arXiv subjects

Anders Johansen

Publications and source records attributed to Anders Johansen.

At least 19 recordsLinked to original sources

arXiv2026

Volatile depletion in rocky planets as a chemical fingerprint of hybrid accretion

Volatile depletion in rocky planets relative to their host stars is commonplace in both the Solar System and exoplanetary systems, yet the connections between planet formation and composition remain elusive. Here we model devolatilization during pebble accretion in combination with collisional growth from volatile-depleted planetesimals to explore the formation pathways of Earth and Mars. Using Bayesian inference, we find that bulk silicate Earth is best reproduced by ${\gtrsim}$75% contribution from two protoplanets formed via pebble accretion, supplemented by up to $\sim$25% material from planetesimals that are compositionally akin to the asteroid Vesta. Using instead a planetesimal volatile-depletion curve that is not observed among known meteorite parent bodies would allow the planetesimal contribution to reach 40$^{+15}_{-14}$%. In comparison, bulk silicate Mars reflects 27$\pm$5% pebble-accreted material and 73${\pm}$5% Vesta-like planetesimals. We identify volatile depletion as a chemical fingerprint of hybrid accretion, in which both pebble accretion and collisional assembly contribute to terrestrial planet growth. By quantitatively linking formation pathways to volatile budgets, our findings demonstrate how planetary accretion histories can be inferred from elemental signatures, with broad implications for interpreting the chemical diversity of rocky exoplanets.

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arXiv2026

Potential sublimating exocomets around the young star PDS 70

Recent observations by the James Webb Space Telescope (JWST) have indicated the presence of water in the inner regions of the disc around the young protostar PDS 70, but the origin of this water remains unclear. Here we report the discovery of variable absorption lines of neutral sodium in archival High Accuracy Radial velocity Planet Searcher (HARPS) spectra of PDS 70. These lines vary strongly and stochastically on a daily basis, in amplitude, number, and radial velocity. Our measurements indicate that this gas tends to be optically thick, and partially cover the stellar disc, meaning this fast-moving gas is often spatially confined. We explore several hypotheses for the origin of these lines, and conclude that a likely source of the observed sodium is sublimation of planetesimals that transit the star on highly elliptical orbits, reminiscent of the exocomet phenomenon seen in other seen in other extrasolar systems. These sublimating exocomets could play a role in sourcing the previously observed water in the inner, terrestrial-planet forming region of the PDS 70 system.

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arXiv2026

Chondrule formation in the outer disk from the primary three-dimensional chemical composition of CM chondrules

Chondrules and their associated fine-grained rims record key processes in the early protoplanetary disk, yet the links between chondrule chemistry, morphology, and matrix complementarity remain poorly constrained. We investigate the major, minor, and trace element compositions of 66 chondrules and FGRs from the relatively unaltered CM carbonaceous chondrites Asuka 12236, Paris, and Maribo, together with their 3D morphology, using LA-ICP-MS and X-ray tomography. CM chondrules record systematic metal loss and evaporation of Si-rich mesostasis, driving initially CI-like precursor compositions toward more Mg- and Si-rich bulk compositions along the CI ratio line and toward increasingly Si-poor forsteritic assemblages. GEMS-like materials in pristine CM matrices closely mirror chondrule compositions and likely represent complementary condensates derived from evaporated mesostasis. Dust accreted onto chondrules is predominantly CI-like but contains about 14 wt.% complementary condensate material represented by chondritic amorphous silicates, reconciling Mg/Si complementarity between chondrules and matrix with the preservation of primordial organics and presolar grains. Morphological observations show no significant sectioning bias, consistent with CM chondrules being dominated by agglomerates of ~100 um microspherules. Many display grape-bunch textures produced by welding of smaller chondrules with metal-rich or CI-like rims. This structure may explain the chondrule moderately volatile-element plateau at about 0.3xCI. We propose a "micro-chondrule-first" scenario in which localized heating events produced small molten droplets that subsequently accreted CI-like dust and ice, aggregated, and experienced limited aqueous alteration. These observations place new constraints on chondrule formation in the outer disk and highlight the importance of localized melting and aggregation processes.

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arXiv2026

Accretion across scales: streamers, surface-layer transport, and rapid replenishment in young protoplanetary discs

Protoplanetary discs evolve around newly-formed stars through an interplay of infall from surrounding turbulent cloud material, accretion towards the young star, and outflow driven mass-loss. It has been challenging to determine if discs are fed predominantly through infall along the disc midplane, or along the poles, and if accretion occurs in a steady or burst-like fashion. Here, we present a suite of 3D ideal magnetohydrodynamical simulations of protoplanetary disc formation and evolution in a dynamic, large-scale molecular cloud environment using the adaptive mesh refinement framework DISPATCH. We focus on nine stellar systems, where we resolve discs down to a scale of 0.8 au. Across the sample, stellar accretion proceeds at rates of $\sim$10$^{-5}$ M$_\odot$ yr$^{-1}$ over 10$^{5}$ yr, with significant variability. Discs grow to 100 au scales and remain gravitationally stable in time, with disc-to-star mass ratios below 10 %. Transient high-density streamers, with 10 kyr infall times, can drive anisotropic mass delivery at rates comparable to the background accretion flow. Their interaction with discs typically results in a temporary reduction of the disc size by half, and disc mass by 40 %. During later quiescent disc evolution stages ($t\gtrsim$50 kyr), accretion predominantly occurs through the midplane and disc surface layers. This is associated with the development of a toroidal magnetic field morphology, which includes field reversals across both disc surfaces. In this way, the full disc mass reservoir is replenished on 10 kyr-timescales. These findings support that the outer parts of very young discs, when well-ionised and close to the ideal MHD regime, are not yet conducive to planet formation, due to high replenishment rates, strong turbulence, and disruptive streamer infall events.

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arXiv2026

Machine-learning clustering of close-in exoplanet populations: links to pebble accretion

Close-in exoplanets exhibit a wide range of orbital architectures and physical properties shaped by both formation conditions and migration processes. Although population-synthesis models predict distinct planetary populations, establishing a quantitative connection between observed exoplanets and synthetic populations remains challenging. We investigate the intrinsic organisation of close-in exoplanets using physically motivated dynamical parameters and connect the resulting populations to pebble-accretion formation pathways. A two-stage Gaussian mixture model (GMM) is applied to an observed sample of close-in exoplanets, performing unsupervised probabilistic clustering in a feature space dominated by dynamical descriptors of planet-star interactions. The resulting clusters are mapped onto a pebble-accretion synthetic population within a statistically motivated three-dimensional parameter space. Formation-related quantities, including gas availability, gas fraction, and ice-rock mass ratio, are then used to interpret the mapped populations. We identify statistically supported sub-populations without imposing predefined classification boundaries, including very-massive gas giants, hot giants, warm-Jupiter-dominated systems, and lower-mass giants. The mapped synthetic populations reveal systematic differences in formation timing, gas accretion, and solid growth histories. In particular, very-massive gas giants are preferentially associated with earlier formation epochs than hot-giant and warm-Jupiter-dominated populations. These results demonstrate that physically motivated machine-learning approaches can provide a statistically robust framework for linking observed exoplanet populations to theoretical planet formation pathways.

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arXiv2026

Exploring the conditions for forming planetesimals by the streaming instability and planetary systems by pebble accretion

The streaming instability and pebble accretion are two physical mechanisms with demonstrated potentials to drive, respectively, the formation of planetesimals and the growth of planetary systems containing a diverse range of planetary types. Here we explore the protoplanetary disc conditions in terms of turbulence strength, Stokes number and initial disc size that are needed to (i) form planetesimals by the streaming instability, (ii) form gas giant planets in cold orbits, (iii) form super-Earths and sub-Neptunes close to the star and (iv) form rocky planet embryos in temperate orbits. We identify an optimum Stokes number range between St= 0.01 and St= 0.03 where all three planetary classes form and where the streaming instability is triggered for a slightly elevated pebble metallicity. Cold gas giants require a turbulence strength of at most $\delta=10^{-4}$ and furthermore need large initial disc sizes to benefit from a prolonged pebble flux; super-Earths and rocky planet embryos tolerate higher turbulence strengths similar to those measured for the vertical shear instability. A higher Stokes number of St=0.1 is detrimental to the formation of cold gas giants due to the short-lived pebble flux. For Stokes numbers below St= 0.003, extremely low values of turbulence ($\delta<10^{-5}$) are required to form cold gas giants. We highlight how loss of gas to disc winds, reduction in the migration speed by thermal or dynamical torques or the presence of pressure bumps in the outer disc could increase the parameter space for the formation of cold gas giants. We derive analytically that the mass of the largest planetesimals formed by the streaming instability is of similar magnitude to the threshold mass beyond which pebble accretion becomes efficient, if planetesimals form in the earliest phases of protoplanetary disc evolution.

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arXiv2026

How leaky? A large parameter study of leaky dust traps to quantify the transport of pebbles and ice in protoplanetary discs

In protoplanetary discs, the presence of dust traps can significantly alter the transport of solids from the outer to the inner regions, and hence they are often invoked as an explanation for the chemical diversity of inner discs observed with JWST (e.g., varying oxygen abundances and C/O ratios). As a detailed treatment of dust transport around dust traps is computationally expensive, earlier works investigating the impact of outer traps on the inner disc composition have often used simplified dust models representing the size distribution with a single effective size and drift speed. In this paper, we revisit the impact of outer traps on dust transport using the state-of-the-art one-dimensional dust evolution code \texttt{DustPy}, which simulates the transport and evolution of dust particles including detailed coagulation and fragmentation. We quantify and map the leakiness of dust traps across a broad parameter space, performing over 300 simulations while varying the disc viscosity, turbulence strength, planet mass and location, and dust fragmentation velocity. We find that dust traps are leakier than previously thought, on a broader parameter space, such that most outer traps (r > 5 au) will result in a long-lived O-rich inner disc with gas-phase C/O < 1. In similar conditions (e.g., carved by the same planet mass), we find inner traps are much leakier than outer traps, though their relative efficiency in reducing the pebble flux is time-dependent. Highly blocking traps altering the inner disc composition dramatically (leading, e.g., to C/O > 1) are possible to set up but necessitate low viscosity and weak turbulence, along with efficient planetesimal formation by the streaming instability. In that case, we find that is the formation of planetesimals, rather than the dust traps themselves, that is capable of significantly altering the inner disc composition.

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arXiv2026

Gas dynamics around dust asymmetries in turbulent disks

High-resolution ALMA observations have revealed asymmetric dust crescents in several protoplanetary disks, suggesting efficient dust trapping mechanisms potentially linked to gas vortices. While such features have been associated with vortices--whether induced by massive planets, turbulence , or other disk processes--their origin remains unclear. In this study, we investigate the viability of dust trapping by vortices that are self-sustained in disks dominated by Vertical Shear Instability (VSI) turbulence. We perform 3D hydrodynamic simulations using the PLUTO code with Lagrangian particles of three sizes (1 mm, 500~$\mu$m, 100~$\mu$m) to analyze the gas-dust dynamics around vortices. Our simulations reveal the formation of multiple vortices, including two characteristic large-scale, long-lived vortices that are able to capture the dust particles. We also find that dust vertical diffusion is reduced within vortices, suggesting that these structures preferentially enhance radial and azimuthal motions. Finally we generate synthetic dust continuum images at different wavelength bands and velocity residuals to compare the observable properties with ALMA observations. No clear spiral features are observed in either the synthetic dust images or the velocity residuals, unlike in vortices triggered by planets. Projection effects at high disk inclinations can obscure dust asymmetries, implying that more disks may host dust crescents than currently reported.

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arXiv2025

Interstellar Ices as Carriers of Supernova Material to the Early Solar System

Planetary materials show systematic variations in their nucleosynthetic isotope compositions that resonate with orbital distance. The origin of this pattern remains debated, limiting how these isotopic signatures can be used to trace the precursors of terrestrial planets. Here we test the hypothesis that interstellar ices carried supernova-produced nuclides by searching for a supernova nucleosynthetic fingerprint in aqueous alteration minerals from carbonaceous and non-carbonaceous chondrite meteorites. We focus on zirconium, a refractory element that includes the neutron-rich isotope $^{96}$Zr formed in core-collapse supernovae. Leaching experiments reveal extreme $^{96}$Zr enrichments in alteration minerals, showing that they incorporated supernova material hosted in interstellar ices. We show that the Solar System's zirconium isotope variability reflects mixing between these ices and an ice-free rocky component. Finally, the presence of supernova nuclides in a volatile carrier supports models where the Solar System's nucleosynthetic variability was imparted by thermal processing of material in the protoplanetary disk and during planetary accretion.

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arXiv2025

The cosmochemistry of planetary systems

Planets form and obtain their compositions from the leftover material present in protoplanetary disks of dust and gas surrounding young stars. The chemical make-up of a disk influences every aspect of planetary composition including their overall chemical properties, volatile content, atmospheric composition, and potential for habitability. This Review discusses our knowledge of the chemical and isotopic composition of Solar System materials and how this information can be used to place constraints on the formation pathways of terrestrial planets. We conclude that planetesimal formation by the streaming instability followed by rapid accretion of drifting pebbles within the protoplanetary disk lifetime reproduces most of the chemical and isotopic observables in Solar System. This finding has important implications for planetary habitability beyond the Solar System because in pebble accretion, volatiles important for life are accreted during the main growth phase of rocky planets as opposed to the late-stage. Finally, we explore how bulk chemical inventories and masses of planetary bodies control the composition of their primordial atmospheres and their potential to develop habitable conditions.

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arXiv2025

How stellar mass and disc size shape the formation and migration of super-Earths

The occurrence rate of close-in super-Earths is higher around M-dwarfs compared to stars of higher masses. In this work we aim to understand how the super-Earth population is affected by both the stellar mass, the size of the protoplanetary disc, and viscous heating. We utilise a standard protoplanetary disc model with both irradiated and viscous heating together with a pebble accretion model to simulate the formation and migration of planets. We find that if the disc is heated purely through stellar irradiation, inwards migration of super-Earths is very efficient, resulting in the close-in super-Earth fraction increasing with increasing stellar mass. In contrast, when viscous heating is included, planets can undergo outwards migration, delaying migration to the inner edge of the protoplanetary disc, which causes a fraction of super-Earth planets to grow to become giant planets instead. This results in a significant reduction of inner super-Earths around high-mass stars and an increase in the number of giant planets, both of which mirror observed features of the planet population around high-mass stars. This effect is most pronounced when the protoplanetary disc is large, since such discs evolve over a longer time-scale. We also test a model when we inject protoplanets at a fixed time early on in the disc lifetime. In this case, the fraction of close-in super-Earths decreases with increasing stellar mass in both the irradiated case and viscous case, since longer disc lifetimes around high-mass stars allows for planets to grow into giants instead of super-Earths for most injection locations.

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arXiv2025

The cosmic journey of dust grains -- from nucleation to planetary system

Dust is essential to the evolution of galaxies and drives the formation of planetary systems. The challenge of inferring the origin of different presolar dust grains from meteoritic samples motivates forward modelling to understand the contributions of low- and high-mass stars to dust in our Solar System. In this work we follow the evolution of dust with tracer particles within a hydrodynamical simulation of a Milky Way-like isolated disc galaxy. We find that nearly half of the grains released from stars lose less than $10\%$ of their initial mass due to thermal sputtering in the interstellar medium (ISM), with an average degree of atomisation $\sim$$10\%$ higher for dust grains released by supernovae relative to asymptotic giant branch (AGB) star grains. We show through supernova remnant model variations that supernova (SN) dust survival is primarily shaped by the supernova bubble environment in the first million years (Myr) after the explosion rather than by its evolution during $10^2-10^3$ Myr in the ISM. The AGB/SN ratio of dust grains incorporated into newly formed stars approaches $0.8$ after a few hundred Myr of galactic evolution. Our analysis also shows that star-forming particles with short ($<$$10$ Myr) free-floating time-scales in the ISM are predominantly released from supernovae rather than AGB stars. This implies that the Solar System budget of short-lived radioactive isotopes such as $^{26}$Al, whose decay contributed to melting and differentiating planetesimals, should have been provided by massive stars with masses $M \gtrsim 8$ M$_{\odot}$.

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arXiv2025

Formation of Planetesimals in the Outer Solar System

The Solar System hosts the most studied and best understood major and minor planetary bodies - and the only extraterrestrial bodies to have been visited by spacecraft. The Solar System therefore provides important constraints on both the initial stages of planetary growth, communicated to us by its surviving planetesimal populations, and for the final result of the planet formation process represented by the architecture of the system and properties of the individual planets. We review here models of planetesimal formation in the outer Solar System as well as the wealth of recent observational constraints that has been used to formulate and refine modern planetesimal formation theory.

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arXiv2025

On Linking Planet Formation Models, Protoplanetary Disk Properties, and Mature Gas Giant Exoplanet Atmospheres

Measuring a single elemental ratio (e.g., carbon-to-oxygen) provides insufficient information for understanding the formation mechanisms and evolution that affect our observations of gas giant planet atmospheres. Although the fields of planet formation, protoplanetary disks, and exoplanets are well established and interconnected, our understanding of how to self-consistently and accurately link the theoretical and observational aspects of these fields together is lacking. To foster interdisciplinary conversations, the Max-Planck Institut für Astronomie (MPIA) hosted a week-long workshop called, "Challenge Accepted: Linking Planet Formation with Present-Day Atmospheres." Here, we summarize the latest theories and results in planet formation modeling, protoplanetary disk observations, and atmospheric observations of gas giant atmospheres to address one of the challenges of hosting interdisciplinary conferences: ensuring everyone is aware of the state-of-the-art results and technical language from each discipline represented. Additionally, we highlight key discussions held at the workshop. Our main conclusion is that it is unclear what the ideal observable is to make this link between formation scenarios and exoplanet atmospheres, whether it be multiple elemental abundance ratios, measuring refractory budgets, or something else. Based on discussions held throughout the workshop, we provide several key takeaways of what the workshop attendees feel need the most improvement and exploration within each discipline.

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arXiv2025

Burned to ashes: How the thermal decomposition of refractory organics in the inner protoplanetary disc impacts the gas-phase C/O ratio

The largest reservoir of carbon in protoplanetary discs is stored in refractory organics, which thermally decompose into the gas-phase at the organics line, well interior to the water iceline. Because this region is so close to the host star, it is often assumed that the released gaseous material is rapidly accreted and plays little role in the evolution of the disc composition. However, laboratory experiments show that the thermal decomposition process is irreversible, breaking macromolecular refractory organics into simpler, volatile carbon-bearing compounds. As a result, unlike the iceline of other volatiles, which traps vapor inwards due to recondensation, the organics line remains permeable, allowing gaseous carbon to diffuse outward without returning to the solid phase. In this paper, we investigate how this process affects the disc composition, particularly the gas-phase C/H and C/O ratios, by incorporating it into a 1D evolution model for gas and solids, and assuming refractory organics dominantly decompose into C$_2$H$_2$. Our results show that this process allows this carbon-rich gas to survive well beyond the organics line (out to $7 \mathrm{~au}$ around a solar-mass star) and for much longer timescales, such that its abundance is increased by an order of magnitude. This has several implications in planet formation, notably by altering how the composition of solids and gas relate, and the fraction of heavy elements available to giant planets. In the framework of our model, refractory organics significantly influence the evolution of the gas-phase C/O ratio, which may help interpreting measurements made with Spitzer and JWST.

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arXiv2025

A primordial radius valley as a consequence of planet formation

The radius distribution of close-in planets has been observed to have a bimodal distribution with a dearth of planets around ~1.5-2.0 $R_\oplus$ commonly referred to as the ''radius valley''. The origin of the valley is normally attributed to mass-loss process such as photoevaporation or core-powered mass loss. Recent work, however, has suggested that the radius valley may instead arise as a consequence of gas accretion by low-mass planets. In this work we therefore aim to investigate the formation of a primordial radius valley from the formation of planet cores through pebble accretion up until the dissipation of the protoplanetary disc and subsequent contraction of accreted atmospheres. The goal of this work is to explore the conditions for forming a primordial radius valley from first principles of planet formation theory, rather than attempting to explain the detailed structure of the observed valley. We use an analytical model with minimal assumptions to estimate the contraction rate of atmospheres and, indeed, find the formation of a primordial radius valley. The planets smaller than the valley did not reach the pebble isolation mass, which is required for the planets to cool down sufficiently to be able to accrete a significant amount of gas. We also estimate the slopes of the radius gap as a function of orbital period for the intrinsic population as well as for planets with orbital periods <100 days. For the intrinsic population, the radius gap follows the pebble isolation mass and increases with increasing orbital period, while for close-in planets the direction of the slope reverses and decreases with increasing orbital period. We find that planets smaller than the radius valley are predominantly rocky while the population of planets larger than the valley consists of a mixture of rocky and water-rich planets.

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arXiv2025

The influence of dust growth on the observational properties of circumplanetary discs

Dust growth is often indirectly inferred observationally in star-forming environments, theoretically predicted to produce mm-sized particles in circumstellar discs, and also presumably witnessed by the predecessors of the terrestrial meteoritic record. For those reasons it is believed that young gas giants under formation in protoplanetary discs with putative circumplanetary discs (CPDs) surrounding them, such as PDS 70c, should be containing mm-sized particles. We model the spectra of a set of CPDs, which we obtained from radiation hydrodynamic simulations at varying Rosseland opacities kappa_R. The kappa_R from the hydrodynamic simulations are matched with consistent opacity sets of ISM-like composition, but grown to larger sizes. Our high kappa_R hydro data nominally corresponds to 10 mum-sized particles, and our low kappa_R-cases correspond to mm-sized particles. We investigate the resulting broad spectral features at first while keeping the overall optical depth in the planetary envelope constant. Dust growth to size distributions dominated by millimeter particles generally results in broad, featureless spectra with black-body like slopes in the far-infrared, while size distributions dominated by small dust develop steeper slopes in the far-infrared and maintain some features stemming from individual minerals. We find that significant dust growth from microns to millimeters can explain the broad features of the PDS 70c data, when upscaling the dust masses from our simulations by x100. Furthermore our results indicate that the spectral range of 30-500 mum is an ideal hunting ground for broadband features arising from the CPD, but that longer wavelengths observed with ALMA can also be used for massive circumplanetary discs

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arXiv2025

The coexistence of the streaming instability and the vertical shear instability in protoplanetary disks: Scale-dependence of dust diffusion

The vertical shear instability and the streaming instability are two robust sources of turbulence in protoplanetary disks. The former has been found to induce anisotropic turbulence that is stronger in the vertical than in the radial dimension and to be overall stronger compared to the largely isotropic turbulence caused by the streaming instability. In this study, we shed light on the dust diffusion by the vertical shear instability and the streaming instability separately and together, and in particular on the direction- and scale-dependence of the diffusion. To this end, we employ two-dimensional global models of the two instabilities either in isolation or in combination. The vertical shear instability in isolation diffuses dust more strongly in the vertical direction than the streaming instability in isolation, resulting in a wave-shaped dust layer in our two-dimensional simulations. Compared with this large-scale diffusion, though, our study highlights that the vertical shear instability causes substantially weaker or even negligible small-scale diffusion. We validate this result using previously published three-dimensional simulations. In particular when simulating centimetre-sized dust, the undulating dust layer becomes internally razor-thin. In contrast, the diffusion owing to the streaming instability exhibits only a marginal scale-dependence, with the dust layer possessing a Gaussian shape. In models including both instabilities, the undulating mid-plane layer is broadened to a width set by the intrinsic diffusion level caused by the streaming instability.

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