We review the key observations and theories relevant to the origin and evolution of the Galilean satellites. Key observations include: the potentially undifferentiated nature of Callisto; the increasing ice fraction with semi-major axis; the present-day existence of the Laplace resonance; the potential resurfacing of Ganymede mid-way through its evolution; and the metal-enriched nature of Jupiter’s envelope. The most widely accepted theory for the formation of the satellites is the so-called “starved disk” model, although newer alternatives including decretion disks and pebble accretion have also been proposed. Models that allow slow satellite formation in a cold disk are preferred, based on the density progression and Callisto’s apparent differentiation state. Major model uncertainties include the angular momentum distribution of the material infalling to the circumplanetary disk, the source of the solids, and the thermal and viscosity structure of the disk. We identify six outstanding questions, some of which will be answered by JUICE, Europa Clipper and Tianwen-4. A major difficulty in answering some questions is overprinting of primordial characteristics by later events.

In protoplanetary disks, the water snowline marks the location where ice-rich pebbles sublimate, releasing silicate grains and water vapor. These processes can trigger pile-ups of solids, making the water snowline a promising site for forming planetesimals. However, previous studies exploring the pile-up conditions typically employ 1D, vertically-averaged and isothermal assumptions. In this work, we investigate how a 2D flow pattern and realistic temperature structure affect the pile-up of pebbles at the snowline and how latent heat effects can leave observational imprints. We perform 2D (R-Z) multifluid hydrodynamic simulations, tracking chemically heterogeneous pebbles and the released vapor. With a recent-developed phase change module, the mass transfer and latent heat exchange during ice sublimation are calculated self-consistently. The temperature is calculated by a two-stream radiation transfer method under various opacities and stellar luminosity. We find that vapor injection at the snowline drives a previously unrecognized outflow, leading to a pile-up of ice outside the snowline. Vapor injection also decreases the headwind velocity in the pile-up, promoting planetesimal formation and pebble accretion. In active disks, we identify a water-cycle: after ice sublimates in the hotter midplane, vapor recondenses onto pebbles in the upper, cooler layers, which settle back to the midplane. This cycle promotes ice-trapping at snowline. Latent heat exchange flattens the temperature gradient across the snowline, broadening the width while reducing the peak solid-to-gas ratio of pile-ups. Due to the water cycle, active disks are more conducive to planetesimal formation than passive disks. The significant temperature dip ( 40K) caused by latent heat cooling manifests as an intensity dip in the dust continuum, presenting a new channel to identify the water snowline in outbursting systems.

The size distribution of trans-Neptunian objects (TNOs) in the Kuiper Belt provides crucial insights into the formation and evolution of the outer Solar System. Recent observational surveys, including the Outer Solar System Origins Survey (OSSOS++), have revealed that dynamically cold and hot TNO populations exhibit similar size distributions for dimmer objects (H-r > 5), which are consistent with planetesimal formation by streaming instability (SI). However, the hot population contains a significantly larger number of massive bodies, including several dwarf planets. In this study, we investigate the role of pebble accretion in shaping the size distribution of hot TNOs, after their formation in the primordial disk (PB) between 20 and 30 au and before these bodies were dynamically implanted into their current orbits by a migrating Neptune. We find that pebble accretion grows the most massive bodies only, consistent with the flattening of the distribution brightwards of H-r = 5. All results point to a correlation (degeneracy) between the pebble aerodynamic size and the intensity of the gas motions. Nevertheless, accretion from an inward-drifting stream of pebbles is unlikely, as it puts extreme demands on the mass budget of pebbles. In particular, the masses of the cold classicals are too low to trigger pebble accretion. Accretion in an environment where pebbles are entrained, as believed to be the case in rings seen with the Atacama Large Millimeter Array (ALMA), is preferable. Combining the constraints obtained from this study with ALMA imagery morphology fitting reveals a typical pebble aerodynamic size of tau(s) similar to 10(-2), a turbulent diffusivity parameter alpha(D) similar to 10(-3), and a total accreted pebble mass of similar to 10 m(circle plus) in the primordial belt. Those TNOs formed through significant pebble accretion with masses exceeding similar to 10(-4) m(circle plus) are likely to satisfy the International Astronomical Union's "round shape" criterion for dwarf planets.

Kepler-221 is a G-type star hosting four planets. In this system, planets b, c, and e are in (or near) a 6:3:1 three-body resonance even though the planets' period ratios show significant departures from exact two-body commensurability. Importantly, the intermediate planet d is not part of the resonance chain. To reach this resonance configuration, we propose a scenario in which there were originally five planets in the system in a chain of first-order resonances. After disk dispersal, the resonance chain became unstable and two planets quickly merged to become the current planet d. In addition, the b/c/e three-body resonance was re-established. We run N-body simulations using REBOUND to investigate the parameter space under which this scenario can operate. We find that our envisioned scenario is possible when certain conditions are met. First, the reformation of the three-body resonance after planet merging requires convergent migration between planets b and c. Second, as has previously pointed out, an efficient damping mechanism must operate to power the expansion of the b/c/e system. We find that planet d plays a crucial role during the orbital expansion phase due to destabilizing encounters of a three-body resonance between c, d, and e. A successful orbital expansion phase puts constraints on the planet properties in the Kepler-221 system including the planet mass ratios and the tidal quality factors for the planets. Our model can also be applied to other planet systems in resonance, such as Kepler-402 and K2-138.

In the past two decades, transit surveys have revealed a class of planets with thick atmospheres—sub-Neptunes—that must have completed their accretion in protoplanet disks. When planets form in the gaseous disk, the gravitational interaction with the disk gas drives their migration and results in the trapping of neighboring planets in mean motion resonances, though these resonances can later be broken when the damping effects of disk gas or planetesimals wane. It is widely accepted that the outer solar system gas-giant planets originally formed in a resonant chain, which was later disrupted by dynamical instabilities. Here, we explore whether the early formation of the terrestrial planets in a resonance chain (including Theia) can evolve to the present configuration. Using N -body simulations, we demonstrate that the giant planet instability would also have destabilized the terrestrial resonance chain, triggering Moon-forming giant impacts in 20%–50% of our simulated systems, dependent on the initial resonance architecture. After the instability, the eccentricity and inclination of the simulated planets match their present-day values. Under the proposed scenario, the current period ratio of 3.05 between Mars and Venus—devoid of any special significance in traditional late-formation models—naturally arises as a relic of the former resonance chain.

Planet formation encompasses processes that span a remarkable 40 magnitudes in mass, ranging from collisions between micron-sized grains inherit from the ISM to accretion of gas by giant planets. The planet formation process takes place in the interior of dusty disks, which offer us only limited observational constraints. Historically, the two main paradigms describing planet formation are the disk instability and core accretion models. In the former giant planets condense directly form a disk massive enough to fragments under its self-gravity. In contrast, the core accretion model follows a bottom-up approach driven by solids. This model includes the direct growth phase, where pebble and possibly planetesimal sized bodies form by surface-driven processes. Alternatively, planetesimal bodies can emerge from instabilities in the solid population, leading to the formation of clumps that ultimately collapse. The later growth phases of the core accretion model involve the creation of a protoplanet out of the planetesimal population, which then accrete smaller bodies|planetesimals or pebbles|before ultimately accreting freely the gas of the disk. In this Chapter I will review these formation mechanisms, aiming, where possible, to build an intuitive understanding from elementary physical principles.

The characterization of super-Earth- to Neptune-sized exoplanets relies heavily on our understanding of their formation and evolution. In this study, we link a model of planet formation by pebble accretion to the planets’ long-term observational properties by calculating the interior evolution, starting from the dissipation of the protoplanetary disk. We investigate the evolution of the interior structure in 5–20 M⊕ planets, accounting for silicate redistribution caused by convective mixing, rainout (condensation and settling), and mass loss. Specifically, we have followed the fate of the hot silicate vapor that remained in the planet’s envelope after planet formation as the planet cools. We find that disk dissipation is followed by a rapid contraction of the envelope from the Hill or Bondi radius to about one-tenth of that size within 10 Myr. Subsequent cooling leads to substantial growth of the planetary core through silicate rainout accompanied by inflated radii, in comparison to the standard models of planets that formed with core-envelope structure. We examined the dependence of rainout on the planet’s envelope mass, on the distance from its host star, on its silicate mass, and on the atmospheric opacity. We find that the population of planets that formed with polluted envelopes can be roughly divided into three groups based on the mass of their gas envelopes: bare rocky cores that have shed their envelopes, super-Earth planets with a core-envelope structure, and Neptune-like planets with diluted cores that undergo gradual rainout. For polluted planets that formed with envelope masses below 0.4 M⊕, we anticipate that the inflation of the planet’s radius caused by rainout will enhance the mass loss by a factor of 2–8 compared to planets with unpolluted envelopes. Our model bridges the gap between the predicted composition gradients in massive planets and the core-envelope structure in smaller planets.

Context. Clouds are ubiquitous in exoplanets' atmospheres and play an important role in setting the opacity and chemical inventory of the atmosphere. Understanding clouds is a critical step in interpreting exoplanets' spectroscopic data. Aims. The aim is to model the multispecies nature of clouds in atmospheric retrieval studies. To this end, we developed ExoLyn - a 1D cloud model that balances physical consistency with computational efficiency. Methods. ExoLyn solves the transport equation of cloud particles and vapor under cloud condensation rates that are self-consistently calculated from thermodynamics. Exolyn is a standalone, open source package capable of being combined with optool to calculate solid opacities and with petitRADTRANS to generate transmission or emission spectra. Results. With ExoLyn we find that the compositional structure of clouds in hot Jupiter planets' atmospheres is layered with a cloud dominated by magnesiumsilicates on top of an iron cloud. This finding is consistent with more complex cloud formation models but can be obtained with ExoLyn in only a few seconds. The composition of the cloud particles can be constrained from the spectrum, for example, MgSiO3 and Mg2SiO4 components give rise to an absorption feature at 8-10 mu m. We investigate the dependence of the cloud structure on the bulk elemental composition of the planet and find that SiO2-dominated clouds form on metal-rich planets and Fe clouds with a strong extinction effect form on C-rich planets. Conclusions. Designed toward maximum flexibility, ExoLyn can also be used in retrieval analysis of sub-Neptunes and self-luminous planets. The efficiency of ExoLyn opens the possibility of joint retrieval of exoplanets' gas and cloud components.

. Introduction:In the traditional core accretion scenario, a planet grows by the subsequent accretion of a solid core and a gaseous envelope [3]. However, the accretion of these solids generates a large amount of heat, which can easily vaporize incoming pebbles and fractured planetesimals before the core has grown massive [1,4,5]. This naturally leads to the formation of planets with polluted envelopes that are characterized by different interior conditions and that follow an altered evolutionary pathway. In this series of papers [1,2+forthcoming], we develop new analytical and numerical models to describe the formation and evolution of polluted planets and link emerging trends in their formation to observations of planetary systems.2. Formation of polluted planets: Fig. 1. A sketch of the four potential evolutionary phases of a polluted planet.We find that envelope pollution substantially alters the structure of proto-planets in a number of ways and we suggest that their evolution can be described by four distinct phases, sketched in Fig. 1.(I). In the first phase of direct core growth, the envelope is still cold enough for solids to reach the central core. As the planet's internal temperatures rise, an increasing fraction of the accreting solids sublimates and is absorbed in the envelope. This slows down the growth of the core until it halts completely when all incoming solids become vaporized and remain in the envelope. We find that in the case of pebble accretion, this limits the size of the central cores to ‹ 1-2 M⊕, depending on conditions.(II). We refer to the second phase as that of envelope growth, as this is where all the accreted solids end up after direct core growth ends. In our analytical model, we assume that it mixes efficiently with the nebular gas but we relax this assumption in a forthcoming numerical work. Regardless, we find that polluted interiors become very hot and dense due to a higher mean molecular weight, lower adiabatic index and smaller core. Interior temperatures can already reach values in excess of 104 K at only a few Earth masses. Traditional models use the critical core mass as a criterion to identify the transition to runaway gas accretion but this term becomes a meaningless in planets that do not grow their cores beyond a certain size. We therefore suggest the critical metal mass (Mz,crit) as an equivalent criterion to supercede it. It is defined as the total mass in solids (core + vapor) that a planet needs to accrete in order to reach runaway growth. We derive the first expression for this mass:where κrcb is the opacity at the radiative-convective boundary, d is the planet's semi-major axis, Tvap is the vaporization temperature of the solids, is their accretion rate and Mc is the mass of the central core. Planets that form beyond the ice-line accrete a larger fraction of volatile materials and therefore form smaller cores with material that is characterized by lower vaporization temperatures. Both these effects reduce the critical metal mass compared to the inner disk where super-Earths and mini-Neptunes are more resistant to runaway gas accretion. (III). If a planet stops accreting solids before it reaches runaway accretion and while the disk is still present, it enters a phase of embedded cooling. This naturally happens in pebble accretion when a planet reaches the pebble isolation mass and begins to perturb the surrounding disk. The continued inward drift of tiny dust allows the planet to maintain a high opacity, limiting the pace of cooling. Besides this, the dilution of the interior can counteract the contraction of the envelope and further limit nebular accretion, although this requires the interior to remain compositionally mixed. We suggest that a combination of these effects can help explain why Uranus and Neptune did not reach runaway gas accretion, even if their solids flux dried up while the disk was still present.(IV). Finally, the proto-planetary disk will dissipate and the planets eventually enter phase IV of isolated cooling. In traditional models, this is mainly associated with contraction and potential mass loss. We suggest that in the case of a polluted planet, the cooling will eventually lead to the rainout of the vapor interior and generate a second phase of (indirect) core growth after several Gyr. While the process of photo-evaporation should not be altered by this, we find that it makes internal energy release an ineffective mass-loss mechanism. This is because most of the energy is only liberated late in the planet's evolution after substantial contraction, when mass loss from winds is far less efficient.2.1 Opacity in pebble accretionFig. 2. Trends in the critical metal mass from the opacity of gas, dust and pebbles.We model the opacity during pebble accretion in a forthcoming work with a combination of molecular, dust and pebble contributions. We find that pebbles can effectively reduce the dust abundance through sweep-up, but only in the early stages when nebular gas accretion is outpaced by the pebble flux. Near the onset of runaway accretion, the opacity displays a dichotomy between the hot inner disk where molecular opacity dominates and the outer disk where dust obscures the envelopes. The result is an opacity valley around 1-3 AU that translates to an equivalent minimum in the critical metal mass at the same location (see Fig. 2), which can help explain the abundance of warm Jupiters in this region.Acknowledgements:This work has benefited from discussions at the ISSI Ice Giants Meetings in Bern 2019 & 2020. Marc Brouwers acknowledges the support of a Royal Society Studentship, RG 160509.References:[1] Brouwers, M. G., Vazan, A., & Ormel, C. W. 2018, A&A, 611, A65[2] Brouwers, M. G. & Ormel, C. W. 2020, A&A, 634, A15[3] Pollack, J. B., Hubickyj, O., Bodenheimer, P., et al. 1996, Icarus, 124, 62[4] Mordasini, C., Mollière, P., Dittkrist, K.-M., Jin, S., & Alibert, Y. 2015, International Journal of Astrobiology, 14, 201[5] Valletta, C. & Helled, R. 2019, ApJ, 871, 127

Protoplanets formed by core accretion can become massive enough to accrete gas from the disk they are born in. If theplanetary proto-atmosphere exceeds a critical mass, runaway gas accretion starts and the planetary atmosphere collapses into a gasgiant. In recent years, many close-in super-Earths have been observed which raises the question on how they avoided becoming hotJupiters. We investigate the recycling hypothesis as a possible mechanism to avoid the collapse of the atmosphere.We use three-dimensional radiation-hydrodynamics to simulate the formation of proto-atmosphere in the local frame aroundthe planet. In post-processing we use tracer particles to calculate the shape of the atmosphere and determine the non-uniform recyclingtimescale in a quantitative manner. Our simulations converge to a quasi-steady state where the velocity field of the gas does not change anymore. For theparameter space explored, a = 0.1 au, m_c ∈ [1, 2, 5, 10] M_Earth, we find that recycling of the atmosphere counteracts the collapse bypreventing the gas from cooling efficiently.

Planets, embedded in their natal discs, harbour hot envelopes. When pebbles are accreted by these planets, the contained volatile components may sublimate, enriching the envelope and potentially changing its thermodynamical properties. However, the envelopes of embedded planets actively exchange material with the disc, which would limit the buildup of a vapour-rich atmosphere. To properly investigate these processes, we have developed a new phase change module to treat the sublimation process with hydrodynamical simultions. Combined with the recently developed multi-dust fluid approach, we conduct 2D self-consistent hydrodynamic simulations to study how pebble sublimation influences the water content of super-Earths and sub-Neptunes. We find the extent and the amount of vapour that a planet is able to hold on to is determined by the relative size of the sublimation front and the atmosphere. When the sublimation front lies far inside the atmosphere, vapour tends to be locked deep in the atmosphere and keeps accumulating through a positive feedback mechanism. On the other hand, when the sublimation front exceeds the (bound) atmosphere, the ice component of incoming pebbles can be fully recycled and the vapour content reaches a low, steady value. Low disc temperature, small planet mass and high pebble flux (omitting accretion heating by pebbles) render the planet atmosphere vapour-rich while the reverse changes render it vapour-poor. The phase change module introduced here can in future studies also be employed to model the chemical composition of the gas in the vicinity of accreting planets and around snowlines.

ABSTRACT In the past decade, ALMA observations have revealed that a large fraction of protoplanetary discs contains rings in the dust continuum. These rings are the locations where pebbles accumulate, which is beneficial for planetesimal formation and subsequent planet assembly. We investigate the viability of planet formation inside ALMA rings in which pebbles are trapped by either a Gaussian-shaped pressure bump or by the strong dust backreaction. Planetesimals form at the mid-plane of the ring via streaming instability. By conducting N-body simulations, we study the growth of these planetesimals by collisional mergers and pebble accretion. Thanks to the high concentration of pebbles in the ring, the growth of planetesimals by pebble accretion becomes efficient as soon as they are born. We find that type-I planet migration plays a decisive role in the evolution of rings and planets. For discs where planets can migrate inward from the ring, a steady state is reached where the ring spawns ∼20 M⊕ planetary cores as long as rings are fed with materials from the outer disc. The ring acts as a long-lived planet factory and it can explain the ‘fine-tuned’ optical depths of the observed dust rings in the DSHARP large program. In contrast, in the absence of a planet removal mechanism (migration), a single massive planet will form and destroy the ring. A wide and massive planetesimals belt will be left at the location of the planet-forming ring. Planet formation in rings may explain the mature planetary systems observed inside debris discs.

Our understanding of planet formation has been rapidly evolving in recent years. The classical planet formation theory, developed when the only known planetary system was our own Solar System, has been revised to account for the observed diversity of the exoplanetary systems. At the same time, the increasing observational capabilities of the young stars and their surrounding disks bring new constraints on the planet formation process. In this chapter, we summarize the new information derived from the exoplanets population and the circumstellar disks observations. We present the new developments in planet formation theory, from dust evolution to the growth of planetary cores by accretion of planetesimals, pebbles, and gas. We review the state-of-the-art models for the formation of diverse planetary systems, including the population synthesis approach which is necessary to compare theoretical model outcomes to the exoplanet population. We emphasize that the planet formation process may not be spatially uniform in the disk and there are preferential locations for the formation of planetesimals and planets. Outside of these locations, a significant fraction of solids is not growing past the pebble-sizes. The reservoir of pebbles plays an important role in the growth of planetary cores in the pebble accretion process. The timescale of the emergence of massive planetary cores is an important aspect of the present models and it is likely that the cores within one disk form at different times. In addition, there is growing evidence that the first planetary cores start forming early, during the circumstellar disk buildup process.

ABSTRACT Due to their aerodynamical coupling with gas, pebbles in protoplanetary discs can drift over large distances to support planet growth in the inner disc. In the past decade, this pebble accretion has been studied extensively for aerodynamically small pebbles (Stokes number St < 1). However, accretion can also operate in the St > 1 mode, e.g. when planetesimals collisionally fragment to smaller bodies or when the primordial gas disc disperses. This work aims to extend the study of pebble accretion to these aerodynamically loosely coupled particles. We integrate the pebble’s equation of motion, accounting for gas drag, stellar and planetary gravity, in the mid-plane of a laminar disc. The accretion probability (ϵ) is calculated as function of Stokes number, disc pressure gradient index, planet mass, and eccentricity. We find that for Stokes number above unity ϵ(St) first rises, due to lower drift and aided by a large atmospheric capture radius, until it reaches a plateau where the efficiency approaches 100 per cent. At high St, the plateau region terminates as particles become trapped in resonance. These results are well described by a semi-analytical ‘kick-and-drift’ model and we also provide fully analytical prescriptions for ϵ. We apply our model to the accretion of ${\sim }30\, \mu \mathrm{m}$ dust particles in a dispersing protoplanetary and secondary (CO-rich) debris disc. It shows that such physically small particles are mainly accreted as aerodynamically large Stokes number pebbles during the debris disc phase. Earth-mass planets may obtain ∼25 per cent of their heavy elements through this late accretion phase.

The theory of Type~I migration has been widely used in many studies. Transiting multi-planet systems offer us the opportunity to examine the consistency between observation and theory, especially for those systems harbouring planets in Mean Motion Resonance (MMR). The displacement these resonant pairs show from exact commensurability provides us with information on their migration and eccentricity-damping histories. Here, we adopt a probabilistic approach, characterized by two distributions -- appropriate for either the resonant or non-resonant planets -- to fit the observed planet period ratio distribution. With the Markov chain Monte Carlo (MCMC) method, we find that ${\approx}15\%$ of exoplanets are in first order ($j+1{:}j$) MMRs, the ratio of eccentricity-to-semi-major axis damping is too high to allow overstable librations and that the results are by-and-large consistent with Type-I migration theory. In addition, our modeling finds that a small fraction of resonant pairs is captured into resonance during migration, implying late planet formation (gas-poor). Most of the resonant pairs park themselves at the migration barrier, indicating early planet formation (gas-rich). Furthermore, after improving the criterion on two-body resonant trapping, we obtain an upper limit of the disc surface density at the time the planets are locked in resonance.

Binary solar system objects are common and range from satellite systems with very large mass ratios $M_1/M_2$ to mass ratios very close to unity. A well-known example of a binary is the Pluto-Charon system. With Charon only eight times less massive than Pluto the question arises as for many other systems, why the mass-ratio is still close to unity. There is much evidence that (binary) planet(esimal) formation happened early, when the protoplanetary gas disk was still around. It is likely that (some of) these binaries grew up together subject to pebble accretion. Here we focus on the question of how the mass arriving in the gravitational influence zone of the binary during pebble accretion, is distributed over the binary components. Does the accretion through time lead to a converging mass ratio, or to a diverging mass ratio? We numerically integrate pebble paths in the same well-known fashion as for a single mass subject to pebble accretion and track what the efficiency of accretion is for the two separate binary components, compared to a single body with the same mass. These numerical simulations are done for a range of binary mass-ratios, mutual separations, Stokes numbers and two orbital distances, 2.5 and 39 au. We find that in the limit where pebbles start to spiral around the primary (this holds for relatively large pebbles), the pebble preferentially collides with the secondary, causing the mass ratio to converge towards unity on Myr timescales. In this regime the total sweep-up efficiency can lower to half that of a pebble-accreting single body because pebbles that are thrown out of the system, after close encounters with the system. The results show that systems such as Pluto-Charon and other larger equal mass binaries could well have co-accreted by means of pebble accretion in the disk phase without producing binaries with highly diverging mass-ratios.

Context. Protoplanetary disks, the birthplaces of planets, commonly feature bright rings and dark gaps in both continuum and line emission maps. Accreting planets interact with the disk, not only through gravity, but also by changing the local irradiation and elemental abundances, which are essential ingredients for disk chemistry. Aims. We propose that giant planet accretion can leave chemical footprints in the gas local to the planet, which potentially leads to the spatial coincidence of molecular emissions with the planet in the ALMA observations. Methods. Through 2D multi-fluid hydrodynamical simulations in Athena++ with built-in sublimation, we simulated the process of an accreting planet locally heating up its vicinity, opening a gas gap in the disk, and creating the conditions for C-photochemistry. Results. An accreting planet located outside the methane snowline can render the surrounding gas hot enough to sublimate the C-rich organics off pebbles before they are accreted by the planet. This locally elevates the disk gas-phase C/O ratio, providing a potential explanation for the C 2 H line-emission rings observed with ALMA. In particular, our findings provide an explanation for the MWC 480 disk, where previous work identified a statistically significant spatial coincidence of line-emission rings inside a continuum gap. Conclusions. Our findings present a novel view of linking the gas accretion of giant planets and their natal disks through the chemistry signals. This model demonstrates that giant planets can actively shape their forming chemical environment, moving beyond the traditional understanding of the direct mapping of primordial disk chemistry onto planets.

Observations and statistical studies have shown that giant planets are rare around M dwarfs compared with Sun-like stars. The formation mechanism of these extreme systems has remained under debate for decades. With the help of the TESS mission and ground-based follow-up observations, we report the discovery of TOI-4201b, the most massive and densest hot Jupiter around an M dwarf known so far with a radius of 1.22 ± 0.04 R J and a mass of 2.48 ± 0.09 M J , about 5 times heavier than most other giant planets around M dwarfs. It also has the highest planet-to-star mass ratio ( q ∼ 4 × 10 −3 ) among such systems. The host star is an early M dwarf with a mass of 0.61 ± 0.02 M ⊙ and a radius of 0.63 ± 0.02 R ⊙ . It has significant supersolar iron abundance ([Fe/H] = 0.52 ± 0.08 dex). However, interior structure modeling suggests that its planet TOI-4201b is metal-poor, which challenges the classical core-accretion correlation of stellar−planet metallicity, unless the planet is inflated by additional energy sources. Building on the detection of this planet, we compare the stellar metallicity distribution of four planetary groups: hot/warm Jupiters around G/M dwarfs. We find that hot/warm Jupiters show a similar metallicity dependence around G-type stars. For M-dwarf host stars, the occurrence of hot Jupiters shows a much stronger correlation with iron abundance, while warm Jupiters display a weaker preference, indicating possible different formation histories.

Sub-Neptune planets formed in the protoplanetary disk accreted hydrogen-helium (H,He) envelopes. Planet formation models of sub-Neptunes formed by pebble accretion result in small rocky cores surrounded by polluted H,He envelopes where most of the rock (silicate) is in vapor form at the end of the formation phase. This vapor is expected to condense and rain-out as the planet cools. In this Letter we examine the timescale for the rainout and its effect on the thermal evolution. We calculate the thermal and structural evolution of a 10 Earth masses planet formed by pebble accretion, taking into account material redistribution from silicate rainout (condensation and settling) and from convective mixing. We find that the duration of the rainout in sub-Neptunes is on Gyr timescale and varies with envelope mass: planets with envelopes below 0.75 Earth mass rainout into a core-envelope structure in less than 1 Gyr, while planets in excess of 0.75 Earth mass of H,He preserve some of their envelope pollution for billions of years. The energy released by the rainout inflates the radius with respect to planets that start out from a plain core-envelope structure. This inflation would result in estimates of the H,He contents of observed exoplanets based on the standard core-envelope structure to be too high.We identify a number of planets in the exoplanet census where rainout may operate, which would result in their H,He contents to be overestimated by up to a factor two. Future accurate age measurements by the PLATO mission may allow the identification of planets formed with polluted envelopes.

Recently, the Molecules with ALMA at Planet-forming Scales (MAPS) ALMA Large Program reported a high number of line-emission substructures coincident with dust rings and gaps in the continuum emission, suggesting a causal link between these axisymmetric line-emission and dust-continuum substructures. To test the robustness of the claimed correlation, we compare the observed spatial overlap fraction in substructures with that from the null hypothesis, in which the overlap is assumed to arise from the random placement of line-emission substructures. Our results reveal that there is no statistically significant evidence for a universal correlation between line-emission and continuum substructures, questioning the frequently made link between continuum rings and pressure bumps. The analysis also clearly identifies outliers. The chemical rings and the dust gaps in MWC 480 appear to be strongly correlated (>4 σ ), and the gaps in the CO isotopologues tend to moderately (∼3 σ ) correlate with dust rings.