths, for several gaseous planets, an unprecedented achievement outside the solar system. These curves and maps will allow one to determine horizontal and vertical, thermal and chemical gradients and exo-cartography.

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. To truly understand, and possibly correct, the biases created by this approach, we propose test the retrieval methods developed here on synthetic data obtained with a 3D atmospheric and radiative transfer model. To that end, we will use the LMDZ Generic Global Climate Model (GCM), a version of a heavily used Earth climate model made as generic as it possible to simulate any planetary or exoplanetary atmosphere with very arbitrary parameters. This model can predict the 3D structure of a planetary atmosphere, with inhomogeneous cloud covers when warranted. To complement this model, the team led by J. Leconte is developing a 3D radiative transfer code that is able to model observables (transits and eclipse lightcurves, orbital phasecurves) directly from the outputs of the aforementioned GCM. This code accounts for molecular absorptions (using either high resolution cross sections from the UCL-ExoMol project of k-correlated coefficients), Rayleigh scattering, collision induces absorptions, and mie scattering by clouds or hazes.

The goal of this particular study is to model the realistic spectrum created by a planet with a realistic thermal and chemical structure and to feed this to retrieval algorithm. This is the only way we can test our retrieval procedures in a more realistic way while still knowing the ground truth about the atmosphere we are trying to characterize. 

Subtask 5.1.2 – Habitability of rocky planets around cool-stars (Jérémy Leconte (University of Bordeaux), Emeline Bolmont (CEA), Giovanna Tinetti (UCL), Manuel Guedel, Colin Johnstone, Kristina Kislyakova (UNIVIE), Helmut Lammer (Space Research Institute Graz), Sacha Brun (CEA).

In the light of the very recent discovery of the TRAPPIST-1 system (Gillon et al., Nature, 2017), the study of the habitability of planets around ultra-cool dwarfs has now become a realistic perspective. The interest of these objects is amplified by the fact that JWST and future IR space instruments such as ARIEL will be able to probe their atmospheres (e.g., Belu et al. 2013, Barstow et al, 2015). The fact that ultra-cool dwarf exoplanets are extremely close-in (including the habitable zone planets) means that they are significantly affected by star-planet interactions: both magnetic, radiative, wind and tidal interactions. These interactions shape the planetary systems and in particular, the rotation state of the planet can be estimated (see WP5.2). Atmospheric escape can, depending on stellar parameters, remove substantial masses of atmospheric gases over time, so that a rocky planet can become completely uninhabitable.

The rotation state of a planet has a strong impact on its climate and in particular on the temperature distribution and winds. TRAPPIST1-type systems are therefore great laboratories to study the impact of orbital dynamics and star-planet interactions on the potential climates of planets, and possibly to assess their habitability. A more basic approach consists on estimating the energy budget of the planet (e.g. Bolmont et al. 2014), more detailed climate simulations can then follow. Indeed, the progress made in the climate simulations of exoplanets allow the community to infer possible climates for these objects. We plan to use the LMDz climate model, which comes from the LMD climate model initially developed for the study of the Earth's climate and which has been generalized for exoplanets (e.g., Wordsworth et al. 2011 for GJ581). To study such close-in planets which are likely to be tidally locked, 3D climate modelling is necessary (Joshi M, Astrobiology, 3 415, 2003).

The deliverables from WP5.2, such as rotation state and tidal heating, together with climate/chemistry simulations, will allow us to estimate the effect of dynamics, chemistry and star-planet interactions on close-in planets. The outputs of the climate/chemistry models can then be used to interpret transit spectra as observed by JWST and future dedicated missions.

Task 5.2 – Star-planet interaction (coordinated by Sacha Brun (CEA), collaborators: Stéphane Mathis, Emeline Bolmont and Antoine Strugarek (CEA) Manuel Guedel, Colin Johnstone, Kristina Kislyakova (UNIVIE), Helmut Lammer (Space Research Institute Graz), postdoc funded through this grant)

During the last two decades, a large population of short-period exoplanets has been discovered around host stars of different masses and ages. Stars, being dynamical rotating and magnetically active objects, dramatically affect their environment. To understand the properties of planetary surfaces and atmospheres, it is therefore necessary to understand the different star-planet interactions, which are not yet taken into account self-consistently in current simulations of exoplanets atmospheres. To start with, magnetic active stars interact with the atmosphere and magnetosphere of their planets through their winds (e.g. Strugsarek et al. 2015, ApJ, 815, 111). Additionally, in the case of short-period planets, strong tides are exerted on the planet by the host star, causing the rapid erosion or even the loss of the planetary atmosphere (e.g. Hansen et al, 2015).

Stars are dynamical objects often possessing intense magnetic activity, strong wind of particles and variability over a broad spectral energy range. Planets embedded in such a dynamical and energetic environment have their exosphere directly impacted and modulated in phase with the activity of the host star.  Depending on whether or not the exoplanet possesses a magnetosphere, the interaction between the host star and the planet's atmosphere will greatly vary. Planetary magnetosphere plays a shielding role to energetic charged particles but – an aspect often neglected – it also provides a larger region for the interaction with the stellar wind, hence funnelling a larger proportion of the wind energy toward the planet. With such a dense time-varying energetic space environment, planetary atmosphere are known to adapt to and evolve through the influence of their host star extended corona.

The study of such "space weather" interactions has several components related to winds, magnetic fields of the star/wind and the planet, and high-energy radiation. Direct radiative interactions with upper atmospheres lead to heating and thermal escape, and this depends on whether a magnetosphere is present, and on its strength and extent. Furthermore, interactions between the ionized/magnetized wind and the atmosphere or magnetosphere or both add various non-thermal escape mechanisms to the picture (charge exchange + Energetic Neutral Atoms, ion escape, sputtering, dissociative recombination etc).

On the largest scales (magnetosphere-wind interactions) the CEA team coordinated by Dr. Brun has the unique capability to compute self consistently in 3-D the stellar dynamo (Brun et al. 2004, Augustson et al. 2015, Brun et al. 2017) and its associated magnetic topology and to feed these results into a 3-D stellar wind / space environment model around the planet (Reville et al. 2016, Strugarek et al. 2015, 2016). As part of the 5.2 task, the realistic simulations of the stellar wind conditions will allow to characterise the plasma conditions into which the planet orbits and evolves and how its exosphere (magnetosphere, ionosphere, atmosphere) reacts to such forcing. Task 5.2 will also assess if the exoplanet is within the Alfvén radius of its host star, which has direct consequences on the existence or not of Alfvén wings (direct star-planet magnetic connectivity) or of a bow shock and extended magneto-tail. Task 5.2 will also estimate the rate of the atmospheric evaporation through processes such as pick up ions, as the ram pressure of the wind and its thermal and magnetic components will be quantified. Available data in X-rays (ref) and UV (ref) will permit the validation of the models. The validated models will then be used to interpret the observations obtained at longer wavelengths as part of WP2.

The boundary conditions for the wind simulations and stellar magnetic field strength and possibly topology will come from stellar constraints delivered by Task 4.3 of WP4.

The upper atmospheres (mesospheres/thermospheres) of planets are subject to short-wavelength radiation driving chemistry (including photodissociation of molecules), heating, and ionization (e.g. for H atmospheres, Johnstone et al. 2015, ApJ, 815 L12). Consequences are thermal loss mechanisms either in the Jeans regime or in a hydrodynamic flow regime. Hydrodynamic codes handling these regimes are available to the team and are further developed, including chemical reactions, eddy/ambipolar/molecular diffusion, chemical networks, chemical and Joule heating, and conduction. Making use of radiation inputs delivered by WP4 and specifically the possible radiative history of the star (in the UV/X-ray regime), this task will therefore allow the thermal mass-loss history to be estimated, setting constraint on the initial planetary mass.

At the exosphere/wind boundary, various non-thermal interaction processes may lead to further loss mechanisms, depending on the presence and strength of a planetary magnetosphere and the wind ram pressure (Kislyakova, et al. 2013, AsBio, 13, 1030). Codes (Monte Carlo and particle codes) are available calculating particle collisions, charge exchange, electron impact ionization, and photoionization in exospheres to estimate loss rates. Again, knowing the possible wind mass-loss history and the radiative history of the host star delimited by WP4, the non-thermal mass-loss history can be assessed. Other process such as sputtering will also be taken into account.

In addition to magnetic interactions and radiation, planetary atmosphere will be exposed to strong tides in close systems. Two types of tides will be considered: gravitational tides and atmospheric thermal tides due to the irradiation of the planetary atmosphere by the host star (e.g. Mathis & Remus 2013, LNP, 857, 111; Arras & Socrates, 2010, ApJ, 714, 1).

When focusing on planetary atmospheres, key information can be obtained by studies of star-planet tidal interactions. First, thanks to new ab-initio evaluation of the dissipation of tides in the host star (Mathis 2015, A&A, 580, L3), we can understand the current position of the planet relatively to the habitable zone (e.g. Bolmont & Mathis 2016, CMDA, 126, 275). Next, the estimate of the tidal dissipation in planets thanks to new realistic models (e.g. Remus et al. 2012, A&A, 541,165; Ogilvie 2014, ARAA, 52, 171) provide key information on their dynamical state, i.e. their rotation and the inclination of their spin, which are critical for their atmospheric general circulation and climate. For example, in the case of a super-Earth with an atmosphere, it has been recently predicted that if the atmospheric layers close to the ground are convective, the torque applied by thermal tides on the atmosphere leads to an asynchronous rotation (Leconte et al. 2015, Science, 347, 632; Auclair-Desrotour et al. 2017, A&A, in press). In the case of a convectively stable atmosphere, the tidal torque applied by thermal tides is weak and the tides applied on the rocky core leads to synchronized planets. Such new predictions will be used in the project for observed targets for which constrains on the temperature profile in the atmosphere will be obtained. In addition, thanks to these constrains, we will estimate tidal heating in the atmosphere and evaluate its impact on the planetary evolution and on the atmospheric general circulation.

The outcome of this effort will enable the development of a new generation of state-of-the-art atmospheric simulations, which will take into account the impact of star-planet tidal interactions which is currently neglected or modelled separately.

WP Leaders: WP5 is chaired by Giovanna Tinetti (UCL) and co-chaired by Sacha Brun (CEA).

Participants: Stéphane Mathis, Emeline Bolmont, Pascale Tremblin (CEA); Michiel Min (SRON), team at UNIVIE; collaborators: Jérémy Leconte (University of Bordeaux) and Helmut Lammer (University of Graz). One postdoc (26 months) based at UCL but collaborating/travelling regularly to CEA will be dedicated to this WP.