Planetary Evolution Research Articles

Grid-based exoplanet atmospheric mass-loss predictions via neural networks

The fast and accurate estimation of planetary mass-loss rates is critical for planet population and evolution modelling. We used machine learning (ML) for fast interpolation across an existing large grid of hydrodynamic upper atmosphere models, providing mass-loss rates for any planet inside the grid boundaries with superior accuracy compared to previously published interpolation schemes. We considered an already available grid comprising about 11,000 hydrodynamic upper atmosphere models for training and generated an additional grid of about 250 models for testing purposes. We developed the ML interpolation scheme, dubbed the `atmospheric Mass Loss INquiry frameworK' ( MLink ), using a dense neural network, further comparing the results with what was obtained employing classical approaches (e.g. linear interpolation and radial-basis-function-based regression). Finally, we studied the impact of the different interpolation schemes on the evolution of a small sample of carefully selected synthetic planets. MLink provides high-quality interpolation across the entire parameter space by significantly reducing both the number of points with large interpolation errors and the maximum interpolation error compared to previously available schemes. For most cases, evolutionary tracks computed employing MLink and classical schemes lead to comparable planetary parameters on gigayear timescales. However, particularly for planets close to the top edge of the radius gap, the difference between the predicted planetary radii at a given age of tracks obtained employing MLink and classical interpolation schemes can exceed the typical observational uncertainties. Machine learning can be successfully used to estimate atmospheric mass-loss rates from model grids, paving the way to exploring future larger and more complex grids of models computed accounting for more physical processes.

What we can learn about Mars from the magnetism of returned samples

The Red Planet is a magnetic planet. The Martian crust contains strong magnetization from a core dynamo that likely was active during the Noachian period when the surface may have been habitable. The evolution of the dynamo may have played a central role in the evolution of the early atmosphere and the planet’s transition to the current cold and dry state. However, the nature and history of the dynamo and crustal magnetization are poorly understood given the lack of well-preserved, oriented, ancient samples with geologic context available for laboratory study. Here, we describe how magnetic measurements of returned samples could transform our understanding of six key unknowns about Mars’ planetary evolution and habitability. Such measurements could i) determine the history of the Martian dynamo field’s intensity; ii) determine the history of the Martian dynamo field’s direction; iii) test the hypothesis that Mars experienced plate tectonics or true polar wander; iv) constrain the thermal and aqueous alteration history of the samples; v) identify sources of Martian crustal magnetization and vi) characterize sedimentary and magmatic processes on Mars. We discuss how these goals can be achieved using future laboratory analyses of samples acquired by the Perseverance rover.

Characterization of seven transiting systems, including four warm Jupiters from SOPHIE and TESS

While several thousand exoplanets are now confirmed, the number of known transiting warm Jupiters ($10 d period d $) remains relatively small. These planets are generally believed to have formed outside the snowline and migrated to their current orbits. Because they are sufficiently distant from their host stars, they mitigate proximity effects and so offer valuable insights into planet formation and evolution. Here, we present the study of seven systems, three of which -- TOI-2295, TOI-2537, and TOI-5110 -- are newly discovered planetary systems. Through the analysis of TESS photometry, SOPHIE radial velocities (RVs), and high-spatial resolution imaging, we found that TOI-2295b, TOI-2537b, and TOI-5110b are transiting warm Jupiters with orbital periods ranging from 30 to 94 d, masses between 0.9 and 2.9 M_ J and radii ranging from 1.0 to 1.5 R_ J . Both TOI-2295 and TOI-2537 harbor at least one additional, outer planet. Their outer planets -- TOI-2295c and TOI-2537c -- are characterized by orbital periods of 966.5^ and 1920^ d, respectively, and minimum masses of 5.61^ and 7.23^ M_ J respectively. We have also investigated and characterized the two recently reported warm Jupiters TOI-1836b and TOI-5076b, which we independently detected in SOPHIE RVs. Our new data allow for further discussion of their nature and refinement of their parameters. Additionally, we study the planetary candidates TOI-4081.01 and TOI-4168.01. For TOI-4081.01, despite our detection in RVs, we cannot rule out perturbation by a blended eclipsing binary, and we thus exercise caution regarding its planetary nature. On the other hand, we identify TOI-4168.01 as a firm false positive; its RV curve exhibits a large amplitude in an antiphase relation with the transit ephemeris observed by TESS, indicating that the detected event is the eclipse of a secondary star rather than a planetary transit. Finally, we highlight interesting characteristics of these new planetary systems. The transits of TOI-2295b are highly grazing, with an impact parameter of 1.056^ . This leaves its radius uncertain but potentially makes it an interesting probe of gravitational dynamics in its two-planet system, as transit shapes for grazing planets are highly sensitive to even small variations in inclination. TOI-2537b, in turn, is a temperate Jupiter with an effective temperature of 307±15 K and can serve as a valuable low-irradiation control for models of hot Jupiter inflation anomalies. We also detected significant transit timing variations (TTVs) for TOI-2537b, which are likely caused by gravitational interactions with the outer planet TOI-2537c. Further transit observations are needed to refine the analysis of these TTVs and enhance our understanding of the system’s dynamics. Finally, TOI-5110b stands out due to its orbital eccentricity of 0.745^ one of the highest planetary eccentricities discovered thus far. We find no conclusive evidence for an external companion, but an unseen planet with a semi-amplitude smaller than 10 m/s could nonetheless still be exciting its eccentricity.

Decoding the Future of Exoplanets: Asteroseismic Confirmation of Subgiant and Red Giant Hosts

Abstract Asteroseismology has emerged as a powerful tool to unravel the intricate relationships between evolved stars and their planetary systems. In this study, we leverage this technique to investigate the evolutionary stages of five exoplanet host stars, each exhibiting solar-like oscillations. Building on our previous work that identified two host stars as red clump and red giant branch (RGB) stars, this study focuses on a new and broader sample. By precisely measuring asteroseismic parameters such as the period spacing of dipole gravity modes (ΔΠ1), we provide definitive confirmation of these stars’ evolutionary states as subgiants or RGB stars. These results are not only crucial for understanding the internal structures of evolved stars but also for predicting the eventual fate of their planetary companions, which may face engulfment as their host stars expand. This research highlights the profound role of asteroseismology in advancing our knowledge of planetary system evolution and opens new pathways for exploring how stellar evolution impacts planetary survival. Our findings set the stage for future studies on the dynamic fates of exoplanets, providing key insights into the intricate processes of stellar and planetary evolution.

Moderate-separation Binary Companions May Influence Young Stellar X-Ray Luminosity

Abstract Young pre-main sequence stars exhibit elevated X-ray levels due to their strong magnetic activity. Understanding young stars’ X-ray activity is essential for contextualizing the forming planets that they host. Binary stars present a unique environment that may influence planet formation and evolution. In this work, we assembled a sample of 65 systems with stellar characterization from the literature and X-ray fluxes from the XMM-Newton Extended Survey of the Taurus Molecular Cloud to investigate the potential relationship between binary separation and X-ray flux. We found that binary stars with separations smaller than the sample median may exhibit elevated X-ray flux compared to single stars. This suggests that binary companions could influence stellar and planetary evolution and warrants further investigation.

The Mass–Radius Relation for Planets in Binary Systems

Abstract Stellar companion within a few hundred astronomical unit alter the structure and shorten the lifetime of protoplanetary disks, influencing planetary formation and evolution. Such systems host fewer close-in planets, and have fewer ~2.3R ⊕ “sub-Neptunes” relative to ~1.3R ⊕ “super-Earths” compared to single-star hosts, observations that can be explained by early dissipation of the gas disk. Here we construct the mass–radius diagram of 15 small (<8R ⊕), well-characterized planets on S-type orbits in systems with projected separations <500 au, and show that it is indistinguishable from that of planets around single stars. This suggests that accretion of the H2-dominated envelopes of sub-Neptunes could be much faster than gas disk dissipation and limited instead by available solids for cores, or that many sub-Neptunes have envelopes of condensible volatiles such as H2O.

Isotope Evolution of the Depleted Mantle

The depleted mantle reservoir is that part of Earth's mantle from which crust has been extracted, leaving the remaining mantle depleted in incompatible elements. Knowing how and when it formed is essential for understanding the chemical evolution of Earth, including formation of continental crust. The best-constrained Hf isotope data presented here indicate that the mantle does not become significantly depleted until as late as 700 million years after Earth's accretion. This onset of mantle depletion coincides with the first appearance of substantial volumes of continental crust in the geological record. These data compel a revision to the reference depleted mantle parameters used in Hf isotope studies of planetary evolution. This new reference line follows chondritic evolution until 3.8 Ga and then describes a linear trajectory to a present-day depleted mid-ocean ridge basalt source mantle composition (εHf = +18). We infer that stabilization of continental crust only occurred in earnest on Earth after 3.8 Ga. ▪ Hf isotopes show that Earth's mantle does not become significantly depleted until 700 million years after planetary accretion. ▪ Most of Earth's oldest rocks formed from mantle sources that had radiogenic isotope compositions similar to those of chondritic meteorites. ▪ Isotope evidence shows that Hadean (>4.0-billion-year-old) crust was not essential for formation of younger crust in Archean terranes. ▪ Growth of Earth's continents only began in earnest after 3.8 Ga.

Rubidium Isotope Measurements of Low‐Rb Geological Materials by MC‐ICP‐MS

The rubidium (Rb) isotope system has the potential to trace planetary evolution, magmatic‐fluid interaction and chemical weathering. These applications are based on Rb isotope measurement results with precisions fit for purpose, but measurements of low‐Rb geological materials are challenging due to large sample consumption and overload of ion‐exchange resin. Here we developed a measurement procedure for Rb isotope data (δ87RbSRM984) of low‐Rb geological materials using MC‐ICP‐MS. Using an Aridus III desolvator and Ni standard sampler + Ni X skimmer cone combination, the Rb loading amount was reduced significantly to 20 ng. A comparison between two methods for instrumental mass‐bias correction, the sample‐standard bracketing and combined sample‐standard bracketing and internal (Zr) normalisation (C‐SSBIN), shows that C‐SSBIN could produce Rb isotope data with better intermediate measurement precisions but strictly restricted to optimal Zr/Rb ratio. The robustness of this method was demonstrated by monitoring δ87RbSRM984 data of two in‐house Rb isotope standards, replicates, some reference materials with δ87RbSRM984 values previously reported, and element‐doped and matrix‐spiked synthetic solutions. Based on repeated measurements of Rb isotope standards and reference materials, the long‐term (over one year) intermediate precision was better than 0.05‰ (2s, standard deviations). We additionally recommend thirteen reliable reference materials for future Rb isotope ratio measurements.

H2–H2O demixing in Uranus and Neptune: Adiabatic structure models

Context. Demixing properties of major planetary constituents influence the interior structure and evolution of planets. Comparing experimental and computational data on the miscibility of hydrogen and water to adiabatic profiles suggests that phase separation between these two components occurs in the ice giants Uranus and Neptune. Aims. We aim to predict the atmospheric water abundance and transition pressure between the water-poor outer envelope and the water-rich deep interior in Uranus and Neptune. Methods. We constructed seven H2–H2O phase diagrams from the available experimental and computational data. We computed interior adiabatic structure models and compared these to the phase diagrams to infer whether demixing occurred. Results. We obtain a strong water depletion in the top layer due to the rain-out of water and find upper limits on the atmospheric water-mass fraction Zatm of 0.21 for Uranus and 0.16 for Neptune. The transition from the water-poor to the water-rich layer is sharp and occurs at pressures PZ between 4 and 11 GPa. Using these constraints on Zatm and PZ, we find that the observed gravitational harmonics J2 and J4 can be reproduced if PZ ≳ 10 GPa in Uranus and ≳5 GPa in Neptune, and if the deep interior has a high primordial water-mass fraction of 0.8, unless rocks are also present. The agreement with J4 is improved if rocks are confined deeper than PZ, for instance, below a rock cloud level at 2000 K (20–30 GPa). Conclusions. These findings confirm classical few-layer models and suggest that a layered structure may result from a combination of primordial mass accretion and subsequent phase separation. Reduced observational uncertainty in J4 and its dynamic contribution, atmospheric water abundance measurements from the Uranus Orbiter and Probe (UOP) or a Neptune mission, and better understanding of the mixing behaviour of constituents are needed to constrain the interiors of ice giants.

Convective Mixing in Gas Giant Planets with Primordial Composition Gradients

Linking atmospheric measurements to the bulk planetary composition and ultimately the planetary origin is a key objective in planetary science. In this work, we identify the cases in which the atmospheric composition represents the bulk composition. We simulate the evolution of giant planets considering a wide range of planetary masses (0.3–2 M J), initial entropies ( 8-11kBmu−1 ), and primordial heavy-element profiles. We find that convective mixing is most efficient at early times (ages ≲107 yr) and that primordial composition gradients can be eroded. In several cases, however, the atmospheric composition can differ widely from the planetary bulk composition, with the exact outcome depending on the details. We show that the efficiency of convection is primarily controlled by the underlying entropy profile: For low primordial entropies of 8-9kBmu−1 , convective mixing can be inhibited and composition gradients can persist over billions of years. The scaling of mixing efficiency with mass is governed by the primordial entropy. For the same primordial entropy, low-mass planets mix more efficiently than high-mass planets. If the primordial internal entropy would increase with mass, however, this trend could reverse. We also present a new analytical model that predicts convective mixing under the existence of composition (and entropy) gradients. Our results emphasize the complexity in the interpretation of atmospheric abundance measurements and show the great need to better understand the planetary formation process as it plays a key role in determining the planetary evolution and final structure.

Detectability of biosignatures in warm, water-rich atmospheres

Context. Warm rocky exoplanets within the habitable zone of Sun-like stars are favoured targets for current and future missions. Theory indicates these planets could be wet at formation and remain habitable long enough for life to develop. However, it is unclear to what extent an early ocean on such worlds could influence the response of potential biosignatures. Aims. In this work we test the climate-chemistry response, maintenance, and detectability of biosignatures in warm, water-rich atmospheres with Earth biomass fluxes within the framework of the planned LIFE mission. Methods. We used the coupled climate-chemistry column model 1D-TERRA to simulate the composition of planetary atmospheres at different distances from the Sun, assuming Earth’s planetary parameters and evolution. We increased the incoming instellation by up to 50% in steps of 10%, corresponding to orbits of 1.00 to 0.82 AU. Simulations were performed with and without modern Earth’s biomass fluxes at the surface. Theoretical emission spectra of all simulations were produced using the GARLIC radiative transfer model. LIFEsim was then used to add noise to and simulate observations of these spectra to assess how biotic and abiotic atmospheres of Earth-like planets can be distinguished. Results. Increasing instellation leads to surface water vapour pressures rising from 0.01 bar (1.31%, S = 1.0) to 0.61 bar (34.72%, S = 1.5). In the biotic scenarios, the ozone layer survives because hydrogen oxide reactions with nitrogen oxides prevent the net ozone chemical sink from increasing. Methane is strongly reduced for instellations that are 20% higher than that of the Earth due to the increased hydrogen oxide abundances and UV fluxes. Synthetic observations with LIFEsim, assuming a 2.0 m aperture and resolving power of a R = 50, show that ozone signatures at 9.6 µm reliably point to Earth-like biosphere surface fluxes of O2 only for systems within 10 parsecs. The differences in atmospheric temperature structures due to differing H2O profiles also enable observations at 15.0 µm to reliably identify planets with a CH4 surface flux equal to that of Earth’s biosphere. Increasing the aperture to 3.5 m and increasing instrument throughput to 15% increases this range to 22.5 pc.

Revisiting the conundrum of the sub-Jovian and Neptune desert

Context. The search for exoplanets has led to the identification of intriguing patterns in their distributions, one of which is the so-called sub-Jovian and Neptune desert. The occurrence rate of Neptunian exoplanets with an orbital period P ≲ 4 days sharply decreases in this region in period-radius and period-mass space. Aims. We present a novel approach to delineating the sub-Jovian and Neptune desert by considering the incident stellar flux F on the planetary surface as a key parameter instead of the traditional orbital period of the planets. Through this change of perspective, we demonstrate that the incident flux still exhibits a paucity of highly irradiated Neptunes, but also captures the proximity to the host star and the intensity of stellar radiation. Methods. Leveraging a dataset of confirmed exoplanets, we performed a systematic analysis to map the boundaries of the sub-Jovian and Neptune desert in the (F, Rp) and (F, Mp) diagrams, with Rp and Mp corresponding to the planetary radius and mass, respectively. By using statistical techniques and fitting procedures, we derived analytical expressions for these boundaries that offer valuable insights into the underlying physical mechanisms governing the dearth of Neptunian planets in close proximity to their host stars. Results. We find that the upper and lower bounds of the desert are well described by a power-law model in the (F, Rp) and (F, Mp) planes. We also obtain the planetary mass-radius relations for each boundary by combining the retrieved analytic expressions in the two planes. This work contributes to advancing our knowledge of exoplanet demographics and to refining theoretical models of planetary formation and evolution within the context of the sub-Jovian and Neptune desert.

The Featherweight Giant: Unraveling the Atmosphere of a 17 Myr Planet with JWST

The characterization of young planets (<300 Myr) is pivotal for understanding planet formation and evolution. We present the 3–5 μm transmission spectrum of the 17 Myr, Jupiter-size (R ∼10R ⊕) planet, HIP 67522b, observed with JWST NIRSpec/G395H. To check for spot contamination, we obtain a simultaneous g-band transit with the Southern Astrophysical Research Telescope. The spectrum exhibits absorption features 30%–50% deeper than the overall depth, far larger than expected from an equivalent mature planet, and suggests that HIP 67522b’s mass is <20 M ⊕ irrespective of cloud cover and stellar contamination. A Bayesian retrieval analysis returns a mass constraint of 13.8 ± 1.0 M ⊕. This challenges the previous classification of HIP 67522b as a hot Jupiter and instead, positions it as a precursor to the more common sub-Neptunes. With a density of <0.10 g cm−3, HIP 67522 b is one of the lowest-density planets known. We find strong absorption from H2O and CO2 (≥7σ), a modest detection of CO (3.5σ), and weak detections of H2S and SO2 (≃2σ). Comparisons with radiative-convective equilibrium models suggest supersolar atmospheric metallicities and solar-to-subsolar C/O ratios, with photochemistry further constraining the inferred atmospheric metallicity to 3 × 10 solar due to the amplitude of the SO2 feature. These results point to the formation of HIP 67522b beyond the water snowline, where its envelope was polluted by icy pebbles and planetesimals. The planet is likely experiencing substantial mass loss (0.01–0.03 M ⊕ Myr−1), sufficient for envelope destruction within a gigayear. This highlights the dramatic evolution occurring within the first 100 Myr of its existence.

Convective mixing in distant and close-in giant planets. Dependences on the initial composition, luminosity, bloating, and semi-convection

Recent structure models of Jupiter suggest the existence of an extended region in the deep interior with a high heavy element abundance, referred to as a dilute core. This finding has led to increased interest in modelling the formation and evolution processes with the goal of understanding how and under what circumstances such a structure is formed and retained, to in turn better understand the relation between atmospheric and bulk metallicity. We modelled the evolution of giant planets, varying various parameters relevant for the convective mixing process, such as the mixing length parameter and the size of the mesh, and parameters related to the general evolution, such as the orbital distance and the initial luminosity. We in particular studied hot Jupiters and find that the effect of bloating on the mixing process is small but can in some cases inhibit convective mixing by lowering the intrinsic luminosity for a given entropy. Semi-convection can significantly lower the extent of a dilute core if it is strong enough. We find that dilute cores are unable to persist for initial luminosities much higher than $ 10^3$ L$_J$ for a Jupiter-like planet for the initial heavy element profiles we studied. From this we conclude that, based on our model, it is unlikely that a large number of giant planets retain a dilute core throughout their evolution, although this is dependent on the assumptions and limitations of our method. Future work should focus on improving the link between formation and evolution models so that the mixing process is accurately modelled throughout a planet's lifetime and on improving the understanding of how to model convection near radiative-convective boundaries.

Searching for GEMS: TOI-6383Ab, a Giant Planet Transiting an M3-dwarf Star in a Binary System* *Based on observations obtained with the Hobby–Eberly Telescope (HET), which is a joint project of the University of Texas at Austin, the Pennsylvania State University, Ludwig-Maximillians-Universitaet Muenchen, and Georg-August Universitaet Goettingen. The HET is named in honor of its principal benefactors, William P. Hobby and

We report on the discovery of a transiting giant planet around the 3500 K M3-dwarf star TOI-6383A located 172 pc from Earth. It was detected by the Transiting Exoplanet Survey Satellite and confirmed by a combination of ground-based follow-up photometry and precise radial velocity measurements. This planet has an orbital period of ∼1.791 days, a mass of 1.040 ± 0.094 M J , and a radius of 1.008−0.033+0.036RJ , resulting in a mean bulk density of 1.26−0.17+0.18 g cm−3. TOI-6383A has an M dwarf companion star, TOI-6383B, which has a stellar effective temperature of T eff ∼ 3100 K and a projected orbital separation of 3126 au. TOI-6383A is a low-mass dwarf star hosting a giant planet and is an intriguing object for planetary evolution studies due to its high planet-to-star mass ratio. This discovery is part of the Searching for Giant Exoplanets around M-dwarf Stars (GEMS) Survey, intending to provide robust and accurate estimates of the occurrence of GEMS and the statistics on their physical and orbital parameters. This paper presents an interesting addition to the small number of confirmed GEMS, particularly notable since its formation necessitates massive, dust-rich protoplanetary discs and high accretion efficiency (>10%).

Tracking the influence of a very close secondary star on planetary growth and evolution

Abstract This work investigated the dynamics of planets in binary systems and provided insights into the stability and evolution of these systems. We explored the influence of a nearby secondary star on planetary growth and evolution, focusing on S-type configurations. We tracked the orbits of the planets and analyzed their stability over long timescales, considering various parameters such as mass, eccentricity, and inclination. Our results show that the presence of a secondary star can significantly impact the growth and evolution of planets, leading to changes in their orbits and potential ejection from the system, however, it was possible to identify stable planets even in systems experiencing multiple disturbances. One of the most significant results of the work was the analysis of the increased material in the disc near the primary star, which contributes to planet growth, driven by the density spirals influenced by the binary star.These findings have important implications for the search for habitable exoplanets and emphasize the need for further studies of planetary systems in binary star environments.

Double-diffusive layering during the evolution of planetary mantles

Summary The degree of layering during planetary mantle evolution has been a key issue and is still heavily debated, especially since the observation of seismic discontinuities within the Earth mantle and their correlation to phase transitions within the mantle mineral assemblage and the detection of geochemical differences in mantle derived basalts (Mid Ocean Ridge Basalts and Ocean Island Basalts). We present the phenomenon of double-diffusive layering in a fluid with strongly temperature dependent viscosity, as relevant for planetary mantle material, to study the influence of an initial compositional gradient with regard to the thermal and chemical evolution of a planet. The numerical experiments show that in a wide parameter range distinct layers are formed self-organized from a continuously stratified state by dynamical fractionation and are thus likely to appear as a generic. Considering this as a plausible model for planetary mantle evolution it provides a dynamical explanation for the existence of distinct chemical reservoirs within the history of a planet's mantle.

Evolution of gas envelopes and outgassed atmospheres of rocky planets that formed via pebble accretion

In this work, we present results of numerical simulations of the formation and early evolution of rocky planets through pebble accretion, with an emphasis on hydrogen envelope longevity and the composition of the outgassed atmosphere. We modelled planets with a range in mass from 0.1 to 5 Earth masses that orbit between 0.7 and 1.7 AU. The composition of the outgassed atmosphere was calculated with the partial pressure of free oxygen fit to geophysical models of magma ocean self-oxidation. The combined X-ray and UV (XUV) radiation-powered photoevaporation is considered as the main driver of atmospheric escape. We modelled planets that remain below the pebble isolation mass and hence accrete tenuous envelopes only. We considered slow, medium, or fast initial stellar rotation for the temporal evolution of the XUV flux. The loss of the envelope is a key event that allows the magma ocean to crystallise and outgas its bulk volatiles. The atmospheric composition of the majority of our simulated planets is dominated by CO2. Our planets accrete a total of 11.6 Earth oceans of water, the majority of which enters the core. The hydrospheres of planets lighter than the Earth reach several times the mass of the Earth’s modern oceans, while the hydrospheres of planets ranging from 1 to 3.5 Earth masses are comparable to those of our planet. However, planets of 4–5 Earth masses have smaller hydrospheres due to the trapping of volatiles in their massive mantles. Overall, our simulations demonstrate that hydrogen envelopes are easily lost from rocky planets and that this envelope loss triggers the most primordial partitioning of volatiles between the solid mantle and the atmosphere.

Dynamics of Binary Planets within Star Clusters

We develop analytical tools and perform three-body simulations to investigate the orbital evolution and dynamical stability of binary planets within star clusters. Our analytical results show that the orbital stability of a planetary-mass binary against passing stars is mainly related to its orbital period. Critical flybys, defined as stellar encounters with energy kicks comparable to the binary binding energy, can efficiently produce a wide range of semimajor axes (a) and eccentricities (e) from a dominant population of primordially tight Jupiter-mass binary objects (JuMBOs). The critical flyby criterion we derived offers an improvement over the commonly used tidal radius criterion, particularly in high-speed stellar encounters. Applying our results to the recently discovered JuMBOs by the James Webb Space Telescope (JWST), our simulations suggest that to match the observed ∼9% wide binary fraction, an initial semimajor axis of a 0 ∼ 10–20 au and a density-weighted residence time of χ ≳ 104 Myr pc−3 are favored. These results imply that the JWST JuMBOs probably formed as tight binaries near the cluster core.

Petrogenesis of the unbrecciated pigeonite cumulate eucrite Northwest Africa 8326: Bridging the gap between eucrites and diogenites

Understanding of the diversity and petrogenesis of achondrites is critical for deciphering magmatic processes and the early evolution of planets and asteroids. Here, we report the detailed petrologic, mineralogical, geochemical, and chronological features of the unbrecciated Vestan meteorite Northwest Africa (NWA) 8326. We found that NWA 8326 is composed of coarse-grained orthopyroxene (∼74 vol%), plagioclase (∼19 vol%), fine-grained augite (∼5 vol%), and many accessory minerals such as chromite, ilmenite, Fe-sulfide, silica phases, K-feldspar, Ca-phosphate phases, zircon, baddeleyite, rutile, and primary Si,Al,K-rich glass, differing from typical howardite-eucrite-diogenite meteorites. Based on textural feature and compositional calculation of pyroxene, we suggest that the coarse-grained orthopyroxene was inverted from primary pigeonite and NWA 8326 should be classified as a pigeonite cumulate eucrite. The oxygen and chromium isotope data (Δ17O = − 0.254 ± 0.009 ‰; ε54Cr = − 0.60 ± 0.06) support this classification. A few zircon aggregates are observed in NWA 8326 and the grains therein show a core-mantle zoned texture in cathodoluminescence (CL) images, with the cores being dark and Al-rich while the mantles being bright and Al-poor. We interpret that the CL-dark cores are xenocrystic zircon grains derived from eucrites, whose presence indicates that NWA 8326 should have formed through partial melting of the Vestan mantle, with assimilation of eucritic material. The presence of xenocrystic zircon and primary Si,Al,K-rich glass and the large compositional variation of plagioclase indicate that NWA 8326 is an unequilibrated cumulate eucrite and hence the zircon 207Pb/206Pb age of 4559.2 ± 5.2 (2σ) Ma represents the crystallization of NWA 8326. Reconciling the cumulative texture with the presence of the chemically evolved glass, NWA 8326 would be excavated during the late stage of its crystallization and escaped the prevalent crustal thermal metamorphism of the eucrite parent body. The Mg isotopic composition of NWA 8326 is higher than most diogenites, which suggests that the parent magma of such a pigeonite cumulate eucrite was derived from a source region with heavier magnesium isotopic composition (μ25Mg: −90 to − 96 ppm).