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\DOI{10.5802/crphys.141}
\datereceived{2022-11-21}
\dateaccepted{2023-01-19}
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\dateposted{2023-03-01}
\begin{document}

\begin{noXML}

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

\title{Exoplanet science with SPIRou: near-infrared precision
velocimetry and spectropolarimetry}

\alttitle{Les exoplan\`etes et leurs \'etoiles vues par SPIRou~:
v\'elocim\'etrie de pr\'ecision dans l'infrarouge proche et
spectropolarim\'etrie}

\author{\firstname{Claire} \lastname{Moutou}\IsCorresp}
\address{IRAP/CNRS/OMP/UPS, 14 avenue Edouard Belin, 31400 Toulouse,
France}
\email[C. Moutou]{claire.moutou@irap.omp.eu}

\author{\firstname{Jean-Fran\c{c}ois} \lastname{Donati}\CDRorcid{0000-0001-5541-2887}}
\addressSameAs{1}{IRAP/CNRS/OMP/UPS, 14 avenue Edouard Belin, 31400
Toulouse, France}
\email[J.-F. Donati]{jean-francois.donati@irap.omp.eu}

\author{\firstname{Florian} \lastname{Debras}\CDRorcid{0000-0002-3113-4840}}
\addressSameAs{1}{IRAP/CNRS/OMP/UPS, 14 avenue Edouard Belin, 31400
Toulouse, France}
\email[F. Debras]{Florian.debras@irap.omp.eu}

%\thanks{We acknowledge funding from the European Research Council
%under the H2020 \& innovation program (grant \#740651 NewWorlds).}

\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}}

\shortrunauthors

\keywords{\kwd{Exoplanets}\kwd{Stars}\kwd{Spectroscopic
techniques}\kwd{Atmospheres}\kwd{Magnetic fields}}

\altkeywords{\kwd{Exoplan\`etes}\kwd{\'Etoiles}\kwd{Techniques
spectroscopiques}\kwd{Atmosph\`eres}\kwd{Champs magn\'etiques}}

\begin{abstract} 
High-precision radial-velocity measurements in the near-infrared allow
one to search for exoplanet signals around the lowest-mass stars,
including young ones. Because these stars can be among the most active
ones, it is essential to characterize their magnetic activity. SPIRou,
installed at the Canada--France--Hawai'i telescope, was designed to
explore such planetary systems and characterize their properties. With
a wide spectral range and large resolving power, SPIRou is also
optimized for transmission spectroscopy and for investigating the
chemical composition of exoplanetary atmospheres. 
\end{abstract}

\begin{altabstract}
Les mesures de vitesse radiale de haute pr\'ecision dans l'infrarouge
proche permettent de rechercher le signal d'exoplan\`etes autour
d'\'etoiles de faible masse, en incluant les \'etoiles pr\'ecoces.
Comme ces \'etoiles sont parmi les plus actives, il est essentiel de
caract\'eriser leur activit\'e magn\'etique. L'instrument
\mbox{SPIRou}, install\'e au t\'elescope Canada--France--Hawai'i, a
\'et\'e con\c{c}u pour explorer de tels syst\`emes extra-solaires et
caract\'eriser leurs propri\'et\'es. Avec un domaine spectral \'etendu
et un pouvoir de r\'esolution \'elev\'e, SPIRou est \'egalement
optimis\'e pour la spectroscopie de transmission, permettant
d'\'etudier la composition chimique des atmosph\`eres d'exoplan\`etes.
\end{altabstract}

\maketitle

\end{noXML}

\section{Introduction}

In 2022, more than five thousands exoplanets are known and only 8\% of
them are hosted by stars cooler than 4000~K approximately corresponding
to the spectral types of M~dwarfs. Being the most abundant stellar type
in the galaxy and in the solar neighbourhood in particular, these
low-mass stars are also the most diverse. They span a very large range
in mass, from 0.08 to 0.6~solar mass, and a factor 200 in luminosity.
Planet search with the radial velocity (RV) method is favoured for
low-mass stars, as the planet signal is enhanced for a given planet
mass compared to a more massive star. Planetary formation and migration
models have been challenged by recent discoveries in the lower-mass end
of the main sequence, that showed that multiple compact systems can
exist around stars as small as Trappist-1
($0.09~\text{M}_{\odot}$,~\cite{gillon2017}) where circumstellar
material is sparse. Transit surveys were the first ones to unveil the
large population of super-earths near the habitable zone of their host
stars, deriving an occurrence rate of at least one planet per star at
orbital periods of less than 100 days~\cite{dressing2015}. Microlensing
surveys, which mainly probe low-mass stars due to their higher
frequency, also find very large occurrences of planets---amounting to
about one planet per star~\cite{cassan}. Since the mid 1990's, radial
velocity surveys with optical spectrographs have been challenged by the
low luminosity of M~stars and have mainly concentrated on solar-mass
stars, although pioneer work already showed a dozen confirmed or
candidate planet signals found around 100 low-mass
stars~\cite{bonfils2013}. This is much less than the expected number of
planets around such stars, given the lower limit on the planet
occurrence rate estimated from transit data, implying that many more
planets are still to be discovered. In this context, it was decided to
design and build a near-infrared cryogenic echelle spectrograph,
capable of high RV precision. 

\section{SPIRou} \label{sec2}

SPIRou (Spectro-Polarim\`etre InfraROUge) is a nIR spectropolarimeter
mounted at the Cassegrain focus of the 3.60~m Canada--France--Hawai'i
telescope (CFHT) atop Maunakea in Hawai'i. The spectral range is
continuous from 950 to 2500~nm (spanning YJHK bands) at a spectral
resolving power of about 70,000, recoded on a 4~K $\times$ 4~K H4RG
detector. The Cassegrain unit is composed of an achromatic polarimeter,
a tip-tilt mechanism with its guiding system, an atmospheric dispersion
corrector, and a calibration device (to inject the light from
calibration lamps through the instrument). The cross-dispersed
spectrograph, located two floors below in a cool and stable
environment, is connected to the Cassegrain unit through fluoride
fibers. A calibration unit is also connected for internal flat-fielding
and spectral calibration purposes. The cryogenic spectrograph is kept
in vacuum and cooled down to a temperature of 70~K, with a relative
thermal stability of better than 1~mK. Such a level of stability is
required to suppress the opto-mechanical variations that would result
from temperature changes and cause the measured RVs to vary with time.
The internal stability of SPIRou has been benchmarked with a suite of
internal lamps (UNe, Fabry-P\'erot and a laser frequency comb) and has
a typical scatter of under 0.2~m/s on a timescale of a night. The
dispersion obtained on stellar spectra of M~stars is of the order of
1.5 to 2~m/s. Regarding efficiency, SPIRou typically achieves a peak
signal to noise ratio per detector pixel of 100 on a H~11 magnitude
star in a 1~h exposure, which corresponds to a peak throughput of about
10\%, everything included. SPIRou has been in operations since February
2019 and is operated in queue mode, mostly during bright time. For a
more detailed description of SPIRou, its science goals and
performances, we refer the user to the review paper~\cite{donati2020}.

\section{Exoplanet detection and characterization with SPIRou}
\label{sec3}

Science goals for SPIRou include the search for exoplanets orbiting
low-mass stars, the mass characterization of transiting candidates
unveiled by TESS space photometry, and the spectroscopic transmission
of exoplanet atmospheres. In addition, spectropolarimetry and
high-resolution nIR spectroscopy allow us to explore stellar physics,
in particular the early phases of planetary system formation and the
evolving magnetic fields at the surface of the host stars.
Characterizing the magnetic field of the host star is of prime
importance in the field of exoplanet{\break} science. With indirect
methods, the intrinsic phenomena occurring at the surface of the host
stars need to be investigated as much as, and simultaneously to the
exoplanet signals themselves (see Section~\ref{sec4}). 

High performance in RV measurements is critical to get competitive
detection limits. In the nIR domain, the ground-based spectra are
plagued by variable Earth atmospheric transmission and emission.
Although most absorption lines can be removed with models and
comparison with a library of hot-star spectra~\cite{artigau2014}, small
residuals of telluric lines may remain at the percent level, and induce
an extra noise in the time series. In addition, nIR detectors are more
prone to persistence, causing low-S/N measurements to exhibit excess
noise in addition to the photon noise. In this context, it is required
to investigate new developments in the extraction pipelines for nIR
velocimeters such as SPIRou in order to get RV precision matching the
expected RV noise (including photon noise and the instrumental RV
precision). SPIRou data are commonly extracted, calibrated, and
corrected for their telluric contamination by the \mbox{APERO}
pipeline~\cite{cook2022}. First RV estimates are obtained primarily
using the cross-correlation method. A new method called line-by-line
was developed to optimize the selection of lines with respect to the
micro-telluric residuals and the extracted spectral
content~\cite{artigau2022}. This method is now commonly used on SPIRou
spectra. It allows to get the RVs for thousands of individual
lines{\break} along with the corresponding error bar, and to combine
them into a single value which is more robust to outliers.

The SPIRou Legacy Survey (SLS) is a 310-night program held at CFHT for
3.5 years held from February 2019 to July 2022. In this framework, 50
nearby M~stars were intensively observed in the double goal of
collecting RV measurements (for exoplanet detection) and circular
polarization sequences (for magnetic field characterization). In
addition, about 20 TESS objects of interest with M~stellar hosts were
monitored in the same way to establish the planetary nature and
characterize the parameters of transiting candidates. Finally, about 40
pre-main sequence stars were monitored, both to explore their stellar
magnetic properties and to search for potential RV signatures of
close-in giant planets. The whole set of time series is being
investigated, since the survey is now finished. It is being continued
and consolidated by another large program extending from 2022 to 2024,
called SPICE.

Among the first highlights in exoplanet detection and characterization,
one can cite the following results:

\begin{itemize}
\item Characterization of a rare substellar companion in close orbit to
a cool star~\cite{artigau2021}.\Tg</p>
\Tg<p>TOI-1278 b has been identified by space photometry as a
transiting body of an early M~star with a 14.5 day orbital period and a
percent-level transit. Its mass has been measured from SPIRou RV
measurements to $18.5 \pm 0.5~\text{M}_{\mathrm{Jup}}$. Such close-in
massive companions---either giant planet or brown dwarf in nature---are
extremely rare and may be considered a challenge for planetary
formation models.

\item Exploring the surrounding of the hot super-Earth desert.\Tg</p>
\Tg<p>TOI-1759 b and TOI-1452 b are two sub-Neptune planets unveiled by
their transit with TESS and whose mass and density were derived from
SPIRou RV measurements. While TOI-1759~b turns out to be a low-mass gas
planet with a bulk density of{\break} $1.3 \pm
0.5~\text{g}{\cdot}\text{cm}^{-3}$, TOI-1452~b is a super-Earth or
water-rich world with a density of $5.6 \pm
1.8~\text{g}{\cdot}\text{cm}^{-3}$. Both planets have M-type stellar
hosts. More details on the properties of these systems, including the
magnetic topology of their host stars, can be found in relevant
Refs.~\cite{martioli2022, cadieux2022}.

\item Multiple systems around low-mass stars.\Tg</p>
\Tg<p>Gl 876 is the nearby M4V star where the first exoplanet was
detected~\cite{delfosse1998}. Since then, three more planets have been
found in the system which turns out to be a very interesting dynamical
system with a chain of resonances~\cite{millholland2018}. Gl~876 has
been repeatedly observed with SPIRou. All four planets are recovered
and further constraints on the system's dynamical evolution and
potential additional planets will be derived over the years (see
Figure~\ref{fig:gl876}), while the stellar magnetic topology will be
investigated for the first time.
\end{itemize}

\begin{figure}[t!]
\includegraphics{fig01}
\caption{\label{fig:gl876}The SPIRou RV measurements of Gl~876 (purple
dots) and the 4-planet model (grey plot) as a function of time. Each
data point has an error of about 1.5~m/s and the scatter of the
residuals is about 3~m/s.}
\end{figure}

\section{Stellar characterization} \label{sec4}

The detection of planetary signals is plagued by the intrinsic
variability of the stellar-host surface. It is thus essential to work
out both questions together. As in the Sun, stellar activity is a
by-product of the magnetic field generated by dynamo processes acting
in the convective interior. SPIRou, with its spectro-polarimetric
capability, is designed to measure and characterize magnetic fields of
stars. The large-scale topology of the field can be probed through the
modulation of circular polarization Stokes~V profiles, while the small
scale magnetic field can be concentrated in spots is measured in the
Zeeman broadening and intensification of lines sensitive to magnetic
fields. Although the planet-search program of the SPIRou Legacy Survey
has been built from a sample of low-activity M~stars~\cite{moutou2017},
one has to be cautious about RV signals induced by stellar intrinsic
variability. Following the pioneering work that first explored
potential correlations between Stokes $V$ signatures and activity RV
signals~\cite{hebrard2016}, we now see, from the abundant SLS data,
that the complex modulation of the longitudinal field can be faithfully
modeled with a quasi-periodic gaussian process~\cite{fouque2022,
donati2022}. This analysis is systematically carried out for all SPIRou
targets, and the method is sensitive down to large-scale fields of only
a few~G, and to rotation periods of more than a year.

One example in the SPIRou early science is the study of AD Leo. AD Leo
is an active M3 star located at 5~pc, with a known stellar rotation
period of 2.23 days~\cite{morin2008}. A stable RV signal seen in the
optical domain with the HARPS spectrograph at a period close to the
rotation period has been interpreted as being the signature of a planet
locked with the stellar spin~\cite{tuomi2018}. Recent~RV measurements
obtained with SPIRou in the nIR domain did not confirm this signal. The
semi-amplitude of RV variations is 20~m/s in the optical and less than
5~m/s in the nIR. Since planetary signatures are not dependent on the
wavelength of observation, the optical-RV signal must thus be
attributed to stellar activity, whose chromaticity is easy to interpret
as caused by the wavelength-dependent spot-to-photosphere contrasts
and/or Zeeman-induced line profile distortions. This combined analysis
is described in~\cite{carmona2022} on the basis of SPIRou and
OHP/SOPHIE data. In less active stars, it is possible that the
chromatic behaviour of stellar activity is less
pronounced~\cite{cortes2022}. A global analysis of the whole SLS sample
will help draw general conclusions about this behaviour.

On the other hand, young stars can be hugely active, with RV
modulations of tens or hundreds of m/s. For instance, AU Mic is a
20-Myr star of M1 spectral type at 9.7~pc and has photometric
variations up to 5\% over its 4.9 day rotation cycle and intense
flaring activity. Two close-in transiting planets have also been found
by TESS, orbiting AU Mic at 8.46 and 18.86 day orbital periods. An
early intensive follow-up campaign with SPIRou allowed to detect with a
4-$\sigma$ significance the gravitation pull by the inner planet, after
the removal of an extra signal of 50~m/s amplitude due to the star's
magnetic activity~\cite{klein2021}. This work yielded the first density
measurement for a Neptune-like planet younger than 200~My and triggered
a number of follow-up studies and additional observing campaigns.
Several tens of other young stars from 2 to 25~My are being monitored
with SPIRou. Together, they will offer a set of unique constraints for
star and planet formation and migration models.\looseness=-1

\section{Exoplanet atmospheres with SPIRou} \label{sec5}

A few years only after the discovery of exoplanets, it was realised
that it is possible to characterize the atmosphere of transiting
exoplanets from Earth: as the planet crosses the disk of its host star,
the stellar light is absorbed in a way that varies with wavelength by
the planet atmosphere as a result of its composition and physical
characteristics (temperature, dynamics, gravity{\ldots}). These
observations were first performed at low spectral resolution from
space, where the very large spectral range and the absence of tellurics
contamination make it possible to detect the large bands of molecular
absorption of, e.g., water (at 1600~nm) and carbon monoxide (2300~nm)
and broad atomic lines of, e.g., sodium (580~nm), hydrogen (121~nm) and
potassium (750~nm).

Since 2010, such observations have also been carried out at high
spectral resolution (HRS) from the ground with visible and infrared
spectrometers like SPIRou. Although limited by tellurics and a smaller
spectral range, such characterization allows one to recover
complementary information from space-borne observations: for example,
the atmospheric dynamics is accessible through Doppler-shifted
absorption and HRS allows to detect absorption features above the cloud
level, thus extending the parameter space of characterisable
atmospheres. In the infrared, SPIRou is expected to be the best
instrument in the world to date because of a combination of (i)~a large
continuous spectral range making it possible to detect absorption from
many different molecules, (ii)~one of the highest resolving power
available in this spectral range in the world (70,000), (iii)~the high
velocimetric stability of the instrument and (iv)~arguably the best
observing conditions in the best temperate astronomical site on the
northern hemisphere (Maunakea). A synthetic model of the atmosphere of
planet HD 209458b as would be observed through HRS with SPIRou is
provided on Figure~\ref{fig:hd209}.

\begin{figure}[t!]
\includegraphics{fig02}
\caption{\label{fig:hd209}Synthetic apparent radius (in Jupiter radius)
of the planet HD209458b as a function of wavelength, with a resolution
equal to that of SPIRou, on a small subset of the SPIRou range. In this
spectral range, the variations are due to the absorption by water vapor
in the atmosphere of the planet.}
\end{figure}

Some early highlights on exoplanet atmospheres with SPIRou include:
\begin{itemize}
\item The characterization of the atmosphere of HD 189733
b~\cite{boucher2021,debras2022}.\Tg</p>
\Tg<p>This hot Jupiter is one of the most studied exoplanet as it was
discovered early and has very favourable conditions for atmospheric
characterisation. With SPIRou, we were able to detect
$\text{H}_2\text{O}$ and CO, thus allowing us to derive its C/O ratio,
a prime quantity to recover information about the formation and
evolution of planets.

\item The surprising lack of water in the atmosphere of $\tau$ Boo
b~\cite{pelletier2021}.\Tg</p>
\Tg<p>This planet is so hot that it was observed in emission (rather
than transmission) spectroscopy, by differentiating the light received
from Earth at different phases of its orbit. Surprisingly, there is a
lack of water compared to what was expected regarding our knowledge of
planetary atmospheres in the solar system and elsewhere. This probably
indicates a large depletion in oxygen, which relates to the region of
the disc where the planet was formed, i.e., beyond the water snowline.

\item Iron and water dynamics in WASP-76 b~\cite{hood2022}.\Tg</p>
\Tg<p>WASP-76 b is a ultra hot Jupiter (temperature higher than 2000~K)
which orbits extremely close to its host star and seems to exhibit an
asymmetric iron absorption~\cite{ehrenreich2020}: the western limb of
the planets is iron rich whereas the eastward one seems to be iron
depleted. Thanks to simultaneous observations with SPIRou (infrared)
and MAROON-X (visible), we are able to explore the different dynamics
between iron and water, probing different pressure levels, in order to
understand better the physics of highly irradiated atmospheres. 
\end{itemize}

\section{Conclusion} \label{sec6}

SPIRou is currently a unique new instrument, providing the full
near-infrared spectral range from 950 to 2500~nm (YJHK bands) at a
resolving power of 70~k, with an RV precision better than 2~m/s, along
with spectropolarimetric capabilities (circular and linear
polarisation). A twin instrument, called SPIP, is currently under
construction for the Telescope Bernard Lyot at Pic du Midi. This will
be convenient to monitor the same stars from both Hawai'i and France,
in order to mitigate the sampling problems due to the diurnal cycle. It
is also envisioned to combine ESPaDOnS and SPIRou at CFHT and their
twins at TBL, to simultaneously observe stars and retrieve the full
spectra and polarimetric information in the range 390 to 2500~nm. The
power of combining RV and magnetic field measurements is a powerful
tool to reach a global view of exoplanetary systems.

In the coming years, all RV and spectropolarimetric time series, and
more generally all high-resolution SPIRou spectra will be available to
the community for their legacy value beyond the main goals of the SLS,
i.e., the study of planetary systems of nearby M dwarfs and of the
impact of magnetic field on star/planet formation.

SPIRou has also proven to be a powerful instrument for the observation
of planetary atmospheres. It already provided several
characterizations, and is expected to deliver many more in the near
future. The complementarity between low resolution from space with JWST
and ARIEL (launched in 2029) and SPIRou at high resolution from the
ground will be essential to understand better the formation and
evolution of planetary systems.

\section*{Conflicts of interest}

Authors have no conflict of interest to declare.

\section*{Acknowledgements}

The authors, on behalf of the SPIRou community, fully appreciate the
very significant cultural role that the summit of Maunakea has always
had within the population of Hawai'i over the centuries, and are most
fortunate to have the opportunity to conduct observations from this
exceptional site.

We acknowledge funding from the European Research Council under the
H2020 \& innovation program (grant \#740651 NewWorlds).

\CDRGrant[ERC]{\#740651}

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