\makeatletter
\@ifundefined{HCode}
{\documentclass[screen, CRPHYS, Unicode, Thematic, HideArticleType]{cedram}
\newenvironment{noXML}{}{}
\def\tsup#1{$^{{#1}}$}
\def\tsub#1{$_{{#1}}$}
\RequirePackage{etoolbox}
\def\jobid{crphys20221218}
%\graphicspath{{/tmp/\jobid_figs/web/}}
\graphicspath{{./figures/}}
\newcounter{runlevel}
\let\MakeYrStrItalic\relax
\csdef{Seqnsplit}{\\}
\def\refinput#1{}
\def\back#1{}
\DOI{10.5802/crphys.143}
\datereceived{2022-12-12}
\dateaccepted{2023-01-27}
\ItHasTeXPublished
\def\sfrac#1#2{{#1}/{#2}}
\makeatletter
\g@addto@macro{\UrlBreaks}{\UrlOrds}
\gappto{\UrlBreaks}{\UrlOrds}
\makeatother
}
{\documentclass[crphys]{article}
\def\CDRdoi{10.5802/crphys.143}
\let\newline\break
\makeatletter
\def\sfrac#1#2{{#1}/{#2}}
}
\makeatother

\usepackage{upgreek}

\dateposted{2023-04-06}
\begin{document}

\begin{noXML}

%\TopicEF{Exoplanets}{Exoplan\`etes}

\title{Characterization and modelling of exoplanetary atmospheres}

\alttitle{Caract\'erisation et mod\'elisation des atmosph\`eres
exoplan\'etaires}

\author{\firstname{Benjamin} \lastname{Charnay}\CDRorcid{0000-0003-0977-6545}\IsCorresp}
\address{LESIA, Observatoire de Paris, Universit\'e PSL, CNRS, Sorbonne
Universit\'e, Universit\'{e} Paris-Cit\'{e}, 5 place Jules
Janssen, 92195 Meudon, France}
\email[B. Charnay]{benjamin.charnay@obspm.fr}

\author{\firstname{Pierre} \lastname{Drossart}\CDRorcid{0000-0001-7261-0228}}
\address{Institut d'Astrophysique de Paris, CNRS, UMR 7095, Sorbonne
Universit\'e, 98 bis bd Arago, 75014 Paris, France}
\email[P. Drossart]{pierre.drossart@iap.fr}

\def\thanksname{Note}
\thanks{Follows up on a conference-debate of the French Academy of
Sciences entitled ``Exoplanets: the new challenges'' held on 18 May 2021,
visible via
\url{https://www.academie-sciences.fr/fr/Colloques-conferences-et-debats/exoplanetes.html}}

\thanks{\textbf{Note.}
Fait suite \`a une conf\'erence-d\'ebat de l'Acad\'emie des sciences
intitul\'ee  \guillemotleft{}~Exoplan\`etes : les nouveaux 
d\'efis~\guillemotright{} tenue le 18 mai 2021, visible via
\url{https://www.academie-sciences.fr/fr/Colloques-conferences-et-debats/exoplanetes.html}}


\keywords{\kwd{Exoplanets}
\kwd{Atmospheres}
\kwd{Models}
\kwd{Retrieval}
\kwd{James Webb space telescope (JWST)}
\kwd{Ariel mission}}

\altkeywords{\kwd{Exoplan\`etes}
\kwd{Atmosph\`eres}
\kwd{Mod\`eles}
\kwd{Inversion}
%\kwd{R\'ecup\'eration}
\kwd{T\'el\'escope spatial James Webb (JWST)}
\kwd{Mission Ariel}}

\makeatletter
\def\cdr@preabstracthook{%
  \vspace*{-5pt}
%\vskip2pt plus 3pt minus 1pt
}
\makeatother

\begin{abstract} 
The spectral characterization of exoplanetary atmospheres is a
relatively new subject. Until recently, space and ground-based
observatories allowed only a few planets to be observed with
spectroscopic instruments. The arrival of JWST data and the development
of new instruments on Earth will drastically modify the situation,
giving access to deeper modelling contraints and to a better
understanding on planet formation and evolution. This paper is devoted
to describe the turning point experienced by the astronomy of
exoplanets that will continue in particular with the launch of the
ESA/Ariel mission at the end of the decade. 
\end{abstract}

\begin{altabstract}
Cet article passe en revue les r\'ecentes avanc\'ees en physique des
atmosph\`eres des exoplan\`etes, dans le cadre de la pr\'eparation de
la mission Ariel de l'ESA. La mod\'elisation des atmosph\`eres, b\^atie
sur les mod\`eles plan\'etologiques d\'evelopp\'es dans l'\'etude du
Syst\`eme Solaire, peut aujourd'hui \^etre extrapol\'ee aux
exoplan\`etes par l'utilisation de bases de donn\'ees mol\'eculaires
adapt\'ees aux temp\'eratures rencontr\'ees. L'une des difficult\'es
des mod\`eles repose sur la forte d\'eg\'en\'erescence dans l'inversion
des param\`etres de structure et composition atmosph\'erique. Les
m\'ethodes bayesiennes d'inversion ont permis des avanc\'ees
r\'ecentes, sur les observations \`a basse r\'esolution du Hubble Space
Telescope, et sont pr\'epar\'ees pour les prochaines observations du
James Webb Space Telescope. Les avanc\'ees attendues dans les
prochaines ann\'ees dans le domaine font de la caract\'erisation par
spectroscopie des exoplan\`etes une des branches les plus actives du
domaine.
\end{altabstract}

\maketitle

\end{noXML}

\vspace*{-6pt}

\section{Introduction}\label{sec1}

After 25 years devoted mostly to the detection and statistical sampling
of general parameters of exoplanets, the spectral characterization
enters in a mature stage, with both modelling improvements and new
observations. 
This article will describe some of the recent
developments in the field, which are important for the preparation of
the Ariel ESA mission currently in development phase. The JWST
observations which are just at their beginning will certainly increase
our knowledge in a near future and this article will also raise the
most important questions to be addressed with both ground-based and
space mission of the decade.

\section{Observations of exoplanetary atmospheres}

\subsection{Observations of the solar system---historical achievements}

The observations of the planets of the solar system are naturally a
first basis on which the models of exoplanets are built. Scientists are
nevertheless immediately confronted with the question of the
specificity of the Solar System. The anthropocentrism encountered in
the history of astronomy must be avoided before extrapolating our
knowledge of planets to exoplanets. Among examples of unsafe
extrapolations are the search for life on Mars, with announcement of
vegetation detection in the 1950's~\cite{Sinton1}. However, the
physical mechanisms can be studied with unparalleled precision on
nearby objects and then applied to different systems: this is the case
in the major fields of radiative transfer, meteorology, Earth--Sun
interactions, etc. The utility of the Solar System observations is
therefore important to describe detailed physical mechanisms, before
extrapolation is made to other planets. The interpretation of planetary
atmospheres of Solar System objects has been developed since the space
era and the first planetary missions of the 1960s--1970s. The questions
were approached from terrestrial models (atmospheric structure,
meteorology, radiative transfer, aeronomy), by pointing out the
Earth--Planet relations in the physics of the atmospheres. It is the
same paradigm that we live today by passing from planets to exoplanets.
The planets, thanks to the close observations, even in situ, allow us
to validate the physical models which will have to be extrapolated to
different conditions in the
%\break
exoplanets.

\subsection{Past and future observations of exoplanetary atmospheres:
which constraints for atmospheric models?}

Observations of exoplanetary atmospheres come almost exclusively from
transiting and directly imaged exoplanets. For transiting exoplanets,
the atmosphere can be probed:  (1) during transit by measuring the
variation of planetary radius caused by molecular/atomic absorption
with wavelength (technique called transit spectroscopy),  (2)~during
the eclipse by measuring the thermal emission or reflected light from
the exoplanet (technique called eclipse spectroscopy),  (3) during the
whole orbit by measuring the variation of thermal emission or reflected
light (technique called phase curve observation).  The first
observation of an exoplanetary atmosphere was obtained by transit
spectroscopy of sodium lines in the atmosphere of HD209458 b, although
this detection is still debated 
\cite{charbonneau00,charbonneau02,Casasayas1,Casasayas2}.  Most of JWST
exoplanet observations will be dedicated to transit/eclipse
spectroscopy and phase curves. For directly imaged exoplanets, one can
measure the thermal emission spectrum of the planet and detect
molecular absorption. Future telescopes, like the Roman Space
Telescopes or the ELTs will detect reflected light of temperate
exoplanets. The characterization of exoplanetary atmospheres by these
different techniques is now done routinely. New data of exoplanetary
atmospheres are published every week. After 20 years, a large list of
molecules/atoms detected in exoplanetary atmosphere has now been
established (H$_2$O, CO, CH$_4$, NH$_3$, CO$_2$, HCN, C$_2$H$_2$, H,
He, Na, K, Li, Mg, Ca, Fe, V, Cr$,\ldots$)~\cite{guillot22,giacobbe21}. A
surprise was to discover that most exoplanets are cloudy or
hazy~\cite{sing16}. The presence of these aerosols is currently one of
the main limitations in our ability to probe the chemical composition
of exoplanetary atmospheres. Another major breakthrough on exoplanetary
climates came from the detection of equatorial superrotation on hot
Jupiters from thermal phase  curves~\cite{knutson07}. The fast
equatorial jet redistributes heat to the nightside and produces an
eastward shift of the hot spot on strongly irradiated exoplanets.
Superrotation was predicted by atmospheric modelers a few years before
its detection~\cite{showman02}. Exquisite measurements of phase curves
will be obtained in the coming years with JWST revealing the horizontal
variations of temperature, chemical composition and clouds. These would
be strong constraints for atmospheric models able to couple atmospheric
dynamics, thermal structure, chemistry and clouds.

One of the main goals is now to measure molecular abundances in order
to provide constraints on exoplanet formation mechanisms. A major
question for the coming decade is to determine the nature of
sub-Neptunes, planets with radii between 1.8 et 3 Earth radii, which
are very abundant in our galaxy but with no equivalent in our solar
system. These planets could be composed of a rocky core surrounded by a
H$_2$-dominated atmosphere or they could be water-worlds with a
H$_2$O-dominated atmosphere. The last case would imply formation of
these planets beyond the ice line and migration toward their host
stars. Another big question concerns the evolution of the atmospheric
metallicity (the fraction elements higher than H and He) as a function
of the planetary mass. Giant planets in the solar system show a
decrease of the atmospheric metallicity with the planetary mass
following a simple scaling law. A generalization of this trend to
exoplanets would be a major constraint for planetary formation models.
However, measurements of atmospheric metallicity based on water vapor
abundance in exoplanet atmospheres do not show such a correlation with
often a low metallicity, sometimes even lower than 
solar~\cite{guillot22}. Better measurements will be required to break the
degeneracies caused by the presence of clouds/hazes but there seems to
be a large diversity in exoplanetary atmospheres. Another motivation
for the chemical characterisation of exoplanetary atmosphere is the
possible link between the atmospheric C/O ratio and the planetary
formation mechanism. Models predict variations of this ratio in the gas
and solids of protoplanetary disks depending on the location of the
different ice lines (H$_2$O, CO$_2$, CO)~\cite{oberg11}. Atmospheric
C/O ratios could potentially be a footprint of where and how planets
formed in the protoplanetary disks.

\subsection{Characterization of host stars and star--planet interactions}
The exoplanets atmospheric characterization has become the principal
target for exploration, in particular with the technique of transit
spectroscopy. The observation of light curves during exoplanets transit
with spectrometers has given important results on the composition of
exoplanets. Nevertheless, many effects can perturb the data reduction:
stellar variability, stellar activity, etc. A recent reevaluation of
sodium emission on HD209458b~\cite{Casasayas1,Casasayas2} has pointed
that the combination of stellar lines with planetary lines, with
varying Doppler shift due to a rapid stellar rotation (called
Rossiter--McLaughlin effect~\cite{Rossiter,McLaughlin}), can perturb
the planetary interpretation of Na or K emission. Similar effects are
expected when molecular lines are present in cold stars~\cite{Cospar1},
with corrections to take into account to disentangle stellar and
planetary spectra. The stellar activity is another important aspect
which begins only to be well enough modelized in order to correct for
{em e.g.} granulation or stellar spot, beyond simple limb darkening
models~\cite{Chiavassa-Brogi}. The accuracy of stellar models for FGK
stars gives today an access to correction in transit spectroscopy for
most effects, when high resolution stellar spectra are available.

\subsection{The ESA-Ariel mission}

The ESA/Ariel mission~\cite{Tinetti1} scheduled for launch in 2029 is
devoted to the transit spectroscopy of a representative sample of a
thousand exoplanets from temperate Super-Earths to Giant planets. Its
main objective is to answer to fundamental questions about planet
formation and evolution in a statistical frame, and to put in
perspective the question about the condition of the emergence of life.
The Ariel mission has been build after the phase A study of the ECHO
mission~\cite{Puig1}. It is a 1~m class space telescope devoted to
the observation by spectroscopy of transiting exoplanets, between 1.1
and 7.8~$\upmu$m with access to a large number of molecular bands present
in planetary atmospheres (e.g. H$_2$O, CH$_4$, CO, NH$_3$, etc.) By
observing a known sample of transit exoplanets, Ariel will focus on the
transit observation from one planet to the other with agility. The
sample target reference list is therefore a major program element to be
built in advance. The observation strategy is built on a four Tier
approach. Tier~1 aims at observing the whole Ariel sample (${\sim}$1000
planets) by transit to detect which planets have a cloud-free,
H$_2$-dominated atmosphere and which planets have a cloudy of secondary
atmosphere. Tier~2 consists in an atmospheric characterisation by
transits and eclipses of ${\sim}$500 exoplanets to mesure chemical
abundances, clouds and thermal structure. Tier~3 is a detailed
atmospheric analysis of ${\sim}$50 planets, cumulating many
transits/eclipses. Tier~4 is dediacted to additional science and to
phase curves. ${\sim}$40 planets will be observed by phase curves to map
the thermal structure, clouds and chemical composition and to constrain
the atmospheric dynamics~\cite{charnay22}. \looseness=1

The Ariel mission relies on the observation of planetary transit of a
large sample of planets. An optimization of the mission process relies
on a good knowledge of ephemeris. Many targets are observed from
previous mission, especially TESS, and a growing uncertainty with time
of the ephemeris accuracy has to be compensated. A program of
ground-based observations has been coordinated~\cite{Kokori1} with the
participation of world-wide consortium of observers on small telescopes
to update and verify the ephemeris of the Ariel targets.

\section{Models for exoplanetary atmospheres}

\subsection{Description of 1D/3D atmospheric models}

Atmospheric models are now essential tools for interpreting
observations of exoplanetary atmospheres. Many scientific teams around
the world have over the past 30 years been developing atmospheric
models for exoplanets with different degrees of complexity. These
models include key physical/chemical processes ruling planetary
atmospheres. A hierarchy of models is available, from simple 1D models,
which can be very efficient for simulating observational spectra and
for atmospheric retrieval, to complex 3D Global Climate Model (GCM),
which are necessary to investigate the atmospheric dynamics of
exoplanets. As much as possible, the processes simulated by the
atmospheric models are based on universal physical/chemical equations.
The ultimate goal is to build virtual planets able to reproduce and
predict all the available observations~\cite{forget13}. Historically,
the development of atmospheric models for exoplanets started before the
detection of the first exoplanet around a Sun-like star~\cite{mayor95}.
One of the first and still one of the most famous models was developed
by Kasting to determine the limits of the habitable zone for
different stellar types~\cite{kasting93}. The exoplanet community has
naturally benefited from developments of atmospheric models for the
Earth and solar system planets. It has also strongly benefited from
developments from the stellar/brown dwarf community, especially as the
first observed exoplanets were hot giant planets. The main processes
included in atmospheric models are the radiative transfer, atmospheric
dynamics, the chemistry, the cloud/haze formation\break (see
Figure~\ref{figure_1}).


\begin{figure}[t!] 
\vspace*{4pt}
\includegraphics{fig01}
\vspace*{6pt}
\caption{Illustration of the different processes included in a 1D or 3D
atmospheric model for gaseous exoplanets.\label{figure_1}}
\vspace*{2pt}
\end{figure} 

\subsubsection{Radiative transfer}
The radiative transfer of stellar and thermal radiation is a central
element of all atmospheric models for exoplanets. It allows us to
compute the heating/cooling rate in the atmosphere, and thus the
thermal structure, as well as spectra (in emission, reflection and
transmission). For exoplanets, atmospheric opacities in the visible and
the infrared includes:
\begin{itemize}
\item Molecular lines which correspond to transitions between
electronic, rotational, vibrational and roto-vibrational energy levels.
A typical planetary spectrum is composed of billions of lines. Beyond
the absorption in the visible wavelengths range by atomic sodium and
potassium in the NaI and KI doublet, the main source of opacity for
gaseous exoplanets are molecules as H$_2$O, CH$_4$, CO, NH$_3$, CO$_2$,
PH$_3$, VO, TiO Na and K.
\item Continuum which varies slowly with wavelength. It can be produced
by the contributions of all molecular line far wings. The line profiles
far from the center are generally not well known and experimental data
are essential to determine the continuum. Continuum can also be
produced by collision-induced absorptions (CIA), which correspond to
the formation of transitory states with a dipole moment when two
molecules collide. This process is fundamental for atmospheres mostly
composed of non-dipolar molecules, such as H$_2$, N$_2$ and O$_2$.
H$_2$--H$_2$ CIA are one of the main opacities at high pressure ($P > 1$
bar) for gaseous exoplanets.
\item Finally, cloud/haze particles produce a continuum opacity for
both absorption and scattering.
\end{itemize}
The most accurate atmospheric models use line-by-line radiative
transfer codes, which allows to spectrally resolve each molecular line
and to generate spectra at high resolution (e.g. 
$R= \sfrac{\lambda}{\Delta \lambda} \sim 10^5$), similar to the resolution
of high-resolution spectrograph (e.g. VLT-CRIRES). However, these
models are very slow and are generally limited to the calculation of
spectra for a given temperature/composition profile. For
self-consistent 1D models computing the thermal structure and for 3D
models, faster methods are used, as the correlated-k method. In this
method, the spectrum is divided into large spectral bands. In a band,
the line intensities are reorganized as a smooth distribution, which
can be easily fitted with a few points and for which the radiative
transfer as monochromatic is applied. The correlated-k method is very
fast and quite accurate. The mixing of k-coefficients for a gas mixture
is possible, and online tools allow to generate grids of k-coefficients
for different atmospheric composition~\cite{leconte21}.

\subsubsection{Dynamics}
Atmospheric dynamics includes the large-scale circulation as well as
more localized atmospheric motions such as gravity waves, convection
and turbulence. It is responsible for redistributing stellar energy
absorbed in the atmosphere. Planets in our Solar System have relatively
small equator-pole and day-night thermal contrasts, contrary to
strongly irradiated exoplanets which should belong to different climate
regimes. These planets are likely tidally-locked with a synchronous
rotation and a permanent dayside/nightside. The strong thermal contrast
drives the atmospheric circulation, leading in most cases to the
development of equatorial superrotation jets which redistribute heat
from the dayside to the nightside. The radiative timescale (defined as
the characteristic time of radiative cooling) and the advective
timescale (defined as the characteristic time of horizontal wind
transport) can be of the same order, allowing us to probe the
atmospheric dynamics by measuring the thermal structure and heat
redistribution through thermal phase curves. Tidal locking might also
affect temperate terrestrial planets in the Habitable Zone of low-mass
stars. The circulation regime on these planets could modify the cloud
distribution and consequently the cloud radiative effects (albedo
cooling effect and greenhouse warming effect), with implications on the
limits of the Habitable Zone~\cite{leconte13a, yang13}. 3D GCMs are
necessary tools to study the atmospheric circulation on exoplanets and
its observational impacts. These models solve the atmospheric fluid
equations in a rotating sphere. Depending on GCMs, different
assumptions are made (e.g. hydrostatic or non-hydrostatic, shallow or
deep atmosphere) and different numerical solvers and grids (e.g.
finite-differences or spectral model, latitude-longitude grid or
icosahedral grid) are used. 3D simulations are computationally
time-consuming, thus the parameter exploration (i.e. the atmospheric
chemical composition) is generally\break limited.

\subsubsection{Chemistry}
The large range of temperature and irradiation of exoplanetary
atmospheres covers different regimes where the chemical composition can
be at chemical equilibrium (when the concentration of chemical species
have reached conditions for no further changes with forward reaction
rates equal to reverse reaction rates) or at chemical disequilibrium.
The latter relies on the kinetics of chemical reactions and can be due
to vertical/horizontal mixing in the atmosphere, UV photolysis, impacts
or other processes. The composition at chemical equilibrium can be
relatively easily computed in atmospheric models by minimizing the
%\pagebreak
Gibbs energy for each vertical level. However, the range of validity of
models only based on chemical equilibrium is very limited. More
accurate models include non-equilibrium chemistry either by using
parameterizations of the quenching level due to vertical
mixing~\cite{zahnle14} or by solving individual reactions of a chemical
network. The development of reduced chemical networks, selecting a
limited number of key reactions, allows their implementation in 3D
models~\cite{venot20}.

\subsubsection{Aerosols}
One of the main discoveries on exoplanetary atmospheres was to realize
that most exoplanets are cloudy or hazy. These particles produce flat
and featureless transit spectra (e.g.~\cite{sing16, kreidberg14a}) and
potentially high reflectivity in secondary eclipses and phase
curves~\cite{demory13, parmentier16}. These particles are either: 
(1)~condensate clouds that grow from an atmospheric specie when the partial
pressure of the vapor exceeds its saturation vapor pressure (e.g. iron,
silicate or water clouds) or (2)~photochemical hazes produced by from
photochemistry in the atmosphere that results in the formation of
involatile solids (e.g. Titan's hazes)~\cite{marley13, gao21}. Clouds
and hazes can have a major impact on spectra and their radiative
effects must be properly taken into account in atmospheric models to
simulate climates or to retrieve molecular abundances and thermal
structures. A few cloud models include detailed microphysics
(e.g.~\cite{helling06, gao20}), while most models use fixed particle
sizes or crude microphysics. This last approach with 1D models appears
sufficient to reproduce at first order the transition between L dwarfs
and T dwarfs, likely due to the disappearance of silicate and iron
clouds~\cite{charnay18}. 3D models predict that clouds preferentially
form on the nightside of irradiated exoplanets, warming the planets by
their greenhouse effect~\cite{parmentier18}. The cloud radiative
effects are also a key process for the limits of the Habitable Zone
(e.g.~\cite{yang13, turbet21a}) with large uncertainties between
models.


\subsection{Atmospheric retrieval}
The principle of atmospheric retrieval is to compare the observed
spectrum and data to several simulated spectra computed with an
atmospheric model. A statistical method is applied to estimate the most
likely atmospheric parameters allowing ``to fit the data''.
Atmospheric models are either self-consistent models, as described in
the previous part, or parametric models. The former include different
physical/chemical processes and are computationally expensive. They can
be used for atmospheric retrieval by computing a grid of models and
applying after a statistical method (generally minimization of chi2).
However, they cannot explore a large parameter space and the best fit
can be biased by the physical/chemical processes included in the model
(e.g. the treatment of clouds). Parametric model assume simple
parametrisations of the temperature and cloud profiles, defined with a
few free parameters, and generally constant profiles of molecular
abundances. In contrast to self-consistent models, they are very fast
to run, can be used to explore a large parameter space, but can allow
degenerated or unphysical solutions.  Parametric models are ideal for
exoplanets for which there still so many unknowns. However, for
low-quality data, they can lead to very degenerated solutions.

The full exploration of the parameter space for the atmospheric
retrieval of an exoplanet would generally require a huge number of
models, not achievable in a reasonable amount of time. This is why
statistical methods are used to quickly explore the parameter space,
focusing on the regions where are the best fits. Most of retrieval
codes are based on a Bayesian approach to explore the parameter space
focusing on the best solutions, to derive posterior distributions. The
random exploration of the parameter spaces is done either with a Markov
chain Monte Carlo (MCMC) or with the method called Nested Sampling.

\subsection{Lessons for atmospheric modelling}

\subsubsection{When models work and do not work}
Our experience of planetary atmospheres in the solar system (Earth,
Mars, Venus, Titan, the giant planets and Pluto) teach us that
atmospheric models based on universal equations and a limited number of
processes can reproduce well planetary climates and spectra. 1D models
have been successful to reproduce thermal emission spectra of brown
dwarfs and young giant planets obtained from ground-based instruments,
validating molecular line lists at high temperatures.
Figure~\ref{figure_2} illustrates how 1D models can perform well under
such conditions. For transiting hot Jupiters, the equatorial
superrotation and the offset in thermal phase curves was predicted by
2D/3D models~\cite{showman02} before it was observed with the Spitzer
phase curve of HD209458 b~\cite{knutson07}. Generally, the main reasons
why models fail to reproduce observations are missing physical
processes or neglected 3D effects, as for instance H$_2$ and H$_2$O
dissociation for ultra-hot Jupiters~\cite{parmentier18} or nightside
clouds on hot Jupiters.

\begin{figure}[t!] 
\vspace*{2pt}
\includegraphics{fig02}
\vspace*{2pt}
\caption{Spectrum of HR8799 b from VLT-SINFONI (in green, $R=5000$)
compared to the best fit from the Exo-REM model (in
red)~\cite{charnay18}. The bottom panel shows the residuals between the
observations and the model spectrum. Figure from Simon Petrus (personal
communication). \label{figure_2}}
\vspace*{2pt}
\end{figure} 

\subsubsection{Biases in atmospheric retrievals}
Biases and degeneracies are currently a strong limitation of
atmospheric retrievals. There is in particular a degeneracy between the
cloud top pressure and molecular abundances in transit spectroscopy
leading to large uncertainties in the measurements of atmospheric
metallicity~\cite{benneke13}. A large spectral range from the visible
to the mid-infared of the combinaison of low and high spectral
resolution could break this degeneracy. Another degeneracy related to
clouds concerns the retrieval of thermal structure of exoplanets.
Indeed, clouds and a reduced thermal gradient similarly reduce
molecular feature in emission spectra. Cloud effect can be clearly
identified by the detection of cloud absorbing bands (e.g. the silicate
feature at 10~$\upmu$m). Here also,  a large spectral range as provided
by JWST or Ariel could break this degeneracy. Finally, the 3D structure
of exoplanet atmospheres, with strong horizontal variations of
temperature and chemical species in particular at the
terminators~\cite{pluriel20}, or inhomogeneous and time-varying cloud
cover~\cite{charnay21}, is a large source of uncertainties. It shows
the need to use a hierarchy of models from 1D retrieval code to 3D GCM
to interpret the observations.

\subsubsection{The interest of intercomparison}
Intercomparison between models is an efficient way to reveal biases and
uncertainties. Inspired by the Coupled Model Intercomparison Project
(CMIP) done by the Earth's climate modelling community, more and more
intercomparisons of exoplanet models are performed. They revealed
relatively small differences between models concerning the radiative
transfer (e.g.~\cite{baudino17}) and the atmospheric dynamics for dry
atmospheres (e.g.~\cite{turbet21b}). However, differences become
significant when clouds are included, highlighting the importance of
cloud radiative feedbacks 
(e.g.~\cite{sergeev21}{)}.
A large intercomparison program for exoplanet models involving many
teams and called CUISINES (Climates Using Interactive Suites of
Intercomparisons Nested for Exoplanet Studies) has recently started.

\subsubsection{Future challenges}

A major challenge for exoplanet atmosphere models is to succeed in
reaching the true converged state of the atmosphere. 3D simulations
showed that the atmospheric dynamics may evolve over 100,000 Earth
days~\cite{wang20}, same as the thermal structure in the deep
atmosphere which is impacted by the circulation~\cite{sainsbury19}. The
challenge for 3D modelling is to cover both the short timescales of
chemistry/radiation/dynamics and the long timescales of evolution of
the thermal structure and winds in the deep atmosphere. 

Finally, another main challenge related to the previous one is to
couple atmospheric models to interior models, to correctly simulate the
thermal evolution, chemical composition, radius and spectra. Such
models could provide new insights into the interior and formation of
exoplanets by simultaneously interpreting spectroscopic measurements
and mass/radius
%\break
measurements.

\section{Conclusion and perspectives}
The exploration of the Solar System by space remote sensing or in situ
probes has renewed the knowledge not only in accuracy, but with a
change of perspective: in some sense, Solar System objects have become
piece of Earth, and the field of specialists has moved from astronomers
to Earth physics (meteorology, geology, geochemistry, etc.) Future
years may provide the same change of perspective for exoplanets: even
if direct exploration is out of reach due to the distance of
exoplanets, a combination of modelling, comparison to Solar System
physical processes, and accurate observations by new space or
ground-based telescopes will provide  a huge amount of data and an
expected maturation of the exoplanets science in the next decade.

\section*{Conflicts of interest}
Authors have no conflict of interest to declare.

\back{}

\bibliographystyle{crunsrt}
\bibliography{crphys20221218}
\refinput{crphys20221218-reference.tex} 

\end{document}
