Runaway Gas Accretion Research Articles

Dust rings trap protoplanets on eccentric orbits and get consumed by them

ABSTRACT We study the orbital evolution and mass growth of protoplanets with masses $M \in [0.1{\!-\!}8]$ M$_{{\oplus }}$ in the vicinity of a dusty ring, using three-dimensional numerical simulations with a two-fluid model and nested-meshes. We find two stable, eccentric orbits that lock the planet in the ring vicinity, thereby inhibiting its migration and allowing it to accrete dust from the ring. One of these orbits has an eccentricity comparable to the aspect ratio of the gaseous disc and has its periastron within the ring, enabling intermittent accretion during each pass. The other orbit has a smaller eccentricity and an apoastron slightly inside the ring. A planet locked at the outer orbit efficiently accretes from the ring and can reach the critical mass for runaway gas accretion on time-scales $\gtrsim 10^5$ yr (for a 10 M$_{{\oplus }}$ dust ring at 10 au), while a planet locked at the inner orbit has a slower growth and might not supersede the super-Earth stage over the disc lifetime. While in our runs a low-mass embryo forming within the ring eventually joins the outer orbit, it is likely that the path taken depends on the specific details of the ring. The trapping on the outer orbit arises from an intermittent, strong thermal force at each passage through the ring, where the accretion rate spikes. It is insensitive to uncertainties that plague models considering planets trapped on circular orbits in rings. It is highly robust and could allow a growing planet to follow an expanding ring over large distances.

The role of drag and gravity on dust concentration in a gravitationally unstable disc

ABSTRACT We carry out three-dimensional smoothed particle hydrodynamics simulations to study the role of gravitational and drag forces on the concentration of large dust grains (St > 1) in the spiral arms of gravitationally unstable protoplanetary discs, and the resulting implications for planet formation. We find that both drag and gravity play an important role in the evolution of large dust grains. If we include both, grains that would otherwise be partially decoupled will become well coupled and trace the spirals. For the dust grains most influenced by drag (with Stokes numbers near unity), the dust disc quickly becomes gravitationally unstable and rapidly forms clumps with masses between 0.15–6M⊕. A large fraction of clumps are below the threshold where runaway gas accretion can occur. However, if dust self-gravity is neglected, the dust is unable to form clumps, despite still becoming trapped in the gas spirals. When large dust grains are unable to feel either gas gravity or drag, the dust is unable to trace the gas spirals. Hence, full physics is needed to properly simulate dust in gravitationally unstable discs. Dust trapping of large grains in spiral arms of discs stable to gas fragmentation could explain planet formation in very young discs by a population of planetesimals formed due to the combined roles of drag and gravity in the earliest stages of a disc’s evolution. Furthermore, it highlights that gravitationally unstable discs are not just important for forming gas giants quickly, it can also rapidly form Earth mass bodies.

TESS Spots a Super-puff: The Remarkably Low Density of TOI-1420b

We present the discovery of TOI-1420b, an exceptionally low-density (ρ = 0.08 ± 0.02 g cm−3) transiting planet in a P = 6.96 days orbit around a late G-dwarf star. Using transit observations from TESS, LCOGT, Observatoire Privé du Mont, Whitin, Wendelstein, OAUV, Ca l’Ou, and KeplerCam, along with radial velocity observations from HARPS-N and NEID, we find that the planet has a radius of R p = 11.9 ± 0.3R ⊕ and a mass of M p = 25.1 ± 3.8M ⊕. TOI-1420b is the largest known planet with a mass less than 50M ⊕, indicating that it contains a sizeable envelope of hydrogen and helium. We determine TOI-1420b’s envelope mass fraction to be , suggesting that runaway gas accretion occurred when its core was at most four to five times the mass of the Earth. TOI-1420b is similar to the planet WASP-107b in mass, radius, density, and orbital period, so a comparison of these two systems may help reveal the origins of close-in low-density planets. With an atmospheric scale height of 1950 km, a transmission spectroscopy metric of 580, and a predicted Rossiter–McLaughlin amplitude of about 17 m s−1, TOI-1420b is an excellent target for future atmospheric and dynamical characterization.

In situ enrichment in heavy elements of hot Jupiters

Context. Radius and mass measurements of short-period giant planets reveal that many of these planets contain a large amount of heavy elements. Although the range of inferred metallicities is broad, planets with more than 100 M⊕ of heavy elements are not rare. This is in sharp contrast with the expectations of the conventional core-accretion model for the origin of giant planets. Aims. The proposed explanations for the heavy-element enrichment of giant planets fall short of explaining the most enriched planets. We look for additional processes that can explain the full envelope of inferred enrichments. Methods. We revisited the dynamics of pebbles and dust in the vicinity of giant planets using analytic estimates and published results on the profile of a gap opened by a giant planet, the radial velocity of the gas with respect to the planet, the Stokes number of particles in the different parts of the disk, and the consequent dust/gas ratio. Although our results are derived in the framework of a viscous α-disk, we also discuss the case of disks driven by angular momentum removal in magnetized winds. Results. When giant planets are far from the star, dust and pebbles are confined to a pressure bump at the outer edge of the planet-induced gap. When the planets reach the inner part of the disk (rp ≪ 2 au), dust instead penetrates into the gap together with the gas. The dust/gas ratio can be enhanced by more than an order of magnitude if the radial drift of dust is not impeded farther out by other barriers. Thus, hot planets undergoing runaway gas accretion can swallow a large amount of dust, acquiring ~100 M⊕ of heavy elements by the time they reach Jupiter masses. Conclusions. Whereas the gas accreted by giant planets in the outer disk is very dust-poor, that accreted by hot planets can be extremely dust-rich. Thus, provided that a large fraction of the atmosphere of hot Jupiters is accreted in situ, a large amount of dust can be accreted as well. We draw a distinction between this process and pebble accretion (i.e., the capture of dust without the accretion of gas), which is ineffective at small stellocentric radii, even for super-Earths. Giant planets farther out in the disk are extremely effective barriers against the flow of pebbles and dust across their gap. Saturn and Jupiter, after locking into a mutual mean motion resonance and reversing their migration, could have accreted small pebble debris.

The interplay between pebble and planetesimal accretion in population synthesis models and its role in giant planet formation

Context. In the core accretion scenario of planet formation, rocky cores grow by first accreting solids until they are massive enough to accrete gas. For giant planet formation, this means that a massive core must form within the lifetime of the gas disk. Inspired by observations of Solar System features such as the asteroid and Kuiper belts, the accretion of roughly kilometre-sized planetesimals is traditionally considered as the main accretion mechanism of solids but such models often result in longer planet formation timescales. The accretion of millimetre- to centimetre-sized pebbles, on the other hand, allows for rapid core growth within the disk lifetime. The two accretion mechanisms are typically discussed separately. Aims. We investigate the interplay between the two accretion processes in a disk containing both pebbles and planetesimals for planet formation in general and in the context of giant planet formation specifically. The goal is to disentangle and understand the fundamental interactions that arise in such hybrid pebble-planetesimal models laying the groundwork for informed analysis of future, more complex, simulations. Methods. We combined a simple model of pebble formation and accretion with a global model of planet formation which considers the accretion of planetesimals. We compared synthetic populations of planets formed in disks composed of different amounts of pebbles and 600 metre-sized planetesimals to identify the impact of the combined accretion scenario. On a system level, we studied the formation pathway of giant planets in these disks. Results. We find that, in hybrid disks containing both pebbles and planetesimals, the formation of giant planets is strongly suppressed, whereas, in a pebbles-only or planetesimals-only scenario, giant planets can form. We identify the heating associated with the accretion of up to 100 kilometre-sized planetesimals after the pebble accretion period to delay the runaway gas accretion of massive cores. Coupled with strong inward type-I migration acting on these planets, this results in close-in icy sub-Neptunes originating from the outer disk. Conclusions. We conclude that, in hybrid pebble-planetesimal scenarios, the late accretion of planetesimals is a critical factor in the giant planet formation process and that inward migration is more efficient for planets in increasingly pebble-dominated disks. We expect a reduced occurrence rate of giant planets in planet formation models that take the accretion of pebbles and planetesimals into account.

Simultaneous gas accretion onto a pair of giant planets: Impact on their final mass and on the protoplanetary disk structure

Several planetary systems are known to host multiple giant planets. However, when two giant planets are accreting from the same disk, it is unclear what effect the presence of the second planet has on the gas accretion process of both planets. In this paper we perform long-term 2D isothermal hydrodynamical simulations (over more than 0.5 Myr) with the FARGO-2D1D code, considering two non-migrating planets accreting from the same gaseous disk. We find that the evolution of the planets’ mass ratio depends on gap formation. However, in all cases, when the planets start accreting at the same time, they end up with very similar masses (0.9 < mp,out/mp,in < 1.1 after 0.5 Myr). Delaying the onset of accretion of one planet allows the planets’ mass ratio to reach larger values initially, but they quickly converge to similar masses afterward (0.8 < mp,out/mp,in < 2 in 105 yr). In order to reproduce the more diverse observed mass ratios of exoplanets, the planets must start accreting gas at different times, and their accretion must be stopped quickly after the beginning of runaway gas accretion (less than 0.5 Myr), for example via disk dispersal. The evolution of the planets’ mass ratio can have an important impact on the dynamics of the system and may constrain the formation history of Jupiter and Saturn.

Recycling of the first atmospheres of embedded planets: Dependence on core mass and optical depth

Context. Recent observations found close-in planets with significant atmospheres of hydrogen and helium in great abundance. These are the so-called super-Earths and mini-Neptunes. Their atmospheric composition suggests that they formed early during the gas-rich phase of the circumstellar disk and were able to avoid becoming hot Jupiters. As a possible explanation, recent studies explored the recycling hypothesis and showed that atmosphere-disk recycling is able to fully compensate for radiative cooling and thereby halt Kelvin-Helmholtz contraction to prevent runaway gas accretion. Aims. To understand the parameters that determine the efficiency of atmospheric recycling, we extend our earlier studies by exploring the effects of the core mass, the effect of circumstellar gas on sub-Keplerian orbits (headwind), and the optical depth of the surrounding gas on the recycling timescale. Additionally, we analyze their effects on the size and mass of the forming atmosphere. Methods. We used three-dimensional (3D) radiation-hydrodynamic simulations to model a local shearing box centered on the planet. Our planet is located at a separation of ap = 0.1 au from its solar-type host star, and we scanned the core mass range from 1 to 10 MEarth. In order to measure and track the recycling of the atmosphere, we employed tracer particles as well as tracer fluids after thermodynamic equilibrium was reached. Results. For the explored parameter space, all simulations eventually reach an equilibrium where heating due to hydrodynamic recycling fully compensates radiative cooling. In this equilibrium, the atmosphere-to-core mass ratio stays well below 10%, preventing the atmosphere from becoming self-gravitating and entering runaway gas accretion. Higher core masses cause the atmosphere to become turbulent, which further enhances recycling. Compared to the core mass, the effect of the headwind on the recycling timescale is negligible. The opacity has no significant effect on the recycling timescale, which demonstrates that the Kelvin-Helmholtz contraction timescale and the atmosphere-disk recycling timescale are independent of each other. Conclusions. Even for our highest core mass of 10 MEarth, atmosphere-disk recycling is efficient enough to fully compensate for radiative cooling and prevent the atmosphere from becoming self-gravitating. Hence, in-situ formation of hot Jupiters is very unlikely, and migration of gas giants is a key process required to explain their existence. Our findings imply that atmosphere-disk recycling is the most natural explanation for the prevalence of close-in super-Earths and mini-Neptunes.

Wide Dust Gaps in Protoplanetary Disks Induced by Eccentric Planets: A Mass-eccentricity Degeneracy

Abstract The tidal perturbation of embedded protoplanets on their natal disks has been widely attributed to be the cause of gap-ring structures in submillimeter images of protoplanetary disks around T Tauri stars. Numerical simulations of this process have been used to propose scaling of characteristic dust-gap width/gap-ring distance with respect to planet mass. Applying such scaling to analyze observed gap samples yields a continuous mass distribution for a rich population of hypothetical planets in the range of several Earth to Jupiter masses. In contrast, the conventional core-accretion scenario of planet formation predicts a bimodal mass function due to (1) the onset of runaway gas accretion above ∼20 Earth masses and (2) suppression of accretion induced by gap opening. Here, we examine the dust disk response to the tidal perturbation of eccentric planets as a possible resolution of this paradox. Based on simulated gas and dust distributions, we show the gap-ring separation of Neptune-mass planets with small eccentricities might become comparable to that induced by Saturn-mass planets on circular orbits. This degeneracy may obliterate the discrepancy between the theoretical bimodal mass distribution and the observed continuous gap width distribution. Despite damping due to planet–disk interaction, modest eccentricity may be sustained either in the outer regions of relatively thick disks or through resonant excitation among multiple super Earths. Moreover, the ring-like dust distribution induced by planets with small eccentricities is axisymmetric even in low viscosity environments, consistent with the paucity of vortices in Atacama Large Millimeter/submillimeter Array images.

No Sub-Saturn-mass Planet Desert in the CORALIE/HARPS Radial-velocity Sample

Abstract We analyze the CORALIE/HARPS sample of exoplanets found by the Doppler radial-velocity method for signs of the predicted gap or “desert” at 10–100 M ⊕ caused by runaway gas accretion at semimajor axes of <3 au. We find that these data are not consistent with this prediction. This result is similar to the finding by the MOA gravitational microlensing survey that found no desert in the exoplanet distribution for exoplanets in slightly longer period orbits and somewhat lower host masses (Suzuki et al. 2018). Together, these results imply that the runaway gas accretion scenario of the core accretion theory does not have a large influence on the final mass and semimajor axis distribution of exoplanets.

Radial Gradients in Dust-to-gas Ratio Lead to Preferred Region for Giant Planet Formation

Abstract The Rosseland mean opacity of dust in protoplanetary disks is often calculated assuming the interstellar medium (ISM) size distribution and a constant dust-to-gas ratio. However, the dust size distribution and dust-to-gas ratio in protoplanetary disks are distinct from those of the ISM. Here we use simple dust evolution models that incorporate grain growth and transport to calculate the time evolution of the mean opacity of dust grains as a function of distance from the star. Dust dynamics and size distribution are sensitive to the assumed value of the turbulence strength α t and the velocity at which grains fragment v frag. For moderate-to-low turbulence strengths of α t ≲ 10−3 and substantial differences in v frag for icy and ice-free grains, we find a spatially nonuniform dust-to-gas ratio and grain size distribution that deviate significantly from the ISM values, in agreement with previous studies. The effect of a nonuniform dust-to-gas ratio on the Rosseland mean opacity dominates over that of the size distribution. This spatially varying—that is, non-monotonic—dust-to-gas ratio creates a region in the protoplanetary disk that is optimal for producing hydrogen-rich planets, potentially explaining the apparent peak in the gas-giant planet occurrence rate at intermediate distances. The enhanced dust-to-gas ratio within the ice line also suppresses gas accretion rates onto sub-Neptune cores, thus stifling their tendency to undergo runaway gas accretion within disk lifetimes. Finally, our work corroborates the idea that low-mass cores with large primordial gaseous envelopes (“super-puffs”) originate beyond the ice line.

How planets grow by pebble accretion

Context. In the theory of pebble accretion, planets form by the subsequent accretion of solids (micron-sized dust and larger pebbles) and gas. The amount of nebular gas that a planet can bind is limited by its cooling rate, which is set by the opacity of its envelope. Accreting dust and pebbles contribute to the envelope opacity and, thus, influence the outcome of planet formation. Aims. Our aim is to model the size evolution and opacity contribution of solids inside planetary envelopes. We then use the resultant opacity relations to study emergent trends in planet formation. Methods. We design a model for the opacity of solids in planetary envelopes that accounts for the growth, fragmentation, and erosion of pebbles during their sedimentation. It also includes a separate dust component, which can be both replenished and swept up by encounters with pebbles, depending on the relative velocities. We formulate analytical expressions for the opacity of pebbles and dust and map out their trends as a function of depth, planet mass, distance, and accretion rate. Results. The accretion of pebbles rather than planetesimals can produce fully convective envelopes, but only in lower-mass planets that reside in the outer disk or in those that are accreting pebbles at a high rate. In these conditions, pebble sizes are limited by fragmentation and erosion, allowing them to pile up in the envelope. At higher planetary masses or reduced accretion rates, a different regime applies, where the sizes of sedimenting pebbles are only limited by their rate of growth. The opacity in this growth-limited regime is much lower and declines steeply with depth and planet mass but is invariant with the pebble mass flux. Our results imply that the opacity of a forming planet’s envelope cannot be approximated by a value that is constant with either depth or planet mass. Applying these results to the Solar System, we argue that Uranus and Neptune could not have maintained a sufficiently high opacity to avoid runaway gas accretion unless they both experienced sufficiently rapid accretion of solids and formed late.

In situ formation of hot Jupiters with companion super-Earths

ABSTRACT Observations have confirmed the existence of multiple-planet systems containing a hot Jupiter and smaller planetary companions. Examples include WASP-47, Kepler-730, and TOI-1130. We examine the plausibility of forming such systems in situ using N-body simulations that include a realistic treatment of collisions, an evolving protoplanetary disc, and eccentricity/inclination damping of planetary embryos. Initial conditions are constructed using two different models for the core of the giant planet: a ‘seed-model’ and an ‘equal-mass-model’. The former has a more massive protoplanet placed among multiple small embryos in a compact configuration. The latter consists only of equal-mass embryos. Simulations of the seed-model lead to the formation of systems containing a hot Jupiter and super-Earths. The evolution consistently follows four distinct phases: early giant impacts; runaway gas accretion on to the seed protoplanet; disc damping-dominated evolution of the embryos orbiting exterior to the giant; a late chaotic phase after dispersal of the gas disc. Approximately 1 per cent of the equal-mass simulations form a giant and follow the same four-phase evolution. Synthetic transit observations of the equal-mass simulations provide an occurrence rate of 0.26 per cent for systems containing a hot Jupiter and an inner super-Earth, similar to the 0.2 per cent occurrence rate from actual transit surveys, but simulated hot Jupiters are rarely detected as single transiting planets, in disagreement with observations. A subset of our simulations form two close-in giants, similar to the WASP-148 system. The scenario explored here provides a viable pathway for forming systems with unusual architectures, but does not apply to the majority of hot Jupiters.

Formation of giant planets with large metal masses and metal fractions via giant impacts in a rapidly dissipating disk

Context. According to planetary interior models, some giant planets contain large metal masses with large metal-mass fractions. HD 149026b and TOI-849b are characteristic examples of these giant planets. It has been suggested that the envelope mass loss during giant impacts plays a key role in the formation of such giant planets. Aims. The aim of the present Letter is to propose a mechanism that can explain the origin of such giant planets. Methods. We investigate the formation of giant planets in a rapidly dissipating disk using N-body simulations that consider pebble accretion. Results. The results show that although the pebble isolation mass is smaller than the metal mass (≳30 Earth masses) in some giant planets, the interior metal mass can be increased by giant impacts between planets with the isolation mass. Regarding the metal fraction, the cores accrete massive envelopes by runaway gas accretion during the disk-dissipation phase of 1−10 Myr in a disk that evolves without photoevaporation. Although a large fraction of the envelope can be lost during giant impacts, the planets can reaccrete the envelope after impacts in a slowly dissipating disk. Here, we demonstrate that, by photoevaporation in a rapidly dissipating disk, the runaway gas accretion is quenched in the middle, resulting in the formation of giant planets with large metal-mass fractions. Conclusions. The origins of HD 149026b and TOI-849b, which are characterized by their large metal-mass fractions, can be naturally explained by a model that considers a disk evolving with photoevaporation.

Influence of grain size and composition on the contraction rates of planetary envelopes and on planetary migration

A crucial phase during planetary growth is the migration, when the planetary core has been assembled but has not yet opened a deep gap. During this phase, the planet is subject to fast type-I migration, which is mostly directed inwards, and the planet can lose a significant fraction of its semi-major axis. The duration of this phase is set by the time required for the planetary envelope to contract before it reaches a mass similar to that of the planetary core, which is when runaway gas accretion can set in and the planet can open a deeper gap in the disc, transitioning into the slower type-II migration. This envelope contraction phase depends crucially on the planetary mass and on the opacity inside the planetary envelope. Here we study how different opacity prescriptions influence the envelope contraction time and how this in turn influences how far the planet migrates through the disc. We find within our simulations that the size distribution of the grains as well as the chemical composition of the grains crucially influences how far the planet migrates before reaches the runaway gas accretion phase. Grain size distributions with larger grain sizes result in less inward migration of the growing planet because of faster gas accretion enabled by more efficient cooling. In addition, we find that planets forming in water-poor environments can contract their envelope faster and therefore migrate less, implying that gas giants forming in water-poor environments might be located further away from their central star compared to gas giants forming in water-rich environments. Future studies of planet formation that aim to investigate the chemical composition of formed gas giants need to take these effects into account self-consistently.

How planets grow by pebble accretion

During their formation, planets form large, hot atmospheres due to the ongoing accretion of solids. It has been customary to assume that all solids end up at the center, constituting a “core” of refractory materials, whereas the envelope remains metal-free. However, recent work, as well as observations by the Juno mission, indicate that the distinction may not be so clear cut. Indeed, small silicate, pebble-sized particles will sublimate in the atmosphere when they hit the sublimation temperature (T ~ 2000 K). In this paper we extend previous analytical work to compute the properties of planets within such a pebble accretion scenario. We conduct 1D numerical calculations of the atmosphere of an accreting planet, solving the stellar structure equations, augmented by a nonideal equation of state that describes a hydrogen and helium-silicate vapor mixture. Calculations terminate at the point where the total mass in metal is equal to that of the H+He gas, which we numerically confirm as the onset of runaway gas accretion. When pebbles sublimate before reaching the core, insufficient (accretion) energy is available to mix dense, vapor-rich lower layers with the higher layers of lower metallicity. A gradual structure in which Z decreases with radius is therefore a natural outcome of planet formation by pebble accretion. We highlight, furthermore, that (small) pebbles can act as the dominant source of opacity, preventing rapid cooling and presenting a channel for (mini-)Neptunes to survive in gas-rich disks. Nevertheless, once pebble accretion subsides, the atmosphere rapidly clears followed by runaway gas accretion. We consider atmospheric recycling to be the most probable mechanism to have stalled the growth of the envelopes of these planets.

Steady state by recycling prevents premature collapse of protoplanetary atmospheres

Context. In recent years, space missions such as Kepler and TESS have discovered many close-in planets with significant atmospheres consisting of hydrogen and helium: mini-Neptunes. This indicates that these planets formed early in gas-rich disks while avoiding the runaway gas accretion that would otherwise have turned them into hot-Jupiters. A solution is to invoke a long Kelvin-Helmholtz contraction (or cooling) timescale, but it has also been suggested that thermodynamical cooling can be prevented by hydrodynamical planet atmosphere-disk recycling. Aims. We investigate the efficacy of the recycling hypothesis in preventing the collapse of the atmosphere, check for the existence of a steady state configuration, and determine the final atmospheric mass to core mass ratio. Methods. We use three-dimensional radiation-hydrodynamic simulations to model the formation of planetary proto-atmospheres. Equations are solved in a local frame centered on the planet. Results. Ignoring small oscillations that average to zero over time, the simulations converge to a steady state where the velocity field of the gas becomes constant in time. In a steady state, the energy loss by radiative cooling is fully compensated by the recycling of the low entropy gas in the planetary atmosphere with high entropy gas from the circumstellar disk. Conclusions. For close-in planets, recycling naturally halts the cooling of planetary proto-atmospheres, preventing them from contracting toward the runaway regime and collapsing into gas giants.

MOA-2009-BLG-319Lb: A Sub-Saturn Planet inside the Predicted Mass Desert

Abstract We present an adaptive optics (AO) analysis of images from the Keck II telescope NIRC2 instrument of the planetary microlensing event MOA-2009-BLG-319. The ∼10 yr baseline between the event and the Keck observations allows the planetary host star to be detected at a separation of 66.5 ± 1.7 mas from the source star, consistent with the light-curve model prediction. The combination of the host star brightness and light-curve parameters yields host star and planet masses of M host = 0.524 ± 0.048M ⊙ and m p = 67.3 ± 6.2M ⊕ at a distance of D L = 7.1 ± 0.7 kpc. The star−planet projected separation is 2.03 ± 0.21 au. The planet-to-star mass ratio of this system, q = (3.857 ± 0.029) × 10−4, places it in the predicted “planet desert” at 10−4 < q < 4 × 10−4 according to the runaway gas accretion scenario of the core accretion theory. Seven of the 30 planets in the Suzuki et al. sample fall in this mass ratio range, and this is the third with a measured host mass. All three of these host stars have masses of 0.5 ≤ M host/M ⊙ ≤ 0.7, which implies that this predicted mass ratio gap is filled with planets that have host stars within a factor of two of 1M ⊙. This suggests that runaway gas accretion does not play a major role in determining giant planet masses for stars somewhat less massive than the Sun. Our analysis has been accomplished with a modified DAOPHOT code that has been designed to measure the brightness and positions of closely blended stars. This will aid in the development of the primary method that the Nancy Grace Roman Space Telescope mission will use to determine the masses of microlens planets and their hosts.

Giant planet formation at the pressure maxima of protoplanetary disks

Context.Recent high-resolution observations of protoplanetary disks have revealed ring-like structures that can be associated to pressure maxima. Pressure maxima are known to be dust collectors and planet migration traps. The great majority of planet formation studies are based either on the pebble accretion model or on the planetesimal accretion model. However, recent studies proposed hybrid accretion of pebbles and planetesimals as a possible formation mechanism for Jupiter.Aims.We aim to study the full process of planet formation consisting of dust evolution, planetesimal formation, and planet growth at a pressure maximum in a protoplanetary disk.Methods.We compute, through numerical simulations, the gas and dust evolution in a protoplanetary disk, including dust growth, fragmentation, radial drift, and particle accumulation at a pressure maximum. The pressure maximum appears due to an assumed viscosity transition at the water ice line. We also consider the formation of planetesimals by streaming instability and the formation of a moon-size embryo that grows into a giant planet by the hybrid accretion of pebbles and planetesimals, all within the pressure maximum.Results.We find that the pressure maximum is an efficient collector of dust drifting inwards. The condition of planetesimal formation by streaming instability is fulfilled due to the large amount of dust accumulated at the pressure bump. Subsequently, a massive core is quickly formed (in ~104yr) by the accretion of pebbles. After the pebble isolation mass is reached, the growth of the core slowly continues by the accretion of planetesimals. The energy released by planetesimal accretion delays the onset of runaway gas accretion, allowing a gas giant to form after ~1 Myr of disk evolution. The pressure maximum also acts as a migration trap.Conclusions.Pressure maxima generated by a viscosity transition at the water ice line are preferential locations for dust traps, planetesimal formation by streaming instability, and planet migration traps. All these conditions allow the fast formation of a giant planet by the hybrid accretion of pebbles and planetesimals.

Giant Planet Formation Models with a Self-consistent Treatment of the Heavy Elements

Abstract We present a new numerical framework to model the formation and evolution of giant planets. The code is based on the further development of the stellar evolution toolkit Modules for Experiments in Stellar Astrophysics. The model includes the dissolution of the accreted planetesimals/pebbles, which are assumed to be made of water ice, in the planetary gaseous envelope, and the effect of envelope enrichment on the planetary growth and internal structure is computed self-consistently. We apply our simulations to Jupiter and investigate the impact of different heavy-element and gas accretion rates on its formation history. We show that the assumed runaway gas accretion rate significantly affects the planetary radius and luminosity. It is confirmed that heavy-element enrichment leads to shorter formation timescales due to more efficient gas accretion. We find that with heavy-element enrichment Jupiter’s formation timescale is compatible with typical disks’ lifetimes even when assuming a low heavy-element accretion rate (oligarchic regime). Finally, we provide an approximation for the heavy-element profile in the innermost part of the planet, providing a link between the internal structure and the planetary growth history.

Heavy-metal Jupiters by major mergers: metallicity versus mass for giant planets

ABSTRACT Some Jupiter-mass exoplanets contain ${\sim}100\, {\rm M}_{\hbox{$\oplus $}}$ of metals, well above the ${\sim}10\, {\rm M}_{\hbox{$\oplus $}}$ typically needed in a solid core to trigger giant planet formation by runaway gas accretion. We demonstrate that such ‘heavy-metal Jupiters’ can result from planetary mergers near ∼10 au. Multiple cores accreting gas at runaway rates gravitationally perturb one another on to crossing orbits such that the average merger rate equals the gas accretion rate. Concurrent mergers and gas accretion implies the core mass scales with the total planet mass as Mcore ∝ M1/5 – heavier planets harbour heavier cores, in agreement with the observed relation between total mass and metal mass. While the average gas giant merges about once to double its core, others may merge multiple times, as merger trees grow chaotically. We show that the dispersion of outcomes inherent in mergers can reproduce the large scatter in observed planet metallicities, assuming $3{-}30\, {\rm M}_{\hbox{$\oplus $}}$ pre-runaway cores. Mergers potentially correlate metallicity, eccentricity, and spin.