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\DOI{10.5802/crphys.139}
\datereceived{2023-01-04}
\dateaccepted{2023-01-10}
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\dateposted{2023-06-08}
\begin{document}

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

\title{Direct imaging of exoplanets: Legacy and prospects}

\alttitle{Imagerie directe des exoplan\`etes : H\'eritage et perspectives}

\author{\firstname{Gael} \lastname{Chauvin}\CDRorcid{0000-0003-4022-8598}}
\address{J.-L. Lagrange Laboratory , C\^ote d'Azur Observatory, UMR 7293 
B\^atiment H. Fizeau, 28, Avenue Valrose, Nice cedex 2, 06108, France}
\email{gael.chauvin@oca.eu}

\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}} 

\def\thanksname{Note}
\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}}

\begin{abstract} 
Understanding how giant and terrestrial planets form and evolve, what
is their internal structure and that of their atmosphere, represents
one of the major challenges of modern astronomy, which is directly
connected to the ultimate search for life at the horizon 2030--2050.
However, several astrophysical (understanding of the formation and
physics of giant and terrestrial exoplanets), biological
(identification of the best biomarkers) and technological (technical
innovations for the new generations of telescopes and instruments)
obstacles must be overcome. From the astrophysical point of view, it is
indeed crucial to understand the mechanisms of formation and evolution
of giant planets, including planet and disk interactions, which will
completely sculpt the planetary architectures and thus dominate the
formation of terrestrial planets, especially in regions around the host
star capable of supporting life. It is also important to develop
dedicated instrumentation and techniques to study in their totality the
population of giant and terrestrial planets, but also to reveal in the
near future the first biological markers of life in the atmospheres of
terrestrial planets. In that perspective, direct imaging from
ground-based observatories or in space is playing a central role in
concert with other observing techniques. In this paper, I will briefly
review the genesis of this observing technique, the main instrumental
innovation and challenges, stellar targets and surveys, to then present
the main results obtained so far about the physics and the mechanisms
of formation and evolution of young giant planets and planetary system
architectures. I will then present the exciting perspectives offered by
the upcoming generation of planet imagers about to come online,
particularly on the future extremely large telescopes. On the timescale
of a human Life, we may well be witnessing the first discovery of an
exoplanet and the first detection of indices of life in the atmosphere
of a nearby exo-Earth!
\end{abstract}

\begin{altabstract} 
Comprendre comment les plan\`etes g\'eantes et terrestres se forment et
\'evoluent, quelle est leur structure interne et celle de leur
atmosph\`ere, repr\'esente l'un des d\'efis majeurs de l'astronomie moderne,
qui est directement li\'e \`a la recherche ultime de la vie \`a l'horizon 
2030--2050. Cependant, plusieurs obstacles astrophysiques (compr\'ehension
de la formation et de la physique des exoplan\`etes g\'eantes et
terrestres), biologiques (identification des meilleurs biomarqueurs)
et technologiques (innovations techniques pour les nouvelles
g\'en\'erations de t\'elescopes et d'instruments) doivent \^etre surmont\'es. Du
point de vue astrophysique, il est en effet crucial de comprendre les
m\'ecanismes de formation et d'\'evolution des plan\`etes g\'eantes, y compris
les interactions entre la plan\`ete et le disque, qui vont compl\`etement
sculpter les architectures plan\'etaires et ainsi dominer la formation de
plan\`etes terrestres, notamment dans les r\'egions autour de l'\'etoile h\^ote
capables d'accueillir la vie. Il est \'egalement important de d\'evelopper
une instrumentation et des techniques d\'edi\'ees pour \'etudier dans leur
totalit\'e la population de plan\`etes g\'eantes et terrestres, mais aussi de
r\'ev\'eler dans un futur proche les premiers marqueurs biologiques de la
vie dans les atmosph\`eres des plan\`etes terrestres. Dans cette
perspective, l'imagerie directe depuis des observatoires au sol ou dans
l'espace joue un r\^ole central, de concert avec d'autres techniques
d'observation. Dans cet article, je rappellerai bri\`evement la gen\`ese de
cette technique d'observation, les principales innovations et d\'efis
instrumentaux, les cibles stellaires et les relev\'es, pour ensuite
pr\'esenter les principaux r\'esultats obtenus jusqu'\`a pr\'esent sur la
physique et les m\'ecanismes de formation et d\'evolution des jeunes
plan\`etes g\'eantes et les architectures des syst\`emes plan\'etaires. Je
pr\'esenterai ensuite les perspectives passionnantes offertes par la
prochaine g\'en\'eration d'imageurs de plan\`etes sur le point d'\^etre mis en
ligne, notamment sur les futurs extr\^emement grands t\'elescopes. A
l'\'echelle d'une vie humaine, nous pourrions bien assister \`a la premi\`ere
d\'ecouverte d'une exoplan\`ete et \`a la premi\`ere d\'etection d'indices de vie
dans l'atmosph\`ere d'une exo-Terre proche !
\end{altabstract}

\keywords{\kwd{Exoplanets}
\kwd{Planetary systems}
\kwd{Atmosphere}
\kwd{Architecture}
\kwd{Direct imaging}
\kwd{Spectroscopy}}

\altkeywords{\kwd{Exoplan\`etes}
\kwd{Syst\`emes plan\'etaires}
\kwd{Atmosph\`ere}
\kwd{Architecture}
\kwd{Imagerie directe}
\kwd{Spectroscopie}}

\maketitle

\end{noXML}

{\vspace*{-6pt}}

\section{Introduction}

\subsection{A rich legacy}

Today's success of direct imaging of exoplanets and planetary systems
is intimately connected to the pioneer work in the late 80's and early
90's dedicated to the development of Adaptive Optics (AO) system,
infrared (IR) detectors, and coronographic techniques for the
instrumentation of ground-based telescopes. Precursor instruments on
4-m class telescopes rapidly demonstrated performances that could
compete with space as illustrated by the emblematic discoveries and
images of the ultracool brown dwarf companion
Gl\,229\,B~\cite{nakajima1995} and the edge-on circumstellar disk
around $\beta$ Pictoris~\cite{mouillet1997}. They soon motivated the
research and development of more sophisticated AO planet imagers on
10 m-class telescopes of first and second generations, now routinely
operating on world-class observatories, and preparing the path forward
the future planet imagers of the extremely large telescopes in the
decade to come.   Joining the small family of planet hunting techniques
with radial velocity, transit, $\mu$-lensing and astrometry, direct
imaging (and interferometry, see~\cite{Lacour2023}) offers a unique
opportunity to explore the outer part of exoplanetary systems beyond
5--10~au to complete our view of planetary architectures, and to explore
the properties of relatively cool self-luminous giant planets
(see Figure~\ref{fig:allplanets}). The
exoplanet's light can indeed be spatially resolved and dispersed to
probe the atmospheric properties of exoplanets (and brown dwarf
companions). As today's imaged exoplanets are young (because they are
hotter, brighter, thus easier to detect than their older counterparts),
their atmospheres show low-gravity features, as well as the presence of
clouds, and non-equilibrium chemistry processes. These physical
conditions are very different and complementary to the ones observed in
the atmospheres of field brown dwarfs or irradiated inflated Hot
Jupiters (studied in spectroscopy or with transit techniques like
transmission and secondary-eclipse, respectively). Finally, direct
imaging enables to directly probe the presence of planets in their
birth environment. Planet characteristics and disk spatial structures
can then be linked to study the planet--disk interactions and the
planetary system's formation, evolution, and stability, which is a
fundamental and necessary path to understand the formation of smaller
terrestrial planets with suitable conditions to host life. After
briefly describing the key components of the current planet imagers,
and the main characteristics of the nearby stars observed, I will
summarize the key results obtained so far and main perspectives offered
for the decade to come.

\begin{figure}[t!]
\includegraphics{fig01}
\caption{Exoplanets (and brown dwarf companions) discoveries
considering the different planet hunting techniques (from
\url{http://exoplanet.eu}). The exoplanets masses are reported as a function
of their distances to stars (with minimum masses for the radial
velocity method). The different categories of exoplanets are reported
including the rocky planets, super-Earths, warm Neptunes, Hot Jupiter
and classical giants. The brown dwarf companions are also reported as
both populations probably overlap although a typical limit at 20--30
Jupiter masses is generally adopted given the observed companion mass
distribution to stars.\label{fig:allplanets}}
\end{figure}

\subsection{Instrumental innovation and challenges}

The success of direct imaging relies on a sophisticated instrumentation
designed to detect faint planetary signals angularly close to a bright
star. A Jupiter-like planet (orbiting at 5~au) around a typical young,
nearby star at 50~pc would lie at an angular separation of 100~mas
setting the order of angular separation we aim at. The typical
planet--star contrast are about $10^{-6}$ for a young Jupiter (today's
performances) and $10^{-8}$ for a mature Jupiter observed in emitted
light. It goes down $10^{-8}$--$10^{-9}$ for a super-Earth in reflected
light. From the early discoveries of young, massive self-luminous giant
planets like $\beta$\,Pic~b ($H$-band contrast of $10^{-4}$ at
200--400~mas) with the first generation of planet imagers at the Very
Large Telescope (VLT), and the Palomar, Subaru, Keck, and Gemini
observatories to the first images of an Exo-Earth with maybe an
Extremely Large Telescope (ELT), several orders of magnitudes in
contrast must be covered. Therefore, innovative  technological
developments are required to meet the ultimate goal of imaging
Exo-Earths (including the construction of extremely large telescope and
the ability of achieving high-quality wavefront control combined with
ultimate coronographic and differential techniques). In that
perspective, the second generation of planet imagers, like the European
Spectro-Polarimetric High-Contrast Exoplanet Resarch
instrument~\cite{2019AandA...631A.155B} (SPHERE, see
Figure~\ref{fig:planetimager} ({Left}), the Gemini Planet 
Imager~\cite{2014SPIE.9148E..0JM} (GPI), and the Subaru Coronagraphic Extreme
Adaptive Optics (SCExAO) instrument, aim at pushing high-contrast
performances down to $10^{-6}$--$10^{-7}$ at typical separation of
200~mas. They validate a dedicated instrumentation  based on a 3-stages
implementation design with: (i) high angular resolution access, 
(ii) stellar light attenuation using coronography, (iii) speckle subtraction
using differential imaging techniques. In the last step can be added
the recent development of powerful post-processing tools to optimize
the stellar signal suppression. These instruments already serve as
references and demonstrators for future terrestrial planet imagers of
the ELTs, like PCS~\cite{kasper2021} for the
European ELT.

\begin{figure}[t!]
\includegraphics{fig02}
\caption{{Left}, the SPHERE planet imager at UT3/VLT.
{Right}, high angular resolution and high contrast
implementation three main steps for direct imaging of exoplanets
including XAO (a/), coronography (b/) and differential imaging
including post-processing (c/) to optimize the subtraction of the
stellar residual while conserving the exoplanet
signal.\label{fig:planetimager}}
\end{figure}

From the ground, the atmosphere turbulence affects the light
propagation and prevents large telescopes from reaching a spatial
resolution at the diffraction limit. Current Extreme-Adaptive Optics
(XAO) systems enable to compensate for atmospheric, but also telescope
and common path defects to routinely reach nowadays Strehl\footnote{The
Strehl ratio can be defined as the ratio between the peak of the AO
corrected PSF and the one of the ideal diffraction-limited Airy
pattern.} correction of 90\% in H-band on bright ($V\le10$~mag)
targets. XAO-instruments like SPHERE, GPI, and SCExAO rely on
high-order deformable mirror, fast-temporal sampling frequency at more
than 1.0~kHz, spatial filtering of the wavefront before sensing, tuned
calibrations of the instrumental defects and enhanced stability to
limit low-order wavefront errors and alignment drift. The main
specifications are driven by the need to get (i) optimal spatial
sampling of the wavefront given the turbulence coherence length
(sampling better than $(D/r_0)^2$; $r_0\sim20$~cm at Paranal) and 
(ii) optimal temporal sampling to beat the turbulence speed (coherence time
$\tau_0$; 2--5~ms at Paranal) to minimize the errors of wavefront
reconstruction and command. The correction stability in terms of
Strehl-correction, control of low-order aberrations and pointing
stability are essential to avoid any leakage or PSF deformation during
the observation that would significantly degrade the planet detection
performances.

With a stable and diffraction-limited Point-Spread Function (PSF), the
motivation for coronography is relatively intuitive as it consists in
blocking the light from the central star to search for fainter objects
in the close stellar environment. Simple occulting masks or classical
Lyot coronographs have been massively used in the past years on
high-contrast imagers to essentially: (i) reduce by a
typical factor of ${\sim}100$ the intensity of the central star
diffracted-limited core and avoid any saturation effects, (ii) reduce the
intensity of the PSF wings without canceling the off-axis planetary
signal, (iii) improve the observing efficiency by reducing readout
overheads, and (iv) reduce the total read-out noise. Classical and more
recent apodized versions of the Lyot coronagraph are typically limited
to 3--4$\lambda/D$ inner working angle (IWA),\footnote{The IWA is
universally defined as the 50\% off-axis throughput point of a
coronagraphic system.} mostly because they rely on amplitude
manipulation to attenuate starlight diffraction, covering a large area
of the PSF at the coronagraph plane. With the improvement of
instrumental stability achieved with the new generation fo planet
imagers, new concepts have emerged, particularly to access smaller
inner working angles down to $\lambda/D$ (see apodized vortex and
phase-induced amplitude coronographs). Pushing the instrumental
limitation to access smaller IWA down to $\lambda/D$ or $2\lambda/D$
(40 to 80~mas for the VLT at $H$-band) can make a significant
scientific breakthrough as the bulk of the giant planet population
around young, nearby stars is predicted to be located beyond the
ice-line at ${\sim}3$~au (i.e.  100~mas for a star at 30~pc). 

As shown in Figure~\ref{fig:planetimager} ({Right}), boiling
atmospheric speckles and instrumental quasi-statics speckles still
remain an important source of limitation in the XAO coronographic
images. Differential imaging techniques are nowadays a must-have for
any high-contrast instruments to address that the residual speckles
limitation. Various strategies have been adopted including: 
(i)~reference differential imaging (RDI) using a reference single star
observed before/after the science target, (ii) angular differential
imaging (ADI) exploiting the field and pupil rotation of alt-az
Telescope, (iii) spectral differential imaging making use of the
different spectral signatures between the planet and the star completed
by molecular mapping techniques combining high-contrast and spectral
cross-correlation methods, (iv) finally, polarimetric differential
imaging well adapted to image the scattered polarized light of
planet-forming and debris disks (or the reflected polarized light of
exoplanets). Please refer to \cite{GalicherMazoyer2023} for more
details on the limitation and mitigation for the detection and
characterization of the exoplanetary signal with planet imagers. 

\subsection{Young, nearby stars identification}

For more than two decades now, planet imagers have been mainly
targeting young (${\le}$500~Myr), nearby (${\le}$100~pc) stars to search
for giant exoplanets and disks. This is motivated by the fact that
young exoplanets are hotter, brighter and easier to detect than older
counterparts, and that the system proximity allows the exploration of
the close physical surrounding. The identification of these comoving
groups of young stars started from an anomaly observed in the 80's. The
TW\,Hya T\,Tauri star was discovered isolated from any dark cloud nor
birth place regions~\cite{rucinski1983}. A few years later, IRAS
excesses combined with H$\alpha$ and Lithium depletion measurements
enabled the identification a handful of young stars in the vicinity of
TW\,Hya. Their origin, runaway stars from some cloud or formed in situ
from a low-mass cloud, their age and their distance remained unclear
for years. X-ray data finally confirmed that they were members of the
first identified young, nearby association, i.e. the nearest known
region of recent star formation, the so-called TW Hydrae
association~\cite{kastner1997} (TWA). Over the next decade, the number
of TWA members drastically increased to more than 30 members, with age
and mean distance estimates converging towards 10~Myr and 50~pc,
respectively. Immediately after the confirmation of the existence of
the TWA association, several research groups became specifically
interested in identifying new young, nearby associations in the
vicinity of the Sun~\cite{zuckerman2004,torres2008}.

The key criteria of identification relies on two main pillars studying:
(i) the galactic position and motion of a star to derive its age. The
galactic kinematics studies have become increasingly sophisticated
using for instance Bayesian analysis to derive membership probability
of a star and its age considering combined photometric and kinematics
catalogs, (ii) the spectro-photometric stellar characteritics in relation
with well calibrated age-sensitive indicators. Various youth
diagnostics can be used depending on the stellar age and spectral type
like color-magnitude diagram, rotation, Lithium depletion, H$\alpha$
emission, Ca H\&K, Coronal X-rays and chromospheric UV activity, or the
presence of IR excess to inter-compare and refine stellar ages.
Nowadays, more than 500 young, nearby (${<}$100~pc) stars have been
identified in the Sun vicinity, mostly F, G, K and M dwarfs. Systematic
studies, including the recent \textit{Gaia} results,  are now pushing
the horizon down to very low-masses with the discovery of a large
population of M-, L- and even T-type members ~\cite{gagne2015}. These
young, nearby stars are gathered in several groups (TW Hydrae, $\beta$
Pictoris, Tucana-Horologium, $\eta$ Cha, AB Dor, Columba, Carinae\,\ldots),
sharing common kinematics, photometric and spectroscopic properties.
This horizon has been extended to members of intermediate-age
associations or star-forming regions located at larger distance like
the so-called Scorpius Centaurus complex composed of thousands of
stars. With typical
contrast of 10--15 magnitudes for separations beyond 200~mas
(10--20~au for a star at 50~pc) achieved with the second generation of
planet imagers, young giant planets down to 1--2 Jupiter masses are
typically detectable. Therefore, without surprise, a significant amount
of telescope time was dedicated to deep imaging surveys of young,
nearby stars to search for exoplanets, brown companions and disks
leading to the first direct imaging discoveries of planetary mass
companions in the early-2000's.

\section{Results}

Today, deep direct imaging campaigns have surveyed several hundred
members of young, nearby associations (TW Hydrae, $\beta$ Pictoris,
Tucana-Horologium, Columba, Argus,\,\ldots) and star-forming regions
(Lupus, Taurus, Ophiuchus, Chamaeleon, and Sco-Cen). Various
motivations were followed for the target selection of these surveys:
(i) complete census of given associations, (ii) selection of young,
intermediate-mass stars, (iii) or very low-mass stars, or (iv) application
of figure of merit considering detection rate with toy models of planet
population. The near-infrared wavelengths have been used intensively,
particularly with SPHERE and GPI lately. They are a good compromise
between low-background noise, angular resolution and good Strehl
correction. However, thermal (3--5~$\rmmu$m) imaging has been very
competitive in terms of detection performances as the planet--star
contrast and the Strehl correction are more favorable at those
wavelengths despite an increased thermal background. The first clues of
the presence of a giant planet directly imaged is revealed by the
presence of a faint point-source, presenting cool atmosphere features
as described below, in the close vicinity of a young star. Very red
near-IR colors or specific spectral features such as a peaked $H$-band
spectrum are indicative of young L dwarfs atmospheres. Methane
absorption indicates a probable young and cool T dwarf. These
diagnostics enable the observers to rapidly estimate the predicted
mass, effective temperature and physical separation of the candidate
using evolutionary model predictions. Robust confirmation then come
with follow-up observations at consecutive epochs to verify that the
candidate is actually co-moving with the central star considering its
parallactic and proper motion. Orbital motion can be resolved to
unambiguously confirm that both objects are physically bound.
Independently of the technique used, these surveys have shown that
imaging exoplanets remains a challenging task as the typical occurrence
rate of giant planets, more massive than a few Jupiter masses and
beyond 10~au, in nearby systems is a few percent at
most~\cite{2021AandA...651A..72V}. It therefore requires the observation
of large samples (typically 500 stars for SPHERE and GPI exoplanet
surveys) with a significant amount of observing time, strongly
contaminated by background stars representing the vast majority of the
point-source detection. They however led to breakthrough discoveries
over the past decades (see illustrative gallery in
Figure~\ref{fig:protoplanets}) and a better understanding of the
processes of planetary formation and evolution, the physics of young
giant planets, and of their demographics as described below. 

\begin{figure}[t!]
\includegraphics{fig03}
\caption{{Left}, SPHERE combined $K1K2$ image of PDS\,70~b
inside the transition disk surrounding PDS\,70~\cite{muller2018}.
{Middle}, MUSE observations of PDS\,70 b and
c~\cite{2019NatAs...3..749H}. {Right}, ALMA observations of the
disk and the CPD around PDS\,70~c~\cite{2021ApJ...916L...2B}.\label{fig:protoplanets}}
\end{figure}


\subsection{Planet-forming disks, protoplanets \& bulk properties}

In addition to the presence of planet in formation, planet imagers
offer us unprecedented information about the morphology of young
planet-forming disks during or right after the planetary formation
phase. Various substructures like large spiral arms, multiple rings,
shadows or cavities can be spatially resolved and in some cases
characterized spectroscopically or
polarimetrically~\cite{2022AandA...658A.137G}. Several sharp, Kuiper-like
belts were also imaged~\cite{2019AandA...626A..54M,2019AJ....157...39G}.
These first studies paved the way towards the first unambiguous
discovery of two recently formed young planets within a transition disk
surrounding the young Sun-like star PDS\,70 ([\citenum{keppler2018}];
see Figure~\ref{fig:discoveries}). In
synergy with ALMA and MUSE at VLT, SPHERE observations offer a fabulous
way to directly explore the planetary formation and evolution, the
atmospheric and orbital properties of young Jupiters~\cite{muller2018},
the physics of planetary
accretion~\cite{2019NatAs...3..749H,2019ApJ...885...94T}, the presence
and properties of circumplanetary disks~\cite{2021ApJ...916L...2B}, and
the global system architecture to study the gas and dust
dynamics~\cite{2019AandA...625A.118K}. These discoveries allow us to
simultaneously connect the giant planet physical and orbital properties
with the disk morphology, and potentially the inner regions where
terrestrial planets might form\break (see~\cite{Boccaletti2023}).

\begin{figure}[t!]
\includegraphics{fig04}
\caption{Emblematic direct imaging discoveries (not exhaustive): 2M1207
\& AB Pic~\cite{2004AandA...425L..29C,2005AandA...438L..29C},
RXJ\,1609~\cite{2008ApJ...689L.153L}, $\kappa$
And~\cite{2013ApJ...763L..32C}, HR\,8799~\cite{2008Sci...322.1348M},
$\beta$ Pic~\cite{2009AandA...493L..21L},
HD\,95086~b~\cite{rameau2013a}, GJ\,504~\cite{2013ApJ...774...11K}, 51
Eri~b~\cite{2015Sci...350...64M},
HIP\,65426~b~\cite{2017AandA...605L...9C}, TYC
8998-760-1~b~\cite{2020ApJ...898L..16B}, and b
Cen~b~\cite{2021Natur.600..231J}.\label{fig:discoveries}}
\end{figure}

Today, one key limitation of direct imaging is that we do not directly
measure the mass of these young exoplanets, but their photometry,
luminosity and spectral energy distribution. Consequently, we have to
rely on evolutionary model predictions that are not well calibrated at
young ages to convert luminosity to mass. In addition to the system age
uncertainty, the predictions highly depend on the formation mechanisms
and the gas-accretion phase that will form the exoplanetary atmosphere.
The way the accretion shock will behave (sub or super-critical) on the
surface of the young accreting proto-planets during the phase of gas
runaway accretion will drive the planet initial entropy or internal
energy, hence its initial physical properties (luminosity, effective
temperature, surface gravity and radius) and their evolution with
time~\cite{2007ApJ...655..541M}. These different physical states are
described by the so-called hot-start (sub-critical shock), cold-start
(super-critical shock), and warm-start (intermediate cases) models.
They predict luminosities that are spread over several order of
magnitudes for young, massive giant planets. Over time, these young
giant planets will then cool down and shrink as they are not massive
enough to sustain Hydrogen nuclear fusion. Consequently, a lot of open
questions remain to be answered about the formation and the evolution
of giant planets and of their atmospheres from the theoretical point of
view. The observations of proto-planets in their birth environment and
the direct measurement of emission lines have just started, and allow
to explore the planetary accretion processes and shocks to constrain
the way the gas is accreted on young forming
planets~\cite{2022AandA...657A..38M}. The synergy of techniques,
combining direct imaging and/or interferometric observations with
radial velocity and/or astrometry with \textit{Gaia} is particularly
powerful to constrain the Mass-Luminosity relation of young Jupiters as
recently done for $\beta$ Pic b \& c~\cite{2020AandA...642L...2N} or
HR\,8799\,e~\cite{2021ApJ...915L..16B}. The future \textit{Gaia} Data
Release 4 (2025), that will soon deliver fully calibrated astrometric
time series, is in that sense very promising as it will pinpoint the
presence of young giant planets between 3--10~au orbiting young, nearby
stars in the vicinity of the Sun, offering prime follow-up targets for
planet imagers to explore the diversity of the bulk properties of young
giant planets in connection with their environment.

\subsection{Exoplanetary atmospheres}

Since 2014, the use of integral field spectrographs with SPHERE and GPI
enabled to acquire for the first time low-resolution ($R_\lambda \sim
50$) emission spectra of tens of young giant planets exhibiting broad
features due to unresolved molecular absorptions (H$_2$O, CH$_4$, VO,
FeH, CO). These spectra can be compared to predictions of atmospheric
models to infer first-order information on the bulk properties of the
atmosphere, mainly the effective temperature, the surface gravity
(pressure) and the properties of clouds.
The current atmospheric models
used by the community are mainly 1D-models involving at least
hydrodynamics, radiative and convective energy transport, and gas-phase
chemistry. They solve the pressure--temperature structure in
radiative-convective equilibrium and determine the chemical species
that are supposed to form. Sophisticated models of clouds of different
composition (silicates, sulfites\,\ldots) have been considered for more than
a decade (refer to~\cite{CharnayDrossart2023}
for more details).
The analysis of these low-resolution spectra of young giant exoplanets
showed that, as for young brown dwarfs, young exoplanets of a given
spectral type with lower surface gravity can be up to 200--500~K
cooler than their older
counterparts~\cite{2014AandA...562A.127B,2015ApJ...810..158F}. This
discontinuity is particularly noticeable at the M--L transition, and
also evidenced by a lack of methane at the L/T transition as seen for
HR\,8799~b~\cite{2018AandA...616A.146P}. These results confirm the early
photometric characterization that showed the peculiar properties of
young L/T type planets like 2M1207~b, HR\,8799~bcde, HD\,95086~b or
HD\,206893~b owing to the low-surface gravity conditions in their
atmospheres,  leading to an enhanced production of clouds probably
composed of sub-$\rmmu$m dust grains made of iron and
silicate~([\citenum{2011ApJ...737...34M}]; see
Figure~\ref{fig:colors}). With cooler temperatures than
typically 1000~K, they start dissipating and fragmenting, modifying the
cloud vertical and spatial distribution as suggested by recent
variability studies showing high-amplitude photometric variations for
several of these young, planetary-mass
companions~\cite{2019AJ....157..128Z}.  The presence of circumplanetary
disks, as directly evidenced for PDS\,70~c by ALMA sub-mm
observations, might also affect the spectral energy distribution of the
youngest planets mixing contribution in infrared from the photosphere
and the (spatially unresolved) circumplanetary
disk~\cite{2019MNRAS.486.5819C,2021ApJ...916L...2B}. 

\begin{figure}[t]
\includegraphics{fig05}
\caption{{Left:} color-magnitude diagrams comparing the young
imaged exoplanets with field M, L, and T dwarfs with known parallax
measurements. {Right:} Sequence of exoplanet's spectra
characterized in the course of the SHINE and GPIES surveys. Adapted
Figures from~\cite{Chauvin2018}.\label{fig:colors}}
\end{figure}

With the increase of spectral resolution ($R_\lambda > 1000$), the line
blending of molecular absorptions start to be resolved yielding
unprecedented accuracy constraints on surface gravity (pressure),
chemical abundances and the complex molecular chemistry of exoplanetary
atmospheres (see Figure~\ref{6fig:spectra}). 
Atmospheric model degeneracy can be better suppressed and
missing opacities identified. Note that the combination of spectral
diversity with higher spectra resolution and high-contrast imaging is
particularly interesting to boost the detection performances of young
exoplanets using the so-called \textit{molecular mapping}
technique~\cite{2018AandA...617A.144H}. Regarding the \mbox{characterization}
part, high-contrast spectroscopic observations at medium resolution
with AO-fed spectrographs like OSIRIS at Keck, SINFONI at VLT, led to
the determination of molecular abundances such as water,
carbon-monoxide or methane for several young planets like HR\,8799~b
and e, $\beta$ Pic b,
HIP\,65426~b~\cite{2015ApJ...804...61B,2020AandA...633A.110G,2020AandA...640A.131M,2021AandA...648A..59P}. 
More recently, even the $^{12}$CO/$^{13}$CO isotopologue ratio,
connected to the carbon-ice fractionation process, was measured for the
first time in the atmosphere of the young exoplanet TYC-998-760-1~b
suggesting a formation beyond the CO
snowline~\cite{2021Natur.595..370Z}. Accessing the planet atmospheric
composition (C/O, D/H, N/O, N/C and isotopologue ratios) and
metallicity is a path forward the exploration of the formation
mechanisms, the birth location and the potential migration/dynamical
history of the exoplanets. The observed atmospheric composition is
however resulting from various physical complex processes. They are
connected to the disk composition, physical and thermal structure,
chemistry and evolution that will define the building blocks
composition of the giant planet, the planetary formation processes that
will affect the solid-gas accretion phase, the vertical
mixing/diffusion between the planet's bulk and atmosphere, or simply
the dynamical evolution over time, which overall will make any direct
interpretation speculative at this stage~\cite{2022arXiv220413714M}. 
An interesting comparison point is provided by the population of young
free floating brown dwarfs (or exoplanetary analogs), that share common
bulk properties in terms of mass, temperature, surface gravity and age.
Less challenging to observe and characterize, they offer a rich and
complementary test bench for atmospheric models. They offer the
opportunity to explore the chemical differences with the population of
young giant planets that could trace different processes of formation. 

\begin{figure}[t!]
\includegraphics{fig06}
\caption{{Left}, Spectral energy distribution for 51 Eri b, a
young, T3-type, 2--3~M$_{\mathrm{Jup}}$ giant planet orbiting at 11.1~au
from 51 Eri (F8V),  constructed from SPHERE and GPI
observations~\cite{2017AandA...603A..57S,2017AJ....154...10R}.
Predictions from the petitCODE cloudy model are reported with the best
fitting solution of the bulk properties (and 32 randomly drawn samples
from the posterior probability distribution in gray). {Right},
K-band SINFONI spectrum of the warm, dusty planet HIP\,65426~b, with
the best ExoREM fit model found.\label{6fig:spectra}}
\end{figure}

\subsection{Orbit, architecture and dynamical evolution}

All young giant planets discovered in direct imaging up to now orbit
beyond 5~au, revolving around their stellar host with typical periods
longer than 10~years. After their discoveries, intense monitoring and
data mining enabled to rapidly derive orbital solutions of the most
favorable cases. For $\beta$\,Pic~b, since its first image in November
2003, almost 20 years of NaCo and SPHERE, GPI, and more recently
VLTI/GRAVITY astrometric measurements have been carried out covering
almost 100\% of its
orbit~\cite{2012AandA...542A..41C,2020AandA...642L...2N}. For HR\,8799, the
longest baseline has been
achieved for the b planet thanks to the
re-analysis of archived \textit{HST/NICMOS} data from 1998 offering a
time coverage of almost 25 years now combining NIRC2, GPI, SPHERE and
more recently GRAVITY measurements (see Figure~\ref{fig:orbit}). Up to
25\% of the orbit for the three closest planets (cde) has been covered
so far~\cite{2016AJ....152...28K}. For the remaining imaged planets, 
the orbital motion is generally resolved but covers a small fraction
(${\lesssim}10\%$) of the orbit, sometimes finding hints of orbital
curvatures as observed for 51\,Eri~b, GJ\,504~b or HD\,95086~b, and
therefore limiting the precise determination of their orbital
parameters. Accurate astrometric monitoring in direct imaging is a
meticulous task due to the relatively small field of view of these
instruments. The second generation of planet imagers like GPI, SCExAO
and SPHERE now regularly achieves astrometric precision down to
1--2~mas with adequate calibration strategies, compared to the
10--20~mas precision achieved with the first generation of planet
imagers~\cite{2021JATIS...7c5004M}. The recent advent of the dual-field
astrometric mode of GRAVITY at VLTI is now out performing all planet
imagers  in terms of astrometric precision by providing measurements at
the 100 to 10~$\rmmu$as level (see~\cite{Lacour2023}). Direct imaging (and
interferometric) orbital monitoring are currently completed by radial
velocity and \textit{Gaia} astrometric studies whenever
possible~\cite{2021ApJ...915L..16B}. They can be combined with direct
measurement of the planet's radial velocity using medium to
high-resolution spectroscopy~\cite{2014Natur.509...63S} to study the
planetary orbits in the three dimensions, and even explore their
obliquities and alignment with the stellar
spin~\cite{2021AJ....162..217B,2021AJ....162..148W}.

\begin{figure}[t!]
\includegraphics{fig07}
\caption{{Left}, Composite ALMA/SPHERE image of the HR8799
system. ALMA at 1.6~mm reveals a cold component of dust, Kuiper
belt-like, beyond 100~au, while the four giant planets are
distinguishable in the central part thank to SPHERE/IRDIS at K1-band in
infrared.  {Right}, Motion of the HR\,8799 bcde planets monitors
by SPHERE (red-filled circles and yellow hexagons), LUCI at LBT (green
hexagons), NIRC2 at Keck (light-blue filled circles and dark-blue
circles), GPI (Diamonds) and a star symbol is for the most accurate
GRAVITY point. The best-fitting solutions to the four-planet model is
reported in red. Figures adapted
from~\cite{2016MNRAS.460L..10B,2022AandA...666A.133Z}.\label{fig:orbit}} 
\end{figure}

The precise determination of the orbital properties of young giant
planets is an important step towards the study of the structure and
dynamical stability of planetary systems, including the planet--planet
and planet--disk interactions. This is directly connected to our
understanding of the formation of giant planets, but also to the
formation of terrestrial planets as the giant\unskip\break 
planets (when present)
will shape the global system architecture and dynamical evolution.
Early studies, in the case of $\beta$\,Pic~b, evidenced for the first
time a clear planet--disk interaction showing that the planet is
responsible for the inner warp deformation of the debris disk around
$\beta$\,Pic, forcing the planetesimals to precess on a 2--5\textdegree\ 
misaligned orbit with respect to the main
disk~\cite{2012AandA...542A..41C,2012AandA...542A..40L}. The planet
location and low, but non-null eccentricity, is also consistent with
the secular dynamical pertubations at the origin of the exocometary
activity monitored for more than 30 years for
$\beta$\,Pic~\cite{2022NatSR..12.5855L}. Signs of interactions are
indirectly seen in the substructures of debris disks (waves,
eccentricity, excess of exozodiacal dust), or at even younger ages
(cavities, vortices, spirals,\,\ldots). However, there are only a few cases
for which the disk sub-structures and the planets are both directly
imaged and can be unambiguously connected, and studied. The three
emblematic systems HR\,8799, HD\,95086 and PDS\,70 are particularly
interesting in that sense. The giant planets have all
low-eccentricities, and orbit between two coplanar dusty belts,
similarly to the giant planets of our Solar System orbiting between the
asteroids and Kuiper belts~\cite{2018AandA...617A..76C}. For the
multiple-planetary systems HR\,8799 and PDS\,70, the giant planets
orbit in a configuration close to mean motion resonances that do favor
dynamical stability~\cite{2015ApJ...803...31P,2019ApJ...884L..41B}.
Interestingly, a correlation between the formation of terrestrial
planets in the inner region of the planetary systems, the presence of
cold debris dust and the existence of stable low-eccentricity giant
planets has been predicted based on a comprehensive set of dynamical
simulations~\cite{2011AandA...530A..62R,2012AandA...541A..11R}. Systems
with high-eccentricity giant planets are on the contrary not expected
to host debris disks and inner terrestrial planets, or could indicate a
stellar-like architecture and formation
pathway~\cite{2020AJ....159...63B}. Consequently, finding more of these
young, stable Solar system analogs, understanding how common they are,
is fundamental to understand if the architecture of our Solar system is
a rare product (or not) of planetary formation, and if it is an
essential condition for the formation of life.


\subsection{Demographics}

Considering the very large number of young, nearby stars observed and
the dozens of imaged exoplanets discovered so far, the rate of
discovery in direct imaging remains relatively low (typically
${\sim}$5--10\%), but relatively similar to the rate of inner giant
planets discovered in radial velocity or transit. A robust statistical
analysis including the survey completeness and the confirmed exoplanets
can be exploited to derive the occurrence of the young giant planet
population beyond typically 10~au. For that purpose, early studies have
developed statistical analysis tools to exploit the complete deep
imaging performances and derive the detection probabilities of their
surveys by simulating synthetic planet population described by various
sets of parametric distributions (mass, eccentricity, semi-major
axes)~\cite{lafreniere2007,nielsen2008,chauvin2010}. They show that the
first generation of surveys were mostly sensitive to giant planets more
massive than 5~M$_{\mathrm{Jup}}$ for semi-major axis between typically 30
to 300~au. They typically derive overall occurrence rate for planetary
systems hosting at least on 5--13~M$_{\mathrm{Jup}}$ exoplanets of less than
4--6\% (assuming hot-start evolutionary models) around FGK-type
stars. Hints of a dependence of this occurrence rate with the stellar
host mass, analogous to the well-established correlation observed at
small separation with radial velocity surveys, was suggested by surveys
specifically targeting intermediate-mass stars and low-mass stars
~\cite{rameau2013a,2016AandA...596A..83L,2016PASP..128j2001B}.

With the improved sensitivity in mass and semi-major axis, GPI and
SPHERE enable a significant step forward by providing meaningful
sensitivity to planetary-mass companions and brown dwarfs down to
10~au, getting close to the snow line region where the bulk of the
giant planet population formed by core accretion is
expected. Ref.~\cite{2019ApJ...874...81F} identify for instance a break in
the radial velocity giant planet distribution between 2 and 3~au after
which the occurrence rate decreases with distance from the star, but
which must be confirmed with direct imaging and future \textit{Gaia}
astrometric results to get a global picture of the population of giant
planets at all separation.\unskip\break With SPHERE and GPI, it finally becomes
realistic to compare direct imaging observations to predictions of both
core accretion models coupled to dynamical planet scattering evolution,
and gravitational instability
models~\cite{2019AJ....158...13N,2021AandA...651A..72V}. The first
statistical analysis on the properties of the population of sub-stellar
companions at wide orbital separation based on a sub-set of {150}
stars for SPHERE gives a frequency of systems with at least one
companion with a typical mass in the range $M_p = 5$--75~M$_{\mathrm{Jup}}$
and a semi-major axis in the range $a = 10$--300~au, to be
{$5.7_{-2.8}^{+3.8}\%$} for FGK stars (see
Figure~\ref{fig:demographics}). Qualitatively, the contribution of the
core accretion part of the model seems enhanced over the gravitational
instability part, which means that core accretion contributes a higher
fraction of the full companion population required to explain the data.
The use of parametric models of planet and brown dwarf population also
confirms (i) that the frequency of sub-stellar companions is
significantly higher around BA stars than around FGK and M stars, (ii)
and that the planet-like formation and brown dwarf binaries-like
formation mechanisms overlap, but dominate over different ranges of
planet-to-star mass ratios, $q$, which is a strong constraint for
formation models.  

\begin{figure}[t!]
\includegraphics{fig08}
\caption{{Left}, Comparison of the sensitivities of the NaCo and
SPHERE exoplanet surveys, based on the average probability of detecting
a companion as a function of its mass and semimajor axis. The range of
semimajor axes spanning the H$_2$O and CO snow lines for the stars in
the sample are overplotted. {Right}, Comparison of the depth of
search of the SPHERE exoplanet survey with the population synthesis
models based on the CA and GI formation scenarios presented. The CA
companions are represented with shades of red (low density of
companions) to yellow (high density of companions), and the GI
companions are represented with shades of white (low density of
companions) to blue (high density of companions). Figure
from~\cite{2021AandA...651A..72V}.\label{fig:demographics}} 
\end{figure}

\section{Perspectives}

In less than two decades, exoplanetary science has become a major field
of fundamental research in physics, recently rewarded with a first
Nobel Prize. This is well illustrated by the pace
of new discoveries and breakthrough results incessantly impacting the
community and the general public. Two decades ago, the only planets we
knew were the ones of our Solar System. Today, thousands of exoplanets
have been discovered since the 51 Peg discovery~\cite{mayor1995}. These
new worlds reveal an incredible diversity (Hot Jupiters, irradiated and
evaporating planets, misaligned planets with stellar spin, planets in
binaries, terrestrial planets in habitable zone, discovery of Mars-size
planet, circumplanetary disks possibly hosting exomoons in
formation\,\ldots), driving current theories of planetary formation and
evolution.  On the timescale of a human Life, we may well be witnessing
the first discovery of an exoplanet and the first detection of
bio-signatures in the atmosphere of a nearby exo-Earth! 

\subsection{A Golden Era for Exoplanets}

Exoplanetary science is today a major workhorse for the astronomical
community, driving the scientific and instrumental roadmap, in Europe,
for the development of future space missions and ground-based
telescopes and instruments for the decades to come. Covering various
domains of expertise from theory, simulation, instrumentation, lab
experiments to observation, exoplanetary science naturally diversified
in synergy with the study of the Solar System and the stellar physics,
but also transversally connecting with other science fields like
biology, chemistry, geophysics, climatology, photonics, signal
processing, artificial intelligence\,\ldots. Despite this success, we do not
have at this stage a global understanding of the formation of giant and
terrestrial planets to explain the diversity of the new worlds
routinely observed, of the place our Solar System in this exoplanetary
bestiary, and of the seminal conditions required for the emergence of
life. Many fundamental questions remain to be answered concerning:
(i) the existence of one or several mechanisms to form giant planets,
(ii) the physics of accretion  driving the physical properties of young
Jupiters and their evolution over time, (iii) the impact of dynamical
evolution in crafting planetary system architectures with favorable
conditions for life, or (iv) the (non-)singularity of Earth within all
habitable worlds. In that perspective, the upcoming decade is rich in
terms of space missions and ground-based instrumentation that will
attempt to answer these fundamental questions (see
Figure~\ref{fig:timeline}).

\begin{figure}[t!]
\includegraphics{fig09}
\caption{Timeline of current and future space and ground-based projects
with a strong invovlement fo the European community and dedicated to
planetary formation and exoplanetary science.\label{fig:timeline}}
\end{figure}

Following the second generation of planet imagers (GPI, SPHERE \&
SCExAO), new instruments are coming on line to push the exploitation of
the spectral diversity in connection with the spatial one. A new set of
medium resolution integral-field spectrographs and upgrades in visible
and/or infrared, like MagAO-X at Magellan (early-2022), ERIS at VLT
(mid-2022), SHARKS-VIS and NIR at LBT (late-2022), MAVIS at VLT (2028),
will start offering new capabilities to image and \mbox{characterize} young
exoplanets in accretion and/or in molecular mapping with upgraded xAO
facilities. They should provide new insights on the connection of disk
sub-structures with young protoplanets in formation, but also on the
chemical abundances of imaged exoplanets. The coupling of (x)AO system
with high-resolution ($R_{\lambda}$~$=$~30,000--100,000) fiber-fed
spectrographs has also started with CRIRES(+) (soon HiRISE at VLT
coupling SPHERE and CRIRES+, and foreseen for late-2023) and KPIC at
Keck (since 2020). They offer unprecedented opportunities to
systematically measure the radial and rotational velocities of the
imaged planets to determine their orbit in three dimensions, their true
obliquity (when coupled to photometric follow-up), and to open new
horizons to map the chemical composition, and the general atmosphere
circulation of young Jupiters. To push the survey capabilities of
SPHERE and GPI to access the bulk of the young giant planet population
down to the snow line (3--10 au), upgrades of their xAO systems and
instruments have been
proposed~\cite{2020arXiv200305714B,2020SPIE11447E..1SC}. Both upgrades
aim also at observing a larger number of fainter, redder stars in the
youngest star-forming regions, building on the synergy with ALMA to
characterize the architectures and properties of young planetary
systems in formation. They want to improve their characterization
capabilities by either increasing the spectral wavelength coverage or
the spectral resolution of their integral-field spectrographs. In
parallel, on-going developments at VLTI with GRAVITY+ (2026) and the
ASGARD visiting instruments will expand the fringe tracking and
sensitivity performances, the wavelength coverage (JHKL-bands) and
spectroscopic modes, and utlimately the interferometric contrast
implementing nulling techniques (see~\cite{Lacour2023}).
\looseness=-1

For the decade to come, this set of new planet spectro-imagers (and
interferometers) will work in concert with complementary facilities
connected to Exoplanetary science. At sub-mm and centimetric
wavelengths, ALMA (and potential upgrades) will pursue the
characterization of the cold dusty and gaseous component of young
planet-forming and debris disks with an exquisite spatial resolution
similar to the one achieved by current planet imagers in infrared (down
to 10~mas), and will be soon completed by the Square-Kilometer Array
(SKA) starting operation in the late-2020. The new generation of
high-resolution spectrographs dedicated to radial velocity studies in
visible ESPRESSO, EXPRES, HARPS3, and infrared CARMENES, SPIROU, NIRPS,
will extend the current horizon to the population of terrestrial
planets around solar and low-mass stars (including young stars) and
dedicated strategies to tackle the problem of stellar activity. In
space, \textit{Gaia} will achieve a final astrometric precision of
10~$\rmmu$as in the context of a systematic survey of a billion of stars
and therefore discover thousands of new planetary systems. The
\textit{Gaia} Data Release 4 (in 2025) will give us a complete census
of the young and old giant planet population between typically 2 and
5~au for stars closer than 200~pc. Dedicated \textit{Gaia} follow-up in
direct imaging have already started based on the proper motion anomaly
measured between \textit{Hipparcos} and \textit{Gaia} indicating the
potential presence of substellar perturbers~\cite{2022AandA...657A...7K}.
Going beyond the the \textit{Corot}, \textit{Kepler}, \textit{TESS} and
\textit{CHEOPS} missions, \textit{PLATO}, foreseen for 2026, will
extend our knowledge on the content of terrestrial planets at longer
periods, up to several years, around relatively bright, nearby stars. 
Within 10 years, the era of large-scale systematics surveys will decay
thanks to a complete census of exoplanetary systems within 100--200~pc
from the Sun. A new Era fully  dedicated to the characterization of
known systems will rise. Already initiated with \textit{Hubble},
\textit{Spitzer} and the first and second generation of planet imagers
and spectrographs, the characterization of the physics of giant and
terrestrial planets, has been done today for about 30 exoplanets from
super-Jupiters, Hot Neptunes to super-Earths. The presence of water,
carbon monoxide and methane molecules, of haze revealed by Rayleigh
scattering, observation of day-night temperature gradients, constraints
on vertical atmospheric structure and atmospheric escape have been
evidenced in the past decade. This will intensify with the James Webb
Space Telescope (\textit{JWST}), that started operation in mid-2022.
\textit{JWST} will start revolutionize transit and secondary eclipse
spectroscopy by exploring a broad diversity of exoplanetary atmospheres
over a wide range of mass, temperature, and composition. Despite a
design not dedicated to \mbox{high-contrast} imaging, the \textit{JWST}
instruments NIRCam and MIRI are equipped with coronographs. NIRSpec and
MIRI can also exploit the spectral diversity provided by their IFS
($R_{\lambda}=2700$ and 1500--4000, respectively) modes for the
detection and characterization of
exoplanets~\cite{2022AandA...658A..72P}. At separations larger than
0.6--1.0~as, \textit{JWST} will be unique for the exploration of the
outer part of young, nearby planetary systems, to detect young
Saturn-analogs beyond typically 30--50~au  down to Saturn-mass thanks
to its unprecedented sensitivity. Finally, \textit{ARIEL} (launched in
2029) will be fully devoted to the systematic characterization of
exoplanetary atmospheres by surveying a diverse sample of about 1000
extrasolar planets, simultaneously in visible and infrared wavelengths,
and will be almost  contemporaneous to the first Lights of the
extremely large telescopes (ELTs).

\subsection{The Age of Extremely Large Telescopes}

With first light foreseen for 2028 and after, the ELTs, the Giant
Magellan Telescope (GMT,~\cite{2010SPIE.7733E..1YS}), the Thirty Meters
Telecope (TMT,~\cite{2010SPIE.7735E..23S}) and the European Extremely
Large Telescope (hereafter E-ELT,~\cite{2012SPIE.8444E..1FM}), will
offer unique  capabilities in terms of sensitivity, spatial resolution
and instrumental versatility (see Figure~\ref{10fig:spectra}).
They represent some of the most
challenging projects in modern astronomy with the realization of 30 to
40~m-class Adaptive Telescopes, i.e. with AO included in the telescope
design, conceived for visible and infrared wavelengths and equipped
with segmented primary mirrors (7 segments of 8.4 diameter  for GMT,
492 segments of 1.44~m for TMT and 800 segments of 1.45~m for E-ELT).
They will rely on a diversity of dedicated instruments offering various
observing modes: from integrated field spectroscopy, high-precision
astrometry, simultaneous multi-object spectroscopy of hundred of
targets, high resolution spectroscopy or high contrast imaging over a
broad range of wavelengths (from 0.33 to 19.0~$\rmmu$m) together with a
high operation efficiency. Various flavors of AO systems will be used
to ultimately provide the community with diffraction limited images
down to ${\sim}10$~mas in $K$-band and/or to exploit the full patrol
field of view of several arcminutes offered by these telescopes.

\begin{figure}[t!]
{\vspace*{2pt}}
\includegraphics{fig10}
{\vspace*{2pt}}
\caption{The VLT 10-m class telescope, together with the James Webb
Space Telescope and the new generation of extremely large telescope,
the Giant Magellan Telescope, the Thirty Meters Telecope and the
European Extremely Large Telescope (hereafter
E-ELT,~\cite{2012AandA...541A..11R}). Telescope diameters, collecting
areas and spatial resolution at 2~$\rmmu$m are
reported.\label{10fig:spectra}}
{\vspace*{2pt}}
\end{figure}

The increased sensitivity and exquisite spatial resolution of the ELTs
will offer an incredible opportunity to directly scrutinize and
characterize the stellar environment at unprecedented physical scales,
from the inner regions of proto-planetary disks, the chemistry of
planet-forming zones, to the characterization of recently formed giant
planets in their birth environment. Figure~\ref{fig:elt-disks}
\mbox{illustrates} the synergy between current 10 m-class telescope
instrumentation, ALMA and the future instrumentation of the ELTs to
characterize the spatial, temporal and chemical evolution of young
planet-forming disks. The spatial distribution of dust and gas,
including asymmetries and over densities in planet-forming zones, will
be explored at a 1--10 mas scale (10--100~$\rmmu$as scale in
spectro-astrometry).  The spectral information with a resolving power
of 100\,000 will enable to directly map the gas dynamics with a
velocity precision of 3~km${\cdot}$s$^{-1}$  (for instance CO with METIS at
4.7~$\rmmu$m). Keplerian rotation will be distinguished from wind,
accretion or Jet components. Deviation from Keplerian might also help
distinguishing the presence of hidden planets. The inner disk
chemistry, the disk atmospheres, the physical transport of volatile
ices either vertically or radially, and the importance of non-thermal
excitation processes in the planet-forming regions will be also
explored. A direct view of the distribution and the dynamics of water,
playing a key role for the planetesimals formation and the disk
cooling, will be possible. The organics content like CH$_4$,
C$_2$H$_2$, HCN in the planet-forming regions and the prebiotic
chemistry will be studied. Ultimately, the study of isotopic
fractionation should enable to probe the chemical and physical
conditions in the planet-forming disks to improve our understanding of
the transfer processes of water on terrestrial planets. 

\begin{figure}[t!]
\includegraphics{fig11}
\caption{Sketch of the parameter space explored with ELTs and other
facilities. Direct spectral imaging of the main planet-forming regions
will be achievable with the ELTs. For a typical young star at 100~pc, a
10~mas spatial resolution corresponds to physical separations of 1~au,
i.e. to the exploration of the warm gas and dust spatial distribution
down to the snow line, the disk evolution and dissipation to ultimately
determine the initial conditions of planetary
formation.\label{fig:elt-disks}}
\end{figure}

By 2028, the ELTs will arrive at a propitious time where thousands of
new planetary systems, in the Solar vicinity, will have been discovered
and characterized by a large set of instruments or space missions,
therefore covering a broad range of the parameter space in terms of
physical properties (planetary masses, semi-major axis, radii, density,
luminosity, atmospheric composition) and stellar host properties (age,
mass, binarity, composition\,\ldots). The ELTs will therefore be mainly used
for the detailed exploration
of known planetary systems, to search for
additional planets or characterize the most interesting ones. First
light instruments, like MORFEO/MICADO and HARMONI at E-ELT, will be
multi-purpose instruments, not designed for high-contrast, and
operating with single-conjugated AO modules offering limited
performances in term of Strehl (60--80\% at K-band), but already
offering the perspective to exploit the spatial and spectral diversity
of their spectro-imagers. They will reach typical contrasts
(5~$\sigma$) of $10^{-6}$ at angular separations of 50--100~mas at H
and K-bands~\cite{2018arXiv180401371P,2021AandA...652A..67H}. This will
enable to completely bridge the gap with indirect techniques like
radial velocity and astrometry for a systematic exploration of the
population of young, self-emitting giant planet at all separation, to
tightly constrain the Mass-Luminosity relation of young Jupiters, and
to boost the exploration of their atmosphere by getting high-fidelity
spectra at medium to high-spectral resolution. Given the ELT
sensitivity, Doppler imaging techniques will be applied to directly map
the spatial distribution of diverse molecular lines (CO, CO$_2$,
H$_2$O, CH$_4$ and possibly NH$_3$), and explore the general
circulation of their atmospheres (see Figure~\ref{12fig:spectra}). 
\looseness=-1

\begin{figure}[t!]
{\vspace*{-1pt}}
\includegraphics{fig12}
{\vspace*{-1pt}}
\caption{{Top-Left:} MICADO simulations of Direct Imaging
observations of the HR\,8799 planetary systems leading to the detection
of putative inner planets $f$ and $g$ to illustrate the contrast
performances already offered with the first Light instruments of ELTs
(Privite communication P. Baudoz, MICDO team). {Top-Right:} Molecular
mapping obeservations with METIS at the E-ELT of Proxima~b.
{Bottom-Right:} Angular separation and I-band flux ratio between
hypothetical exoearths (Earth size and insolation, one per star) and
parent stars within 10 pc (from the Hipparcos catalogue) observable
from Cerro Armazones. The symbol size indicates the planet's apparent
brightness, and the colours indicate stellar spectral types (red:
M-stars, yellow: solar-type stars). The dotted lines indicate the
approximate contrast boundaries for PCS. {Bottom-Right:} Spectra
of Earth, Venus, and Mars the visible to near infrared wavelength range
(the reflected flux of Mars has been multiplied by 10 for better
visibility). Credit: ESO/Baudoz \etal, Absil \etal, Kasper \etal,
and adapted from~\cite{Kaltenegger2017}.\label{12fig:spectra}}
{\vspace*{-1pt}}
\end{figure}

For very nearby planetary systems, a change of paradigm will begin by
directly imaging the reflected light of exoplanets. Planet to star
contrast of $10^{-5}$ to $10^{-9}$ are typically expected in reflected
light for exoplanets, cold Jupiters down to super-Earths, orbiting at
separation down to 10~mas. For our nearest neighbors like Alpha Cen A
and Proxima, METIS and ANDES might take the first steps towards the
direct detection and characterisation of temperate, rocky terrestrial
planets (given their favorable planet-to-star contrast or large angular
separation; see Figure~\ref{12fig:spectra}). However, to routinely
image and characterize super-Earths and Exo-Earths in the Habitable
Zone (0.02~au at 10~pc around M dwarfs), the ultimate goal will be to
reach the necessary contrast of about $10^{-9}$ at angular separations
of 10--20~mas around nearby dwarfs with a dedicated instrument like the
Planetary Camera and Spectrograph (PCS) at E-ELT, combining xAO,
coronagraphy and spectroscopy. It represents a very ambitious
specification directly linked with the motivation to detect
bio-signatures like O$_2$, O$_3$, CH$_4$, CO$_2$ and H$_2$O in the
atmospheres of terrestrial planets in habitable zones, and possibly the
first ``indicative'' discovery of exo-life, a revolutionary
breakthrough of great scientific and philosophical value.

\section*{Conflicts of interest}

The author has no conflict of interest to declare.

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