EXOA1
Hence, in this session, we aim to bring together recent results and studies performed by observers, modelers, and experimentalists, and a combination of them to open discussions about the pathways that the community needs to follow to understand the exoplanetary variety fully and promote and inspire the collaboration between teams with different expertise. In particular, this session welcomes any abstract related to the following topics:
(1) groundbreaking discoveries of planets and systems of special interest (due to peculiar physical properties/orbital architectures, amenable targets for atmospheric investigations, etc.);
(2) cutting-edge measurements of exoplanet properties (tidal distortions, spin rates, and angles) and first tentative detections of satellites or rings;
(3) contributions to the general picture of exoplanets (precise measurements of radii, masses and internal planetary compositions, observed/theoretical populations, occurrences, etc.);
(4) demographics studies out of detection surveys from the ground- and/or space-based observatories;
(5) synergies between different techniques for comprehensive exoplanet characterization (photometry, low and high-resolution spectroscopy, radial velocities, transit timing variations, radio observations, etc.);
(6) new tools and software developments for exoplanet searches and characterization (Artificial Intelligence tools, new methodologies and computational architectures, open-source initiatives, etc.)
Session assets
The large number and diversity of exoplanets found in the last decades has raised fundamental questions about their formation and evolution mechanisms.
Many of these are giant planets orbiting very close to their host star, hence their name Hot Jupiters, despite the low occurrence rate of such population (Stevenson 1982; Mayor et al. 2011, Fressin et al. 2013, Santerne et al. 2016). This is mainly driven by the inherent observational bias in their detection, given their small periods, which results in a dominant population of Hot Jupiters which is about 75% of the detected exoplanets. Most of such discoveries arose from wide-field ground-based observatories (Bakos et al. 2006, Mandushev et al. 2005, Udalski et al. 2002), from the Kepler and K2 space missions (Borucki 2017, Howell et al. 2014) and, more recently, from the Transiting Exoplanet Survey Satellite mission (TESS, Ricker et al. 2015).
Giant planets are thought to form via two different mechanisms: either by accreting its mass from the proto-planetary disk (Pollack et al. 1996), or by gravitational instabilities, in which the proto-planetary disk fragments into bound clumps to form a planet (Cameron 1978). Both mechanisms predict low formation rate at close-in orbits. Close to the star gas conditions could prevent the formation of bound clumps (Rafikov 2005), while at the same time such conditions may prevent the formation of sufficient large cores to accrete gas from the proto-planetary disk (Schlichting 2014).
Therefore, the observed population of Hot Jupiters strongly suggests that they might have formed further out, in regions where the conditions for core accretion and gravitational instabilities are more favorable, and migrated inwards into their currently close-in circular orbits.
Possible migration mechanisms consider interactions with the disc (e.g. Walsh et al. 2011) and tidal migration produced by changes in the orbit, which can be induced by planet-planet scattering (Rasio & Ford 1996) or cyclic and/or chaotic secular interactions (Kozai 1962, Lidov 1962, Wu & Lithwick 2011).
Therefore, the observed population of Hot Jupiters strongly suggests that they might have formed further out, in regions where the conditions for core accretion and gravitational instabilities are more favorable, and migrated inwards into their currently close-in circular orbits.
Both mechanisms predict different orbital parameters for the planet population. While disc migration predict mainly circular orbits aligned with the stellar spin, high eccentricity migration predict a wide distribution of eccentriticies and obliquities (Chatterjee et al. 2008).
In this context, the estimation of the bulk structures and atmospheric parameters of such population of exoplanets could provide crucial information about their formation locations in the disc and possible migration paths (Madhusudhan et al. 2014, Mollière et al. 2022).
However, the characterization of the formation history of Hot Jupiters, widely defined as planets with periods smaller than 10 days, can be problematic, as their proximity to their host star may induce radiative and tidal interactions which can affect the interpretation of the origins of the observed population (e.g. Albrecht et al. 2012).
Therefore, giant planets within the snow-line, but at farther distances from their host star, represent a unique opportunity to study possible formation and migration mechanisms in giant planets, as they may not be affected by their proximity to their host star, allowing a more direct comparison of their orbital properties with formation and migration models.
Warm giants, planets broadly defined as planets orbiting their host star with periods longer than 10 days, remained elusive to ground-based transit searches, mainly due to the constrains imposed by the daily cycle. While some detections were possible with the Kepler+K2 missions (i.e. Huang et al. 2016), only with TESS it has been possible to systematically characterize them, either through the detection of periodic transiting signals or single transiters (i.e. Brahm et al. 2019, Gill et al. 2020).
By complementing transit observations with radial velocity (RV) measurements, it is possible to provide a detailed characterization of the dynamical properties of the planetary system.
The Warm gIaNts with TESS (WINE) collaboration is a dedicated survey to identify, confirm and characterize warm giants using TESS data and ground-based photometric and spectroscopic follow-up facilities, with the main goal of building a giant planets database to constrain theories of planetary formation and evolution. We have detected more than 20 Warm giants (Kossakowski et al. 2019, Jordán et al. 2020, Brahm et al. 2020, Schleker et al. 2020, Hobson et al. 2021, Trifonov et al. 2021 and 2023, Brahm, et al. 2023, Hobson et al. 2023, Jones et al. 2024) and we have been following-up dozens of exoplanet candidates. Among our discoveries we highlight the detection of several high eccentricity exoplanets, which are ideal candidates to measure their obliquity using the Rossiter-Maclaughlin effect, and determine the 3D exoplanetary orbits and properly constrain planetary migration theories. The figures show the orbital period-eccentricity diagram, the orbital period-planet radius diagram and the planetary mass-radius diagram where we highlight the discoveries made by our collaboration.
How to cite: Tala Pinto, M., Brahm, R., Jones, M., and Jordán, A.: The WINE collaboration: a Warm Giants planets survey, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-578, https://doi.org/10.5194/epsc2024-578, 2024.
JWST has opened a completely new spectral range in the mid IR for the study of exoplanetary systems which promise to offer unprecedented results both in terms of characterization but also discovery space.
Two observing modes in MIRI are of particular interest : 1) the coronagraphic mode installed in the imager made of four independent coronagraphic masks paired with dedicated filters covering the ~10 to ~20 microns window, and 2) the Medium Resolution Spectrograph which has a much wider spectral range from 5 to 28 microns but without coronagraphic capacity.
We will first describe these two modes and their respective performances, based on actual observations or from simulations. Then we will present the first results from the exoplanet Guaranteed Time Observations team of the MIRI consortium, focusing for instance on the multiplanet system HR8799 (see figure 1 and 2), and highlight some results obtained from General Observers programs. Finally we will discuss the broad interest of MIRI for science cases like cold planets at large orbital separations as well as protoplanets.
Figure 1 : Raw coronagraphic and reference star subtracted images of the multiple planet systems HR8799 obtained with MIRI in 4 different filters at mid IR wavelenghts.
Figure 2: spectral energy distribution of planet HR8799 b compared to 2 atmospheric models fitted on near IR and mid IR photometric data points.
How to cite: Boccaletti, A., Mâlin, M., Rouan, D., Perrot, C., Lagage, P.-O., Lagrange, A.-M., Charnay, B., and Pierre, B.: Direct imaging of exoplanets with JWST/MIRI, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1187, https://doi.org/10.5194/epsc2024-1187, 2024.
We present a population study of 20 transiting exoplanets, currently the largest sample of exoplanet atmospheres observed to date in the spectral range near-UV to near-IR. Archive data obtained with Hubble STIS and WFC3 instruments are uniformly analysed using the Iraclis pipeline and a suite of spectral retrieval plugins integrated in TauREx 3.1, including the stellar activity plugin ASteRA and the cloud microphysics plugin YunMa. Most exoplanets included in the sample were observed multiple times with the STIS G430L and/or G750L gratings (Fig. 1), allowing for the study of temporal variability caused by changes in the exoplanet atmosphere, due to e.g. cloud/haze variability and/or stellar magnetic activity.
Fig. 1: The spectral data resulting from our data analysis, colour-coded based on the observation timestamps of each STIS visit. The dashed lines centred at 0.59 μm and 0.77 μm indicate where we expect the peaks of the absorption features of sodium and potassium to appear respectively.
Our analysis reveals significant differences among the observations obtained with STIS at various epochs for about half of the planets included in the sample. These data show amplitudes and patterns that are suggestive of time-varying stellar surface heterogeneities that cause contamination in the exoplanetary spectra. At a population level, our analysis reveals that for planets on which stellar activity exerts the most significant influence on their spectrum, even employing the simplest stellar contamination model is sufficient to align water abundance values to the average of the whole sample.
To statistically assess the preference for an active model over a quiet star model, we compute the Bayes Factor resulting from the difference in Bayesian evidence between two retrieval models: one model representing a primary atmosphere with water vapour as the sole opacity source, assuming a uniform stellar photosphere, and the other incorporating the same atmospheric model but considering the star to have a heterogeneous surface. Despite the active star model being statistically penalised for its increased complexity compared to the simpler model, it remains significantly favoured across many spectral cases. This result emphasises the critical nature of accounting for the potential presence of unocculted stellar spots and faculae when performing spectral retrievals in the optical regime. Among the total number of targets investigated, nearly half of them exhibit a strong preference towards the model incorporating an active star, while only one planet strongly favours a quiet star.
Furthermore, we establish new, complementary metrics to assess the extent of stellar activity contamination; the Stellar Activity Distance metric (SAD) and the Stellar Activity Temporal metric (SAT). The SAD, by corroborating the findings of the Bayes Factor analysis, indicates that an active star model better describes the optical observations in approximately half of the planetary spectra. Similarly, the SAT results reveal a bimodal distribution, indicating that spectral variations in the visible regime within this limited planet sample are either minimal or moderately significant.
We analyse in more detail three case studies: WASP-6 b, WASP-39 b and HAT-P-11 b. Our findings align with previous literature results for the first two planets, for which we find an active stellar environment and a quiet star respectively. However, HAT-P-11, expected to be active, is found to show no significant signs of stellar contamination. Across the population, stellar activity contaminates up to half of the studied exoplanet atmospheres albeit at varying extents (Table 1).
Table 1: For each planet, we report the preference (✓) or rejection (×) for stellar contamination based on the metrics outlined in our investigation. The 15 planets with consistent sets of observations are ordered based on their ranking within the sample over all of the metrics from most likely to be contaminated (WASP-6 b) to least likely (WASP-39 b). Colour coding reflects the consensus among metrics: green signifies unanimous or nearly unanimous indications of significant activity; yellow denotes uncertain outcomes, with some metrics leaning towards stellar activity and others against it; red indicates a preference for an inactive star. The 5 planets that do not have consistent data sets are reported separately below the main table. They are not included in the ranking but are colour coded in the same way. For an explanation of the data sets considered in each Case, see Table 2.
Table 2: Data sets utilised to define each Case number. Each STIS visit indicated here is colour-coded in Fig. 1.
Overall, our findings demonstrate the significant role that stellar contamination may have in all exoplanet spectra observations: therefore, comprehending, modelling, and correcting for the impact of stellar activity is important for a complete characterisation of exoplanet atmospheres. This issue becomes particularly relevant with the ongoing JWST observations of small planets, statistically more likely to orbit active stars. Additionally, addressing stellar activity becomes even more pertinent in anticipation of the extensive chemical surveys to be conducted by next-generation space telescopes in the coming decade. For these reasons, targeted campaigns aimed at investigating the activity of exoplanets' stellar hosts through time would significantly contribute to the success of the exoplanet field.
How to cite: Saba, A., Thompson, A., Yip, K. H. (., Ma, S., Tsiaras, A., Al-Refaie, A., and Tinetti, G.: A Population Analysis of 20 Exoplanets Observed from the Optical to the Near-infrared Wavelengths with HST: Evidence for Widespread Stellar Contamination, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-171, https://doi.org/10.5194/epsc2024-171, 2024.
Introduction
The detection and confirmation of long-period transiting planets present significant challenges due to the infrequency of their transits. Such planets are crucial for enhancing our understanding of exoplanet formation and evolutionary dynamics. TOI-4409 b, initially identified by TESS, represents a prototypical case for studying these phenomena.
This study aims to confirm and further characterise TOI-4409 b, a long-period planetary candidate. Utilising a combination of photometric and radial velocity (RV) data, the research focuses on verifying the planet's existence and describing its orbital and physical properties.
Method
The comprehensive analysis incorporated photometric data from TESS, ASTEP, CHAT, and OMES lightcurves, along with RV measurements from HARPS, FEROS, and PFS. In total, the study analysed eight lightcurves from TESS, four from ASTEP, one from CHAT, and three from OMES. RV data spanned from 2018 to 2023, as shown in Figure 1, offering a longitudinal perspective essential for detecting and confirming the planetary signals of TOI-4409 b.
The TESS spacecraft provided the initial detection of TOI-4409 b through its wide-field, high-precision photometry, which is particularly adept at identifying transiting exoplanets. Subsequent observations by the ASTEP telescope, strategically located in Antarctica, leveraged the prolonged periods of darkness and stable atmospheric conditions to observe additional transits. This was critical for capturing the infrequent transits of TOI-4409 b. Lightcurves from CHAT and OMES telescopes further supplemented these observations, enhancing the data's robustness through additional transit captures and minimising gaps in the observational data.
Radial velocity data were obtained using three high-precision spectrometers: HARPS, FEROS, and PFS. These instruments are renowned for their sensitivity in detecting the subtle stellar wobbles induced by orbiting planets, which are crucial for confirming the planetary nature of transit signals. The combined RV dataset enabled a comprehensive analysis of the star's motion, aiding in the verification of TOI-4409 b and suggesting the presence of additional planetary bodies through observed anomalies in the RV signals.
Results
The integrated analysis confirmed TOI-4409 b as an exoplanet with a planetary radius of 7.28±0.20 Earth radii, orbiting a low-mass main sequence star with an effective temperature of approximately 4945±10 K and a stellar radius of 0.720±0.018 solar radii. The orbital period of TOI-4409 b was determined to be 92.4±0.8 days. Two of the processed light curves are shown in Figure 2.
Consistent transit signals across multiple datasets, combined with observed Transit Timing Variations (TTVs), reinforced the candidate’s robustness. These TTVs (showcased in Figure 3), in conjunction with the radial velocity data, suggested the gravitational influence of a second planet in a 20-day orbit around the same star.
The integration of diverse observational methods and data from multiple instruments was pivotal in confirming the planetary status of TOI-4409 b. This approach not only facilitated a thorough characterisation of the planet but also underscored the potential complexities of its orbital dynamics, hinted at by the TTV and RV analyses.
The evidence for a second planet, suggested by anomalies in TTV measurements, points to a potentially rich multi-planetary system, warranting further investigation. This discovery highlights the importance of multi-modal and multi-instrumental data analysis in the field of exoplanet research, where single-method approaches may not suffice due to the complex nature of planetary systems and the limitations of individual observational techniques.
Conclusion
The successful confirmation of TOI-4409 b as a long-period exoplanet and the indication of a secondary nearby planet demonstrates the critical role of global cooperation and technological integration in exoplanetary science. The unique capabilities of the Antarctica-based ASTEP telescope, combined with the extensive data provided by TESS, CHAT, OMES, HARPS, FEROS, and PFS, exemplify how diverse observational assets can come together to enhance our understanding of the universe.
Continued monitoring and future investigations into the TOI-4409 system are essential. They will not only confirm the characteristics and existence of the second suggested planet but will also provide deeper insights into the dynamics and potential habitability of planets in long-period orbits. This study sets a precedent for future research into similar exoplanetary systems, using refined methods and international collaboration to further our understanding of planetary formation and evolution.
Figure 1: All unique photometric observation midtimes, spanning the years 2018 to 2023. This distribution of dates enabled a robust TTV analysis.
Figure 2: TESS (left) lightcurve and ASTEP (right) model fit result, showcasing the transiting nature and the TTV of the signals analysed. All 16 light curves were fitted using the same method.
Figure 3: All the unique transit midtimes of our observations (in BJD - 2458000), alongside their calculated Transit Timing Variations.
How to cite: Reller, P. and the UCL / ESA / Université Côte d’Azur / Max-Planck-Institut für Astronomie / Millennium Institute for Astrophysics & other groups: Confirmation of the long-period Exoplanet TOI-4409 b, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-908, https://doi.org/10.5194/epsc2024-908, 2024.
Introduction
Rocky airless bodies in the solar system, e.g., Mercury, the Moon, and many asteroids, host a rough silicate regolith layer. The roughness of the surfaces influences the reflectance and emission phase curves of these bodies. At small phase angles, the “opposition effect” increases the reflected radiation and “thermal beaming” leads to higher emitted radiation compared to smooth surfaces (see Figure 1). At higher phase angles, surface roughness leads to “shadowing”, the measured thermal emission is smaller than predicted by the equilibrium model for a smooth surface. The rocky exoplanets LHS 3844 b [1], GJ 1252 b [2], TRAPPIST-1 b [3], and TRAPPIST-1 c [4] are roughly Earth-sized (see Figure 2), likely have no atmospheres, and their surfaces largely remain mysterious. We previously developed and validated a thermal roughness model for airless solar system bodies (the Moon and Mercury) [5]. In this work, we adapt our model to airless exoplanets and analyze how surface roughness and albedo affect the infrared phase curves. The resulting model can then impose constraints on the surface structure of the exoplanets. We will use this approach to analyze upcoming phase curve measurements of LHS 3844 b in the James Webb Space Telescope (JWST) General Observer Program, cycle 2, ID 4008 [6].
Figure 1. The Moon's emission phase curve demonstrates the thermal beaming effect. Adopted from [6].
Figure 2. Radii, temperatures, and stellar distances of the exoplanets studied in this work compared to Mercury, Earth, and the Moon. The radii are not to scale to the distances. The color denotes the dayside surface temperature.
Figure 3. Comparison of the different host star radii and surface temperatures.
Methods
We model the reflectance component of the radiation with the Hapke model [7] and fix all parameters except for the spectral single-scattering albedo. We derive two sets of albedo values from lunar satellite and returned sample data. The “mare” set models a darker surface and represents a more basaltic composition. The “highland” set resembles a comparatively brighter surface and more feldspathic composition. We use the thermal roughness model from [5] for the emission component, which is mainly controlled by the albedo and the roughness parameter (average roughness angle). We simulate disk-resolved radiances of the planets for different phase angles and integrate them over the disk to retrieve the phase curves. We focus on the wavelength region of the JWST NIRSpec G395H/F290LP disperser/filter combination (2.7 μm-5.2 μm).
Results
Figure 4 shows the reflectance and emission phase curves for LHS 3844 b for different roughness angles and albedos. The reflected component is smaller than the emitted component in the infrared wavelength region but not negligible. The thermal beaming and shadowing effects are clearly visible. The opposition effect is most likely not detectable due to the occultation of the planet during the secondary eclipse. The simulations of all four exoplanets show that the albedo influences the amplitude of the phase curve, and the surface roughness alters the shape of the phase curve. Figure 5 shows that the two parameters appear interchangeable at small phase angles (close to the secondary eclipse). However, considering the entire phase curve allows albedo and surface roughness to be separated. Figure 6 shows the combined phase curves for the other three exoplanets. The phase curve of GJ 1252 b is like that of LHS 3844 b and the parameter influences can likely be separated. TRAPPIST-1 b and TRAPPIST-1 c have much lower relative flux densities, which makes parameter retrieval difficult.
Figure 4. Reflectance and emission phase curves for LHS 3844 b. The upper row corresponds to the shorter wavelength region of NIRSpec, and the lower row corresponds to the longer wavelength region. The columns represent the two albedo sets.
Figure 5. Combined reflectance and emission phase curves for LHS 3844 b for different surface roughness angles and albedos.
Figure 6. Combined phase curves for GJ 1252, TRAPPIST-1 b and TRAPPIST-1 c.
Conclusion
The surface roughness and albedo effects are detectable and most likely separable for LHS 3844 b and GJ 1252 b with NIRSpec on JWST. This would allow the retrieval of one or both parameters from phase curve measurements, which can provide information about the nature of extrasolar planetary surfaces. Additional albedo constraints can further improve the retrieval scheme. Due to extensive space weathering, the closely orbiting exoplanets likely have a low albedo, similar to Mercury or the maria of the Moon. The two planets of the TRAPPIST-1 system are less suited for characterization with NIRSpec. Still, MIRI measurements might enable the parameter retrieval.
References
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[6] S. Zieba, L. Kreidberg, R. Hu, C. Morley, K. Wohlfarth, M. Tenthoff, and C. Wöhler. JWST Proposal 4008. https://www.stsci.edu/jwst/science-execution/program-information?id=4008. (2023)
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How to cite: Tenthoff, M., Wohlfarth, K., Wöhler, C., Zieba, S., and Kreidberg, L.: Reflectance and Emission Modelling of Airless Exoplanets, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-649, https://doi.org/10.5194/epsc2024-649, 2024.