The PLATO mission is scheduled for launch early 2027. In this paper we present an overview of the performance drivers for the mission at the time where all flight models of the cameras have been tested and integrated on the optical bench. The PLATO consortium needs an estimate of the planet detection yield to dimension the ground-based radial velocity follow-up resources. We provide updated estimates on the yield of planet detections that can be expected from the mission under certain assumptions. As of today, large uncertainties remain on the planet occurrence rates, especially for small planets in long-period orbits, and on our ability to detect these planets in the presence of stellar variability and instrumental noise. To partially overcome these limitations, we compare results using different planet occurrence rates, detectability rates, and we include an estimate on the expected contribution of stellar variability to the noise budget. The final detection yield of PLATO will provide constraints to planet occurrence rates which in turn will help constraining planet formation models.

The radii of small exoplanets form two populations, super-Earths and sub-Neptunes, separated by a gap known as the radius valley. This feature could be produced by the removal of atmospheres by stellar or internal heating or by the lack of an initial envelope. We used transit photometry and radial velocity measurements to detect and characterize four exoplanets orbiting LHS 1903, a red dwarf star in the Milky Way's thick disk. These four planets have orbital periods ranging from 2.2 to 29.3 days and span the radius valley within a single planetary system. The derived densities indicate that LHS 1903 b is rocky, whereas LHS 1903 c and LHS 1903 d have extended atmospheres. The most distant planet from the host star, LHS 1903 e, has no gaseous envelope, indicating that it formed from gas-depleted material.

In order to meet the science goals of the PLATO space mission, an extensive science calibration and validation plan has been designed. This paper describes this plan, as well as the methodology adopted to select the science calibration and validation stars that have entered its input catalogue. This is the so-called scvPIC, which is part of the general PLATO Input Catalogue (PIC) for the first selected long pointing field in the Southern Hemisphere known as LOPS2. While many of PLATO's science requirements needed dedicated stars as calibrators as discussed here, its most stringent requirement is the delivery of the age of the host stars of exoplanetary systems with an accuracy better than 10% for a G0V star of V = 10 mag, i.e. a nearby Sun-like star. This is presently not within reach for large populations of dwarfs and subgiants in the Milky Way as it requires the models of their stellar interiors to be improved. We discuss how this ambitious age requirement led to the selection of tens of thousands of red giants, and of thousands of main-sequence early F-type gravity-mode pulsators in order to deduce their internal rotation profile across stellar evolution. This asteroseismic observable will then be imported as key information into improved models of dwarfs and subgiants in the Milky Way as optimal modelling tools for ever better age-dating of the exoplanet hosts as the PLATO mission moves along. Additional calibrators and validators included in the scvPIC are a few thousands of binaries, a few hundreds of legacy and benchmark stars, a few hundred photometrically stable stars, and six transiting brown dwarfs.

A few weeks after launch, the PLATO spacecraft is expected to start its payload commissioning, which will be completed within the first three months of the mission. This phase includes the in-orbit verification, calibration, and configuration of the instrument prior to nominal science operations. During this mission-critical period, and again later during regular spacecraft rotations and re-pointings, a set of reference stars is required to complete various calibration steps. This set, referred to as the calibration PLATO Input Catalog (cPIC), is part of the PIC. The cPIC comprises various stellar samples, each serving a dedicated technical calibration purpose, and it contains 71671 unique stellar targets across PLATO's entire field of view (FoV). Once the spacecraft commences science observations, the on-board Fine Guidance System (FGS) will rely on a small set of guide stars. These stars must be particularly bright and will be observed with the two fast cameras, which cover only a smaller central region of PLATO's FoV. This target list, referred to as the fine-guidance PLATO Input Catalog (fgPIC), contains 2640 unique targets, of which about 30 are used by the FGS at any given time. In this paper, we present the selection criteria for both the cPIC and the fgPIC, and asses their impact on the construction of these calibration catalogs for PLATO.

The Habitable Worlds Observatory (HWO) offers a unique opportunity to revolutionize our understanding of planetary formation and evolution. The goal of this Science Case Development Document (SCDD) is to investigate the physical and chemical processes that shape the composition and atmospheric mass loss in exoplanets. We review the key observables currently known as diagnostics of mass loss via transit observations, i.e., absorption lines of escaping hydrogen (Lyman-alpha), helium, and metals (Fe, Mg, C, O). We also explore the challenges to infer planetary formation processes based on atmospheric composition characterization. HWO could enable a broad, continuous coverage from far-ultraviolet to near-infrared spectroscopy ( 100–1600 nm) at high resolution (R > 60, 000), which is essential to make these measurements, disentangle their planetary origin from stellar activity, and ultimately, contextualize the escape rates by simultaneously characterizing the composition, cloud predominance, and thermal structure of exoplanet atmospheres.

PLAnetary Transits and Oscillations of stars is an ESA M-class satellite planned for launch by the end of 2026 and dedicated to the wide-field search of transiting planets around bright and nearby stars, with a strong focus on discovering habitable rocky planets hosted by solar-like stars. The choice of the fields to be pointed at is a crucial task since it has a direct impact on the scientific return of the mission. In this paper, we describe and discuss the formal requirements and the key scientific prioritization criteria that have to be taken into account in the Long-duration Observation Phase (LOP) field selection, and apply a quantitative metric to guide us in this complex optimization process. We identify two provisional LOP fields, one for each hemisphere (LOPS1 and LOPN1), and we discuss their properties and stellar content. While additional fine-tuning shall be applied to LOP selection before the definitive choice, which is set to be made two years before launch, we expect that their position will not move by more than a few degrees with respect to what is proposed in this paper.

With ongoing missions such as the James Webb Space Telescope and planned initiatives such as the Large Interferometer For Exoplanets (LIFE), the detection and attribution of biosignatures in exoplanetary atmospheres increasingly becomes a point of focus. However, in order to assess how different biosignatures manifest themselves in the atmospheres of rocky exoplanets in contrast to our temperate Earth, improved insights into the maintenance of earthlike atmospheric biosignatures in different atmospheres are necessary. Here, we identify and investigate the main processes and possible couplings between atmospheric climate and photochemistry for earthlike planets across the inner habitable zone. We also study the detectability of the modelled spectral features with the LIFE simulator to assess how the atmospheres of planets with an earthlike biosphere may appear to future missions like the LIFE interferometer. We use the global-mean, stationary, coupled climate-chemistry column model, 1D-TERRA, to simulate the climate and chemistry of planetary atmospheres at different distances from the Sun, initially assuming Earth's planetary parameters and evolution. We run six scenarios: we assume rocky exoplanets with Earth’s biomass fluxes forming around the Sun with insolation from 100% to 150% in steps of 10%. From the resulting output of temperature and composition profiles, we calculate theoretical transmission and emission spectra using a radiative transfer model (GARLIC).The models show moderate ocean evaporation as the planet moves closer to the Sun, which results in water-vapour-rich atmospheres with the partial pressures of steam ranging from about 0.01 bar (modern Earth insolation, S=1) up to 0.6 bar (S=1.5). In the latter model, the global mean surface temperature increases to 365.4K. This is mainly due to the higher energy input and the enhanced greenhouse effect due to increased amounts of water vapour in the atmosphere. Regarding key atmospheric biosignatures, ozone, surprisingly, mostly survives in the middle atmosphere in all scenarios, mainly because hydrogen oxide abundances, a catalytic sink for ozone, are prevented from strongly increasing due to reactions with nitrogen oxides. Methane is strongly removed for insolations above 20% those of Earth, because rising water abundances strongly increase hydroxyl (OH) (via UV photolysis) the main sink for methane. Nitrous oxide (N2O) generally survives, mainly due to trade-off effects where enhanced photolytic loss on upper layers due to higher insolation is counterbalanced by stronger absorption of photons on the lower layers due to enhanced water from evaporation. Hydrogen escape rates are 0.690 Tg/yr for the highest insolation scenario. Abiotic oxygen production associated with atmospheric escape of atomic hydrogen as well as catalytic in-situ recycling of oxygen atoms present in HOx species, lead to an increase in the O2 vmr to 0.35 mol/mol on increasing solar insolation from S = 1.0-1.3. For all scenarios, the simulated transmission and emission spectra show clearly evident H2O and CH4 features in the near to mid IR, strong CO2 absorption around 15 microns, and O3 absorption at around 9.6 microns.

Aims: We aim to observe the transits and occultations of WASP-33b, which orbits a rapidly-rotating δ Scuti pulsator, with the goal of measuring the orbital obliquity via the gravity-darkening effect, and constraining the geometric albedo via the occultation depth. Methods: We observed four transits and four occultations with CHEOPS, and employ a variety of techniques to remove the effects of the stellar pulsations from the light curves, as well as the usual CHEOPS systematic effects. We also performed a comprehensive analysis of low-resolution spectral and Gaia data to re-determine the stellar properties of WASP-33. Results: We measure an orbital obliquity 111.3 +0.2 -0.7 degrees, which is consistent with previous measurements made via Doppler tomography. We also measure the planetary impact parameter, and confirm that this parameter is undergoing rapid secular evolution as a result of nodal precession of the planetary orbit. This precession allows us to determine the second-order fluid Love number of the star, which we find agrees well with the predictions of theoretical stellar models. We are unable to robustly measure a unique value of the occultation depth, and emphasise the need for long-baseline observations to better measure the pulsation periods.

The European Space Agency has selected PLATO (PLAnetary Transits and Oscillations of stars) for its M3 launch which is scheduled for 2026. With its extremely large field of view, PLATO is designed to obtain photometric measurements over an extended period for bright stars in order to detect and characterise (primarily) rocky planets in the habitable zones of solar type stars. The PLATO measurements will have sufficient sensitivity to determine the mass, radius and age of the host stars with unprecedented accuracy. The PLATO planet database will provide the first large-scale catalogue of accurately and homogeneously characterised small planets at intermediate orbital periods, which will can be used to severely constraint planet formation theories. This would facilitate large scale comparative exo-planetology. In addition the bright PLATO host stars will be ideal targets for atmospheric study with next generation facilities such as the ELT. The PLATO sensitivity will be sufficient to detect pulsations from stars across the HR diagram allowing a deep understanding of stellar structure and evolution to be developed using parameters determined from asteroseismology.

Context. Recent developments in exoplanetary research highlight the importance of Love numbers in understanding the internal dynamics, formation, migration history, and potential habitability of exoplanets. Love numbers represent crucial parameters that gauge how exoplanets respond to external forces such as tidal interactions and rotational effects. By measuring these responses, insights into the internal structure, composition, and density distribution of exoplanets can be gained. The rate of apsidal precession of a planetary orbit is directly linked to the second-order fluid Love numbers. Thus, Love numbers can also offer valuable insights into the mass distribution of a planet. Aims. In this context, we aim to re-determine the orbital parameters of WASP-43b - in particular, the orbital period, eccentricity, and argument of the periastron - and its orbital evolution. We study the outcomes of the tidal interaction with the host star in order to identify whether tidal decay and periastron precession occur in the system. Methods. We observed WASP-43b with HARPS, whose data we present for the first time, and we also analysed the newly acquired JWST full-phase light curve. We jointly fit new and archival radial velocity and transit and occultation mid-times, including tidal decay, periastron precession, and long-term acceleration in the system. Results. We detected a tidal decay rate of P-a = (-1.99 +/- 0.50) ms yr(-1) and a periastron precession rate of omega = 0.1727(-0.0089)(+0.0083))(omicron) d(-1) = (621.72 (+29.88)(-32.04))"d(-1). This is the first time that both periastron precession and tidal decay are simultaneously detected in an exoplanetary system. The observed tidal interactions can neither be explained by the tidal contribution to apsidal motion of a non-aligned stellar or planetary rotation axis nor by assuming a non-synchronous rotation for the planet, and a value for the planetary Love number cannot be derived. Moreover, we excluded the presence of a second body (e.g. a distant companion star or a yet undiscovered planet) down to a planetary mass of greater than or similar to 0.3 M-J and up to an orbital period of less than or similar to 3700 days. We leave the question of the cause of the observed apsidal motion

PLATO (PLAnetary Transits and Oscillations of stars)1 is the M3 class ESA mission dedicated to the discovery and study of extrasolar planetary systems by means of planetary transits detection. PLATO Payload Camera units are integrated and vibrated at CSL before being TVAC tested for thermal acceptance and performance verification at 3 different test facilities (SRON, IAS and INTA). 15 of the 26 Flight Cameras were integrated, tested and delivered to ESA for integration by the Prime between June 2023 and June 2024, with the remaining flight units to be tested by the end of 2024. In this paper, we provide an overview of our serial testing approach, some of the associated challenges, key performance results and an up-to-date status on the remaining planned activities.

Aims.In this work, we determine the expected yield of detections of solar-like oscillations for the targets of the foreseen PLATO ESA mission. Our estimates are based on a study of the detection probability, which takes into account the properties of the target stars, using the information available in the PIC 1.1.0, including the current best estimate of the signal-to-noise ratio (S/N). The stellar samples, as defined for this mission, include those with the lowest noise level (P1 and P2 samples) and the P5 sample, which has a higher noise level. For the P1 and P2 samples, the S/N is high enough (by construction) that we can assume that the individual mode frequencies can be measured. For these stars, we estimate the expected uncertainties in mass, radius, and age due to statistical errors induced by uncertainties from the observations only.Methods.We used a formulation from the literature to calculate the detection probability. We validated this formulation and the underlying assumptions withKeplerdata. Once validated, we applied this approach to the PLATO samples. Using againKeplerdata as a calibration set, we also derived relations to estimate the uncertainties of seismically inferred stellar mass, radius, and age. We then applied those relations to the main sequence stars with masses equal to or below 1.2M⊙belonging to the PLATO P1 and P2 samples and for which we predict a positive seismic detection.Results.We found that we can expect positive detections of solar-like oscillations for more than 15 000 FGK stars in one single field after a two-year observation run. Among them, 1131 main sequence stars with masses of ≤1.2 M⊙satisfy the PLATO requirements for the uncertainties of the seismically inferred stellar masses, radii, and ages. The baseline observation programme of PLATO consists of observing two fields of similar size (one in the southern hemisphere and one in the northern hemisphere) for two years apiece. Accordingly, the expected seismic yields of the mission amount to over 30 000 FGK dwarfs and subgiants, with positive detections of solar-like oscillations. This sample of expected solar-like oscillating stars is large enough to enable the PLATO mission’s stellar objectives to be amply satisfied.Conclusions.The PLATO mission is expected to produce a catalog sample of extremely well seismically characterized stars of a quality that is equivalent to theKeplerLegacy sample, but containing a number that is about 80 times greater, when observing two PLATO fields for two years apiece. These stars are a gold mine that will make it possible to make significant advances in stellar modelling.

Love numbers measure the reaction of a celestial body to perturbing forces, such as the centrifugal force caused by rotation, or tidal forces resulting from the interaction with a companion body. These parameters are related to the interior density profile. The non-point mass nature of the host star and a planet orbiting around each other contributes to the periastron precession. The rate of this precession is characterized mainly by the second-order Love number, which offers an opportunity to determine its value. We collected all available radial velocity (RV) data, along with the transit and occultation times from the previous investigations of the system. We supplemented the data set with 19 new RV data points of the host star WASP-19A obtained by HARPS. Here, we summarize the technique for modeling the RV observations and the photometric transit timing variations (TTVs) to determine the rate of periastron precession in this system for the first time. We excluded the presence of a second possible planet up to a period of ~4200 d and with a radial velocity amplitude bigger than ~1 m/s. We show that a constant period is not able to reproduce the observed radial velocities. We also investigated and excluded the possibility of tidal decay and long-term acceleration in the system. However, the inclusion of a small periastron precession term did indeed improve the quality of the fit. We measured the periastron precession rate to be 233 $^{+25}_{-35}$''/. By assuming synchronous rotation for the planet, it indicates a k2 Love number of 0.20 $^{+0.02}_{-0.03}$ for WASP-19Ab. The derived k2 value of the planet has the same order of magnitude as the estimated fluid Love number of other Jupiter-sized exoplanets (WASP-18Ab, WASP-103b, and WASP-121b). A low value of k2 indicates a higher concentration of mass toward the planetary nucleus.

AbstractThe coronagraph instrument aboard the Nancy Grace Roman Space Telescope is a technology demonstrator that will perform the first reflected-starlight direct imaging observations of exoplanets. This instrument will pave the way for future missions such as LUVOIR or HabEx, which have the goal of characterizing the atmospheres of Earth-like exoplanets. In this work we develop a statistical method to compute which of the confirmed exoplanets in the NASA Exoplanet Archive would be accessible in reflected starlight to a direct-imaging telescope. By applying our method to the Roman Telescope’s coronagraph, we show that an eventual science phase of the Roman Telescope’s coronagraph has a remarkable potential to study cold and temperate exoplanets and initiate their atmospheric characterization. IntroductionThe years until the expected launch of the Roman Telescope[1] in 2025 should be used to improve the orbital characterization of the most interesting targets through radial velocity or astrometry campaigns. For this, a target list of known exoplanets that could be observed with the Roman Telescope’s coronagraph is needed. Additional figures describing the detectability and scientific interest of each accessible exoplanet are useful to prioritize the observations of the targets. Statistical methodWe use the NASA Exoplanet Archive[2] as the main source of information for the planetary and stellar properties of each of the about 4300 confirmed exoplanets. For each exoplanet, we compute 10,000 orbital realizations letting each of the parameters involved vary within its reported upper and lower uncertainties. When the orbital inclination (i), the eccentricity (e) or the argument of periastron of the planet (ωp) are unconstrained, we randomly draw their value at each orbital realization from uniform distributions. When the planet radius is unknown, we compute it by means of published mass-radius relationships[3][4]. By discretizing each orbital realization into 360 positions, we compute at each position the angular separation between the planet and the star (Δθ) and the planet-to-star contrast ratio (Fp/F*). We define a planet as Roman-accessible if at a certain orbital position its angular separation is within the inner and the outer working angles (IWA, OWA) of the coronagraph and, additionally, the planet-to-star contrast is brighter than the minimum contrast (Cmin) that the coronagraph can detect.Our main outputs are the probability of a planet to be Roman-accessible (Paccess), the range of observable phase angles (αobs), the number of days per orbit in which the planet is accessible (tobs) and its transit probability. Due to its interest for atmospheric modelling, we also compute the equilibrium temperature (Teq) at each orbital position. This allows us to compute the mean value of Teq throughout the orbit, its variation due to orbital eccentricity and the variation of Teq that takes place while the planet is accessible, which could result in detectable atmospheric variability.With this method, we computed the accessibility of each planet at wavelengths 575, 730 and 825 nm, consistent with the coronagraph filters that are currently commissioned. We repeated this study for three plausible configurations of the coronagraph because its final design is not yet completed. We label these configurations as optimistic (IWA=3λ/D, OWA=9λ/D, Cmin=1×10−9), intermediate (IWA=3.5λ/D, OWA=8.5λ/D, Cmin=3×10−9) and pessimistic (IWA=4λ/D, OWA=8λ/D, Cmin=5×10−9). ResultsWe find up to 26 exoplanets Roman-accessible exoplanets in the optimistic scenario with Paccess>25% and orbiting stars brighter than V=7 mag[5]. We apply the latter two vetting criteria throughout our work due to the particular constraints of the Roman Telescope mission timeline and the sensitivity of its coronagraph instrument. This number of Roman-accessible exoplanets is reduced to 10 and 3 in the intermediate and pessimistic scenarios, respectively.For the Roman-accessible exoplanets in the optimistic scenario we carried out a population study and found that this set of planets is dominated by giant exoplanets more massive than Jupiter. Interestingly, it also includes the low-mass planets tau Cet e and f, which orbit near the habitable zone of their host star. These two planets are however barely accessible in the intermediate or pessimistic coronagraph scenarios (Fig. 1). Thirteen of the 26 Roman-accessible exoplanets are part of multi-planet systems and three of them have inner companions observed in transit, which would enable the simultaneous characterization of the inner and the outer regions of these planetary systems. The mean equilibrium temperatures of the Roman-accessible planets range from values in the order of that of Uranus to values above 400 K, including some targets at ~300 K (Fig. 2).For a selection of particularly interesting targets, we analysed in more detail the prospects for observing and eventually characterizing these planets. For instance, we discussed how the detectability prospects may change if additional constraints on the orbital inclination are set e.g. with astrometry (Fig. 1).We find some exoplanets with remarkably wide ranges of observable phase angles, which makes them interesting for atmospheric characterization with reflected-starlight phase curves[6]. In this regard, we also discussed the importance of consistently reporting the planet and stellar parameters in exoplanet catalogues such as the NASA Archive and the misleading detectability results that might be achieved if an A standardization process is not performed.Overall, we find that a science phase of the Roman Telescope’s coronagraph would have an extraordinary potential to perform one-of-a-kind observations before next-generation missions that are not expected at least until the mid-2030s. References[1] Spergel et al. (2013), https://arxiv.org/abs/1305.5422 [2] Akeson et al. (2013), PASP, 125, 989 [3] Hatzes & Rauer (2015), ApJL, 810, L25 [4] Otegi et al. (2020), A&A, 634, A43 [5] Carrión-González et al. (2021), A&A accepted [6] Carrión-González et al. (2021), in prep.

There is much still to learn about giant planets, in particular those at relatively long orbital period. Present (TESS, CHEOPS) and future planetary missions (PLATO, Ariel) will make a significant contribution to our understanding of these systems.One unsolved problem is the origins of warm Jupiters (WJs). If WJs formed beyond the snow line, far from their host stars, then migration is required to bring them to their current orbits. There are several hypotheses explaining the migration history and we are exploring observational tests for these hypotheses. In particular, obliquity (the angle between the stellar rotation and planetary orbital axes) is a key tracer of migration history. Dynamically violent, high-eccentricity migration leads to planets in significantly misaligned orbits with large obliquities, whereas disc-driven migration should result in orbits coplanar with the stellar equator. In contrast to the hot Jupiters, the imprint of dynamical migration in WJs should not be erased through tidal interactions with the convective zone of their stars, because they are tidally detached.Only around 60 transiting warm Jupiters are currently known, only 16 of which have a measured obliquity. We have a VLT/ESPRESSO programme to measure the obliquities of an unbiased sample of eleven WJs, which will greatly increase the size of the measured sample. Our first observations were made earlier this year, and here we present those data, and our preliminary interpretation.

The Large Interferometer For Exoplanets (LIFE) is a proposed space mission that enables the spectral characterization of the thermal emission of exoplanets in the solar neighborhood. The mission is designed to search for global atmospheric biosignatures on dozens of temperate terrestrial exoplanets and it will naturally investigate the diversity of other worlds. Here, we review the status of the mission concept, discuss the key mission parameters, and outline the trade-offs related to the mission's architecture. In preparation for an upcoming concept study, we define a mission baseline based on a free-formation flying constellation of a double Bracewell nulling interferometer that consists of 4 collectors and a central beam-combiner spacecraft. The interferometric baselines are between 10-600 m, and the estimated diameters of the collectors are at least 2 m (but will depend on the total achievable instrument throughput). The spectral required wavelength range is 6-16 mu m (with a goal of 4-18.5 mu m), hence cryogenic temperatures are needed both for the collectors and the beam combiners. One of the key challenges is the required deep, stable, and broad-band nulling performance while maintaining a high system throughput for the planet signal. Among many ongoing or needed technology development activities, the demonstration of the measurement principle under cryogenic conditions is fundamentally important for LIFE.

CHEOPS is a space telescope specifically designed to monitor transiting exoplanets orbiting bright stars. In September 2023, CHEOPS completed its nominal mission and remains in excellent operational conditions. The mission has been extended until the end of 2026. Scientific and instrumental data have been collected throughout in-orbit commissioning and nominal operations, enabling a comprehensive analysis of the mission's performance. In this article, we present the results of this analysis with a twofold goal. First, we aim to inform the scientific community about the present status of the mission and what can be expected as the instrument ages. Secondly, we intend for this publication to serve as a legacy document for future missions, providing insights and lessons learned from the successful operation of CHEOPS. To evaluate the instrument performance in flight, we developed a comprehensive monitoring and characterisation programme. It consists of dedicated observations that allow us to characterise the instrument's response. In addition to the standard collection of nominal science and housekeeping data, these observations provide input for detecting, modelling, and correcting instrument systematics, discovering and addressing anomalies, and comparing the instrument's actual performance with expectations. The precision of the CHEOPS measurements has enabled the mission objectives to be met and exceeded. Careful modelling of the instrumental systematics allows the data quality to be significantly improved during the light curve analysis phase, resulting in more precise scientific measurements. CHEOPS is compliant with the driving scientific requirements of the mission. Although visible, the ageing of the instrument has not affected the mission's performance.

AbstractDirect-imaging observations of exoplanets in reflected starlight are expected to be available this decade. This will improve our knowledge about cold and temperate exoplanets and their atmospheres. Current theoretical efforts related to such planets aim to understand the effects that planet and atmospheric properties have on the spectra to be measured. This will help predict the science outcome of direct-imaging missions and identify the key needs for models used in the interpretarion of future measurements. In this work, we have investigated the information contained in reflected-light exoplanetary spectra and the role played by the planet radius in the atmospheric characterization.IntroductionLong-period exoplanets are a population that remains substantially unexplored because of the biases introduced by the technology currently available. These planets have small transit probabilities due to their large orbital distances and hence the direct-imaging technique will be key to analyse their atmospheres. Space missions such as NGRST (formerly WFIRST), LUVOIR or HabEx will allow us to study this population of long-period exoplanets, providing a more complete picture of exoplanet diversity.ModelWe set up an atmospheric model with hydrogen and helium as the main components. We include methane, in a volume-mixing-ratio fCH4, as the only absorbing gaseous species. We add a cloud layer, described by its optical thickness (τc), its geometrical extension and the position of the cloud top. The aerosols are modelled by their single-scattering albedo and their effective radius, which determines the scattering phase function through Mie theory. This model is motivated by previous modelling of the atmospheres of Solar System gas giants. Apart from the six atmospheric parameters, we include the planet radius (Rp) as another model parameter. We apply our analysis to a particular target, Barnard's Star b candidate super-Earth[1], although our conclusions are generally planet-independent.RetrievalWe built a grid of ~300,000 synthetic reflected-light spectra for a range of possible atmospheric configurations. The spectra were computed at phase angle α=0º (that is, with the exoplanet fully illuminated). The wavelength interval under study is 500-900 nm and the spectral resolution, R~125-225. The multiple-scattering radiative-transfer problem was solved with a previously validated code[2].Observations were simulated by adding wavelength-independent noise at S/N=10. Three atmospheric configurations were considered to simulate observations and carry out retrievals: a cloud-free one (τc=0.05), one with a thin-cloud (τc=1.0) and one with a thick-cloud (τc=20.0). We developed an MCMC-based retrieval package achieving a continuous sampling of the parameter space by interpolating within the pre-computed grid of spectra.The retrievals were carried out for cases where the planet radius was either known (and hence there were only 6 free parameters) or completely unconstrained (7 free parameters). We also analysed intermediate scenarios in which estimates of Rp with different uncertainties were assumed.ResultsThe retrievals of atmospheric properties degrade as the uncertainties in the value of Rp increase. Indeed, the correlations between model parameters triggered by adding Rp as a free parameter make it challenging to distinguish between cloudy- and cloud-free atmospheres. Fig. 1 shows that, even if the atmosphere contains a thick-cloud, the evidence for the cloud disappears as the uncertainties in Rp grow. When the planet radius is a priori unconstrained, the retrieval of τc shows a nearly-flat posterior probability distribution. This indicates that the evidence is equal for both thick clouds or cloud-free atmospheres. On the other hand, if Rp is known we can generally distinguish between cloudy or cloud-free atmospheres in all of the scenarios analysed in this work. The retrieval results for other parameters such as the methane abundance also improve if the planet radius is known.Fig. 1 also shows that a priori estimates on the value of Rp improve the retrievals. This result encourages the development of synergies between direct-imaging and other techniques in order to reduce the uncertainties in the mass and radius of long-period exoplanets.Besides, we find that, if Rp is completely unconstrained, direct-imaging observations can constrain its value to within a factor of ~2 for all the cases explored. This might help start addressing the bulk composition of an exoplanet.Several works have addressed the possible science return of future direct-imaging observations[3]-[5]. Since the exoplanets observed in direct-imaging will generally lack a measurement of Rp, we conclude that this parameter will play an important role in the retrievals and therefore should be included in this type of retrieval exercises.References[1] Ribas et al. (2018), Nature, 563, 365[2] García Muñoz & Mills (2015), A&A, 573, A72[3] Lupu et al. (2016), AJ, 152, 217[4] Nayak et al. (2017), PASP, 129, 973[5] Damiano & Hu (2019), AJ, 159, 175

We report the discovery and characterization of a small planet, TOI-1408 c, on a 2.2 day orbit located interior to a previously known hot Jupiter, TOI-1408 b ( P = 4.42 days, M = 1.86 ± 0.02 M Jup , R = 2.4 ± 0.5 R Jup ) that exhibits grazing transits. The two planets are near 2:1 period commensurability, resulting in significant transit timing variations (TTVs) for both planets and transit duration variations for the inner planet. The TTV amplitude for TOI-1408 c is 15% of the planet’s orbital period, marking the largest TTV amplitude relative to the orbital period measured to date. Photodynamical modeling of ground-based radial velocity (RV) observations and transit light curves obtained with the Transiting Exoplanet Survey Satellite and ground-based facilities leads to an inner planet radius of 2.22 ± 0.06 R ⊕ and mass of 7.6 ± 0.2 M ⊕ that locates the planet into the sub-Neptune regime. The proximity to the 2:1 period commensurability leads to the libration of the resonant argument of the inner planet. The RV measurements support the existence of a third body with an orbital period of several thousand days. This discovery places the system among the rare systems featuring a hot Jupiter accompanied by an inner low-mass planet.

PLATO (PLAnetary Transits and Oscillations) mission is a space-based optical multi-camera photometer mission of the European Space Agency (ESA) to identify and characterize exoplanets and their hosting stars using two main techniques: planetary transit and asteroseismology. Selected(1) as the M3 (third Medium class mission) of the ESA 2015-2025 Cosmic Vision program, PLATO is scheduled to launch end of 2026 and designed for 4 years of nominal observation. The PLATO spacecraft is composed of a Service Module and a Payload Module. The Service Module comprises all the conventional spacecraft subsystems and the sun shield with attached solar arrays. The Payload Module consists of a highly stable optical bench, equipped with 26 optical cameras covering a global field of view of > 2232deg(2). The PLATO spacecraft data is complemented by ground-based observations and processed by a dedicated Science Ground Segment. We describe the mission and spacecraft architecture and provide a view of the current status of development.