Camilla Danielski

The rapidly growing census of exoplanets now includes over a thousand multi-planet systems, revealing architectures that span more than four orders of magnitude in mass and five in orbital scale. Explaining the origin of this striking diversity remains a central challenge for planet formation theory, compounded by observational biases that limit access to Solar System-like regimes. Here we introduce architecture-agnostic metrics to uniformly analyse four independently defined samples of multi-planet systems, uncovering unifying patterns across their diversity. We identify a clear bimodality in system masses, dividing known architectures into two distinct families. The lower-mass family, predominantly hosted by subsolar-to-solar stars, exhibits tightly packed configurations with mass distributions consistent with the ‘peas-in-a-pod’ pattern and modal dynamical excitation comparable with that of the Solar System. By contrast, the high-mass family forms in metal-rich environments around solar-to-supersolar stars, displays gravitationally compact architectures and is an order of magnitude dynamically hotter than both the Solar System and the lower-mass counterparts. Their high level of dynamical excitation points to violent evolutionary pathways driven by global instabilities and planet–planet scattering. Together, these results show that the architectures of planetary systems are born from the combined influence of formation processes and host star properties. Signatures of these roots are preserved across the post-formation dynamical evolution that reshaped most known multi-planet systems.

Context . The characterisation of exoplanetary systems depends on the accurate determination of host star parameters. The Ariel mission will probe the atmospheres of a statistically significant sample of exoplanets, and so requires a precise characterisation of the stellar properties well before its launch in 2031. The homogeneous determination of stellar parameters for Ariel will enable both the optimisation of the final target list and set roots for a reliable interpretation of the formation and evolution of planetary systems. Such a homogeneous characterisation has thus far only been carried out for the cool ( T _eff ≲ 7000 K) host stars among the Ariel target candidates. Aims . We present a uniform determination of fundamental stellar parameters for 18 hot stars ( T _eff ≳ 7000 K) in the Tier 1 candidate list of the Ariel mission candidate sample. Methods . We adopted an iterative spectro-trigonometric approach optimised for high-temperature stars ( T _eff ≳ 7000 K). Highresolution spectra were analysed using the ZEEMAN code with χ ² minimisation, combining model fits to metal and Balmer lines. Surface gravity was refined using photometry-based radii and masses from stellar evolutionary tracks. Results . We derived effective temperatures, surface gravities, projected rotational velocities, microturbulent velocities, overall metal-licities, iron abundances, stellar masses, and radii for our sample of 18 hot stars. Our results were validated against a set of benchmark stars previously presented in the literature, confirming that our methods yield results consistent with previous studies. This ensures an internally consistent parameter scale and maintains continuity across the transition from cool to hot stars within the Ariel sample. Conclusions . The derived parameters provide an internally consistent basis for studying the link between stellar properties and planetary characteristics in intermediate-mass stars (1.5 < M < 2.32 M _⊙ ). Building on our previous work on FGK host stars, we show that correlations between stellar mass, metallicity, and planetary radii also extend to early-type stars, and stellar properties influence the architecture of multi-planet systems.

VHS 1256 b was the first planetary-mass companion to be observed with the James Webb Space Telescope’s Mid-Infrared Instrument (JWST/MIRI) using the Medium-Resolution Spectrometer (MRS). The MRS provides high-quality integral-field spectral data in the mid-infrared (IR) wavelengths from 4.9–18 μ m. This data set serves as a testbed for applying cross-correlation techniques to characterize exoplanet atmospheres. We implement the so-called molecular mapping approach, which consists of performing a spectral cross-correlation between each spectral pixel and atmospheric model templates. We compare these results with those obtained from cross-correlation of the extracted spectrum. Using a self-consistent Exo-REM atmospheric model grid, we constrain the temperature, surface gravity, C/O ratio, and metallicity, finding values consistent with those obtained from other analysis methods. We detect CO (S/N ∼ 25) and H ₂ O (S/N ∼ 76), with tentative detections of NH ₃ and CH ₄ (S/N ∼ 3). We test cross-correlation to measure trace-species abundances and isotopic ratios. We measure a volume mixing ratio (VMR) of [ NH 3 ] = − 5.7 3 − 0.14 + 0.15 and an isotopic ratio 12 C / 13 C = 77 . 8 − 10 + 13 , both consistent with free-chemistry retrievals. The derived NH ₃ VMR, combined with the measured temperature and radius, is consistent with VHS 1256 b having a mass above the deuterium-burning limit. These results demonstrate the diagnostic power of mid-IR spectroscopy and highlight cross-correlation as a robust method for characterizing directly imaged exoplanets, even in future higher-contrast regimes where spectral extraction becomes challenging. Future MIRI MRS observations across a wider range of temperatures and masses will further expand our understanding of planetary atmospheric chemistry.

Context . Planetary formation might occur at different stages of the stellar evolution. In particular, theoretical studies have been focusing on addressing whether formation can occur around compact binaries that evolved beyond the main sequence. Formation of second-generation planets has been tested in circumbinary disks formed by the ejection of stellar material from binaries composed of either a main-sequence star and a white dwarf or a double white dwarf (DWD). In the latter case, formation appears to be common and to create sub-Neptunian, Neptunian, and giant planets that can migrate within 1 au of the central binary. Nevertheless, the orbital stability of these systems has yet to be studied. Aims . We investigate whether planetary systems that formed around compact DWDs in nonresonant and resonant configurations can be dynamically stable over a timescale of a few million years. Methods . We performed N -body simulations of circumbinary multiplanetary systems that initially hosted two, three, four or five planets by employing a hybrid symplectic integrator made specifically for circumbinary systems. We recorded the catastrophic events that planetary systems experience and employed a variety of metrics, such as orbital spacing, variation in the center of mass, and normalized angular momentum deficit, to explore the outcomes of their long-term evolution. Furthermore, we evaluated the potential for detecting these systems in their final configurations with the Laser Interferometer Space Antenna mission by measuring the overall amplitude shift in the gravitational-wave frequency induced by their planets. Results . Our results show that planets orbiting DWDs can be stable over the studied timescales. While planetary systems starting with two planets are more likely to survive unaltered, planetary systems with three, four, or five planets experience catastrophic events that cause them to lose some of their original planets. At the end of their phases of dynamical instability, the five-planet population is completely disrupted, and most of the systems host only two surviving planets. This increases the number of two-planet systems by 122% with respect to their initial abundance and creates a single-planet population of 7% of all systems. Additionally, the four-planet population decreases by 56.1% and the three-planet population by 22.5%. Finally, 7.7% of the systems are disrupted; they initially hosted more than two planets. Most of the systems that in the end only host a single planet are potential candidates for the Laser Interferometer Space Antenna mission. A handful of multiplanet systems might be detected. Finally, we provide a formula for estimating the amplitude shift in the gravitational-wave frequency for multiplanet systems orbiting DWDs. Conclusions . Throughout our analysis, we highlight the importance of characterizing the system orbits and estimating their normalized angular momentum deficit in order to distinguish between the different dynamical scenarios presented above. Ultimately, second-generation systems might represent crucial targets for the Laser Interferometer Space Antenna mission because they reside within its observability range.

Aims . We update the status of the spectropolarimetric campaign dedicated to characterise the magnetic field properties of a sample of known exoplanet-hosting stars included in the current target list of the Ariel mission. The main aims are to inform observing strategies and subsequent analysis of the data of the Ariel mission, and to provide background information on the magnetic properties of the target and their variability on timescales of at least a few years. Methods . We analysed spectropolarimetric data collected for 15 G-M type stars with Neo-Narval, HARPSpol, and SPIRou to assess the detectability of the large-scale magnetic field. For three stars we reconstructed the magnetic field topology and its temporal evolution via Zeeman-Doppler imaging (ZDI). Such reconstructions were then used to perform three-dimensional magnetohydrodynamical simulations of the stellar wind and environment impinging on the hosted exoplanets. Results . We detected the magnetic field of six stars. Of these, we performed ZDI reconstructions for the first time of TOI-1860 and DS Tuc A, and for the second time of HD 63433, providing temporal information of its large-scale magnetic field. Consistently with previous results on young (~50–100 Myr) solar-like stars, the large-scale magnetic field is moderately strong (30–60 G on average) and complex, with a significant fraction of magnetic energy in the toroidal component and high-order poloidal components. From the simulations of the stellar wind, we found the orbit of TOI-1860 b to be almost completely sub-Alfvénic, the orbits of DS Tuc A b and HD 63433 d to be trans-Alfvénic, and the orbits of HD 63433 b and c to be super-Alfvénic. We obtained marginal detections of the magnetic field for TOI-836 and TOI-2076, and detections for TOI-1136, but the number of observations is not sufficient for magnetic mapping. Conclusions . A magnetic star-planet connection can occur for most of TOI-1860 b’s orbit. This can happen more sporadically for DS Tuc A b and HD 63433 c given the lower fraction of their orbit in the sub-Alfvénic regime. The orbit of HD 63433 c is nevertheless more sub-Alfvénic than previously simulated owing to the temporal evolution of the stellar magnetic field. For HD 63433 b and c, we expect the formation of a bow shock between the stellar wind and the planet despite the evolution of the stellar magnetic field.

We present JWST NIRCam coronagraphic observations of the HD 141569A circumstellar disk, obtained as part of the JWST Early Release Science program. The observations recover the multi-ringed structure seen in previous shorter-wavelength observations, but at filters centered on the ∼3 μ m water ice absorption feature and a complementary continuum region (F300M and F360M, respectively). The observations reveal apparent absorption between the F300M and F360M filters that decreases with radius, with a notable change around 200 au, between the innermost and outermost two rings. These results are consistent whether the data is reduced via deconvolution or through a forward-modeling approach. We demonstrate that these changes suggest a radial decrease in the water ice mass fraction by a factor of ∼3–10 and possibly a change in minimum grain size. We do not detect any point sources within the system and can exclude planetary companions 2 Jupiter masses and greater beyond 1 ″ radius (∼111 au). These observations and the subsequent analysis illustrate a robust pathway for using JWST/NIRCam to characterize the distribution of water ice in other circumstellar disks. We highlight some of the early lessons learned from this work that we hope will be useful for future circumstellar disk observation planning and analysis.

In exoplanet research, studies are increasingly focussed on identifying planets that similar in density and habitability potential to our planet, known as Earth analogs. As the number of known rocky exoplanets grows, parallel discussions have emerged on system architectures and galactic environments that may support life, drawing comparisons to our own planet. This has brought renewed attention to the concept of the Galactic habitable zone (GHZ) as a broader context for interpreting the diversity of planetary environments. This study is the first to use detailed chemical evolution models to investigate the impact of stellar migration, modelled through a parametric approach, on the GHZ. Our findings reveal that stellar migration significantly enhances the number of stars capable of hosting habitable planets in the outer Galactic regions, with an increase of up to a factor of five at 18 kpc relative to a baseline value of unity at 6 kpc. Furthermore, we have explored a novel scenario where the presence of gas giant planets increases the probability for the formation of terrestrial ones. We find that this increased probability is higher in the inner Galactic disc, but is also mitigated by stellar migration. In particular, at the present time, the number of FGK stars hosting terrestrial planets with minimum habitability conditions in the ring centred at 4 kpc is approximately 1.4 times higher than in scenarios where gas giants are assumed to hinder the formation and evolution of Earth-like planets. Without stellar migration, this factor increases to 1.5. Even larger ratios are predicted for terrestrial planets orbiting retired A stars, reaching 2.8 in models with stellar migration and 3.3 in models without it. In conclusion, this study shows that stellar migration predominantly influences the GHZ in the outer Galactic regions, while assuming a positive contribution from gas giants to terrestrial planet formation increases the number of stars capable of hosting habitable planets in the Galactic ring centred at 4 kpc.

In exoplanet research, the focus is increasingly on identifying Earth analogs, planets similar in density and habitability potential. As the number of rocky exoplanets grows, parallel discussions have emerged on system architectures and Galactic environments that may support life, drawing comparisons to our own Earth. This has brought renewed attention to the concept of the Galactic Habitable Zone (GHZ) as a broader context for interpreting the diversity of planetary environments. This study is the first to use detailed chemical evolution models to investigate the impact of stellar migration, modeled through a parametric approach, on the GHZ. Our findings reveal that stellar migration significantly enhances the number of stars capable of hosting habitable planets in the outer Galactic regions, with an increase of up to a factor of five at 18 kpc relative to a baseline value of unity at 6 kpc. Furthermore, we explore a novel scenario where the presence of gas giant planets increases the probability for the formation of terrestrial ones. We find that this increased probability is higher in the inner Galactic disc, but is also mitigated by stellar migration. In particular, at the present time, the number of FGK stars hosting terrestrial planets with minimum habitability conditions in the ring centered at 4 kpc is approximately 1.4 times higher than in scenarios where gas giants are assumed to hinder the formation and evolution of Earth-like planets. Without stellar migration, this factor increases to 1.5. Even larger ratios are predicted for terrestrial planets orbiting retired A stars, reaching 2.8 in models with stellar migration and 3.3 in models without it.

Context. The next mission dedicated to the study of planetary atmospheres is the Ariel space mission, planned for launch in 2029, which will observe a variety of planetary systems belonging to different classes around stars with spectral types from M to A. To optimise the scientific outcome of the mission, such stars need to be homogeneously characterised beforehand. Aims. In this work, we focus on a methodology based on spectral synthesis for the characterisation of FGK-type stars from the Ariel Tier 1 mission candidate sample (MCS) that exhibit fast rotation. In addition, we analyse 108 slow-rotating FGK-type stars, with either new observations or archival spectra available, consistently as in our previous work using the equivalent width (EW) analysis. Methods. To ensure consistency between our methods, we re-analysed a sample of FGK-type stars with the spectral synthesis method and compared it to our previous work. The results of our analysis show excellent agreement with the previous set of derived parameters. Results. We provide homogeneous effective temperature, surface gravity, metallicity, projected rotational velocity, and stellar mass for a sample of 36 fast rotators with the spectral synthesis technique, and we include 108 FGK-type dwarfs with the EW analysis. An additional 25 stars were analysed with the spectral synthesis method because their EW analysis did not converge on the final parameters. We computed their orbital parameters establishing whether they belong to the Galactic thin or thick discs. With the current set of stellar parameters, we almost double the analysed hosts in the Ariel MCS to 353 stars in total. Conclusions. Using our homogeneous set of stellar parameters, we studied the correlations between stellar and planetary properties for the Ariel MCS analysed so far. We confirmed a close relationship between stellar mass (up to 1.8 M_⊙) and giant planet radius, with more inflated planets at lower metallicity. We confirm that giant planets are more frequent around more metal-rich stars that belong to the thin disc, while lower-mass planets are also found in more metal-poor environments, and are more frequent than giant planets in the thick disc as also seen in other works in the literature.