Planetesimal Formation Research Articles

Self-consistent model for dust-gas coupling in protoplanetary disks

Various physical processes that ensue within protoplanetary disks - including vertical settling of icy and rocky grains, radial drift of solids, planetesimal formation, as well as planetary accretion itself - are facilitated by hydrodynamic interactions between H/He gas and high-Z dust. The Stokes number, which quantifies the strength of dust-gas coupling, thus plays a central role in protoplanetary disk evolution and its poor determination constitutes an important source of uncertainty within the theory of planet formation. In this work, we present a simple model for dust-gas coupling and we demonstrate that for a specified combination of the nebular accretion rate, Ṁ, and turbulence parameter a, the radial profile of the Stokes number can be calculated in a unique way. Our model indicates that the Stokes number grows sublinearly with the orbital radius, but increases dramatically across the water-ice line. For fiducial protoplanetary disk parameters of Ṁ = 10−8 M⊙ per year and α = 10−3, our theory yields characteristic values of the Stokes number on the order of St ~ 10−4 (corresponding to ~mm-sized silicate dust) in the inner nebula and St ~ 10−1 (corresponding to icy grains of a few cm in size) in the outer regions of the disk. Accordingly, solids are expected to settle into a thin subdisk at large stellocentric distances, while remaining vertically well mixed inside the ice line.

Nonlinear Evolution of Streaming Instabilities in Accreting Protoplanetary Disks

The streaming instability (SI) is one of the most promising candidates for triggering planetesimal formation by producing dense dust clumps that undergo gravitational collapse. Understanding how the SI operates in realistic protoplanetary disks (PPDs) is therefore crucial to assess the efficiency of planetesimal formation. Modern models of PPDs show that large-scale magnetic torques or winds can drive laminar gas accretion near the disk midplane. In a previous study, we identified a new linear dust-gas instability, the azimuthal drift SI (AdSI), applicable to such accreting disks and is powered by the relative azimuthal motion between dust and gas that results from the gas being torqued. In this work, we present the first nonlinear simulations of the AdSI. We show that it can destabilize an accreting, dusty disk even in the absence of a global radial pressure gradient, which is unlike the classic SI. We find the AdSI drives turbulence and the formation of vertically extended dust filaments that undergo merging. In dust-rich disks, merged AdSI filaments reach maximum dust-to-gas ratios exceeding 100. Moreover, we find that even in dust-poor disks the AdSI can increase local dust densities by 2 orders of magnitude. We discuss the possible role of the AdSI in planetesimal formation, especially in regions of an accreting PPD with vanishing radial pressure gradients.

Primordial dusty rings and episodic outbursts in protoplanetary discs

ABSTRACT We investigate the formation and evolution of ‘primordial’ dusty rings occurring in the inner regions of protoplanetary discs, with the help of long-term, coupled dust-gas, magnetohydrodynamic simulations. The simulations are global and start from the collapse phase of the parent cloud core, while the dead zone is calculated via an adaptive α formulation by taking into account the local ionization balance. The evolution of the dusty component includes its growth and back reaction on to the gas. Previously, using simulations with only a gas component, we showed that dynamical rings form at the inner edge of the dead zone. We find that when dust evolution, as well as magnetic field evolution in the flux-freezing limit are included, the dusty rings formed are more numerous and span a larger radial extent in the inner disc, while the dead zone is more robust and persists for a much longer time. We show that these dynamical rings concentrate enough dust mass to become streaming unstable, which should result in a rapid planetesimal formation even in the embedded phases of the system. The episodic outbursts caused by the magnetorotational instability have a significant impact on the evolution of the rings. The outbursts drain the inner disc of grown dust, however, the period between bursts is sufficiently long for the planetesimal growth via streaming instability. The dust mass contained within the rings is large enough to ultimately produce planetary systems with the core accretion scenario. The low-mass systems rarely undergo outbursts, and, thus, the conditions around such stars can be especially conducive for planet formation.

Turbulent Dust-trapping Rings as Efficient Sites for Planetesimal Formation

Recent observations of protoplanetary disks (PPDs) at submillimeter wavelengths have revealed the ubiquity of annular substructures that are indicative of pebble-sized dust particles trapped in turbulent ringlike gas pressure bumps. This major paradigm shift also challenges the leading theory of planetesimal formation from such pebbles by means of the streaming instability, which operates in a pressure gradient and can be suppressed by turbulence. Here, we conduct 3D local shearing box nonideal magnetohydrodynamic simulations of dust trapping in enforced gas pressure bumps, including dust backreaction. Under a moderate level of turbulence generated by the magnetorotational instability with ambipolar diffusion, which is suitable for outer disk conditions, we achieve quasi-steady states of dust trapping balanced by turbulent diffusion. We find strong dust clumping in all simulations near the gas pressure maxima, reaching a maximum density well above the threshold for triggering gravitational collapse to form planetesimals. A strong pressure bump concentrates dust particles toward the bump’s center. With a weak pressure bump, dust can also concentrate in secondary filaments off the bump’s center, due to dust backreaction, but strong clumping still occurs mainly in the primary ring around the bump’s center. Our results reveal dust-trapping rings to be robust locations for planetesimal formation in outer PPDs, while they may possess diverse observational properties.

Nonlinear Outcome of Coagulation Instability in Protoplanetary Disks. I. First Numerical Study of Accelerated Dust Growth and Dust Concentration at Outer Radii

Our previous linear analysis presents a new instability driven by dust coagulation in protoplanetary disks. The coagulation instability has the potential to concentrate dust grains into rings and assist dust coagulation and planetesimal formation. In this series of papers, we perform numerical simulations and investigate the nonlinear outcome of coagulation instability. In this paper (Paper I), we first conduct local simulations to demonstrate the existence of coagulation instability. Linear growth observed in the simulations is in good agreement with the previous linear analysis. We next conduct radially global simulations to demonstrate that coagulation instability develops during the inside-out disk evolution owing to dust growth. To isolate the various effects on dust concentration and growth, we neglect the effects of back-reaction to a gas disk and dust fragmentation in Paper I. This simplified simulation shows that neither back-reaction nor fragmentation is a prerequisite for local dust concentration via the instability. In most runs with weak turbulence, dust concentration via coagulation instability overcomes dust depletion due to radial drift, leading to the formation of multiple dust rings. The nonlinear development of coagulation instability also accelerates dust growth, and the dimensionless stopping time τ s reaches unity even at outer radii (>10 au). Therefore, coagulation instability is one promising process to retain dust grains and to accelerate dust growth beyond the drift barrier.

Athor asteroid family as the source of the EL enstatite meteorites

The identification of meteorite parent bodies provides the context for understanding planetesimal formation and evolution as well as the key Solar System events they have witnessed. However, identifying such links has proven challenging and some appear ambiguous. Here, we identify that the family of asteroid fragments whose largest member is (161) Athor is the unique source of the rare EL enstatite chondrite meteorites, the closest meteorites to Earth in terms of their isotopic ratios. The Athor family was created by the collisional fragmentation of a parent body 3 Gyr ago in the inner main belt. We calculate that the diameter of the Athor family progenitor was 64 km in diameter, much smaller than the putative size of the EL original planetesimal. Therefore, we deduce that the EL planetesimal that accreted in the terrestrial planet region underwent a first catastrophic collision in that region, and one of its fragments suffered a more recent catastrophic collision in the main belt, generating the current source of the EL meteorites.

Global Modeling of Nebulae with Particle Growth, Drift, and Evaporation Fronts. III. Redistribution of Refractories and Volatiles

Formation of the first planetesimals remains an unsolved problem. Growth by sticking must initiate the process, but multiple studies have revealed a series of barriers that can slow or stall growth, most of them due to nebula turbulence. In a companion paper, we study the influence of these barriers on models of fractal aggregate and solid, compact particle growth in a viscously evolving solar-like nebula for a range of turbulent intensities α t = 10−5–10−2. Here, we examine how the disk composition in these same models changes with time. We find that advection and diffusion of small grains and vapor, and radial inward drift for larger compact particles and fractal aggregates, naturally lead to diverse outcomes for planetesimal composition. Larger particles can undergo substantial inward radial migration due to gas drag before being collisionally fragmented or partially evaporating at various temperatures. This leads to enhancement of the associated volatile in both vapor inside, and solids outside, their respective evaporation fronts, or snowlines. In cases of lower α t, we see narrow belts of volatile or supervolatile material develop in the outer nebula, which could be connected to the bands of pebbles seen by the Atacama Large Millimeter/submillimeter Array. Volatile bands, which migrate inwards as the disk cools, can persist over long timescales as their gas phase continues to advect or diffuse outward across its evaporation front. These belts could be sites where supervolatile-rich planetesimals form, such as the rare CO-rich and water-poor comets; giant planets formed just outside the H2O snowline may be enhanced in water.

No Self-shadowing Instability in 2D Radiation Hydrodynamical Models of Irradiated Protoplanetary Disks

Theoretical models of protoplanetary disks including stellar irradiation often show a spontaneous amplification of scale height perturbations, produced by the enhanced absorption of starlight in enlarged regions. In turn, such regions cast shadows on adjacent zones that consequently cool down and shrink, eventually leading to an alternating pattern of overheated and shadowed regions. Previous investigations have proposed this to be a real self-sustained process, the so-called self-shadowing or thermal wave instability, which could naturally form frequently observed disk structures such as rings and gaps, and even potentially enhance the formation of planetesimals. All of these, however, have assumed in one way or another vertical hydrostatic equilibrium and instantaneous radiative diffusion throughout the disk. In this work we present the first study of the stability of accretion disks to self-shadowing that relaxes these assumptions, relying instead on radiation hydrodynamical simulations. We first construct hydrostatic disk configurations by means of an iterative procedure and show that the formation of a pattern of enlarged and shadowed regions is a direct consequence of assuming instantaneous radiative diffusion. We then let these solutions evolve in time, which leads to a fast damping of the initial shadowing features in layers close to the disk surface. These thermally relaxed layers grow toward the midplane until all temperature extrema in the radial direction are erased in the entire disk. Our results suggest that radiative cooling and gas advection at the disk surface prevent a self-shadowing instability from forming, by damping temperature perturbations before these reach lower, optically thick regions.

DustPy: A Python Package for Dust Evolution in Protoplanetary Disks

Many processes during the evolution of protoplanetary disks and during planet formation are highly sensitive to the sizes of dust particles that are present in the disk: the efficiency of dust accretion in the disk and volatile transport on dust particles, gravoturbulent instabilities leading to the formation of planetesimals, or the accretion of pebbles onto large planetary embryos to form giant planets are typical examples of processes that depend on the sizes of the dust particles involved. Furthermore, radiative properties like absorption or scattering opacities depend on the particle sizes. To interpret observations of dust in protoplanetary disks, a proper estimate of the dust particle sizes is needed. We present DustPy: a Python package to simulate dust evolution in protoplanetary disks. DustPy solves gas and dust transport including viscous advection and diffusion as well as collisional growth of dust particles. DustPy is written with a modular concept, such that every aspect of the model can be easily modified or extended to allow for a multitude of research opportunities.

Formation of Comets

Questions regarding how primordial or pristine the comets of the solar system are have been an ongoing controversy. In this review, we describe comets’ physical evolution from dust and ice grains in the solar nebula to the contemporary small bodies in the outer solar system. This includes the phases of dust agglomeration, the formation of planetesimals, their thermal evolution and the outcomes of collisional processes. We use empirical evidence about comets, in particular from the Rosetta Mission to comet 67P/Churyumov–Gerasimenko, to draw conclusions about the possible thermal and collisional evolution of comets.

The size of monomers of dust aggregates in planet-forming disks

Context. The size of the constituent particles (monomers) of dust aggregates is one of the most uncertain parameters directly affecting collisional growth of aggregates in planet-forming disks. Despite its importance, the monomer size has not yet been meaningfully constrained by disk observations. Aims. We attempt to derive the monomer size from optical and near-infrared (IR) polarimetric observations of planet-forming disks. Methods. We performed a comprehensive parameter survey on the degree of linear polarization of light scattered by dust aggregates, using an exact numerical method called the T-matrix method. We investigated the effect of the monomer size, aggregate size, porosity, and composition on the degree of polarization. The obtained results were then compared with observed polarization fractions of several planet-forming disks at optical and near-IR wavelengths. Results. We show that the degree of polarization of aggregates acutely depends on the monomer size unless the monomer size parameter is smaller than one or two. Comparing the simulation results with the disk observations, we find that the monomer radius is no greater than 0.4 μm. The inferred monomer size is therefore similar to subunit sizes of the solar system dust aggregates and the maximum size of interstellar grains. Conclusions. Optical and near-IR quantitative polarimetry will provide observational grounds on the initial conditions for dust coagulation and, thereby, planetesimal formation in planet-forming disks.

The Nature of Low-albedo Small Bodies from 3 μm Spectroscopy: One Group that Formed within the Ammonia Snow Line and One that Formed beyond It

We present evidence, via a large survey of 191 new spectra along with previously published spectra, of a divide in the 3 μm spectral properties of the low-albedo asteroid population. One group (“sharp types,” or STs, with band centers <3 μm) has a spectral shape consistent with carbonaceous chondrite meteorites, while the other group (“not sharp types,” or NSTs, with bands centered >3 μm) is not represented in the meteorite literature but is as abundant as the STs among large objects. Both groups are present in most low-albedo asteroid taxonomic classes, and, except in limited cases, taxonomic classifications based on 0.5–2.5 μm data alone cannot predict whether an asteroid is an ST or NST. Statistical tests show that the STs and NSTs differ in average band depth, semimajor axis, and perihelion at confidence levels ≥98% while not showing significant differences in albedo. We also show that many NSTs have a 3 μm absorption band shape like comet 67P and likely represent an important small-body composition throughout the solar system. A simple explanation for the origin of these groups is formation on opposite sides of the ammonia snow line, with the NST group accreting H2O and NH3 and the ST group only accreting H2O, with subsequent thermal and chemical evolution resulting in the minerals seen today. Such an explanation is consistent with recent dynamical modeling of planetesimal formation and delivery and suggests that much more outer solar system material was delivered to the main asteroid belt than would be thought based on the number of D-class asteroids found today.

The Streaming Instability Cannot Form Planetesimals from Millimeter-size Grains in Pressure Bumps

We present evidence that it is unlikely that the streaming instability (SI) can form planetesimals from millimeter grains inside axisymmetric pressure bumps. We conducted the largest simulation of the SI so far (7 million CPU hours), consisting of a large slice of the disk with millimeter grains, a solar-like dust-to-gas ratio (Z = 0.01), and the largest pressure bump that does not cause gravitational instability (GI) in the particle layer. We used a high resolution of 1000/H to resolve as many SI unstable modes as possible. The simulation produced a long-lived particle overdensity far exceeding the SI criteria (i.e., a critical solid abundance to headwind parameter ratio Z/Π) where strong clumping would occur if these conditions were present over an extended region of the disk; yet we observed none. The likely reason is that the time it takes particles to cross the high-Z/Π region (t cross) is shorter than the growth timescale of the SI (t grow). We propose an added criterion for planetesimal formation by the SI—that t cross > t grow. We show that any bump larger than the one in this run would form planetesimals by the GI instead of the SI. Our results significantly restrict the pathways to planet formation: either protoplanetary disks regularly form grains larger than 1 mm, or planetesimals do not form by the SI in axisymmetric pressure bumps. Since bumps large enough to induce the GI are likely Rossby-wave unstable, we propose that millimeter grains may only form planetesimals in vortices.

Prevalence of short-lived radioactive isotopes across exoplanetary systems inferred from polluted white dwarfs

ABSTRACT In the Solar system, short-lived radioisotopes, such as 26Al, played a crucial role during the formation of planetary bodies by providing a significant additional source of heat. Notably, this led to early and large-scale melting and iron core formation in planetesimals and their loss of volatile elements, such as hydrogen and carbon. In the context of exoplanetary systems therefore the prevalence of short-lived radioisotopes is key to interpreting the observed bulk volatile budget and atmospheric diversity among low-mass exoplanets. White dwarfs that have accreted planetary material provide a unique means to infer the frequency of iron core formation in extrasolar planetesimals, and hence the ubiquity of planetary systems forming with high short-lived radioisotope abundances. Here, we devise a quantitative method to infer the fraction of planetary systems enriched with short-lived radionuclides upon planetesimal formation from white dwarf data. We argue that the current evidence from white dwarfs point towards a significant fraction of exoplanetesimals having formed an iron core. Although the data may be explained by the accretion of exomoon or Pluto-sized bodies that were able to differentiate due to gravitational potential energy release, our results suggest that the most likely explanation for the prevalence of differentiated material among polluted white dwarfs is that the Solar system is not unusual in being enriched in 26Al. The models presented here suggest a ubiquitous pathway for the enrichment of exoplanetary systems by short-lived radioisotopes, disfavouring short-lived radioisotope enrichment scenarios relying on statistically rare chance encounters with single nearby supernovae, Wolf–Rayet, or AGB stars.

Nucleation and growth of iron pebbles explains the formation of iron-rich planets akin to Mercury

The pathway to forming the iron-rich planet Mercury remains mysterious. Its core makes up 70% of the planetary mass, which implies a significant enrichment of iron relative to silicates, while its mantle is strongly depleted in oxidised iron. The high core mass fraction is traditionally ascribed to evaporative loss of silicates, for example following a giant impact, but the high abundance of moderately volatile elements in the mantle of Mercury is inconsistent with reaching temperatures significantly above 1000 K during its formation. Here we explore the nucleation of solid particles from a gas of solar composition that cools down in the hot inner regions of the protoplanetary disc. The high surface tension of iron causes iron particles to nucleate homogeneously (i.e. not on a more refractory substrate) under very high supersaturation. The low nucleation rates lead to depositional growth of large iron pebbles on a sparse population of nucleated iron nanoparticles. Silicates in the form of iron-free MgSiO3 nucleate at similar temperatures but obtain smaller sizes because of the much higher number of nucleated particles. This results in a chemical separation of large iron particles from silicate particles with ten times lower Stokes numbers. We propose that such conditions lead to the formation of iron-rich planetesimals by the streaming instability. In this view, Mercury formed by accretion of iron-rich planetesimals with a subsolar abundance of highly reduced silicate material. Our results imply that the iron-rich planets known to orbit the Sun and other stars are not required to have experienced mantle-stripping impacts. Instead, their formation could be a direct consequence of temperature fluctuations in protoplanetary discs and chemical separation of distinct crystal species through the ensuing nucleation process.

The Geophysical Environment of (486958) Arrokoth—A Small Kuiper Belt Object Explored by New Horizons

AbstractNASA's New Horizons mission performed the first flyby of a small Kuiper Belt Object (KBO), (486958) Arrokoth on 1 January 2019. The fast flyby revealed a fascinating, flattened, contact binary replete with a variety of unexpected geologic terrains. However, the irregular shape and constraints imposed by the fast flyby makes it a challenge to understand these features. Here we use the latest New Horizons shape models of Arrokoth to investigate its geophysical environment, including its surface slopes, gravity field, and moments of inertia—which are critical context for understanding Arrokoth's formation, evolution, and peculiar geology. We find that Arrokoth's surface features have a complicated relationship to its geophysical environment. For example, bright material tends to be concentrated in geopotential lows (like the neck), consistent with mass wasting—however, this trend is not consistently observed across Arrokoth. Mass wasting may naturally explain some aspects of Arrokoth's geology, but the actual dynamics of material transport may be complicated owing to Arrokoth's unique shape, spin‐rate, and inferred density. While New Horizons's fast and distant flyby precluded directly measuring Arrokoth's mass, we used techniques previously pioneered for comets and asteroids to infer its density. We find that Arrokoth has a low bulk density of ρ = 235 kg/m3 (1σ range: 155–600 kg/m3). This density is low compared to previously explored small bodies, but is comparable to comets, select binary KBOs, and the ring‐moons of Saturn. This low density may be a critical data‐point for understanding the formation of planetesimals at the dawn of the Solar System.

Early planet formation in embedded protostellar disks

Recent surveys of young star formation regions have shown that the dust mass of the average class II object is not high enough to make up the cores of giant planets. Younger class O/I objects have enough dust in their embedded disk, which raises the question whether the first steps of planet formation occur in these younger systems. The first step is building the first planetesimals, which are generally thought to be the product of the streaming instability. Hence the question can be restated to read whether the physical conditions of embedded disks are conducive to the growth of the streaming instability. The streaming instability requires moderately coupled dust grains and a dust-to-gas mass ratio near unity. We model the collapse of a dusty proto-stellar cloud to show that if there is sufficient drift between the falling gas and dust, regions of the embedded disk can become sufficiently enhanced in dust to drive the streaming instability. We include four models to test a variety of collapse theories: three models with different dust grain sizes, and one model with a different initial cloud angular momentum. We find a sweet spot for planetesimal formation for grain sizes of a few 10s of micron because they fall sufficiently fast relative to the gas to build a high dust-to-gas ratio in the disk midplane, but their radial drift speeds are slow enough in the embedded disk to maintain the high dust-to-gas ratio. Unlike the gas, which is held in hydrostatic equilibrium for a time as a result of gas pressure, the dust can begin to collapse from all radii at a much earlier time. The dust mass flux in class O/I systems can thus be higher than the gas flux. This builds an embedded dusty disk with a global dust-to-gas mass ratio that exceeds the inter-stellar mass ratio by at least an order of magnitude. The streaming instability can produce at least between 7 and 35 M⊕ of planetesimals in the class O/I phase of our smooth embedded disks, depending on the size of the falling dust grains. This mass is sufficient to build the core of the first giant planet in the system, and could be further enhanced by dust traps and/or pebble growth. This first generation of planetesimals could represent the first step in planet formation. It occurs earlier in the lifetime of the young star than is traditionally thought.

Structure of differentiated planetesimals: A chondritic fridge on top of a magma ocean

Meteorites are interpreted as relics of early formed planetary bodies, and they provide information about the processes that occurred in the first few Myrs of our solar system. The ages measured for some differentiated meteorites (achondrites), indicate that planetesimals formed a differentiated silicate crust as early as ≈3Myr after the beginning of the solar system. The composition of the recently discovered achondrite Erg Chech 002 (EC002), the oldest andesitic rock known so far, betokens partial melting of a chondritic source taking place as early as ≈1Myr before all other known achondrites. However, thermal models of early accreted planetesimals predict massive melting of the planetesimal during core/mantle differentiation and cannot account for the preservation of a substantial amount of chondritic material. In this paper, we propose a way to interpret petrological and geochemical constraints provided by differentiated meteorites by introducing a refined thermal model of planetesimals formation and evolution. We demonstrate that continuous, protracted accretion of cold undifferentiated material upon a magma ocean over a timescale 2 times larger than the lifetime of the 26Al heat source leads to the preservation of a few km thick chondritic crust. During accretion, the heat released by radioactive decay further induces episodes of partial melting at the base of the crust, which can led to the formation of andesitic rocks such as EC002. Using the available constraints on the age of EC002 and its cooling rate, the application of our model constraints the terminal radius of its parent body between 70 and 130km.

The Prospects for Hurricane-like Vortices in Protoplanetary Disks

When ice on the surface of dust grains in protoplanetary disk sublimates, it adds its latent heat of water sublimation to the surrounding flow. Drawing on the analogy provided by tropical cyclones on Earth, we investigate whether this energy source is sufficient to sustain or magnify anticyclonic disk vortices that would otherwise fall victim to viscous dissipation. An analytical treatment, supported by exploratory two-dimensional simulations, suggests that even modestly undersaturated flows can extend the lifetime of vortices, potentially to a degree sufficient to aid particle trapping and planetesimal formation. We expect the best conditions for this mechanism to occur will be found near the disk’s water ice line if turbulent motions displace gas parcels out of thermodynamic equilibrium with the dust midplane.

A Highly Settled Disk around Oph163131

High dust density in the midplane of protoplanetary disks is favorable for efficient grain growth and can allow fast formation of planetesimals and planets, before disks dissipate. Vertical settling and dust trapping in pressure maxima are two mechanisms allowing dust to concentrate in geometrically thin and high-density regions. In this work, we aim to study these mechanisms in the highly inclined protoplanetary disk SSTC2D J163131.2-242627 (Oph 163131, i ∼ 84°). We present new high-angular-resolution continuum and 12CO ALMA observations of Oph 163131. The gas emission appears significantly more extended in the vertical and radial direction compared to the dust emission, consistent with vertical settling and possibly radial drift. In addition, the new continuum observations reveal two clear rings. The outer ring, located at ∼100 au, is well-resolved in the observations, allowing us to put stringent constraints on the vertical extent of millimeter dust particles. We model the disk using radiative transfer and find that the scale height of millimeter-sized grains is 0.5 au or less at 100 au from the central star. This value is about one order of magnitude smaller than the scale height of smaller micron-sized dust grains constrained by previous modeling, which implies that efficient settling of the large grains is occurring in the disk. When adopting a parametric dust settling prescription, we find that the observations are consistent with a turbulent viscosity coefficient of about α ≲ 10−5 at 100 au. Finally, we find that the thin dust scale height measured in Oph 163131 is favorable for planetary growth by pebble accretion: a 10 M E planet may grow within less than 10 Myr, even in orbits exceeding 50 au.