arXiv subjects

Alessandro Morbidelli

Publications and source records attributed to Alessandro Morbidelli.

At least 19 recordsLinked to original sources

arXiv2026

Magnetically Driven Obliquity in Circumplanetary Disks and Twisted Bipolar-jet Formation

Circumplanetary disks (CPDs) regulate gas accretion onto forming giant planets and provide the environment in which their satellites may form. We use high-resolution, global three-dimensional simulations to investigate the early formation, orientation, and outflows of a CPD around a Jupiter-mass planet embedded in a turbulent magnetized protoplanetary disk. Within a locally isothermal, ideal-MHD framework, we evolve disks threaded by net vertical magnetic fields, corresponding to initial plasma parameters $875\leq\beta\leq3500$, until magnetorotational-instability-driven turbulence is established before inserting the planet. We also perform a hydrodynamic control simulation. In the most strongly magnetized model, with $\beta=875$, the CPD forms already highly inclined and reaches a maximum tilt of approximately $87^\circ$. By contrast, the hydrodynamic CPD and the MHD models with $\beta\gtrsim1000$ remain nearly coplanar. A control simulation in which the planet is inserted before global MRI turbulence develops also remains coplanar, despite producing local turbulence and bipolar outflows. The large tilt is therefore associated with the pre-existing global turbulent state and its evolved velocity and toroidal magnetic-field structure, although our current diagnostics do not distinguish between a direct magnetic torque and the accretion of misaligned angular momentum. All MHD models launch bipolar outflows; in the highly tilted case, these develop a curved, helical morphology that persists until the end of our short-term simulations. These results identify pre-existing global magnetized turbulence as a viable route to generating strongly inclined CPDs and twisted planetary outflows.

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arXiv2026

Oxidation Constraints on Terrestrial Planet Formation from a Ring

The present-day solar system comprises meteorites with varying oxidation levels, derived from different parent bodies. Previous studies (e.g. Rubie et al., 2011) of the partitioning of siderophile elements between mantle and core during planetary growth and differentiation showed that Earth must accrete reduced bodies first and oxidised bodies later. Here we show that, if the terrestrial planets formed from a narrow ring of planetesimals, this condition is not fulfilled, whatever heliocentric gradient of oxidation is assumed in the ring. The reason is that planetary embryos quickly accrete planetesimals from the whole width of the ring, incorporating both reduced and oxidised material. The partially oxidised state of all planetary embryos leads to mismatches with the composition of the bulk silicate Earth (BSE) because oxygen fugacity strongly affects the partitioning of siderophile elements. We demonstrate that reproducing the BSE composition requires reduced and oxidised reservoirs to remain segregated until embryo formation is almost complete. The delivery of oxidised material to the terrestrial planet-forming ring towards the end of the disc's lifetime is therefore a key requirement of any successful dynamical model of terrestrial planet formation.

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arXiv2026

Dust dynamics in disk dust traps and late planetesimal formation

The streaming instability is the leading model for planetesimal formation in protoplanetary disks, but it typically operates within the first ~Myr. In the Solar System, however, some planetesimals (the chondrite parent bodies) formed 2-4 Myr after disk formation, implying that dust must have been retained for extended periods. Pressure bumps efficiently trap dust, but trapping alone does not guarantee planetesimal formation: even modest gas turbulence can inhibit vertical settling and radial concentration, preventing dust density from reaching Hill density. This motivates the study of alternative dust-gas instabilities, such as the Dusty Rossby Wave Instability (DRWI). We investigate the viability of such instabilities in global disk simulations using the multi-fluid code fargOCA. We first reproduce previous 2D shearing-box results in a global 2D viscous disk and characterize the dust clumping produced by the DRWI. We find that the instability is suppressed in fully 3D viscous disks by unperturbed high-z gas layers caused by dust settling near the midplane. We then explore the inviscid limit and find that multiple dust sub-rings form, concentrating solids into thin ring structures. These would appear observationally as a single radially broad, vertically thin ring, explaining observed protoplanetary disk rings without invoking anisotropic turbulence. Dust concentrations in the sub-rings may remain below the threshold for gravitational collapse, but gas photoevaporation enhances dust settling and radial concentration, eventually forming dense dust clumps in both viscous and inviscid cases. We conclude that planetesimal formation within dust-trapping pressure bumps is favored in very low-viscosity disks at late evolutionary stages, after sufficient gas removal by photoevaporation. This is consistent with the inferred late formation of chondrite parent bodies in the Solar System.

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arXiv2026

Suppression of Resonant Overstability at Sharp Migration Gradients

Mean-motion resonances are expected to frequently arise at the inner edges of protoplanetary disks, where planet-disk interactions facilitate large-scale orbital convergence. Under certain conditions, however, the same dissipative forces that promote resonant capture can drive resonant librations overstable, ultimately breaking commensurabilities. Here we examine the onset of overstability near disk torque reversals and show that it can be subdued when the transition is sufficiently sharp. Adopting the dissipative circular restricted three-body problem as a paradigm, we present a WKB-style analysis that reduces the resonant dynamics to a damped, driven harmonic oscillator. Within this framework, we obtain an effective frictional term that is proportional to the local migration-rate gradient, parameterized by a dimensionless coefficient $\beta$ that encodes the steepness of the local torque reversal. Our analytical theory predicts that overstability is quenched once $\beta \gtrsim \tau_a/\tau_e$, where $\tau_a$ and $\tau_e$ denote the characteristic disk-driven evolution timescales of semi-major axis and eccentricity. We verify and refine our analytic results with direct $N$-body integrations. Simple estimates based on conventional type-I scalings suggest that the competition between overstability and its mitigation at disk inner edges is a borderline outcome that is sensitive to the detailed structure of planet-disk interactions.

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arXiv2026

A distant brown dwarf coplanar to a warm Jupiter and a hot super-Earth

In transiting planetary systems, in which planetary sizes are accurately determined from transit observations, the presence of transit-timing variations (TTVs), especially when combined with radial velocity (RV) data, provides powerful constraints on masses and orbital eccentricities. Together, these measurements offer crucial insights into system architecture, formation mechanisms and dynamical evolution. We present long-term RV and transit/TTV monitoring of the relatively young star (age approximately 1 Gyr) TOI-201, revealing an exceptional multi-planet system composed of a hot super-Earth (SE) size planet transiting every 5.8 days, a warm Jupiter (WJ) on a 53-day orbit and an eccentric (e = 0.62) low-mass brown dwarf (BD) on an approximately 8-year orbit, with an estimated mass of about 16 Jupiter masses. The BD is the longest-period transiting substellar object ever characterized by means of RVs and the only one known to be coplanar with inner planets. The architecture of this system suggests that the SE was formed isolated and in the innermost region of the gaseous disk. On the other hand, the orbital configuration of the outer companions suggests a nearly in situ formation of both objects, with the WJ forming in a dense inner disk. Alternatively, the BD might have formed farther out and migrated inward, while increasing its eccentricity owing to interactions with the disk.

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arXiv2026

Origins of Compact Mean-Motion Resonances: Evidence for Long-Range Migration and the Case of Kepler-36

The observed census of resonant extrasolar planets spans a tantalizing display of orbital architectures, ranging from familiar 2:1 and 3:2 mean-motion commensurabilities to nearly co-orbital configurations characterized by period ratios close to unity. While mean-motion resonances are widely recognized as signposts of convergent disk-driven migration, the process through which the most compact systems are established remains puzzling, since resonance capture must repeatedly fail at a series of first-order commensurabilities before finally succeeding at a high resonant index. Motivated by this discrepancy, here we develop an analytic theory that fuses the stability-based resonance capture criterion with the conventional paradigm of active accretion disks and the standard model of type-I migration. Within this framework, we derive an expression for the stellocentric radius of resonance capture, $r_{\rm{c}}$, and show that it depends only on the product of the disk viscosity parameter, $\alpha$, and the opacity-contributing small-grain mass fraction, $f_\mu$. Applying this formalism to Kepler-36 - the most compact known resonant system with a 7:6 period ratio - we find that resonance locking could not have been established near the disk's inner edge. Instead, capture must have occurred at $r_{\rm{c}}\approx 1-4$ AU, implying orbital decay of the planetary pair by approximately an order of magnitude. Viewed in this light, compact resonant architectures provide the clearest evidence for long-range migration among sub-Jovian planets. Moreover, the emerging picture is fully consistent with formation models in which super-Earths accrete within localized rings of planetesimals at orbital distances comparable to those that gave rise to the terrestrial planets of the Solar System.

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arXiv2026

Numerical Insights into Disk Accretion, Eccentricity, and Kinematics in the Class 0 phase

The formation and early evolution of protoplanetary disks are governed by a wide variety of physical processes during a gravitational collapse. Observations have begun probing disks in their earliest stages, and have favored the magnetically-regulated disk formation scenario. Disks are also expected to exhibit ellipsoidal morphologies in the early phases, an aspect that has been widely overlooked. We aim to describe the birth and evolution of the disk while accounting for the eccentric motions of fluid parcels. Using 3D radiative magnetohydrodynamic (MHD) simulations with ambipolar diffusion, we self-consistently model the collapse of isolated $1~\mathrm{M_\odot}$ and $3~\mathrm{M_\odot}$ cores to form a central protostar surrounded by a disk. We account for dust dynamics, and employ gas tracer particles to follow the thermodynamical history of fluid parcels. We find that magnetic fields and turbulence drive highly anisotropic accretion onto the disk via dense streamers. This streamer-fed accretion, occurring from the vertical and radial directions, drives vigorous internal turbulence that facilitates efficient angular momentum transport and rapid radial spreading. Crucially, the anisotropic inflow delivers material with an angular momentum deficit that continuously generates and sustains significant disk eccentricity ($e\sim 0.1$). Our results reveal ubiquitous eccentric kinematics in Class 0 disks, with direct implications for disk evolution, planetesimal formation, and the interpretation of cosmochemical signatures in Solar System meteorites.

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arXiv2026

Two-source terrestrial planet formation with a sweeping secular resonance

The models that most successfully reproduce the orbital architecture of the Solar System terrestrial planets start from a narrow annulus of material that grows into embryos and then planets. However, it is not clear how this ring model can be made consistent with the chemical structure of the inner Solar System, which shows a reduced-to-oxidized gradient from Mercury to Mars and a parallel gradient in the asteroid belt. We propose that there were two primary reservoirs in the early inner Solar System: a narrow, refractory enriched ring inside of 1 au, and a less massive, extended planetesimal disk outside of 1 au with oxidation states ranging from enstatite chondrites to ordinary chondrites. We show through a suite of N-body simulations that an inwardly sweeping secular resonance, caused by aerodynamic drag and perturbations from a mean-motion resonant Jupiter and Saturn, gathers the outer planetesimal disk into a narrow ring that migrates radially, forms Mars, and contributes oxidized material to proto-Earth. Remaining unaccreted planetesimals can be implanted into the asteroid belt as the parent bodies of aubrites and non-carbonaceous iron meteorites, while the most reduced material is not implanted and thus unsampled in the meteorite collection. This model explains the oxidation and isotopic gradients within the inner Solar System within the context of a low-viscosity, magnetic wind-driven disk.

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arXiv2025

Physicochemical Controls on the Compositions of the Earth and Planets

Despite the fact that the terrestrial planets formed from the protoplanetary disk, their compositions show marked departures from that of solar nebula condensates. Metallic cores fix oxygen fugacities ($f$O$_2$s) of the planets to 5 (Mercury) to 1 log units below the iron-w\"ustite (IW) buffer, orders of magnitude higher than the nebular gas. Their oxidised character is coupled with a lack of volatile elements with respect to the solar nebula. Condensates from a solar gas at different temperatures ($T_0$) have Fe/O (by mass) of 0.93 ($T_0$ = 1250 K) to 0.81 ($T_0$ = 400 K), far lower than that of Earth (1.06). Because the reaction Fe(s) + H2O(g) = FeO(s) + H2(g) proceeds <600 K, temperatures at which most moderately volatile elements (MVEs) have condensed, oxidised planets should be volatile-rich, and vice-versa. That this is not observed suggests that planets did not accrete from equilibrium nebular condensates and/or underwent additional volatile depletion/$f$O$_2$ changes. Indeed, MVEs in small telluric bodies (Moon, Vesta) indicate near equilibrium evaporation/condensation at IW-1 and 1400-1800 K. Volatile-depleted elemental yet near-chondritic isotopes of larger telluric bodies (Earth, Mars) reflect mixing of bodies of variable volatile depletion, overprinted by volatile-undepleted material. From the Cr- and Ti isotopes in the BSE, such undepleted matter has been proposed to be CI chondrites. 6% CI added late to an enstatite chondrite-like proto-Earth would match the Earth. However, because Earth is an end-member in isotopic anomalies of heavier elements, no combination of existing meteorites alone can account for its chemical- and isotopic composition. Instead, the Earth is made partially or essentially entirely from an NC-like missing component. If so, the oxidised-, yet volatile-poor nature of inner solar system bodies, including Earth and Mars, is intrinsic to the NC reservoir.

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arXiv2025

A massive primordial atmosphere on early Mars

Mars finished forming while the solar nebula was still present, and acquired its primordial atmosphere from this reservoir. The absence of a detectable cometary xenon signature in the present-day Martian atmosphere suggests that the capture of solar nebular gas was significant enough to dilute later cometary contributions. By quantifying the mass of cometary material efficiently retained on Mars, we place a lower bound on the mass of the primordial Martian atmosphere. To test the robustness of our conclusions, we use cometary bombardment data from two independent studies conducted within a solar system evolutionary model consistent with its current structure. Our calculations show that, even under the most conservative scenario, the minimal mass of the primordial martian atmospheres would yield a surface pressure of no less than 2.9 bar. Such a massive nebular envelope is consistent with recent models in which atmospheric capture is strongly enhanced by the presence of heavier species on Mars - due to outgassing or redox buffering with a magma ocean.

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arXiv2025

Terrestrial Planet Formation from Two Source Reservoirs

This work describes new dynamical simulations of terrestrial planet formation. The simulations started at the protoplanetary disk stage, when planetesimals formed and accreted into protoplanets, and continued past the late stage of giant impacts. We explored the effect of different parameters, such as the initial radial distribution of planetesimals and Type-I migration of protoplanets, on the final results. In each case, a thousand simulations were completed to characterize the stochastic nature of the accretion process. In the model best able to satisfy various constraints, Mercury, Venus, and Earth accreted from planetesimals that formed early near the silicate sublimation line near 0.5 au and migrated by disk torques. For Venus and Earth to end up at 0.7-1 au, Type-I migration had to be directed outward, for example as the magnetically driven winds reduced the surface gas density in the inner part of the disk. Mercury was left behind near the original ring location. We suggest that Mars and multiple Mars-sized protoplanets grew from a distinct outer source of planetesimals at 1.5-2 au. While many migrated inwards to accrete onto the proto-Earth, our Mars was the lone survivor. This model explains: (1) the masses and orbits of the terrestrial planets, (2) the chemical composition of the Earth, where ~70% and ~30% come from reduced inner-ring and more-oxidized outer-ring materials, and (3) the isotopic differences of the Earth and Mars. It suggests that the Moon-forming impactor Theia plausibly shared a similar isotopic composition and accretion history with that of the proto-Earth.

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arXiv2025

On inertial forces (indirect terms) in problems with a central body

Gravitational systems in astrophysics often comprise a body -- the primary -- that far outweights the others, and which is taken as the centre of the reference frame. A fictitious acceleration, also known as the indirect term, must therefore be added to all other bodies in the system to compensate for the absence of motion of the primary. In this paper, we first stress that there is not one indirect term but as many indirect terms as there are bodies in the system that exert a gravitational pull on the primary. For instance, in the case of a protoplanetary disc with two planets, there are three indirect terms: one arising from the whole disc, and one per planet. We also highlight that the direct and indirect gravitational accelerations should be treated in a balanced way: the indirect term from one body should be applied to the other bodies in the system that feel its direct gravitational acceleration, and only to them. We point to situations where one of those terms is usually neglected however, which may lead to spurious results. These ideas are developed here for star-disc-planets interactions, for which we propose a recipe for the force to be applied onto a migrating planet, but they can easily be generalized to other astrophysical systems.

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arXiv2025

Very-wide-orbit planets from dynamical instabilities during the stellar birth cluster phase

Gas giant planets have been detected on eccentric orbits several hundreds of astronomical units in size around other stars. It has been proposed that even the Sun hosts a wide-orbit planet of 5-10 Earth masses, often called Planet Nine, which influences the dynamics of distant Trans-Neptunian objects. However, the formation mechanism of such planets remains uncertain. Here we use numerical simulations to show that very wide-orbit planets are a natural byproduct of dynamical instabilities that occur in planetary systems while their host stars are still embedded in natal stellar clusters. A planet is first brought to an eccentric orbit with an apoastron of several hundred au by repeated gravitational scattering by other planets, then perturbations from nearby stellar flybys stabilise the orbit by decoupling the planet from the interaction with the inner system. In our Solar System, the two main events likely conducive to planetary scattering were the growth of Uranus and Neptune, and the giant planets instability. We estimate a 5-10% likelihood of creating a very wide-orbit planet if either happened while the Sun was still in its birth cluster, rising to 40% if both were. In our simulated exoplanetary systems, the trapping efficiency is 1-5\%. Our results imply that planets on wide, eccentric orbits occur at least $10^{-3}$ per star.

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arXiv2025

Compositional Outcomes of Earth Formation from a Narrow Ring

We address Earth formation from an elemental perspective, using a method similar to Rubie et al. (2015) but with updates from Dale et al. (2023) to simulate the chemical evolution of Earth's mantle during metal-silicate equilibration events from accretional collisions. Our model introduces two key differences: (1) Earth forms from a dense ring of planetesimals and planetary embryos near 1 AU, extending into the asteroid belt, and (2) we divide this population into four zones. The innermost zone contains planetesimals enriched in refractory elements relative to Si and depleted in volatiles. The remaining zones represent enstatite, ordinary, and CI chondrites. We fit the Earth's bulk silicate composition by adjusting the boundaries of these zones and the refractory enrichment in the inner zone, giving us four compositional free parameters. A fifth parameter relates to the depth of planetesimal equilibration after a giant impact. We examined twenty-two ring model simulations, expanded to forty-eight based on hot or cold targets during collisions. Seventeen simulations resulted in a mantle chemistry resembling the bulk silicate Earth (BSE), despite differences in growth sequences. These variations lead to different fitting parameter values, altering the proportions of different meteorite types required to match the BSE. However, findings show Earth must accrete 60-80% of material from the innermost refractory-enriched zone. This indicates that, with the right growth sequence, multiple ring model structures can yield an Earth-anaologue composition consistent with the BSE.

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arXiv2025

The refractory-to-ice ratio in comet 67P: implications on the composition of the comet-forming region of the protoplanetary disk

Comets, asteroids, and other small bodies are thought to be remnants of the original planetesimal population of the Solar System. As such, their physical, chemical, and isotopic properties hold crucial details on how and where they formed and how they evolved. Yet, placing precise constraints on the formation region of these bodies has been challenging. Data from spacecraft missions have a particularly high potential of addressing the question of the origin of the visited bodies. ESA's Rosetta mission to comet 67P/Churyumov-Gerasimenko returned data from the comet for two years on its journey around the Sun. This extensive data set has revolutionised our view on comets and still holds unsolved problems. We use the Rosetta/ROSINA (Rosetta Orbiter Spectrometer for Ion and Neutral Analysis) measurement of the volatile/ice composition and the Rosetta/COSIMA (COmetary Secondary Ion Mass Analyzer) measurements of the refractory composition of comet 67P. These measurements are combined using a Monte Carlo method. The refractory-to-ice ratio is a free parameter that is constrained a posteriori. Using only the composition, we constrain the refractory-to-ice ratio to $0.5<\chi<1.7$, and derive the bulk elemental abundances for 67P of H, C, N, O, Na, Mg, Al, S, K, Ar, Ca, Cr, Mn, Fe, Kr, and Xe. We find the noble gas xenon in near solar elemental abundance in comet 67P. Krypton is slightly depleted, while argon is heavily depleted. Comet 67P is enriched in all three noble gases by up to 2.5 orders of magnitude compared to CI chondrites. We show this is consistent with a formation region between 25 and 35 au in a protoplanetary disk region with temperatures between 30 and 40 K and with the trapping of dust for a long time in rings of the protoplanetary disk.

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arXiv2025

Reassessing the origin and evolution of Ecliptic Comets in the Planet-9 Scenario

A group of newly observed extreme trans-Neptunian objects exhibit unexpected orbital confinement, characterized by the alignment of orbital angular momentum vectors and apsidal lines. It is proposed that an undiscovered giant planet, named Planet-9, exists in the solar system's outer regions and causes this clustering. Initial studies suggested Planet-9 could have a mass of 15 Earth masses. However, such a massive planet strongly interacts with scattered disk objects (SDOs; 50 < a < 1000 au) and influences the orbits of short-period comets, resulting in orbital inclinations inconsistent with observations. This study models the formation and long-term evolution of trans-Neptunian object populations and the Oort cloud during the solar system's dynamical instability, using revised parameters for Planet-9. Simulations assume Planet-9 has a mass of 7.5 Earth masses, an inclination of ~20 degrees, a semi-major axis of ~600 au, and an eccentricity of ~0.3. Results suggest a less massive Planet-9 aligns with observed trans-Neptunian object inclinations and the number of ecliptic comets (D > 10 km). Distant Kuiper belt objects with 40 < q < 100 au and 200 < a < 500 au, particularly with significant inclinations, are more likely to align apsidally with Planet-9, with an anti-aligned-to-aligned ratio of 0.5-0.7. Lower inclination objects (<20 degrees) exhibit significant apsidal anti-alignment, with an anti-aligned-to-aligned ratio of 2-4. These findings offer a new observational direction to refine the search for Planet-9.

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arXiv2024

Did the terrestrial planets of the Solar System form by pebble accretion?

The dominant accretion process leading to the formation of the terrestrial planets of the Solar System is a subject of intense scientific debate. Two radically different scenarios have been proposed. The classic scenario starts from a disk of planetesimals which, by mutual collisions, produce a set of Moon to Mars-mass planetary embryos. After the removal of gas from the disk, the embryos experience mutual giant impacts which, together with the accretion of additional planetesimals, lead to the formation of the terrestrial planets on a timescale of tens of millions of years. In the alternative, pebble accretion scenario, the terrestrial planets grow by accreting sunward-drifting mm-cm sized particles from the outer disk. The planets all form within the lifetime of the disk, with the sole exception of Earth, which undergoes a single post-disk giant impact with Theia (a fifth protoplanet formed by pebble accretion itself) to form the Moon. To distinguish between these two scenarios, we revisit all available constraints: compositional (in terms of nucleosynthetic isotope anomalies and chemical composition), dynamical and chronological. We find that the pebble accretion scenario is unable to match these constraints in a self-consistent manner, unlike the classic scenario.

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arXiv2024

Mechanisms and timing of carbonaceous chondrite delivery to the Earth

The nucleosynthetic isotope signatures of meteorites and the bulk silicate Earth (BSE) indicate that Earth consists of a mixture of "carbonaceous" (CC) and "non-carbonaceous" (NC) materials. We show that the fration of CC material recorded in the isotopic composition of the BSE varies for different elements, and depends on the element's tendency to partition into metal and its volatility. The observed behaviour indicates that the majority of material accreted to the Earth was NC-dominated, but that CC-dominated material enriched in moderately-volatile elements by a factor of ~10 was delivered during the last ~2-10% of Earth's acccretion. The late delivery of CC material to Earth contrasts with dynamical evidence for the early implantation of CC objects into the inner solar system during the growth and migration of the giant planets. This, together with the NC-dominated nature of both Earth's late veneer and bulk Mars, suggests that material scattered inwards had the bulk of its mass concentrated in a few, large CC embryos rather than in smaller planetesimals. We propose that Earth accreted a few of these CC embryos while Mars did not, and that at least one of the CC embryos impacted Earth relatively late (when accretion was 90-90% complete). This scenario is consistent with the subsequent Moon-formign impact of a large NC body, as long as this impact did not re-homogenize the entire Earth's mantle.

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