EXOA12
unconstrained despite the huge progress of the field since the discovery of the first planets in 1995. Understanding planet formation processes is key to improve our knowledge on the origin of planetary systems and our Solar System. The study of exoplanets at young ages (below the age of the Hyades ~600 Myr) is fundamental to determine the timescale of the formation of planets and constrain the dynamical models of planet evolution.
Many exoplanets orbiting young stars have been reported over the past few years in young stellar forming regions, open clusters and moving groups have well-constrained ages and well-determined physical properties (mass, radius, density, temperature), essential to refine evolutionary models based on orbital migration or photo-evaporation. The availability of space-borne missions (Kepler, TESS, Gaia, JWST), high-resolution and high-precision spectrographs (e.g. HARPS, CARMENES, ESPRESSO, SPIRou, etc...), and (sub)-millimetre facilities (e.g. JCMT, SMA, ALMA) have revolutionised the field over the past years.
The goals of this session are to offer a review on the observational techniques and theoretical modelling efforts in the field of young exoplanets, highlight the latest discoveries, and propose a way forward to gain a better understanding of how planetary systems form and evolve. We will divide the session with dedicated contributions on direct imaging, radial velocity, transit timing variation, and transit spectroscopy as well as computer modelling.
K2-33b is the second youngest known transiting planet, orbiting a pre-main-sequence M dwarf member of the Upper Scorpius association at an age of 5-10 Myr. With a radius of about 5 Rearth and an orbital period of 5.42 days (David et al. 2016, Mann et al. 2016), it represents a unique opportunity to probe the physical state of a close-in planet near the epoch at which migration, atmospheric contraction, and volatile loss are expected to be most active.
We aim to place the first dynamical constraint on the mass of K2-33b, which is essential for interpreting its bulk density, atmospheric scale height, and the competing physical explanations for its chromatic transit depth. We collected 113 high-resolution spectra with the ESPRESSO spectrograph on the ESO VLT across two observing campaigns spanning 2023 and 2025, complemented by photometry from K2, TESS, and LCO. We jointly modelled the transit photometry and the radial velocity time series using Gaussian processes to account for the large activity-induced variability of this rapidly rotating young M dwarf.
We constrain the mass of K2-33b to an upper limit of 12 Mearth at the 3-sigma level. Notably, for a planet with a radius as large as ~5 Rearth, a higher mass would have been expected from the mass–radius relations of the older exoplanet population (Chen & Kipping 2017), which would in principle have rendered the Keplerian signal more accessible to detection. The fact that the mass falls below this threshold is therefore itself significant: it implies that K2-33b is less massive than its radius would suggest, confirming that the planet is genuinely inflated relative to its older counterparts of comparable mass. This result is consistent with recent findings for other very young close-in planets, including AU Mic b (~20 Myr, Mallorquín et al. 2024), HIP67522b (~17 Myr, Thao et al. 2024), and V1298 Tau b and e (~20 Myr; Livingston et al. 2026). In the context of the young planet population, the extended radius of K2-33b at only 5 Myr support a scenario in which sub-Neptunes descend from initially larger, low-density progenitors whose atmospheres are eroded on short timescales by the intense high-energy irradiation of their host stars.
How to cite: Lodieu, N., Mallorquin, M., and Bejar, V.: Dynamical mass constraints on K2-33 b: a 5 Myr-old possible precursor to sub-Neptunes, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1353, https://doi.org/10.5194/epsc2026-1353, 2026.
In the era of JWST, high-precision transmission spectra are now available for a growing number of sub-Neptunes, shedding light on the present-day atmospheric compositions of these planets. Many of these spectra have been flat, with a degeneracy between possible high mean molecular weight or cloudy atmospheres. There remain key questions surrounding the sub-Neptune regime, including the formation and evolution processes that sculpt the population. Observations of young sub-Neptunes are an important avenue for understanding such processes, with distinct diagnostics of water-rich versus water-poor formation scenarios. The larger scale heights of young sub-Neptunes yield larger signals in transmission spectroscopy than mature sub-Neptunes, providing confident detections and avoiding degeneracies from flat spectra. The recent observations of V1298 Tau b revealed strong detections of key molecules in its thick hydrogen-rich atmosphere, including CO2, CO, H2O, and CH4, the latter at significantly lower abundance than predicted by equilibrium chemistry. High internal temperatures (Tint ~ 600 K) and strong vertical mixing were invoked to explain these findings. However, this extreme Tint is difficult to reconcile with predictions of planetary evolution models and the planet’s energy budget; the planet’s bulk measurements and constraints on atmospheric escape require a low internal temperature consistent with boil-off. This therefore points to a problem with the assumptions behind either atmospheric or interior models. A common simplifying assumption in radiative transfer modelling is the plane-parallel set-up, valid for atmospheric scale heights much less than the planetary radius – a condition that is violated for the highly extended atmospheres of young sub-Neptunes. This could have significant implications for interpreting the thermal structure and chemistry of their atmospheres and interiors. We explore the effect of relaxing this assumption on the inferred Tint and predicted methane abundances for young sub-Neptunes, demonstrated using V1298 Tau b. Using a radiative transfer model configured in both plane-parallel and spherical coordinates we compare the resulting atmospheric pressure-temperature profiles for equivalent internal luminosities. We proceed to consider the implications of using spherical coordinates on models of atmospheric chemistry, by modifying the photochemical code VULCAN, and self-consistently including our updated temperature structures. Due to geometric dilution, a spherical atmosphere has a higher temperature at deeper pressures for fixed internal temperature compared to a plane-parallel one, suppressing the methane. Young sub-Neptunes are crucial windows into the origins of the most abundant planetary population and closely linked to the origin of the radius valley. Our results demonstrate the importance of challenging modelling assumptions when interpreting observations of young and low-gravity exoplanets with JWST.
How to cite: Rigby, F. and Owen, J.: Beyond Plane-Parallel: Modelling the Atmospheres of Young Sub-Neptunes in the JWST Era, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-537, https://doi.org/10.5194/epsc2026-537, 2026.
The scarcity of giant planets around M dwarfs raises important questions about the physical processes that regulate planet formation around the least massive stars. In this work, we investigate planet formation around very low-mass stars using N-body simulations that incorporate pebble accretion, gas accretion, planet-disk interactions, star-planet tidal interactions, and stellar evolution during the pre-main-sequence phase. We explore how disk properties influence planetary growth and migration by assuming a compact dust disk of 30 au.
Our simulations show that low-viscosity disks naturally favor the formation of close-in super-Earths, while the scarcity of larger planets around M dwarfs emerges as a natural outcome of the planet formation process. In cases where planet-planet collisions efficiently assemble cores of 2–3 Earth masses, close-in mini-Neptunes and Neptune-mass planets may form. Moreover, if planetary cores reach masses of ~5 Earth masses within the first ~1 Myr, cold giant planets with masses between Saturn and Jupiter can form at distances of 1–5 au, provided that the disks are massive (~10% of the stellar mass), weakly turbulent (αt = 10-4), and long-lived (10 Myr).
We further investigate the role of fragmentation in the formation of planetary cores by introducing an evolving pebble flux linked to different fragmentation velocities associated with distinct grain compositions across the disk. These results provide new insights into the diversity of planetary systems around M dwarfs and the conditions required for giant planet formation in the low-stellar-mass regime.
How to cite: Sanchez, M., Drążkowska, J., van der Marel, N., Lambrechts, M., and Mulders, G. D.: Forming giant planets around M dwarfs via pebble accretion: the role of fragmentation, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-226, https://doi.org/10.5194/epsc2026-226, 2026.
Coagulation of dust particles in protoplanetary disks is the first step on the journey to the formation of planets. The surface free energy (SFE) of the dust particles determines the effectiveness of particles sticking to each other after collision, as well as the critical collision velocity above which fragmentation will occur. Studies of SFE have focused on the simplest silicate, silica, usually at standard temperature and pressure. Previously reported values show orders of magnitude in difference, creating significant uncertainty in how planetesimals are forming. However, protoplanetary dust grains have a wide variety of compositions and temperatures, and a low-pressure environment lacking in water vapor. We perform molecular dynamics simulations using a ReaxFF-type potential to quantify the SFE minerals and ice at temperatures ranging from 30 to 700 K in both true vacuum along with water vapor. We find that the SFE drops by tens of percent with increasing temperature or shifting to more complex silicate compositions. More dramatically, we find that the values of the SFE in a vacuum are 2 orders of magnitude higher than those usually measured in terrestrial laboratories. We then quantify the unique role surface coverage of water vapor plays in drastically reducing the SFE, even at low coverages. Results confirm previous work that suggests that hydroxylation by monolayers of water produces this reduction in SFE in experiments. We suggest that SFEs for minerals and ices should be sampled as a function of the specific environment along the disk and provide these values for future models.
How to cite: Morrissey, L., Clouter-Gergen, B., Mac-Lowe, M., Verkercke, S., Ebel, D., Ericksson, L., and Pfeil, T.: Dust Collisions in Protoplanetary Disks: Atomic Modelling to Better Understand Surface Free Energies in Unique Environments, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-692, https://doi.org/10.5194/epsc2026-692, 2026.
A large amount of evidence accumulated over recent years has suggested that planet formation must start during the earliest phases of star formation, when the circumstellar disc is still embedded in its envelope, but how this occurs is still not clear. In the core accretion model, a critical step for planet formation is the formation of planetesimals. Yet, the dust growth to these km-sized solid aggregates is hindered by several growth barriers.
One of the most promising mechanisms to overcome these barriers is the streaming instability (SI), a drag instability caused by the aerodynamic coupling of gas and dust. However, the optimal conditions for strong dust clumping by SI are still debated. As an alternative, much effort has been devoted to the study of the formation of planetesimals via direct gravitational collapse in gravitationally unstable discs, with several works highlighting its potential efficiency in accumulating solids in dense aggregates.
The two aforementioned processes are usually investigated separately: in SI analyses, the self-gravity of the perturbations is neglected, while gravitational instability (GI) studies do not include some of the necessary ingredients for the onset of the SI. In this work, we aim to combine these phenomena to understand how the influence of the disc’s self-gravity modifies the classical SI. Since they occur on quite different scales, catching both processes in hydrodynamical simulations is very challenging. For this reason, we propose a local, linear stability analysis of a protoplanetary disc where gas and dust interact through both gravitational and drag forces. The background state is a radially drifting, differentially rotating flow, and the self-gravity of the perturbations is included.
We will discuss how our analysis contributes to understanding whether these combined mechanisms (SI and GI) are efficient enough to explain planetesimal formation in young, embedded discs.
How to cite: Bucci, F., Rice, K., and Nipoti, C.: Early Planetesimal Formation from Combined Streaming and Gravitational Instabilities, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-573, https://doi.org/10.5194/epsc2026-573, 2026.
Disc instability remains a leading formation pathway for a subset of observed giant planets. In particular, it naturally accounts for giant planets at wide separations, around M-type stars, and in very young systems. Despite this, many aspects of the mechanism remain poorly understood, and the resulting planetary population is still uncertain. We have developed a comprehensive model that follows the formation and evolution of a star–disc system from the collapse of a molecular cloud core through disc dispersal and beyond. The model incorporates the possibility of disc fragmentation as well as the subsequent evolution of any resulting fragments. We apply this framework to conduct a population synthesis within the disc-instability paradigm (DIPSY). We present the results of the baseline population and discuss the emerging distribution of companions around different stellar types. Our findings indicate that, although fragmentation—i.e., the formation of gravitationally bound clumps within the disc, a necessary step for planet formation via disc instability—occurs in only a minority of systems, it often produces at least one companion when it does occur. The resulting population (Figure) spans a wide mass range, from planetary to stellar companions. These outcomes provide new insights for both theoretical studies of planet formation (e.g., hydrodynamic simulations) and future observational searches for companions. Overall, DIPSY advances our understanding of planet formation independently of any particular formation pathway.
How to cite: Schib, O., Mordasini, C., Emsenhuber, A., and Helled, R.: DIPSY: A new Disc Instability Population SYnthesis, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-979, https://doi.org/10.5194/epsc2026-979, 2026.
The elemental composition of exoplanetary atmospheres provides a unique opportunity to investigate the formation and evolutionary history of giant planets. While carbon- and oxygen-bearing species have traditionally been used to constrain formation pathways, sulphur chemistry remains comparatively unexplored despite its sensitivity to planetary accretion history, formation location, and atmospheric evolution. In this work, we investigate the potential of atmospheric sulphur abundances as tracers of planet formation histories across a diverse population of giant exoplanets.
Using synthetic planet populations generated with the SPONCHpop framework at University College London together with atmospheric forward modelling and retrievals using TauREx 3, we investigate the detectability of atmospheric sulphur abundances across diverse formation environments, migration pathways, and planetary temperatures. Our study includes both highly irradiated hot Jupiters and colder giant planets spanning a wide range of masses, sulphur abundances, and orbital separations (See the picture down below). We assess the recoverability of sulphur-bearing species under realistic observational conditions expected from the upcoming Ariel mission.
Our first results show that the uncertainty in sulphur abundance strongly decreases with increasing signal-to-noise ratio (SNR), with precisions of ~0.2 dex typically achievable at SNR ≈ 20–40, while ~0.1 dex generally requires SNR ≳ 40–60 depending on planetary properties. Sulphur retrieval is most successful for hotter, larger planets with extended atmospheres, whereas cooler planets require substantially higher SNR to reach comparable precision.
Our results demonstrate that sulphur abundances can provide complementary constraints on giant planet formation and migration histories beyond traditional diagnostics such as metallicity and C/O ratios. In particular, sulphur-bearing species may help distinguish planets formed in chemically distinct regions of the protoplanetary disc and provide new insight into volatile accretion and atmospheric evolution.
Figure: Selected planets (coloured and open circles; 100 out of ∼ 2 500 objects) from the viscous irradiated disc model with pratio = 0.5. Grey
circles show the full simulated population. Open circles denote planets with S/H = 0 (Somerville-Thomas et al. 2026). The dashed horizontal line
indicates the mass of Jupiter, whose orbital distance is marked by the Jupiter symbol (Wong et al. 2004).).
How to cite: Lehtmets, A., Kama, M., Thomas-Sommerville, A., Yip, G., and Ran, J.: SPONCHpop: Tracing Planet Formation Histories with Atmospheric Sulphur, Europlanet Science Congress 2026, The Hague, The Netherlands, 7–11 Sep 2026, EPSC2026-1120, https://doi.org/10.5194/epsc2026-1120, 2026.
