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Physics Astrophysics Earth and Planetary Astrophysics

astro-ph.EP Watch Earth and Planetary Astrophysics

Tuesday 8 September

Earth and Planetary Astrophysics

The GAPS programme at TNG LXXVIII. Phase-resolved detection of multiple atomic species in the atmosphere of KELT-20b/MASCARA-2b

F. Biassoni, F. Borsa, L. Fossati et al. (+15 more)

Among the different classes of exoplanets, ultra-hot Jupiters represent prime laboratories for atmospheric characterisation. With an equilibrium temperature of ~2300 K, KELT-20b/MASCARA-2b (HD 185603 b) is a well-studied ultra-hot Jupiter, for which several atomic species have already been detected in both transmission and emission spectroscopy. Nevertheless, important open questions remain regarding the completeness of the chemical inventory and the interpretation of spurious aliasing signals that may mimic genuine atmospheric signatures. In addition, asymmetries in the detected signals across transits can encode valuable information on the spatial structure and dynamics of the planetary atmosphere. In this work, we analyse seven transits of KELT-20b/MASCARA-2b observed with the high-resolution spectrograph HARPS-N at the Telescopio Nazionale Galileo. We apply the cross-correlation function technique to a large set of atomic and ionic species, accounting for aliasing effects that can lead to false detections. We confirm previous detections of HI, NaI, MgI, CrI, FeI, and FeII, and tentatively detect CrII. We further report new detections of KI, CaI, VI, MnI, and BaII, and a tentative detection of PrI in the planetary atmosphere. By performing a phase-resolved analysis and splitting the transit into pre- and post-mid-transit phases, we investigate longitudinal variations in the detected signals. For several species, we observe a double-peaked structure in the Kp-Vsys maps, notably in CaI, VI, CrI, and FeI, consistent with absorption arising from two distinct atmospheric limbs of the planet.
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All authors F. Biassoni, F. Borsa, L. Fossati, M.C. D'Arpa, A.S. Bonomo, G. Guilluy, T. Zingales, D. Sicilia, A. Sozzetti, M. Rainer, M. Basilicata, I. Carleo, R. Cosentino, R. Claudi, P. Giacobbe, A. Harutyunyan, L. Malavolta, L. Mancini

Earth and Planetary Astrophysics

The Discovery of K2-232c: Divergent Formation Histories for Hot and Warm Jupiters Based on Outer Companion Eccentricity

Jessica A. Ranshaw, Xian-Yu Wang, Jennifer A. Burt et al. (+6 more)

Ever since their discovery, hot Jupiters have been one of the most studied types of exoplanets to exist thanks to their significant size, their proximity to their host star, and their significant departure from anything present in our solar system. Yet, the details of their formation and evolution remain unclear, including their connection, if any, to the wider-orbiting warm Jupiter population. In this work, we present the discovery of K2-232c, an eccentric cold Jupiter ($P = 1950 ^{+140}_{-120}$ days, $e=0.352^{+0.095}_{-0.076}$, $M\sin{i} = 5.31^{+0.48}_{-0.45} {M_{\rm Jup}}$) companion in a known warm Jupiter ($P = 11.1684377 \pm 0.0000010$ days, $e=0.245^{+0.023}_{-0.024}$, $M = 0.427^{+0.039}_{-0.036} {M_{\rm Jup}}$) system. Placing this system in context with the literature, we find that cold Jupiter eccentricities are generally higher in hot Jupiter systems as compared to warm Jupiter systems, suggesting the formation of hot Jupiters is more dynamically violent than warm Jupiters. This adds further evidence to the claim that these two populations are independent from one another with cold Jupiters appearing to play a crucial role in shaping the formation of both.
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All authors Jessica A. Ranshaw, Xian-Yu Wang, Jennifer A. Burt, Juan I. Espinoza-Retamal, Brandon T. Radzom, R. Paul Butler, Bradford P. Holden, Steven S. Vogt, Songhu Wang

Earth and Planetary Astrophysics

The size and mass distribution of cold classical TNOs for $5<H<13$

Gary M. Bernstein, Wesley C. Fraser, Marielle R. Eduardo et al. (+7 more)

The cold classical trans-Neptunian objects (CCs) are the only observable in situ population of planetesimal remnants believed to have escaped collisional grinding. Recent JWST observations make it possible to fit the differential absolute magnitude distribution $dN/dH$ of the CCs from $5<H_r<13,$ and infer the differential mass distribution $dN/dM$ across $\ge5$ orders of magnitude in mass. We find $dN/dM$ well fit by a lognormal distribution, and equally well by a generalized $\Gamma$ distribution or a double power law. The maximum fraction of the total CC mass per log interval in $M$ is in bodies near 75~km diameter, or $\approx10^{-7.5}M_\oplus.$ Extrapolation of the fitted $dN/dH$ functions to $H_r>13$ is unwise, as the different analytic forms diverge. It remains unclear if $dN/dH$ turns over at faint $H.$ The uncertainty in the total mass of the CC belt is dominated by uncertainty in the relation between $M$ and $H$. A calibration using CC binaries suggests a total CC mass of 1.7–2.7$\times10^{-3}\,M_\oplus.$ A trend toward lower density and/or higher albedo for smaller bodies may be present in the data, and would lower the estimated total CC mass. Qualitative comparison of the derived mass distribution to the results of numerical simulations of the streaming instability (SI) suggest the simulations produce $dN/dM$ distributions that are more sharply peaked, and steeper at the bright end, than the CCs. Such differences could be ascribed to inhomogeneous formation conditions in the classical belt that are not yet included in modeling. The variety and uncertainty of $dN/dM$ derived from state-of-the-art SI simulations currently preclude any definitive test of the SI hypothesis.
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All authors Gary M. Bernstein, Wesley C. Fraser, Marielle R. Eduardo, William M. Grundy, Bryan Hilbert, Matthew J. Holman, Anastasia N. Morgan, Kevin J. Napier, John A. Stansberry, David E. Trilling

Earth and Planetary Astrophysics

The Luminosity Function of Ultra-Faint Trans-Neptunian Objects Detected by JWST

Marielle R. Eduardo, Anastasia N. Morgan, Wesley C. Fraser et al. (+8 more)

We present a definitive discovery of 27 trans-Neptunian objects (TNOs) using the Near-Infrared Camera (NIRCam) aboard the James Webb Space Telescope (JWST). By employing a shift-and-stack technique and a machine learning network geared specifically to identifying false-positive detections in JWST images produced through the shift-and-stack process, we achieved a 40% detection threshold of $m_{F150W2}=28.8$ mag (corresponding to $m_r\sim29.8$ mag) across a sky area of $0.05 \ \text{deg}^2$. This marks the deepest Solar System survey to date, reaching magnitudes that allow us to explore never-before-seen regions of the TNO size distribution. Our faintest detection has $m_{F150W2}=29.3$ mag and diameter of $\sim10$ km (assuming 15% albedo). Within our sample, we find that both the Cold and Hot TNO subpopulations exhibit a power-law slope. The distribution of apparent magnitudes of our nominal sample (detections at all epochs) are well fit by a single power law $dN/dm \propto 10^{\alpha m}$ with $\alpha=0.29^{+0.08}_{-0.07}$. The dynamically hot and cold subsamples in our discovery set are consistent with the same power law, suggesting that the planetesimal formation process yields similar slopes despite the differing disk conditions at the presumed $\sim25$ and $\sim45$~au formation regions of the two populations.
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All authors Marielle R. Eduardo, Anastasia N. Morgan, Wesley C. Fraser, David E. Trilling, Gary M. Bernstein, John A. Stansberry, Bryan Hilbert, Matthew J. Holman, William M. Grundy, Thomas L. Storer, Kevin J. Napier

Earth and Planetary Astrophysics

Combined JWST and HST Deep Imaging to Characterize the Smallest Known Trans-Neptunian Objects

Anastasia N. Morgan, Marielle R. Eduardo, David E. Trilling et al. (+9 more)

We present optical-near-infrared (NIR) color measurements for small ($\leq 40$ km) Trans-Neptunian Objects (TNOs) using coordinated and nearly simultaneous observations from the James Webb Space Telescope (JWST) and the Hubble Space Telescope (HST). JWST/NIRCam data provided detections and NIR photometry for faint TNOs, while HST/ACS and WFC3 imaging enabled recovery in the optical, together yielding optical-NIR colors spanning 0.35-3.2 $\mu$m. Thirteen JWST-detected TNOs were recovered in the HST observations, and trailed PSF photometry was used to derive mean magnitudes, colors, and rotational lightcurves. The color distribution of our small cold classical TNO discoveries is narrow and consistent with the occupation of a single reflectance (color) sequence previously identified for larger cold classical TNOs. We find no evidence for a change in this sequence at smaller sizes. In contrast, the dynamically excited TNOs we discovered exhibit a broader range of colors consistent with multiple compositional classes seen at larger sizes. Lightcurve amplitudes were generally low for both dynamical groups in our sample. 2015 GK56, a previously known TNO in our field, displays a large-amplitude, structured lightcurve consistent with a contact binary. Our results indicate that the characteristic color distribution of TNOs extends to smaller sizes than previously studied, suggesting a primordial origin rather than size-dependent collisional processing.
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All authors Anastasia N. Morgan, Marielle R. Eduardo, David E. Trilling, Wesley C. Fraser, John A. Stansberry, Gary M. Bernstein, Bryan Hilbert, Matthew J. Holman, William M. Grundy, Kevin J. Napier, Stephen C. Tegler, Cesar Fuentes

Earth and Planetary Astrophysics

TEMPUS: Relativistic coordinate time scales for any solar-system body from arbitrary ephemerides

Yan Seyffert and Agnès Fienga

Precise time-scale transformations between solar-system bodies are an essential requirement for deep-space science and solar system navigation. The post-Newtonian relativistic expression relating a body-centered coordinate time TCX to the Barycentric Coordinate Time TCB depends on the gravitational environment and velocity of the target body, and must be evaluated consistently with the chosen planetary ephemerides. So far only INPOP has provided such a solution as part of its ephemerides releases, albeit only for Earth with TCG-TCB. We present TEMPUS, a tool that numerically integrates the IAU 2000 time-transformation for any solar-system body using positions and velocities from an arbitrary ephemerides. We provide a validated, ephemerides-agnostic implementation of TCX-TCB and assess its accuracy against published independent solutions. The rate $d\,\mathrm{TCX}/d\,\mathrm{TCB}$ is integrated with an Adams-Moulton order-12 predictor-corrector. The gravitational potential sum tested here includes the Sun, eight planets, Pluto, up to 343 main-belt asteroids, and 30 trans-Neptunian objects. Outputs can be stored in readable format tables of sampled values or as Chebyshev coefficient files. Comparisons are performed against the TU Dresden Klioner time solutions, the TCG-TCB solution provided with INPOP19a and INPOP21a, and the LTE440 lunar solution. For the planetary bodies TEMPUS agrees with Klioner at the $10^{-21} s\,s^{-1}$ level over 200 yr. Against the INPOP19a released TCG-TCB we also agree to the same level. The main-belt asteroids as well as the trans-Neptunian objects shift the planetary time scales at the $10^{-18} s\,s^{-1}$ level, and the choice of planetary and lunar ephemerides contributes differences on the $10^{-18} s\,s^{-1}$ level as well. TEMPUS provides a validated, ephemeris-agnostic post-Newtonian time-transformation for any solar-system body.
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Earth and Planetary Astrophysics

Interplanetary Dust Ablation Seeds Silicate Clouds in Exoplanet Atmospheres

Sven Kiefer, Gregory J. Gosselin, Caroline V. Morley et al. (+1 more)

Interplanetary dust particles (IDPs) are present throughout the Solar System, with Earth experiencing a mass flux of roughly 100 tonnes of IDPs per day. These particles ablate in the upper atmosphere, delivering aerosols that form noctilucent and stratospheric clouds. Since IDPs are present in exoplanet systems as exozodiacal dust disks (exozodis), it is likely that exoplanets experience a constant infall of IDPs as well. In this work, we investigate the effect of IDPs on the cloud structure of gas-giant exoplanets and brown dwarfs, and determine their observability. We apply the microphysical cloud model Nimbus to the Sonora Diamondback model grid to cover a wide range of effective temperatures, surface gravities, atmospheric mixing constants, and IDP mass fluxes. Our results show that IDP infall increases cloud particle number densities, decreases cloud particle radii, and enhances silicate features in transmission and thermal emission spectra. Exoplanets with lower atmospheric mixing rates, effective temperatures, and gravity are more sensitive to IDPs. Most notably, they lead to observable silicate absorption features in colder planets where silicate clouds would otherwise not be observable. WASP-107 b is an example of such an exoplanet with observational evidence of silicate particles. Using Nimbus, we found that a 100 times Earth-like IDP mass flux with a Solar-System-like IDP composition of quartz (SiO2), enstatite (MgSiO3), and iron (Fe) can explain the observations of high altitude silicate particles in WASP-107 b.
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All authors Sven Kiefer, Gregory J. Gosselin, Caroline V. Morley, Sean P. S. Gulick

Earth and Planetary Astrophysics

Centaur Longevity Revisited: Lifetime Dispersion and the Bailey & Malhotra Classifications under Modern Orbital Solutions

Naresh Prasanna and Chrisphin Karthick

Centaurs occupy dynamically unstable orbits between Jupiter and Neptune and are typically removed by giant-planet encounters within a few Myr. Bailey and Malhotra (hereafter BM09) classified the known Centaurs, from 2007 orbits, into a short-lived diffusing class (D), a resonance-hopping class (R), and a long-lived, quasi-stable class (Q); their sole Q object, 2005 TH173, was fit to a 17\,d arc. We revisit both halves of that analysis on modern Jet Propulsion Laboratory Small-Body Database (SBDB) orbits (retrieved 2026-07-26), integrating clone ensembles of each object with the REBOUND WHFast $N$-body integrator. We ask (i) whether tighter orbital precision narrows the dispersion in clone lifetimes, and (ii) whether the BM09 labels survive on updated elements. Among 65 genuine Centaurs with arc $\ge 30$\,d and no censored clones, $\sigma(\log_{10}\mathrm{lifetime})=0.54$\,dex (a factor of 3.5). Conditioned on present-day $a$, $e$, $i$, and $q$, this dispersion is insensitive to relative semimajor-axis uncertainty (slope $=-0.002$\,dex per decade in $\sigma_a/a$; 95% confidence interval $-0.034$ to $+0.030$) — a chaos floor. On the D/Q axis, 49/50 comparable objects retain their BM09 class; 2005 TH173 is the exception (Q$\to$D): 80/100 clones escape before 10\,Myr (median 2.99\,Myr), and a 40\,Myr integration of the same initial conditions returns 88\,D / 12\,R / 0\,Q. None of the 30 multi-opposition 2007 orbits changes class; two short-arc objects leave the Centaur zone.}
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Earth and Planetary Astrophysics

XUE. ProDiMo models of internally and externally irradiated planet-forming disks around 0.3-4.0 solar mass stars (The IRIS project I)

Jenny Frediani, Konstantin V. Getman, Peter Woitke et al. (+18 more)

Most stars and planets form in massive star-forming regions, where disks are exposed to external far-ultraviolet (FUV) radiation from nearby O- and B-type stars. The combined effects of stellar irradiation and external FUV fields on terrestrial planet-forming regions (< 10 au) across stellar masses remain unclear. We investigate how internal UV and X-ray irradiation and external FUV fields affect mid-infrared (mid-IR) gas emission and the atmospheric carbon-to-oxygen (C/O) ratios inferred in T Tauri and Herbig Ae/Be disks. We compute disk structures with ProDiMo and synthetic spectra with FLiTs, convolved to a representative JWST/MIRI-MRS resolution (R$\sim$2680). (1) We present the Internal and external irRadIation of diSks (IRIS) grid: four model sets spanning stellar masses of 0.3-4.0 solar masses, including stellar X-ray flares and an external FUV field of 1e4 G0 (Habing units). (2) We predict increasing flux densities with stellar mass for key atomic and molecular mid-IR tracers. (3) External FUV irradiation enhances CH3+ and H2 emission, whereas FUV-induced disk truncation yields inner disk chemistry resembling that of disks irradiated only by their host stars. (4) Mid-IR H2O, CO2, and C2H2 line ratios imply carbon-rich compositions (C/O$\sim$1-10) in the warm emitting layers of T Tauri and Herbig Ae/Be disks, primarily reflecting stellar irradiation, with little sensitivity to external FUV irradiation. The IRIS grid provides a flexible framework for interpreting JWST and future Extremely Large Telescope (ELT) infrared disk observations across a broad range of stellar properties and irradiation conditions. Future models should include FUV-driven photoevaporative winds, X-ray radiative transfer, and time-dependent X-ray irradiation.
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All authors Jenny Frediani, Konstantin V. Getman, Peter Woitke, Bayron Portilla-Revelo, Eric D. Feigelson, Christian Rab, Arjan Bik, María Claudia Ramírez-Tannus, Rens Waters, Thomas Henning, Inga Kamp, Germán Chaparro, Sebastián Hernández, Simón Rodríguez, Pablo Cuartas-Restrepo, Andrew Winter, Thomas J. Haworth, Thomas Preibisch, Sierk E. van Terwisga, Veronica Roccatagliata, Alexis Brandeker

Earth and Planetary Astrophysics

Radial velocity follow-up of Gaia astrometric substellar companions: six confirmed brown dwarfs and 13 impostor binaries

Marcus L. Marcussen, Simon H. Albrecht, Kamil K. Kalinowski et al. (+7 more)

With microarcsecond precision, Gaia has made the astrometric discovery of substellar companions feasible. However, follow-up observations are needed to check on orbital solutions and rule out astrophysical false positives. We validate and characterise a sample of 20 Gaia Data Release 3 (DR3) astrometric candidates for substellar companions, with the dual goal of identifying false positives and deriving robust physical parameters for genuine companions. We performed high-resolution spectroscopy using FIES, NIRPS, and NEID at the Nordic Optical Telescope, the ESO 3.6m telescope, and the WIYN Telescope, respectively. Double-lined spectroscopic binaries were identified as false positives, and surviving systems were characterised through joint Gaia astrometry and radial velocity modelling. Thirteen of the candidates proved to be near-twin binary stars for which the astrometric motion of the center of light is small enough to mimic that of a single star in response to a substellar companion. We confirm that six companions are brown dwarfs with masses in the range ${\sim}26$–$68\,M_\mathrm{Jup}$, two of them for the first time. In contrast to earlier studies of lower-mass Gaia candidates, we find that the astrometric orbital solutions are generally reliable to within the stated uncertainties.
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All authors Marcus L. Marcussen, Simon H. Albrecht, Kamil K. Kalinowski, Joshua N. Winn, Gu{\dh}mundur Stefánsson, Jens Reersted Larsen, Julie Gadeberg, Evan Fitzmaurice, Kevin Schlaufman, Suvrath Mahadevan

Earth and Planetary Astrophysics

MAD-LEO: A Maneuver-Annotated Orbital Dataset for LEO Satellites with Tiered Multi-Source Evidence

Zhixin Guo, Qi Shi, Xiaofan Xu et al. (+6 more)

With the rapid development of aerospace technology and the large-scale deployment of low Earth orbit (LEO) constellations, the risk of orbital collisions has increased, creating a growing demand for reliable observations of satellite maneuvers. However, public datasets containing real maneuver records remain scarce. We present MAD-LEO, a Maneuver-Annotated orbital Dataset for LEO satellites. The mission-reported subset contains 1,134 maneuver events from eleven geodetic and altimetry satellites spanning 1992 to 2026, with labels taken directly from mission-published maneuver histories. Each event is checked against two-line element (TLE) data, precise orbit products, and satellite laser ranging (SLR) observations, with evidence tiers assigned according to data availability. The operational subset pairs operator-published ephemerides for 6,785 Starlink satellites with cataloged TLE records over a continuous 107-hour period. Technical validation across seven machine-readable experiment suites confirms the cross-source consistency of the labels and the evidence products.
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All authors Zhixin Guo, Qi Shi, Xiaofan Xu, Linqiang Ge, Hua Zhu, Liyan Ben, Bendian Nie, Yuanrui Zhao, Xiaohan Li

Monday 7 September

Earth and Planetary Astrophysics

Sea Ice as an Origin of Life Location for Hycean and Ocean Worlds

Edouard F. L. Barrier, Nikku Madhusudhan and Frances E. Rigby

Water worlds with potential deep oceans are at the forefront of currently observable exoplanetary habitability. The standard requirements for habitability are the presence of an energy source, a source of nutrients, and a solvent, all of which can be met in exoplanets with deep liquid water oceans. Such planets may be numerous, ranging from hycean worlds and ocean worlds to terrestrial-size planets with enhanced water content. However, abiogenesis requires additional conditions, namely a mechanism to accumulate nutrients to the high concentrations required for prebiotic chemistry, and an energy source to drive the chemistry. A deep ocean, and in some cases the high-pressure ice layers underneath, has traditionally been thought to prevent the required concentrations. Here we outline a location where nutrients could be concentrated in sufficiently high quantities to make abiogenesis possible – at the interface between the open ocean and the surface ice shelf. Using a 3D General Circulation Model, we explore cases corresponding to several candidate habitable exoplanets which might have dayside sea ice. We discuss how the planet's climate influences the nature and behaviour of the sea ice. We investigate the ability of small-scale ice formation and melting processes to concentrate nutrients to prebiotically useful levels, and also explore the potential for meteoritic impactor fragments to provide a platform for prebiotic chemistry. We find that both of these mechanisms may plausibly provide concentrated chemical feedstock for prebiotic chemistry, which adds an important new perspective when evaluating the prospects of habitability in planets with deep oceans.
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Earth and Planetary Astrophysics

Long-baseline ATLAS photometry of the seven main-belt asteroid targets of the Emirates Mission to the Asteroid Belt

Nicolas Erasmus, Josef Ďurech, David E. Trilling et al. (+6 more)

The Emirates Mission to the Asteroid Belt (EMA) will use the MBR Explorer spacecraft to fly past six main-belt asteroids – (10253) Westerwald, (623) Chimaera, (13294) Rockox, (88055) Ghaf, (23871) Ousha and (59980) Moza – before rendezvousing with, and deploying a lander onto, the extremely red (269) Justitia. We present a homogeneous analysis of a decade of sparse photometry of all seven targets from the Asteroid Terrestrial-impact Last Alert System (ATLAS). For each object we derive absolute magnitudes and phase slopes in the ATLAS c and o bands, the c-o colour on a common phase slope, a rotation period from a Lomb-Scargle periodogram of the phase-corrected light curve, and a light-curve amplitude setting a lower limit on elongation. Five periods are secure – 3.243 h for Rockox, 33.1 h for Justitia, 3.64 h for Westerwald, 14.6 h for Chimaera and 4.36 h for Moza – and agree with independent work; for Ousha aliasing leads us to adopt the published 8.35 h, while Ghaf remains unresolved. Lightcurve inversion gives a unique convex shape and spin state for Rockox and two pole solutions for Chimaera, Moza and Justitia; no model was obtained for Ghaf, Ousha or Westerwald, though Westerwald's spin axis must be retrograde. Combining the c-o colour with an albedo class from the phase slope, we classify the sample as one dark C-type (Chimaera), three probable S-types (Westerwald, Ghaf, Moza), two X-complex objects (Rockox, plausibly M-type, and Ousha, possibly E-type), and the exceptionally red L/D-type Justitia, whose (c-o) = 0.48 is redder than typical Jupiter Trojans. This only partly matches the mission's family-based expectation of a predominantly primitive target set, so we present these as testable predictions. Those of Ghaf and Ousha are least secure – both disagree with published work and have unresolved or aliased periods – and warrant follow-up.
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All authors Nicolas Erasmus, Josef Ďurech, David E. Trilling, Larry Denneau, John L. Tonry, Henry Weiland, Ken W. Smith, Javier Licandro, Miguel R. Alarcon

Earth and Planetary Astrophysics

A semi-analytical Ring-Arc model for Plutino perturbations

Yue Chen and Jian Li

Neptune's 2:3 mean motion resonance induces an asymmetric spatial distribution of Plutinos. We previously proposed a 9000-point-mass arc model that successfully captures the resulting influence on planetary ephemerides. However, integrating thousands of point masses involves significant computational cost. In this paper, to more efficiently simulate the total perturbation of Plutinos, we develop a semi-analytical Ring-Arc model described by gravitational potentials. This model consists of an outer ring and two inner arcs, representing the resonant spatial distribution of Plutinos. The model parameters are derived from observed Plutinos and theoretical sampling, yielding consistent structures. Numerical simulations show that the perturbations on the Sun-Neptune and Sun-Saturn distances from the semi-analytical Ring-Arc model agree with those from the 9000-point-mass arc model within 4% over 50 years, while outperforming a simple ring model in comparison with the bias-corrected Plutino population. Our semi-analytical approach preserves the dynamical characteristics while significantly enhancing computational efficiency, offering a state-of-the-art tool for ephemeris modelling.
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Earth and Planetary Astrophysics

That's no moon: Exomoon survival and detection limitations

Alicia Pérez-Rodrigo and Isabel Rebollido

While moons are ubiquitous in the Solar System, exomoons have not yet been unambiguously confirmed despite the discov- ery of over 5,600 exoplanets. We investigated the reasons for this lack of detections by reviewing high-impact literature searches, assessing dynamical stability and detectability constraints, and comparing these with the moon population in the Solar System. We compiled a homogeneous catalogue of 24 proposed exomoon candidates, including stellar, planetary, and inferred lunar parameters. For each case we applied Hill, Roche, and Laplace stability criteria and estimated detectability limits for current and forth- coming instruments. In addition, we tested 6038 confirmed exoplanets against the same stability and mission-specific observational filters for a given range of moon parameters. A large fraction of the potential exomoon candidates investigated in the literature are either inconsistent with long-term stability or produce signals below Kepler sensitivity. Only 8 of the 13 candidates meet at least one of the stability criteria. Our extended analysis highlights 27 promising targets for exomoon searches with PLATO, while moons with Solar-System-like properties remain below current thresholds. The lack of exomoon detections is likely due to a combination of the inefficient formation of stable massive moons and limited instrumental sensitivity. Other processes, such as early moon loss during planetary migration or giant impacts, may also influence moon survival. Future searches should prioritise systems that combine long-term dynamical stability with favourable observational conditions for next-generation photometric and astrometric facilities.
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Earth and Planetary Astrophysics

The effect of the atmospheric C/O ratio on interiors of hot Jupiters

Esther van Dijk, Yamila Miguel and Paul Mollière

The atmosphere is the outer boundary of a gas giant exoplanet's interior. Therefore, compositional changes in the atmosphere can affect interior inferences. Typically, only metallicity is considered in atmospheric boundary conditions, while other elemental ratios, in particular the C/O ratio, are assumed to be solar. In light of the observational constraints now achievable with JWST, this assumption might no longer be justified. In this work, we investigate the effect of the C/O ratio on atmospheric boundary conditions, interiors and radii of hot Jupiters. We construct an atmospheric boundary grid, consisting of one-dimensional atmospheric models in radiative-convective and thermochemical equilibrium. This grid is coupled to a static interior structure model at the radiative-convective boundary (RCB). Because in the temperature regime of hot Jupiters, the C/O ratio determines the dominant species in the atmosphere, it can significantly alter the opacity and, consequently, the pressure and temperature at the RCB. This temperature change propagates into the convective interior, thus affecting the calculated radius. The radius difference relative to a solar C/O-atmosphere significantly exceeds observed radius uncertainties of 3 per cent for planets with equilibrium temperatures $\gtrsim$ 1500 K and super-solar atmospheric metallicities. As an example, we demonstrate that, for WASP-19~b, higher C/O ratios result in higher inferred intrinsic temperatures. These results highlight the importance of treating atmospheric composition and interior structure as a coupled system. As atmospheric constraints improve, incorporating measured C/O ratios into atmospheric boundary conditions is necessary to obtain robust inferences of hot Jupiter interiors.
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Earth and Planetary Astrophysics

The First Observations of Moonlit Satellites

Sarah E. Caddy, Stephen E. Catsamas, Michele Trenti et al. (+2 more)

Optical Space Domain Awareness (SDA) operations have traditionally relied on Sunlit passes near the terminator, limiting continuous tracking of low-Earth orbit (LEO) satellites. In this work, we show that by accounting for all four major illumination sources: Sunlight, Earthshine, Moonlight, and Lunar-Earthshine, large LEO satellites like the ISS can be optically monitored for a full 24 hour period. We present what we believe to be the first quantitative observations of the International Space Station (ISS) and Chinese Space Station (CSS) illuminated only by Moonlight and Lunar-Earthshine under night-time conditions. Using a 0.6 m telescope, we obtain 147 detections of the ISS with a median brightness of V = $12.02 \pm 0.17$ mag, approximately $13.1 \pm 1.3$ magnitudes fainter than in daylight. For interpretation, we extend a satellite brightness model based on reflected Sun light (lumos-sat) to include Moonlight and Lunar-Earthshine, and validate this model both against the new observations and independent Ansys Systems Tool Kit simulations, achieving residuals of $0.03 \pm 0.80$ mag. The simulations confirm that Lunar-Earthshine dominates the illumination of nadir- facing components, boosting their radiance by at least 100 times relative to Starlight alone. Applying this validated model to existing and proposed large satellites, we show that Moonlight and Lunar-Earthshine will make these future spacecraft well within detection limits of even modest ($\sim 0.6$ m) optical SDA systems for most of the night, and for up to $\sim 11$ nights per lunar month at local midnight. When Moonlit observations are combined with daytime and twilight observations, this enables near-continuous ($\sim 24$ h) optical monitoring of LEO satellites from a single site, significantly increasing SDA tracking capabilities.
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All authors Sarah E. Caddy, Stephen E. Catsamas, Michele Trenti, Lee R. Spitler, Patrick North

Earth and Planetary Astrophysics

Constraining the orbit of the retrograde planet in the $\nu$ Octantis system

Alan Cefali Signor, Alexandre C.M. Correia, Maria Helena Moreira Morais et al. (+4 more)

The $\nu$ Octantis system is composed of a compact stellar binary and a retrograde S-type planet orbiting near the edge of stability. Here, we investigate the dynamically viable architectures of this system by exploring the planetary orbital elements with a Monte Carlo method. To handle the large number of free parameters, we employed a novel approach in which all relevant orbital parameters were varied simultaneously, which we refer to as corner stability maps. Furthermore, since the osculating orbital period of the planet undergoes large oscillations, we combined these maps with frequency analysis to identify the stable configurations that remain consistent with the observational data. We find that the retrograde planetary orbit most likely lies in the region of influence of the high-order 28/$-$11 mean-motion resonance, where its long-term stability is enhanced by secular apsidal alignment.
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All authors Alan Cefali Signor, Alexandre C.M. Correia, Maria Helena Moreira Morais, Timothée Vaillant, Ho Wan Cheng, Man Hoi Lee, Trifon Trifonov

Sunday 6 September

Earth and Planetary Astrophysics

The Effect of X-Ray and Accretion Variability on Mid-Infrared Lines in the Young Disk-Bearing Binary DQ Tau

Bayron Portilla-Revelo, Konstantin V. Getman, Ágnes Kóspál et al. (+5 more)

A star's radiation field is the main energy source of a planet-forming disk. Stellar photons—from X-rays to optical—are reprocessed in the inner protoplanetary disk enabling mid-IR diagnostics of the disk's physical conditions. Most pre-main-sequence stars accrete at moderate rates and emit X-rays efficiently. Such activity is expected to drive stochastic variability in the stellar spectrum. It is not clear to what extent such variability affects the thermochemical structure of protoplanetary disks. We aim to examine the effect of routine accretion variability and X-ray flares on the thermochemical structure and mid-IR line emission spectra of protoplanetary disks. We combine multiepoch, contemporaneous X-ray and NUV/optical observations of the DQ Tau binary with JWST/MIRI spectra of its circumbinary disk. We interpret the data and assess the effect of a varying stellar spectrum on the disk using a thermochemical model illustrative of the system. Synthetic fluxes of CO, CO2, HCN, and H2O are systematically stronger at periastron than at apastron due to an enhanced accretion luminosity. Variations in the integrated fluxes lie within a factor of two between epochs, in agreement with the JWST/MIRI spectra of DQ Tau. Routine stellar variability induces only modest changes in the disk's structure, yet it can still shift the snowline outwards by 17 percent relative to its apastron position. Because the JWST observations did not coincide with X-ray flares, our flare results rely on simulations: moderate-intensity flares leave most mid-IR molecular emission unchanged; in contrast, X-ray flares strongly enhance the abundances and mid-IR luminosities of H I, Ar II, and Ne II in the disk surface. Our results imply that the Spitzer/JWST line variability observed in non-outbursting systems is consistent with moderately variable stellar spectra driven by accretion and magnetospheric activity.
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All authors Bayron Portilla-Revelo, Konstantin V. Getman, Ágnes Kóspál, Dmitry Semenov, Vitaly Akimkin, Lis Zwicky, Jan Forbrich, Sierk van Terwisga

Earth and Planetary Astrophysics

Calibration of CNEOS Fireball Velocities and the Robustness of Nominally Hyperbolic Events

Volkan Duran and Abraham Loeb

The NASA/JPL CNEOS Fireball Data catalogue provides a near-global record of bright atmospheric entries but does not report per-event velocity covariance. We calibrate its kinematic accuracy using 19 events with independent reference trajectories and use optical, satellite, radar, and infrasound catalogues as source-specific controls. For nine modern high-quality comparisons, the CNEOS-minus-reference speed residual has mean -1.00 km s$^{-1}$ and standard deviation 0.86 km s$^{-1}$; variance decomposition assigns 0.77 km s$^{-1}$ to the CNEOS component. The radiant residuals have a 0.74 degree per-axis core plus one 16.3 degree outlier, so the extreme tail remains poorly determined. Seven of 354 complete CNEOS states are nominally hyperbolic without correction. A +0.9935 km s$^{-1}$ atmospheric-stage shift produces five additional crossings, but these are treated only as sensitivity cases. Polar-IM (2026 April 1) is the strongest post-2018 CNEOS anomaly, whereas independent GLM-LI stereo solutions yield 57.3 +/- 2.0 and 56.7 +/- 3.0 km s$^{-1}$, close to their direction-specific parabolic thresholds. We conclude that CNEOS can identify priority candidates, but secure interstellar classification requires an independent, uncertainty-bearing three-dimensional trajectory for the same event.
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Earth and Planetary Astrophysics

An Outer Giant Excites and Constrains the Orbit of $\pi$ Mensae d

Qunfeng Jiang

The nearby $\pi$ Mensae system contains the eccentric giant planet b on a 5.7 yr orbit, the 6.27 day transiting sub-Neptune c, and a candidate planet d on a 123 day orbit between them. Published estimates place the eccentricity of d between approximately 0.2 and 0.4. We reanalyze 772 published velocities with three-Keplerian fits and explore three-dimensional b-d dynamics with N-body integrations. The high-eccentricity solution persists when the phase or apsidal prior is widened; exact importance reweighting to a population-informed Beta eccentricity prior gives $e_{d}=0.378^{+0.050}_{-0.049}$. In 20 kyr integrations, 84.5% of configurations near the prograde and retrograde coplanar endpoints survive, compared with 21.3% across $60^\circ \leq I_{bd} < 120^\circ$; stellar collisions dominate the failures. Secular forcing by b moves 53 of 62 one-Myr survivors initialized with $e_{d} \leq 0.1$ into $0.3 \leq e_{d} \leq 0.5$. Representative b+d configurations remain viable for 100 Myr, whereas controls with c and general relativity retain fewer systems. The outer giant can therefore excite the observed eccentricity of d while constraining its viable orbit toward prograde or retrograde near-coplanar configurations.
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Earth and Planetary Astrophysics · Cross-listed from astro-ph.IM

Mechanisms of Electrostatic Charge Formation and Retention in Lunar Regolith

Minhyeok Kim, Hyun Jung Kim, Sang H. Choi et al. (+1 more)

As the Artemis program advances toward the lunar south pole and permanently shadowed regions (PSRs), understanding lunar charging is increasingly important for protecting astronauts, robotic systems, instruments, and infrastructure. Persistent darkness, cryogenic temperatures, low regolith conductivity, and long charge-relaxation times may allow energetic-particle-induced charge to accumulate beneath the surface. This Technical Memorandum addresses a key unresolved question: how is electrostatic charge generated, separated, retained, and accumulated within lunar regolith? Existing models predict that solar energetic particles and galactic cosmic rays may produce subsurface electric fields approaching dielectric breakdown thresholds, but the microscopic connection between incident particles and macroscopic volumetric charge sources remains unclear. Energy deposition alone does not determine retained charge. Incident particles and their secondary particles may stop, implant, backscatter, transmit, recombine, become trapped, or escape. This memorandum therefore defines the required microscopic input as the average signed retained-charge distribution per unit depth and incident particle, resolved by particle species and energy. Combined with incident flux and energy spectra, this response provides a depth-dependent volumetric charge-source rate that can be coupled with charge continuity, conduction, dielectric relaxation, and Poisson's equation. Key uncertainties include the effects of mineralogy, grain and pore geometry, temperature, and pre-existing potential on charge retention. Particle-resolved modeling and cryogenic high-vacuum irradiation experiments are needed to constrain these processes and assess subsurface electric fields, dielectric breakdown, dust transport, contamination, and charge-mitigation requirements for sustained lunar polar exploration.
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All authors Minhyeok Kim, Hyun Jung Kim, Sang H. Choi, Ashley Daeun Jung

Saturday 5 September

Earth and Planetary Astrophysics

Shaving the Outskirts of Planetary Systems Probed by Roman via Stellar Flybys

Donald Liveoak, Tim Hallatt and Sarah Millholland

The recently launched Nancy Grace Roman Space Telescope (Roman) will soon begin observations that promise to revolutionize our understanding of exoplanet demographics. Roman's Galactic Bulge Time-Domain Survey (GBTDS) will observe the Galactic Bulge via transit and microlensing techniques, providing a unique opportunity to study hot Jupiters as well as long-period/free-floating exoplanets in a dense and kinematically hot stellar environment. In this Letter, we quantify how repeated, impulsive stellar flybys truncate the outskirts of planetary systems in the Bulge. We find that flybys result in ejections on $\sim$Gyr timescales for wide orbits (${\gtrsim} 250\ \text{au}$) around low-mass hosts. Provided these wide orbits are populated, these ejections likely contribute to the population of free-floating planets probed by Roman. Surviving planets outside of ${\sim} 10$ au are placed on eccentric and misaligned orbits. We show that, though systems suffer many close encounters, the evolution is dominated by the single strongest perturbation; planetary orbits (even those that evade ejection) undergo superdiffusion (i.e. a Lévy flight). We comment on the implications of our results for the origin of hot Jupiters amenable to $\textit{Roman}$'s transit search. Our work serves as a quantitative baseline from which future dynamical studies can build to better understand the evolution of planetary systems in the Galactic Bulge.
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Friday 4 September

Earth and Planetary Astrophysics · Cross-listed from astro-ph.SR

Deep H$\alpha$ Imaging Survey of IC 348 with the Hubble Space Telescope: I. Accretion Properties of Stellar and Substellar Objects

Lillian Yushu Jiang, Brendan P. Bowler, Yifan Zhou et al. (+5 more)

Accretion governs the growth of young stars and the early evolution of their circumstellar disks, yet population-level measurements of accretion are often hampered by heterogeneous diagnostics and by samples preferentially selected toward disk-bearing or accreting objects. This can impact the mass accretion rate-stellar mass ($\dot{M}$-$M_\star$) relation, particularly at substellar masses. We present a uniform analysis of accretion in the $\sim$2 Myr-old star-forming cluster IC 348 based on deep Hubble Space Telescope F656N imaging. Using H$\alpha$ excess as a single, homogeneous accretion tracer, we derive accretion rates and robust upper limits for 200 cluster members spanning the stellar to substellar mass regime ($3\ M_\odot$ to $4\ M_{\rm Jup}$). Accretion is detected in $37\pm3\%$ of the sample, with fractions of $34\pm4\%$ among stellar members and $46\pm6\%$ among substellar objects. For accretors alone, the inferred $\dot{M}$-$M_\star$ relation is consistent with those measured in similarly aged regions such as Lupus. In contrast, including weak accretors and non-detections increases the scatter and flattens the slope while lowering the intercept, demonstrating the strong influence of the low-accretion tail on population-level accretion relations. For free-floating planetary-mass objects in IC 348, extrapolating the accretors-only fit overpredicts $\dot{M}$ by approximately an order of magnitude compared to the fit that includes upper limits, which instead implies mass accretion rates of $<10^{-12}$ $M_{\odot}\ \mathrm{yr}^{-1}$. These results show that sample selection, specifically whether the sample is restricted to disk-bearing or accreting targets, or instead is drawn from a complete membership census, is a dominant factor shaping population-level accretion relations across a wide dynamic range in stellar mass.
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All authors Lillian Yushu Jiang, Brendan P. Bowler, Yifan Zhou, Adam L. Kraus, Sean M. Andrews, Lynne A. Hillenbrand, Michael J. Ireland, Zhaohuan Zhu

Earth and Planetary Astrophysics

Rotational broadening of exoplanet spectra in arbitrarily oriented systems: Application to reflected light

Sophia R. Vaughan and Laura Kreidberg

Exoplanet reflection spectra contain a wealth of information, which we are now beginning to access. At a high spectral resolution, the rotation of both the star and the planet affects the shape of the reflected spectral lines. This effect must be accounted for in high-resolution analyses and could be used to measure the orbital alignment of systems. In this work, we derive the rotational broadening kernels for arbitrarily aligned systems, both for the stellar spectrum viewed by the planet and for the reflected component of the planet's spectrum. We then determine the relation between the parameters of the kernels and the standard orbital parameters of an exoplanet system. Finally, we show examples of these kernels, compare them with previous models, and investigate the limitations imposed by our assumptions. We find that the difference in the rotational broadening of a non-aligned system relative to the broadening expected from a prograde aligned orbit can be of the order of 50 km s$^{-1}$. We also highlight how the equations and kernels presented in this work could be modified for other uses, such as thermal emission spectra.
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Earth and Planetary Astrophysics

POSTELLAR: Posterior Stellar Spectrum Sampling - An Alternative to Approximate Stellar Spectra for Exoplanetary Analysis

Dhvani Doshi, Nicolas B. Cowan, Yashar Hezaveh et al. (+2 more)

We present a novel approach to perform posterior sampling of the underlying stellar spectrum in high-resolution spectroscopic observations. Our method, postellar, coherently combines information from empirical observations and physics-based models, enabling more accurate spectral recovery while providing uncertainties that can be propagated into downstream analyses. This is accomplished by treating the intrinsic stellar spectrum as a latent variable and performing posterior sampling under a Gaussian likelihood with an informative prior constructed using a score-based diffusion model trained on PHOENIX stellar models. We validate the framework on synthetic SPIRou radial velocity (RV) observations generated from the empirical spectra of Barnard's Star and Proxima Centauri. Spectra inferred with postellar more accurately recover the ground truth than standard empirical templates. For medium signal-to-noise observations, postellar improves RV accuracy by up to a factor of three, while RVs derived from empirical templates tend to be biased. This method performs particularly well in low signal-to-noise and low-cadence regimes. The postellar framework is broadly applicable to other high-resolution spectroscopy science cases including stellar abundance analyses and exoplanet atmospheric characterization.
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All authors Dhvani Doshi, Nicolas B. Cowan, Yashar Hezaveh, Gabriel Missael Barco, Étienne Artigau

Earth and Planetary Astrophysics

Two extremely irradiated volatile-rich sub-Neptunes with companions in the TOI-426 and TOI-1839 systems: Insights into arrival and survival near the lower edge of the Neptunian desert

A. Castro-González, O. Barragán, D. J. Armstrong et al. (+26 more)

Using TESS photometry and 147 HARPS radial velocities, we confirm two extremely irradiated, volatile-rich sub-Neptunes near the lower edge of the Neptunian desert: TOI-426 b and TOI-1839 b, orbiting the solar-type stars HD 34390 and TYC 304-865-1. We modelled stellar activity with shared-timescale and multidimensional Gaussian processes. The planets have orbital periods of approximately 1.32 and 1.42 days, radii of $2.19\pm0.09$ and $2.27\pm0.10$ Earth radii, and masses of $6.7\pm1.8$ and $7.10\pm0.78$ Earth masses, respectively. They receive approximately 1700 and 1050 times Earth's irradiation, yet their densities require substantial volatile content. We also detect a transiting sub-Neptune, TOI-1839 c, at approximately 4.02 days and a giant companion, TOI-426 c, with a period of $235.8^{+9.2}_{-8.5}$ days and a minimum mass of $444^{+18}_{-17}$ Earth masses, assuming a circular orbit. Interior-structure and atmospheric-escape models yield mass-loss timescales of 1-100 Myr for hydrogen/helium envelopes, compared with 100-1000 Gyr for water-dominated envelopes, favouring a steam-world interpretation for all three transiting planets. A population analysis reveals a striking excess of detected outer giant companions to low-mass sub-Neptunes near the desert edge, where high-eccentricity tidal migration is expected to circularize surviving planets: 36% (9/25) within the tidal survival band versus 6% (9/148) outside it (Fisher exact test $p=1.4\times10^{-4}$). This excess is robust to changes in sample selection and companion definitions. The survival of these volatile-rich planets challenges a boundary set solely by evaporation of primordial hydrogen/helium envelopes, while the companion excess supports an important role for high-eccentricity tidal migration in shaping the lower desert edge.
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All authors A. Castro-González, O. Barragán, D. J. Armstrong, A. Aguichine, V. Bourrier, D. Ehrenreich, E. X. Tao, J. Lillo-Box, M. R. Standing, S. G. Sousa, E. Delgado-Mena, A. Moya, V. Adibekyan, M. Lendl, C. Hellier, S. B. Howell, E. Furlan, C. Ziegler, A. C. M. Correia, K. Cui, P. Figueira, J. M. Jenkins, M. A. Fetzner Keniger, B. Merín, A. Osborn, L. Parc, F. Pepe, P. J. Wheatley, J. N. Winn

Earth and Planetary Astrophysics

Ion abundances in the plasma tail of 3I/ATLAS show that it is N2-rich

Lea Ferellec, Cyrielle Opitom and Colin Snodgrass

Interstellar objects allow us to compare the early planetary formation process in the Solar System and around other stars. The second and third interstellar objects, 2I/Borisov and 3I/ATLAS, showed visible signs of activity, making it possible to assess their composition via spectroscopic analysis of their gas coma. While neutral volatiles surrounding comets are easily probed by common spectrographs, ion emissions emanate mostly from the tail, making them easier to detect with integral-field units. We present WHT/WEAVE-LIFU observations of 3I/ATLAS, performed post-perihelion on 2025 November 30 and 2025 December 02. We simultaneously detect the ions N2+, CO+, CO2+, H2O+ and CH+ located in an anti-solar tail. Using common fluorescence factors, we determine abundance ratios of these species in the tail. In particular, the N2+/CO+ ratio allows us to constrain the lower-limit N2/CO>0.023+/-0.001, indicating that 3I/ATLAS is N2-rich compared to Solar System comets, a property which can be linked to cold formation conditions. Abundance ratios between other ions cannot directly be used to constrain abundances of neutrals but seem consistent with the fact that 3I/ATLAS is enriched in hypervolatiles. We also investigate whether these ratios vary along the tail, and only detect a marginal decrease of CH+ with cometocentric distance.
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Earth and Planetary Astrophysics

Dust Substructures and Line Perturbations driven by a Forming Planet in J16120

Anibal Sierra, Andrés F. Izquierdo, Paola Pinilla et al. (+18 more)

Hints of planet formation have been independently reported within the gap of the disc around 2MASS J16120668-301027 from millimetre continuum, infrared, and H-alpha observations. In this work, we present new evidence for ongoing planet formation based on Atacama Large Millimeter/submillimeter Array (ALMA) Band 7 observations, detecting 0.87 mm dust continuum emission together with 12CO (J=3-2) and 13CO (J=3-2) line emission. Visibility modelling of the continuum data reveals an inner disc and two dust rings peaking at 23 and 75 au. The continuum morphology is better reproduced by an eccentric disc model (e approximately 0.1) than by an axisymmetric disc. We further investigate the gas kinematics through modelling of the 12CO channel maps. The residual line-width map shows a localised increase in velocity dispersion at the position of a previously reported circumplanetary disc candidate (deprojected radius approximately 32 au, position angle approximately 170 degrees) and along its orbit. This signal is spatially coincident with kink-like features and a transition from sub-Keplerian to super-Keplerian velocities. In addition, the velocity residual map exhibits an arc-like structure extending outward from the planet candidate, while the gas kinematics, despite substantial uncertainties, is consistent with inflow towards the candidate's orbital radius. The observed increase in velocity dispersion agrees with predictions from planet-disc interaction simulations, which produce enhanced turbulence both at the planet location and along its orbital path. Taken together, the continuum morphology and gas kinematic signatures provide compelling new evidence for ongoing planet formation within the disc gap.
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All authors Anibal Sierra, Andrés F. Izquierdo, Paola Pinilla, Kan Chen, Myriam Benisty, Laura Pérez, Teresa Paneque-Carreño, Carolina Agurto-Gangas, Jaehan Bae, John Carpenter, Luca Cieza, Dingshan Deng, Stefano Facchini, Camilo González-Ruilova, Nicolás Kurtovic, Aleksandra Kuznetsova, Carlo F. Manara, James Miley, Álvaro Ribas, Giovanni Rosotti, Miguel Vioque

Earth and Planetary Astrophysics

Contrasting C/O ratios in Uranus and Neptune from disequilibrium chemistry: A clue to distinct evolutionary pathways?

T. Briand, V. Hue, O. Mousis et al. (+4 more)

The formation of Uranus and Neptune remains poorly constrained largely due to uncertain deep elemental abundances. Carbon monoxide (CO), a disequilibrium species in the upper troposphere, provides an indirect constraint on the deep oxygen abundance. We investigate the deep O/H and C/O ratios of the ice giants and their formation history by accounting for meridional variations in atmospheric structure and uncertainties in chemical kinetics. We extended a 1D thermochemical and diffusion model into a pseudo-2D model by including latitudinal variations in key model parameters. The O/H ratio was inferred by matching the modeled upper-tropospheric CO mole fractions to observations, and combined with the deep carbon abundance to derive the C/O ratio. For Uranus, varying tropospheric methane alone yielded O/H $\sim [47-57] \times$ protosolar, whereas allowing $K_{zz}$ to vary expanded the range to O/H $\sim [62-177] \times$ protosolar. For Neptune, the corresponding ranges are O/H $\sim [182-215] \times$ protosolar and O/H $\sim [222-342] \times$ protosolar, respectively, supporting lower oxygen enrichment in Uranus than Neptune. Chemical-network uncertainties have a more modest effect, amounting to at most $\sim 10\%$ of the retrieved oxygen ranges, comparable to the uncertainty associated with the measured CO abundance on Neptune, but larger on Uranus. We computed C/O latitudinal ranges and found C/O $\sim [0.06-0.52]$ on Uranus and $\sim [0.02-0.12]$ on Neptune. Comparison with a protoplanetary disk model suggests different formation or evolutionary pathways for the two planets. Our results highlight the dominant role of vertical mixing in constraining deep oxygen abundance and the importance of accounting for meridional variability and chemical uncertainties. This work also provides a framework for selecting the entry latitude of a future Uranus Orbiter and Probe mission.
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All authors T. Briand, V. Hue, O. Mousis, T. Cavalié, T. Benest Couzinou, A. Schneeberger, M. D. Hofstadter