arXiv subjects

Fred J. Ciesla

Publications and source records attributed to Fred J. Ciesla.

At least 19 recordsLinked to original sources

arXiv2025

Soot Planets instead of Water Worlds

Some low-density exoplanets are thought to be water-rich worlds that formed beyond the snow line of their protoplanetary disc, possibly accreting coequal portions of rock and water. However, the compositions of bodies within the Solar System and the stability of volatile-rich solids in accretionary disks suggest that a planet rich in water should also acquire as much as 40% refractory organic carbon (``soot''). This would reduce the water mass fraction well below 50%, making the composition of these planets similar to those of Solar System comets. Here we show that soot-rich planets, with or without water, can account for the low average densities of exoplanets that were previously attributed to a binary combination of rock and water. Formed in locations beyond the soot and/or snow lines in disks, these planets are likely common in our galaxy and already observed by JWST. The surfaces and interiors of soot-rich planets will be influenced by the chemical and physical properties of carbonaceous phases, and the atmospheres of such planets may contain plentiful methane and other hydrocarbons, with implications for photochemical haze generation and habitability.

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arXiv2025

Oxygen Isotope Constraints on the Importance of Photochemical Processing in Protoplanetary Disks

Observations have revealed evidence of photochemical processing in protoplanetary disks. This processing occurs in the photon dominated layer, the optically thin regions of the disk high above the disk midplane. It remains unclear, however, how much this photochemical processing impacts the compositions of the planets and their building blocks within the disk. Here we use the oxygen isotopic compositions of Solar System solids, which has been attributed to photochemistry in the solar nebula, to quantitatively evaluate whether this processing could have produced the conditions needed to provide the diversity of compositions seen in the Solar System. We do this by modeling the chemical evolution while fine dust grows into the building blocks of the planets. We find that the oxygen isotopic evolution cannot be attributed to processing in the solar nebula and must instead be inherited from the parent molecular cloud. Further, our results indicate that the observed photochemical processing in protoplanetary disks does not significantly impact the compositions of planets that form within.

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arXiv2025

Dust Recycling and Icy Volatile Enhancement (DRIVE): A Novel Method of Volatile Enrichment in Cold Giant Planets

Giant planet atmospheres are thought to reflect the gas phase composition of the disk when and where they formed. However, these atmospheres may also be polluted via solid accretion or ice sublimation in the disk. Here, we propose a novel mechanism for enriching the atmospheres of these giant planets with volatiles via pebble drift, fragmentation, and ice sublimation. We use a combination of 3D hydrodynamic simulations, radiative transfer, and particle tracking to follow the trajectories and resulting temperatures of solids in a disk containing an embedded planet forming outside the CO snowline. We show that small dust can become entrained in the meridional flows created by the giant planet and advected above the disk midplane where temperatures are well above the sublimation temperature of CO. This transport of small grains occurs over 10 kyr timescales, with individual micron-sized grains cycling between the midplane and surface of the disk multiple times throughout the planetary accretion stage. We find that this stirring of dust results in sublimation of CO gas above the snow surface in the dust trap created exterior to the giant planet, leading to super-solar CO abundances in the pressure bump. This mechanism of Dust Recycling and Icy Volatile Enhancement in cold giant planets, which we call the DRIVE effect, may explain enhanced metallicities of both wide separation exoplanets as well as Jupiter in our own Solar System.

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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

Dynamics of small, constant size particles in a protoplanetary disk with an embedded protoplanet

Hydrodynamical simulations of protoplanetary disk dynamics are useful tools for understanding the formation of planetary systems, including our own. Approximations are necessary to make these simulations computationally tractable. A common assumption when simulating dust fluids is that of a constant Stokes number, a dimensionless number that characterizes the interaction between a particle and the surrounding gas. Constant Stokes number is not a good approximation in regions of the disk where the gas density changes significantly, such as near a planet-induced gap. In this paper, we relax the assumption of constant Stokes number in the popular FARGO3D code using semi-analytic equations for the drag force on dust particles, which enables an assumption of constant particle size instead. We explore the effect this change has on disk morphology and particle fluxes across the gap for both outward- and inward-drifting particles. The assumption of constant particle size, rather than constant Stokes number, is shown to make a significant difference in some cases, emphasizing the importance of the more accurate treatment.

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arXiv2025

Three-dimensional transport of solids in a protoplanetary disk containing a growing giant planet

We present the results of combined hydrodynamic and particle tracking post-processing modeling to study the transport of small dust in a protoplanetary disk containing an embedded embryo in 3D. We use a suite of FARGO3D hydrodynamic simulations of disks containing a planetary embryo varying in mass up to 300 $M_\oplus$ on a fixed orbit in both high and low viscosity disks. We then simulate solid particles through the disk as a post-processing step using a Monte Carlo integration, allowing us to track the trajectories of individual particles as they travel throughout the disk. We find that gas advection onto the planet can carry small, well-coupled solids across the gap opened in the disk by the embedded planet for planetary masses above the pebble isolation mass. This mixing between the inner and outer disk can occur in both directions, with solids in the inner disk mixing to the outer disk as well. Additionally, in low viscosity disks, multiple pile-ups in the outer disk may preserve isotopic heterogeneities, possibly providing an outermost tertiary isotopic reservoir. Throughout Jupiter's growth, the extent of mixing between isotopic reservoirs varied depending on dust size, gas turbulence, and the Jovian embryo mass.

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arXiv2024

Quantifying the high early solar cosmic ray flux with cosmogenic neon isotopes in refractory minerals

An enhancement in the activity of the early young Sun resulting in a high charged particle flux has been invoked to explain excesses in spallation-induced nuclides in primitive planetary materials. Astronomical observations of energetic outbursts of young stellar objects (YSOs) also support the idea of an active young Sun. However, the early solar cosmic-ray (SCR) flux has not been well constrained. Here we use measured concentrations of SCR-produced nuclides that formed and are preserved in meteoritical hibonite and spinel, some of the oldest solids formed in the Solar System, and physical models for dust transport in the early protoplanetary disk to determine the magnitude of the early SCR flux. We focus our attention on cosmogenic neon which cannot have been inherited from precursors and can only be produced in situ in solids. Our modeled effective exposure time to SCRs for these solids is very short on the order of years. This indicates the early SCR flux recorded in refractory mineral hibonite was up to ~7 orders of magnitude higher than the contemporary level. Our flux estimate is consistent with the >10^5 enhanced flux inferred from astronomical observations of greatly enhanced flare activities of YSOs.

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arXiv2022

Chemical Feedbacks of Pebble Growth: Impacts on CO depletion and C/O ratios

Observations of protoplanetary disks have revealed them to be complex and dynamic, with vertical and radial transport of gas and dust occurring simultaneously with chemistry and planet formation. Previous models of protoplanetary disks focused primarily on chemical evolution of gas and dust in a static disk, or dynamical evolution of solids in a chemically passive disk. In this paper, we present a new 1D method for modelling pebble growth and chemistry simultaneously. Gas and small dust particles are allowed to diffuse vertically, connecting chemistry at all elevations of the disk. Pebbles are assumed to form from the dust present around the midplane, inheriting the composition of ices at this location. We present the results of this model after 1 Myr of disk evolution around a 1$M_\odot$ star at various locations both inside and outside of the CO snowline. We find that for a turbulent disk ($α= 10^{-3}$), CO is depleted from the surface layers of the disk by roughly 1-2 orders of magnitude, consistent with observations of protoplanetary disks. This is achieved by a combination of ice sequestration and decreasing UV opacity, both driven by pebble growth. Further, we find the selective removal of ice species via pebble growth and sequestration can increase gas phase C/O ratios to values of approximately unity. However, our model is unable to produce C/O values of $\sim$1.5-2.0 inferred from protoplanetary disk observations, implying selective sequestration of ice is not sufficient to explain C/O ratios $>1$.

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arXiv2022

Thermal Processing of Solids Encountering a Young Jovian Core

Jupiter's enhancement in nitrogen relative to hydrogen when compared to the Sun has been interpreted as evidence that its early formation occurred beyond the N$_{2}$ snowline ($\sim$ 20-40 AU). However, the rapid growth necessary to form Jupiter before the dissipation of the solar nebula would lead to the forming planet's core reaching very high temperatures ($>$1000 K), which would lead to it warming its surroundings. Here, we explore the effects of a luminous planetary core on the solids that it ultimately accretes. We find that a critical transition occurs where very hot (rapidly accreting) cores drive off volatiles prior to accretion, while cool cores (slowly accreting) are able to inherit volatile rich solids. Given Jupiter's nitrogen enrichment, if it formed beyond the N$_{2}$ snowline, its core could not have accreted solids at a rate above 10$^{-10}$ M$_{\odot}$ yr$^{-1}$. Our results suggest that either Jupiter formed in more distal regions of the solar nebula, or nitrogen loss was suppressed, either by its incorporation in more refractory carriers or because it was trapped within ices which devolatilized at higher temperatures.

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arXiv2021

Why do M dwarfs have more transiting planets?

We propose a planet formation scenario to explain the elevated occurrence rates of transiting planets around M dwarfs compared to sun-like stars discovered by Kepler. We use a pebble drift and accretion model to simulate the growth of planet cores inside and outside of the snow line. A smaller pebble size interior to the snow line delays the growth of super-Earths, allowing giant planet cores in the outer disk to form first. When those giant planets reach pebble isolation mass they cut off the flow of pebbles to the inner disk and prevent the formation of close-in super-Earths. We apply this model to stars with masses between 0.1 and 2 solar mass and for a range of initial disk masses. We find that the masses of hot super-Earths and of cold giant planets are anti-correlated. The fraction of our simulations that form hot super-Earths is higher around lower-mass stars and matches the exoplanet occurrence rates from Kepler. The fraction of simulations forming cold giant planets is consistent with the stellar mass dependence from radial velocity surveys. A key testable prediction of the pebble accretion hypothesis is that the occurrence rates of super-Earths should decrease again for M dwarfs near the sub-stellar boundary like Trappist-1.

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arXiv2021

The Mass Budgets and Spatial Scales of Exoplanet Systems and Protoplanetary Disks

Planets are born from disks of gas and dust, and observations of protoplanetary disks are used to constrain the initial conditions of planet formation. However, dust mass measurements of Class II disks with ALMA have called into question whether they contain enough solids to build the exoplanets that have been detected to date. In this paper, we calculate the mass and spatial scale of solid material around Sun-like stars probed by transit and radial velocity exoplanet surveys, and compare those to the observed dust masses and sizes of Class II disks in the same stellar mass regime. We show that the apparent mass discrepancy disappears when accounting for observational selection and detection biases. We find a discrepancy only when the planet formation efficiency is below 100%, or if there is a population of undetected exoplanets that significantly contributes to the mass in solids. We identify a positive correlation between the masses of planetary systems and their respective orbital periods, which is consistent with the trend between the masses and the outer radii of Class II dust disks. This implies that, despite a factor 100 difference in spatial scale, the properties of protoplanetary disks seem to be imprinted on the exoplanet population.

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arXiv2021

Earth's carbon deficit caused by early loss through irreversible sublimation

Carbon is an essential element for life but its behavior during Earth's accretion is not well understood. Carbonaceous grains in meteoritic and cometary materials suggest that irreversible sublimation, and not condensation, governs carbon acquisition by terrestrial worlds. Through astronomical observations and modeling we show that the sublimation front of carbon carriers in the solar nebula, or the soot line, moved inward quickly so that carbon-rich ingredients would be available for accretion at 1 au after the first million years. On the other hand, geological constraints firmly establish a severe carbon deficit in Earth, requiring the destruction of inherited carbonaceous organics in the majority of its building blocks. The carbon-poor nature of the Earth thus implies carbon loss in its precursor material through sublimation within the first million years.

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arXiv2021

Early volatile depletion on planetesimals inferred from C-S systematics of iron meteorite parent bodies

During the formation of terrestrial planets, volatile loss may occur through nebular processing, planetesimal differentiation, and planetary accretion. We investigate iron meteorites as an archive of volatile loss during planetesimal processing. The carbon contents of the parent bodies of magmatic iron meteorites are reconstructed by thermodynamic modelling. Calculated solid/molten alloy partitioning of C increases greatly with liquid S concentration and inferred parent body C concentrations range from 0.0004 to 0.11 wt.%. Parent bodies fall into 2 compositional clusters characterized by cores with medium, and low C/S. Both of these require significant planetesimal degassing, as metamorphic devolatilization on chondrite-like precursors is insufficient to account for their C depletions. Planetesimal core formation models, ranging from closed system extraction to degassing of a wholly molten body, show that significant open system silicate melting and volatile loss is required to match medium and low C/S parent body core compositions. Greater depletion in C relative to S is the hallmark of silicate degassing, indicating that parent body core compositions record processes that affect composite silicate/iron planetesimals. Degassing of bare cores stripped of their silicate mantles would deplete S with negligible C loss, and could not account for inferred parent body core compositions. Devolatilization during small-body differentiation is thus a key process in shaping the volatile inventory of terrestrial planets derived from planetesimals and planetary embryos.

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arXiv2020

CO Depletion in Protoplanetary Disks: A Unified Picture Combining Physical Sequestration and Chemical Processing

The gas-phase CO abundance (relative to hydrogen) in protoplanetary disks decreases by up to 2 orders of magnitude from its ISM value ${\sim}10^{-4}$, even after accounting for freeze-out and photo-dissociation. Previous studies have shown that while local chemical processing of CO and the sequestration of CO ice on solids in the midplane can both contribute, neither of these processes appears capable of consistently reaching the observed depletion factors on the relevant timescale of $1{-}3\mathrm{~Myr}$. In this study, we model these processes simultaneously by including a compact chemical network (centered on carbon and oxygen) to 2D ($r+z$) simulations of the outer ($r>20\mathrm{~au}$) disk regions that include turbulent diffusion, pebble formation, and pebble dynamics. In general, we find that the CO/H$_2$ abundance is a complex function of time and location. Focusing on CO in the warm molecular layer, we find that only the most complete model (with chemistry and pebble evolution included) can reach depletion factors consistent with observations. In the absence of pressure traps, highly-efficient planetesimal formation, or high cosmic ray ionization rates, this model also predicts a resurgence of CO vapor interior to the CO snowline. We show the impact of physical and chemical processes on the elemental (C/O) and (C/H) ratios (in the gas and ice phases), discuss the use of CO as a disk mass tracer, and, finally, connect our predicted pebble ice compositions to those of pristine planetesimals as found in the Cold Classical Kuiper Belt and debris disks.

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arXiv2020

Earths in Other Solar Systems N-body simulations: the Role of Orbital Damping in Reproducing the Kepler Planetary Systems

The population of exoplanetary systems detected by Kepler provides opportunities to refine our understanding of planet formation. Unraveling the conditions needed to produce the observed exoplanets will sallow us to make informed predictions as to where habitable worlds exist within the galaxy. In this paper, we examine using N-body simulations how the properties of planetary systems are determined during the final stages of assembly. While accretion is a chaotic process, trends in the ensemble properties of planetary systems provide a memory of the initial distribution of solid mass around a star prior to accretion. We also use EPOS, the Exoplanet Population Observation Simulator, to account for detection biases and show that different accretion scenarios can be distinguished from observations of the Kepler systems. We show that the period of the innermost planet, the ratio of orbital periods of adjacent planets, and masses of the planets are determined by the total mass and radial distribution of embryos and planetesimals at the beginning of accretion. In general, some amount of orbital damping, either via planetesimals or gas, during accretion is needed to match the whole population of exoplanets. Surprisingly, all simulated planetary systems have planets that are similar in size, showing that the "peas in a pod" pattern can be consistent with both a giant impact scenario and a planet migration scenario. The inclusion of material at distances larger than what Kepler observes has a profound impact on the observed planetary architectures, and thus on the formation and delivery of volatiles to possible habitable worlds.

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arXiv2019

The Exoplanet Population Observation Simulator. II -- Population Synthesis in the Era of Kepler

The collection of planetary system properties derived from large surveys such as Kepler provides critical constraints on planet formation and evolution. These constraints can only be applied to planet formation models, however, if the observational biases and selection effects are properly accounted for. Here we show how epos, the Exoplanet Population Observation Simulator, can be used to constrain planet formation models by comparing the Bern planet population synthesis models to the Kepler exoplanetary systems. We compile a series of diagnostics, based on occurrence rates of different classes of planets and the architectures of multi-planet systems, that can be used as benchmarks for future and current modeling efforts. Overall, we find that a model with 100 seed planetary cores per protoplanetary disk provides a reasonable match to most diagnostics. Based on these diagnostics we identify physical properties and processes that would result in the Bern model more closely matching the known planetary systems. These are: moving the planet trap at the inner disk edge outward; increasing the formation efficiency of mini-Neptunes; and reducing the fraction of stars that form observable planets. We conclude with an outlook on the composition of planets in the habitable zone, and highlight that the majority of simulated planets smaller than 1.7 Earth radii have substantial hydrogen atmospheres. The software used in this paper is available online for public scrutiny at https://github.com/GijsMulders/epos

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arXiv2019

Tracking Dust Grains During Transport and Growth in Protoplanetary Disks

Protoplanetary disks are dynamic objects, within which dust grains and gas are expected to be redistributed over large distances. Evidence for this redistribution is seen both in other protoplanetary disks and in our own Solar System, with high-temperature materials thought to originate close to the central star found in the cold, outer regions of the disks. While models have shown this redistribution is possible through a variety of mechanisms, these models have generally ignored the possible growth of solids via grain-grain collisions that would occur during transit. Here we investigate the interplay of coagulation and radial and vertical transport of solids in protoplanetary disks, considering cases where growth is limited by bouncing or by fragmentation. We find that in all cases, growth effectively limits the ability for materials to be carried outward or preserved at large distances from the star. This is due to solids being incorporated into large aggregates which drift inwards rapidly under the effects of gas drag. We discuss the implications for mixing in protoplanetary disks, and how the preservation of high temperature materials in outer disks may require structures or outward flow patterns to avoid them being lost via radial drift.

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arXiv2019

Dust Transport and Processing in Centrifugally Driven Protoplanetary Disk Winds

There is evidence that protoplanetary disks--including the protosolar one--contain crystalline dust grains on spatial scales where the dust temperature is lower than the threshold value for their formation through thermal annealing of amorphous interstellar silicates. We interpret these observations in terms of an extended, magnetocentrifugally driven disk wind that transports grains from the inner disk--where they are thermally processed by the stellar radiation after being uplifted from the disk surfaces--to the outer disk regions. For any disk radius $r$ there is a maximum grain size $a_\mathrm{max}(r)$ that can be uplifted from that location: grains of size $a$$\ll$$a_\mathrm{max}$ are carried away by the wind, whereas those with $a$$\lesssim$$a_\mathrm{max}$ reenter the disk at larger radii. A significant portion of the reentering grains converge to--and subsequently accumulate in--a narrow region just beyond $r_\mathrm{max}(a)$, the maximum radius from which grains of size $a$ can be uplifted. We show that this model can account for the inferred crystallinity fractions in classical T Tauri and Herbig Ae disks and for their indicated near constancy after being established early in the disk evolution. It is also consistent with the reported radial gradients in the mean grain size, crystallinity, and crystal composition. In addition, this model yields the properties of the grains that remain embedded in the outflows from protoplanetary disks and naturally explains the inferred persistence of small grains in the surface layers of these disks.

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