arXiv subjects

Sho Shibata

Publications and source records attributed to Sho Shibata.

18 recordsLinked to original sources

arXiv2026

Beyond Pebble Isolation: Diverse Pathways to Giant Planet Formation Across Stellar and Orbital Scales

Context. Giant planet formation requires reaching crossover mass, i.e., when the mass of the gaseous envelope becomes equal to the mass of the core, within the disc's lifetime. The formation process depends critically on the orbital distance and stellar mass. Aims. We simulate planet formation via pebble accretion up to crossover mass around stellar hosts with masses of 0.1-1.5 Msun, considering a range of formation locations, with and without Type I migration. Methods. We use a modified version of MESA that couples pebble accretion, gas accretion, and disc evolution. Results. We find that cold/warm Jupiters form, whereas in-situ formation fails at short orbital separations: viscous heating raises the isolation mass enough to assemble adequate cores, but the accompanying high disc's temperature prevents cooling and suppresses gas giant formation. This supports migration-based explanations for the origin of hot Jupiters. At large orbital distances, crossover can be reached before pebble isolation mass. This is possible due to efficient envelope contraction in the cold, low-opacity outer disc. Inferred core masses at crossover range between 0.7 and 20 M_Earth. Conclusions. Pebble accretion accommodates multiple formation pathways. Giant planets can also have very small cores. Overall, different formation conditions significantly influence planetary growth and can explain the diversity in compositions and internal structures observed in the exoplanet population.

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arXiv2026

The metallicity of giant exoplanets around low-mass stars and how Ariel can help

Giant planets orbiting low-mass stars represent a unique population of giant planets that can be studied to constrain planet formation theory. Surveys of transiting giant exoplanets around M-dwarfs (GEMS) allow for measurement of their masses and radii, which in turn can be used to estimate their bulk densities. Coupling these observations to interior structure and evolution models can yield the bulk metallicity of these planets, however there are degeneracies to this that are improved by measurements of atmospheric metallicity. Estimates of bulk metallicity can be crucial to our understanding of planet formation timescales and mass budgets, particularly in these extreme (planet-to-star) mass ratio systems. Here we show that GEMS are expected to have a low bulk metallicity as a natural result of the planet formation process, and the low efficiency of planetesimal capture post-formation. To test this empirically, we need a larger sample of GEMS with atmospheric measurements, which can reduce some of the degeneracies with the interior modelling of planets. Measuring the atmospheric composition of GEMS with JWST and Ariel will be crucial for better understanding this unique planetary type, which sits at the tail of standard planet formation conditions.

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arXiv2026

Elevated Eccentricities in the Radius Valley Hint at Water-Rich Mini-Neptunes

While recent planet-formation models broadly reproduce the observed population of super-Earths and mini-Neptunes, as well as the bimodal radius distribution (the ``radius valley''), it remains unclear whether all these planets share a common rocky composition (a single popoulation of planets) or instead comprise two distinct populations -- rocky planets and icy planets (two populations of planets). The inferred eccentricity-radius relation, which shows a modest peak near the radius valley, provides a useful diagnostic for distinguishing between these scenarios. Here we use N-body simulations to examine how the radii and eccentricities of close-in planets depend on the masses and orbital configurations of their progenitor protoplanets. We find that final planetary eccentricities scale with the system initial Safronov number. In two-population systems, energy equipartition between rocky and relatively more massive icy protoplanets creates a strong eccentricity contrast between the two groups, which appears as a peak near the radius valley. This signature does not appear if planetary systems are composed exclusively of rocky planets (with or without H-rich atmospheres), as assumed in photoevaporation and core-powered mass loss models. Because the eccentricity-radius relation traces a dichotomy in the underlying protoplanet mass distribution -- most plausibly arising from formation at different disk locations -- our results suggest that a significant fraction of mini-Neptunes are water-worlds. The observed radius and eccentricity distributions may reflect a mixture of systems that host exclusively rocky planets, systems dominated by icy planets, and systems with both rocky and icy planets.

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arXiv2025

The Role of Magnetospheric Rebound in Breaking Resonant Chains of Super-Earths and Mini-Neptunes

Stellar magnetic fields are thought to truncate the inner regions of protoplanetary disks around T Tauri stars, creating a magnetospheric cavity near the star. As the disk evolves and disperses, the truncation radius is expected to move outward as the balance between magnetic and viscous forces shifts. Planets migrating inward can become trapped near the inner edge, but as the edge itself moves outward, the evolving disk torques can drive planets to migrate outward as well. We employ N-body simulations to assess the influence of magnetospheric cavity expansion on the dynamical evolution and orbital architectures of compact resonant chains of super-Earths and mini-Neptunes. Our results show that rebound-driven expansion of the disk's inner edge plays a pivotal role in destabilizing resonant chains by spreading planetary systems outward, thereby triggering early dynamical instabilities and giant impacts. Despite this dynamical evolution, key observable properties of close-in planetary systems -- such as the distribution of orbital period ratio, the intra-system similarity in planet sizes (``radius uniformity''), and the bimodal distribution of planet radii known as the ``radius valley'' -- remain largely consistent with those of systems formed without the rebound effect, in which the inner edge of the disk remains fixed. Thus, the primary consequence of the rebound appears to be the early disruption of resonant chains, rather than any significant alteration to the statistical properties of the resulting super-Earth and mini-Neptune populations.

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arXiv2025

Giant planet formation via pebble accretion across different stellar masses

The occurrence rate of cold Jupiters was found to depend on stellar mass. The formation environment in the protoplanetary disks regulates core formation and the subsequent gas accretion. In this study, we simulate giant planet formation via pebble accretion accounting for various stellar masses, core formation times, disk turbulent viscosities, and grain opacities. We use a self-consistent formation model that calculates the solid accretion rate and gas accretion rate of growing protoplanets. We investigate how the planetary formation, in particular, the contraction of the envelope, and the formation timescale change under different conditions. We find that to reproduce the observed occurrence rate of cold Jupiters, giant planets must undergo slow envelope contraction after they reach pebble isolation, which lasts for several Myrs. Such a slow contraction phase can be achieved when the grain opacity is assumed to be as high as that of the interstellar medium (ISM). If the grain opacity is smaller than the ISM opacity by a factor of ten or more, the growing protoplanets reach crossover mass within 3 Myrs and form too many cold Jupiters around stars of >0.4Msun. Protoplanets around low-mass stars <0.4Msun take >10 Myrs to reach crossover mass also with low grain opacity. If the grain opacity in the planetary envelope is much lower than that of ISM, other mechanisms, such as atmospheric recycling or planetesimal accretion, is required for cold Jupiter formation. We next explore how the deposition of the accreted heavy elements to the planetary envelope changes the formation timescale. Our model suggests that the formation timescale could be longer due to heavy-element enrichment, resulting from the lower core mass at pebble isolation. We conclude that the details of the formation processes have a significant effect on the planetary growth and therefore, the formation of gaseous planets.

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arXiv2025

A tale of dynamical instabilities and giant impacts in the radius valley

The size distribution of planets with radii between 1 and $4 R_\oplus$ peaks near 1.4 and $2.2R_\oplus$, with a dip around $1.8 R_\oplus$ -- the so-called "radius valley." Recent statistical analyses suggest that planets within this valley ($1.5 < R < 2R_\oplus$) tend to have slightly higher orbital eccentricities than those outside it. The origin of this dynamical signature remains unclear. We revisit the "breaking the chains" formation model and propose that late dynamical instabilities -- occurring after disk dispersal -- may account for the elevated eccentricities observed in the radius valley. Our simulations show that sub-valley planets ($R < 2 R_\oplus$) are generally rocky, while those beyond the valley ($R > 2 R_\oplus$) are typically water-rich. Rocky planets that undergo strong dynamical instabilities and numerous late giant impacts have their orbits excited and their radii increased, ultimately placing them into the radius valley. In contrast, the larger, water-rich planets just beyond the valley experience weaker instabilities and fewer impacts, resulting in lower eccentricities. This contrast leads to a peak in the eccentricity distribution within the valley. The extent to which planets in the radius valley are dynamically excited depends sensitively on the orbital architecture before the orbital instability. Elevated eccentricities among radius valley planets arise primarily in scenarios that form a sufficiently large number of rocky planets within 100 days (typically $\gtrsim 5$) prior to instability, and that also host external perturbers ($P > 100$ days), which further amplify the strength of dynamical instabilities.

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arXiv2025

On the origin of Jupiter's fuzzy core: constraints from N-body, impact and evolution simulations

It has been suggested that Jupiter's fuzzy core could be a result of a giant impact. Here, we investigate the expected impact conditions from N-body simulations. We then use state-of-the-art SPH simulations to investigate the results of impacts with different conditions including various impactor masses and composition, different formation stages in Jupiter's growth, and different resolutions. We next simulate the long-term thermal evolution of Jupiter post-impact. We find that 3D N-body simulations predict rather oblique impacts, and that head-on collisions are rare. Moreover, our results show that even under a head-on collision, Jupiter's fuzzy core cannot be formed. We next simulated Jupiter's thermal evolution and showed that unless post-impact temperatures are extremely low, a giant impact would not lead to an extended dilute core as inferred by interior models. We conclude that Jupiter's fuzzy core is not caused by an impact and is likely to be an outcome of its formation process.

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arXiv2025

Formation of super-Earths and mini-Neptunes from rings of planetesimals

The solar system planetary architecture has been proposed to be consistent with the terrestrial and giant planets forming from material rings at ~1 au and ~5 au, respectively. Here, we show that super-Earths and mini-Neptunes may share a similar formation pathway. In our simulations conducted with a disk alpha-viscosity of 4e-3, super-Earths accrete from rings of rocky material in the inner disk, growing predominantly via planetesimal accretion. Mini-Neptunes primarily originate from rings located beyond the water snowline, forming via pebble accretion. Our simulations broadly match the period-ratio distribution, the intra-system size uniformity, and the planet multiplicity distribution of exoplanets. The radius valley constrains the typical total mass available for rocky planet formation to be less than 3-6 Earth masses. Our results predict that planets at ~1 au in systems with close-in super-Earths and mini-Neptunes are predominantly water-rich. Though relatively uncommon, at ~1% level, such systems might also host rocky Earth-sized planets in the habitable zone that underwent late giant impacts, akin to the Moon-forming event.

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arXiv2023

Can Uranus and Neptune form concurrently via pebble, gas and planetesimal accretion?

The origin of Uranus and Neptune has long been challenging to explain, due to the large orbital distances from the Sun. After a planetary embryo has been formed, the main accretion processes are likely pebble, gas and planetesimal accretion. Previous studies of Uranus and Neptune formation typically don't consider all three processes; and furthermore, do not investigate how the formation of the outer planet impacts the inner planet. In this paper we study the concurrent formation of Uranus and Neptune via both pebble, gas and planetesimal accretion. We use a dust-evolution model to predict the size and mass flux of pebbles, and derive our own fit for gas accretion. We do not include migration, but consider a wide range of formation locations between 12 and 40au. If the planetary embryos form at the same time and with the same mass, our formation model with an evolving dust population is unable to produce Uranus and Neptune analogues. This is because the mass difference between the planets and the H-He mass fractions become too high. However, if the outer planetary embryo forms earlier and/or more massive than the inner embryo, the two planets do form in a few instances when the disk is metal-rich and dissipates after a few Myr. Furthermore, our study suggests that in-situ formation is rather unlikely. Nethertheless, giant impacts and/or migration could potentially aid in the formation, and future studies including these processes could bring us one step closer to understanding how Uranus and Neptune formed.

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arXiv2022

Heavy-element Accretion by Proto-Jupiter in a Massive Planetesimal Disk, Revisited

Planetesimal accretion is a key source for heavy-element enrichment in giant planets. It has been suggested that Jupiter's enriched envelope is a result of planetesimal accretion during its growth assuming it formed in a massive planetesimal disk. In this study, we simulate Jupiter's formation in this scenario. We assume in-situ formation and perform N-body simulations to infer the solid accretion rate. We find that tens-Earth masses of planetesimals can be captured by proto-Jupiter during the rapid gas accretion phase. However, if several embryos are formed near Jupiter's core, which is an expected outcome in the case of a massive planetesimal disk, scattering from the embryos increases the eccentricity and inclination of planetesimals and therefore significantly reduces the accretion efficiency. We also compare our results with published semi-analytical models and show that these models cannot reproduce the N-body simulations especially when the planetesimal disk has a large eccentricity and inclination. We show that when the dynamical evolution of planetesimals is carefully modelled, the total mass of captured planetesimals $M_\mathrm{cap,tot}$ is $2 M_\oplus \lesssim M_\mathrm{cap,tot}\lesssim 18 M_\oplus$. The metallicity of Jupiter's envelope can be explained by the planetesimal accretion in our massive disk model despite the low accretion efficiency coming from the high eccentricity and inclination of planetesimals. Our study demonstrates the importance of detailed modelling of planetesimal accretion during the planetary growth and its implications to the heavy-element mass in gaseous planets.

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arXiv2022

Exploring the Ability of HST WFC3 G141 to Uncover Trends in Populations of Exoplanet Atmospheres Through a Homogeneous Transmission Survey of 70 Gaseous Planets

We present the analysis of the atmospheres of 70 gaseous extrasolar planets via transit spectroscopy with Hubble's Wide Field Camera 3 (WFC3). For over half of these, we statistically detect spectral modulation which our retrievals attribute to molecular species. Among these, we use Bayesian Hierarchical Modelling to search for chemical trends with bulk parameters. We use the extracted water abundance to infer the atmospheric metallicity and compare it to the planet's mass. We also run chemical equilibrium retrievals, fitting for the atmospheric metallicity directly. However, although previous studies have found evidence of a mass-metallicity trend, we find no such relation within our data. For the hotter planets within our sample, we find evidence for thermal dissociation of dihydrogen and water via the H$^-$ opacity. We suggest that the general lack of trends seen across this population study could be due to i) the insufficient spectral coverage offered by HST WFC3 G141, ii) the lack of a simple trend across the whole population, iii) the essentially random nature of the target selection for this study or iv) a combination of all the above. We set out how we can learn from this vast dataset going forward in an attempt to ensure comparative planetology can be undertaken in the future with facilities such as JWST, Twinkle and Ariel. We conclude that a wider simultaneous spectral coverage is required as well as a more structured approach to target selection.

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arXiv2022

Constraining the origin of giant exoplanets via elemental abundance measurements

The origin of close-in giant planets is a key open question in planet formation theory. The two leading models are (i) formation at the outer disk followed by migration and (ii) in situ formation. In this work we determine the atmospheric composition of warm Jupiters for both formation scenarios. We perform N-body simulations of planetesimal accretion interior and exterior to the water ice-line for various planetary formation locations, planetary masses, and planetesimal sizes to estimate the accreted heavy-element mass and final planetary composition. We find that the two models differ significantly: migrating giant planets have 2-14 times higher metallicities than planets that form in situ. The ratio between refractories and volatiles is found to be above one for migrating planets but below 0.4 for planets that form in situ. We also identify very different trends between heavy-element enrichment and planetary mass for these two formation mechanisms. While the metallicity of migrating planets is found to increase with decreasing planetary mass, it is about constant for in situ formation. Our study highlights the importance of measuring the atmospheric composition of warm Jupiters and its connection to their formation and evolutionary paths.

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arXiv2022

Five key exoplanet questions answered via the analysis of 25 hot Jupiter atmospheres in eclipse

Population studies of exoplanets are key to unlocking their statistical properties. So far the inferred properties have been mostly limited to planetary, orbital and stellar parameters extracted from, e.g., Kepler, radial velocity, and GAIA data. More recently an increasing number of exoplanet atmospheres have been observed in detail from space and the ground. Generally, however, these atmospheric studies have focused on individual planets, with the exception of a couple of works which have detected the presence of water vapor and clouds in populations of gaseous planets via transmission spectroscopy. Here, using a suite of retrieval tools, we analyse spectroscopic and photometric data of 25 hot Jupiters, obtained with the Hubble and Spitzer Space Telescopes via the eclipse technique. By applying the tools uniformly across the entire set of 25 planets, we extract robust trends in the thermal structure and chemical properties of hot Jupiters not obtained in past studies. With the recent launch of JWST and the upcoming missions Twinkle, and Ariel, population based studies of exoplanet atmospheres, such as the one presented here, will be a key approach to understanding planet characteristics, formation, and evolution in our galaxy.

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arXiv2022

Enrichment of Jupiter's atmosphere by late planetesimal bombardment

Jupiter's atmosphere is enriched with heavy elements by a factor of about 3 compared to proto-solar. The origin of this enrichment and whether it represent the bulk composition of the planetary envelope remain unknown. Internal structure models of Jupiter suggest that its envelope is separated from the deep interior and that the planet is not fully mixed. This implies that Jupiter's atmosphere was enriched with heavy elements just before the end of its formation. Such enrichment can be a result of late planetesimal accretion. However, in-situ Jupiter formation models suggest the decreasing accretion rate with increasing planetary mass, which cannot explain Jupiter's atmospheric enrichment. In this study, we model Jupiter's formation and show that an migration of proto-Jupiter from $\sim$ 20 AU to its current location can lead to a late planetesimal accretion and atmospheric enrichment. Late planetesimal accretion does not occur if proto-Jupiter migrates only a few AU. We suggest that if Jupiter's outermost layer is fully-mixed and is relatively thin (up to $\sim$ 20\% of its mass), such late accretion can explain its measured atmospheric composition. It is therefore possible that Jupiter underwent significant orbital migration followed by late planetesimal accretion.

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arXiv2021

The origin of the high metallicity of close-in giant exoplanets II The nature of the sweet spot for accretion

The composition of giant planets reflects their formation history. Planetesimal accretion during the phase of planetary migration could lead to the delivery of heavy elements into giant planets. In our previous paper (Shibata et al. 2020) we showed that planetesimal accretion during planetary migration occurs in a rather narrow region of the protoplanetary disk, which we refer as "the sweet spot for accretion". The goal of this paper is to reveal the nature of the sweet spot and investigate the role of the sweet spot in determining the composition of gas giant planets. We analytically derive the required conditions for the sweet spot. Then, we compare the derived equations with the numerical simulations. We find that the conditions required for the sweet spot can be expressed by the ratio of the gas damping timescale of the planetesimal orbits and the planetary migration timescale. If the planetary migration timescale depends on the surface density of disk gas inversely, the location of the sweet spot does not change with the disk evolution. The mass of planetesimals accreted by the planet depends on the amount of planetesimals that are shepherded by mean motion resonances. Our analysis suggests that tens Earth-mass of planetesimals can be shepherded into the sweet spot without planetesimal collisions. However, as more planetesimals are trapped into mean motion resonances, collisional cascade can lead to fragmentation of planetesimals. This could affect the location of the sweet spot and the population of small objects in planetary systems. We conclude that the composition of gas giant planets depends on whether the planets crossed the sweet spot during their formation. Constraining the metallicity of cold giant planets, that are expected to be outer than the sweet spot, would reveal key information for understanding the origin of heavy elements in giant planets.

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arXiv2021

Exploring the link between star and planet formation with Ariel

The goal of the Ariel space mission is to observe a large and diversified population of transiting planets around a range of host star types to collect information on their atmospheric composition. The planetary bulk and atmospheric compositions bear the marks of the way the planets formed: Ariel's observations will therefore provide an unprecedented wealth of data to advance our understanding of planet formation in our Galaxy. A number of environmental and evolutionary factors, however, can affect the final atmospheric composition. Here we provide a concise overview of which factors and effects of the star and planet formation processes can shape the atmospheric compositions that will be observed by Ariel, and highlight how Ariel's characteristics make this mission optimally suited to address this very complex problem.

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arXiv2019

The origin of the high metallicity of close-in giant exoplanets: Combined effect of the resonant and aerodynamic shepherding

Context.Recent studies suggest that many giant exoplanets are highly enriched with heavy elements compared to their host star andcontain several tens of Earth masses or more of heavy elements. Such enrichment is considered to have been brought by accretionof planetesimals in late formation stages. Previous dynamical simulations, however, show that planets are unable to collect so muchheavy elements throughin situplanetesimal accretion. Aims.We investigate whether a giant planet migrating inward can capture planetesimals efficiently to significantly increase its metal-licity. Methods.We performed orbital integrations of a migrating giant planet and planetesimals in a protoplanetary gas disc to infer theplanetesimal mass that is accreted by the planet. Results.We find that the two shepherding processes of mean motion resonances trapping and aerodynamic gas drag inhibit plan-etesimal capture of a migrating planet. However, the amplified libration allows the highly-excited planetesimals in the resonances toescape from the resonance trap and be accreted by the planet. Consequently, we show that a migrating giant planet captures planetes-imals with total mass of several tens of Earth masses, if the planet forms at a few tens of AU in a relatively massive disc. We alsofind that planetesimal capture occurs efficiently in a limited range of semi-major axis, and that the total captured planetesimal massincreases with increasing migration distances. Our results have important implications for understanding the relation between giantplanet metallicity and mass, as we suggest that it reflects the formation location of the planet, or more precisely, the location whererunaway gas accretion occurred. We also suggest the observed metal-rich close-in Jupiters migrated to their present locations fromafar, where they formed.

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arXiv2019

Capture of Solids by Growing Proto-gas Giants: Effects of Gap Formation and Supply-limited Growth

Studies of internal structure of gas giant planets suggest that their envelopes are enriched with heavier elements than hydrogen and helium relative to their central stars. Such enrichment likely occurred by solid accretion during late formation stages of gas giant planets in which gas accretion dominates protoplanetary growth. Some previous studies performed orbital integration of planetesimals around a growing protoplanet with the assumption of an uniform circumstellar disc to investigate how efficiently the protoplanet captures planetesimals. However, not only planetesimals but also disc gas are gravitationally perturbed by the protoplanet in its late formation stages, resulting in gap opening in the circumstellar disc. In this study, we investigate the effects of such gap formation on the capture of planetesimals by performing dynamical simulations of planetesimals around a growing proto-gas giant planet. Gap formation reduces the surface density of disc gas, makes a steep pressure gradient and limits the growth rate of the protoplanet. We find that the first effect enhances the capture of planetesimals, while the others reduce it. Consequently the amount of planetesimals captured during the gas accretion is estimated to be at most ~3 Mearth. We conclude that the in-situ capture of planetesimals needs the initial solid surface density more than five times higher than that of the minimum mass solar nebula for explaining the inferred large amount of heavy element in Jupiter. For highly dense warm Jupiters, we would need additional processes enhancing the capture and/or supply of planetesimals.

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