Haiyang S. Wang

Previous studies report apparent chemical abundance discrepancies between M dwarfs and FGK stars in the solar neighborhood, raising questions about the origin of these differences. Using a statistical pipeline, we revisit whether M dwarf abundance ratios are consistent with non–local thermodynamic equilibrium (non-LTE) measurements of FGK stars, and independently validate these results with predictions from state-of-the-art Galactic chemical evolution (GCE) models of the Milky Way’s thin disk. Abundance trends for [C, O, Na, Mg, Al, Si, Ca, Ti, Ni/H] versus [Fe/H] are derived for a benchmark sample of 47 M dwarfs from the APOGEE, CARMENES, and Subaru/InfraRed Doppler (IRD) surveys. These are compared to non-LTE measurements of FGK stars and to GCE predictions calculated with the open-source code OMEGA+ . Our predictions take into account the intrinsic metallicity scatter of the solar neighbourhood and different stellar yield sets. We find that the M dwarf trends align with FGK stars within uncertainties for most elements (C, O, Na, Al, Si, Ni), resolving prior discrepancies. The observational trends also agree well with GCE predictions. However, the Ti trends are not consistent between spectroscopic samples, likely due to systematic differences in abundance determination techniques. We also show that M dwarf abundances differ significantly between different surveys in the literature. The abundance ratios of M dwarfs and FGK stars are mostly chemically consistent in the solar neighborhood and can be effectively modeled. GCE slopes provide an independent benchmark of these compositional trends.

Linking the composition of gas giant planets to their formation paths has long been a goal in exoplanet science. Cold gas giants with temperatures below ~500K have been especially out of reach for detailed atmospheric characterization. With JWST, however, we can reach high signal-to-noise (S∕N) spectra for such cool worlds and can measure not only their main trace gas abundances but even their isotopic content, thus unlocking new possibilities in linking them to their formation paths. In this study, we present the spectrum of one of the coldest planetary-mass companions, COCONUTS-2b (T _eff ≈ 480 K, separation of ~6400 au from its M dwarf host star), obtained with the Mid-InfraRed Instrument Medium Resolution Spectrometer (MIRI/MRS) on board JWST. We received a high S/N spectrum of up to 40 at ~11.8 μm. By combining the MIRI and archival Gemini/FLAMINGOS-2 datasets, we aim to characterize the chemical composition and physical structure of its frigid atmosphere in order set the stage to uncover insights on the formation of COCONUTS-2b. For the first time on a MIRI/MRS dataset, we used the full spectral resolution of MIRI/MRS and performed atmospheric retrievals to enable a search for the faint absorption features of rare molecules and isotopologs. The latter are identified using a leave-one-out analysis and Bayes factor comparison. We robustly detected three isotopologs, namely ¹⁵ NH ₃ , H ₂ ¹⁸ O, and H ₂ ¹⁷ O, in the atmosphere of COCONUTS-2b. We find the first clear evidence of oxygen isotopes in water in a cold companion, complementing previous CO isotope detections. We constrained the ¹⁶ O/ ¹⁸ O and ¹⁶ O/ ¹⁷ O isotope ratios as being significantly enriched compared to the solar and interstellar medium (ISM) values. However, the value for ¹⁸ O/ ¹⁷ O is consistent with the ISM value. We found a nitrogen ratio ¹⁴ N/ ¹⁵ N compatible with the value of the ISM and with previous observations of Y brown dwarfs. The derived effective temperatures as well as the subsolar C/O and subsolar to solar metallicity are in line with previous results. This dataset demonstrates the capability of MIRI/MRS to characterize such cold planetary-mass companion’s atmospheres with respect to their compositional and isotopic content. In the future, the constrained elemental and isotope ratios will provide a unique avenue for comparisons with the host star’s abundances and eventually the tracing of formation scenarios.

Close-in exoplanets span a wide range of orbital architectures and physical properties, consistent with the combined influence of migration processes and formation conditions. Although population synthesis models predict the emergence of distinct planetary populations, establishing a statistically robust connection between observed samples and synthetic populations remains challenging, particularly in high-dimensional parameter spaces dominated by dynamical quantities. Intrinsic population-level organisation within the observed sample of close-in exoplanets is investigated through physically motivated parameters, with the aim of establishing a quantitative link to formation pathways predicted by pebble-accretion models. A two-stage Gaussian mixture model (GMM) was applied to an observed sample of close-in exoplanets, with unsupervised probabilistic clustering performed in a feature space dominated by dynamical descriptors of planet--star interactions. The resulting clusters were mapped onto a pebble-accreted synthetic population within a statistically motivated 3D parameter space. Formation-related quantities, including formation timing, gas fraction, and ice--rock mass ratio, were then employed to establish a probabilistic interpretation and comparison of the mapped clusters. Statistically supported sub-populations were identified without the imposition of predefined classification boundaries, including very-massive gas giants, hot giants, warm-Jupiter-dominated systems, and lower-mass giants. When mapped onto pebble-accreted synthetic populations, these observational clusters exhibited systematic differences in formation timing, gas fraction, and solid growth histories across the dominant populations. Data-driven clustering in a dynamical-parameter space enables a direct and statistically robust comparison between observed exoplanet sub-populations and pebble-accretion synthetic populations. The inferred cluster-level trends point to systematic differences in formation timing and accretion histories among giant populations, with very-massive gas giants preferentially associated with earlier formation epochs than hot-giant and warm-Jupiter-dominated systems. This demonstrates the potential of physically motivated machine-learning approaches to connect observed exoplanets with theoretical formation scenarios at the population level.

Context. The relative abundances of refractory elements in planets are widely assumed to reflect those of their host stars. However, because elements are classified according to their behaviour in the solar nebula, this implicitly assumes that condensation is independent of nebular chemistry, despite contradictory evidence in chemically reduced systems with high molar carbon-to-oxygen (C/O) ratios. Aims. We investigated how variations in stellar C/O ratio and disk pressure modify condensation chemistry, and assessed the reliability of mapping stellar compositions to planetary building blocks in reduced environments. Methods. For a sample of FGK stars with C/O ratios spanning 0.65–0.95 (solar = 0.59±0.08), we computed the equilibrium phase stability using FactSage over 1900–400 K at total pressures of 10 ⁻² , 10 ⁻⁴ , and 10 ⁻⁶ bar. We tracked the phase evolution and key chemical transitions across C/O, temperature, and pressure. Bulk planet(esimal) compositions were derived using a stochastic accretion framework that aggregates condensates from temperature-dependent feeding zones. Results. We identified three distinct condensation regimes: (i) solar-like (C/O ≲ 0.7), (ii) transitional (C/O ~ 0.7–0.91), and (iii) reduced (C/O ≳ 0.92). Relative to solar-like sequences, oxygen-bearing silicates condense at lower temperatures in transitional and reduced regimes, while carbides, silicides, and sulfides appear. Bulk planetesimal Fe/Mg, Fe/Si, and Fe/O ratios deviate substantially from their host stellar values in transitional and reduced sequences, thus producing more diverse rocky building blocks within the same disk, ranging from metal-rich C- and S-bearing bodies to more Earth-like compositions. Conclusions. Condensation sequences are not universal across stellar compositions. In reduced disks, elemental ratios commonly treated as refractory based on the solar system condensation temperatures may not reliably trace planetary bulk composition. The distinct building blocks produced in high C/O systems thus provide potential formation pathways for metal-enriched super-Mercury analogues and distinct C- and S-rich rocky planets, expanding the diversity of terrestrial compositions beyond solar system analogues.

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.

The chemical abundance of host stars plays a pivotal role in shaping the formation history of planetary systems, yet the influence of elements beyond iron remains poorly understood. Here, we investigate the relationship between the carbon-to-oxygen (C/O) ratio of host stars and the orbital periods of giant planets. By analyzing high-resolution spectroscopic data from 598 planet-hosting stars (hosting 929 planets) across the Sloan Digital Sky Survey, Keck, and HARPS surveys, we identify a correlation; stars with higher C/O ratios are more likely to host longer-period giant planets. Theoretical models of pebble-driven planet formation and migration further support this observation, demonstrating that elevated C/O ratios enhance solid material availability at outer disk regions, promoting giant planet formation at larger distances and subsequent moderate inward migration. Our findings establish stellar C/O as a critical factor in shaping the orbital architecture of giant planets, bridging disk chemistry to planetary system evolution.

Aims . The inferred compositions of the Solar System’s terrestrial (rocky) bodies are fractionated from that of the Sun, where elemental depletions in the bulk rocky bodies correlate with element volatility, expressed in its 50% condensation temperature. However, because element volatility depends on disk gas composition, it is not mandated that elemental fractionation trends derived from the solar-terrestrial scenario apply to other planetary systems. Here, we expand upon previous efforts to quantify elemental volatility during disk condensation, and how this affects rocky planet compositional diversity. Methods . We simulated condensation sequences for a sample of 1000 initial disk compositions based on observed stellar abundances. Based on these simulations, we present parametrisations of how element 50% condensation temperatures depend on disk composition and apply element fractionation trends with appropriate element volatility to stellar abundances to simulate compositions of rocky exoplanets with the same volatile depletion pattern as the Earth, providing a robust and conservative lower limit to the compositional diversity of rocky exoplanets. Results . Here we show that Earth-like planets emerge from low-C/O disks (C/O ≤ 0.75) and graphite-bearing planets from medium-to-high-C/O disks (C/O > 0.75). Furthermore, we identify an intermediate-C/O (0.84–1.04) class of planets characterised by Mg and Si depletion, leading to relatively high abundances of Fe, Ca, and A1. We show that devolatilisation patterns could be adapted potentially with disk composition-dependent condensation temperatures to make predictions of rocky planet bulk compositions within individual systems, although such patterns could be further modified by the dynamics of planetary accretion, which remains under-constrained for most exoplanetary systems. The outcomes of our analysis suggest that accounting for disk composition-dependent condensation temperatures means that we can expect an even broader range of possible rocky planet compositions than has previously been considered.

Earth and Mars are both depleted in moderately volatile elements with sublimation temperatures between 400 K and 1300 K, such as F, Na and Li. A similar depletion pattern is observed in the spectra of both white dwarfs and Sun-like stars suspected of recently having accreted rocky planetary materials. Here we model the sublimative loss of volatiles (devolatilization) during pebble accretion, while simultaneously considering growth by collisions with volatile-predepleted planetesimals. We find that bulk silicate Earth composition is well-explained by a mixture of at least 70% contribution from two protoplanets that grew by pebble accretion together with up to 30% contribution from planetesimals with composition resembling the asteroid Vesta. Bulk silicate Mars, in contrast, is best described by 20-40% contribution from pebble accretion and 60-80% contribution from compositionally Vesta-like planetesimals. Our results lend support to a hybrid formation scenario where terrestrial planets around the Sun and other stars grow by a combination of rapid pebble accretion and a prolonged period of planetesimal accretion and giant impacts.