arXane › Physics › Astrophysics
Wednesday, 4 November 2026: 9 new papers, 5 cross-lists and 2 replacements in Earth and Planetary Astrophysics (astro-ph.EP), in arXiv's order, with full abstracts. The same papers as arXiv's own listing, set as a newspaper; every title links to arXiv.
Recent exoplanet demographics studies have uncovered the Neptune ridge: an overdensity of intermediate-size planets at orbital periods between 3--6~d. The origin of this pile-up remains unclear, but it could be a consequence of high-eccentricity migration (HEM). Here, we confirm TOI-4129\,b, a planet that may help to reveal the origin of the Neptune ridge. Using radial velocities from HARPS-N and transit photometry from TESS and four ground-based telescopes, we found that TOI-4129\,b has a relatively low density ($R_p = 5.7 \pm 0.2\, R_\oplus$, $M_p = 26\pm 6 \,M_\oplus$, $\rho_p = 0.8 \pm 0.2 ~\textrm{g}~\textrm{cm}^{-3}$) and orbits a $V = 12.9$ K dwarf star every 3.27~d, placing it at the inner edge of the Neptune ridge. We detect the stellar rotation period at 12.7~d in the photometry, implying a fairly young gyrochronal age of $1.20^{+0.16}_{-0.30}~$Gyr for the system. With a Transmission Spectroscopy Metric of $112\pm 30$, this adolescent Neptune is a good target for atmospheric observations with \textit{JWST} and \textit{Ariel}, and we also find that it is amenable to future constraints on the (three-dimensional) stellar obliquity. TOI-4129\,b will therefore be an important testing ground for the idea that planets in the Neptune ridge underwent HEM to arrive at their current locations.
Carbon monoxide (CO) isotopologues are widely used to estimate protoplanetary disk gas masses, but their reliability is often compromised by CO freeze-out and subsequent chemical processing. The disk around the luminous Herbig star HD~179218 ($L_* \sim 100\,L_\odot$, $M_* \sim 3.0\,M_\odot$) offers a rare test case of CO-based mass measurements. Its CO snowline lies beyond the outer disk edge, and thus CO remains in the gas phase throughout. We present ALMA and NOEMA observations of six CO isotopologue lines ($^{13}$CO, C$^{18}$O, and C$^{17}$O, $J$=2--1 and 1--0). Fitting the spectral energy distribution yields $M_{dust} \approx 3 \times 10^{-5}\, M_\odot$. Modeling CO isotopologue emission with the thermochemical code DALI yields the gas-disk characteristic radius ($R_{c} \approx 90$~au) and the gas mass ($M_{gas} \approx 8 \times 10^{-3}\, M_\odot$). These results yield a gas-to-dust ratio of $\sim$270 for our fiducial dust opacity ($a_{\rm max} = 1$~mm), above the canonical interstellar value of $\sim$100. The thermal structure of our best-fitting model confirms that CO never freezes out in this disk, so the elevated ratio is not an artifact of CO depletion; together with the compact millimeter dust disk relative to the gas disk, it is consistent with substantial inward radial drift of millimeter dust. In addition, we find a gas cavity inside 50\,au that encloses the compact millimeter dust disk---the inverse of a typical transition disk---which we attribute to photoevaporative clearing of the inner gas after the pebbles drifted inward. HD~179218 thus provides a clean benchmark for studying the gas mass distribution and gas--dust interaction in protoplanetary disks, free from the CO-depletion biases that affect colder disks.
Classifying small exoplanets as rocky, volatile-rich, or iron-enriched is a prerequisite for efficient allocation of atmospheric characterization resources. The Nominally Rocky Planet Zone (NRPZ) provides a framework for this classification by comparing observed bulk densities against the expected density range for volatile-free rocky planets, calibrated from stellar abundance distributions. However, the efficacy of classifying a planet using the NRPZ is critically dependent on the precision of the underlying mass and radius measurements. Here, we perform an analytical propagation of observational uncertainties through the NRPZ classification framework for 37 candidate rocky exoplanets with radii below 2 Earth radii orbiting stars with visual magnitudes <11. Among the 37 planets, we find 12 already possess sufficiently precise mass and radius measurements to be confidently classified as Mini-Neptunes or Super-Mercuries. The remaining planets require approximately a ~60% reduction in the combined mass and radius uncertainties before they can be reliably classified. We find that 1-1,000 new observations are necessary to achieve these lower uncertainties, and these are likely lower limits. We argue that, while possible, this level of intense observation is likely not feasible for many of these planets and that mass-radius models alone are not likely to allow us to classify all but those planets with outlier densities. We therefore recommend a staged strategy in which improved mass and radius measurements are used to identify and prioritize ambiguous systems, followed (where feasible) by atmospheric observations with JWST or future facilities as a complementary test of whether selected planets retain substantial atmospheres.
Intact chains of mean-motion resonances may preserve information about the formation and early dynamical evolution of multiplanet systems. Although most mature multiplanet systems are non-resonant, the resonant fraction may have been substantially higher at young ages. Establishing whether exoplanet data are consistent with a resonant configuration is often challenging because the planetary orbits must be sufficiently well constrained. Moreover, while resonance configurations promote stability, standard orbital fitting methods do not impose such a dynamical constraint, with stability tested only a posteriori. As the first step in a series of papers aimed at favoring stable orbital solutions during the fitting process, we develop a method that restricts solutions to stable fixed-point orbits (fPOs)---coplanar, exactly periodic configurations associated with resonant dynamics. We show that dissipative evolution drive resonant systems toward an fPO state, motivating for fPO-constrained fitting for those planet systems that have avoided large-scale dynamical rearrangements since formation. The fPO model greatly reduces the number of free parameters and yields stable solutions by construction. We determine the best-fitting fPos through $\chi^2$ minimization and apply the method to astrometric data for the directly imaged PDS 70 and HR 8799 systems and to radial-velocity data for GJ 876. For HR 8799 we independently recover a previously reported resonant solution, illustrating the fidelity of our method. For PDS 70 we find solutions fall in two groups, with the group where the outer planet c mass exceeds planet b more aligned with previous measurements. Finally, For GJ 876 our fPO fit is on par with previous unconstrained fitting, which strengthens the case that its planets reside deep in resonance.
Close-in planets orbiting core-helium-burning red-clump (RC) giants directly challenge tidal-engulfment theory, which predicts that planets within about 1 au of Sun-like stars should be engulfed during red-giant-branch (RGB) expansion. The existence of such survivors would imply missing physical processes in the late stages of stellar and planetary evolution. Yet only one such "forbidden" planet had been identified to date. Here we report three systems-24 Boo b, HD 12648 b, and TYC 4282-605-1 b-whose host stars are confirmed red-clump giants through asteroseismology, showing that each has already undergone dramatic expansion and contraction following helium ignition. Their planets now reside at orbital separations smaller than the maximum stellar radii attained during the RGB phase, implying that they should not have survived. Among them, 24 Boo b orbits at only 0.183 au with a 30-day period, making it the closest planet known around a red-clump giant. These discoveries demonstrate that this paradox is not an isolated anomaly but signals the emergence of a new class of evolved planetary systems. Their survival defies current models of stellar-planetary coevolution and points to unexplored physics shaping planetary architectures around post-RGB stars.
The exoALMA survey has revealed candidate gas pressure bumps at radii beyond the observed millimetre continuum discs in several protoplanetary systems, raising the question of whether these regions could host undetected dust structures. We investigate this for nine of the exoALMA discs by reconstructing gas surface density profiles from midplane pressure gradient measurements derived from $^{12}$CO and $^{13}$CO ALMA Band 7 observations. We evolve the dust using DustPy across a grid of turbulence and fragmentation parameters ($\alpha \in \{10^{-4}, 10^{-3}\}$; $v_\mathrm{frag} \in \{1, 10\}\,\mathrm{m\,s^{-1}}$), followed by radiative transfer calculations with RADMC-3D to give predictions for the continuum emission. We find that the dominant outcome across the explored parameter space is efficient outer dust trapping, leading to prominent outer continuum rings that should be detectable at ALMA Band 7, in tension with the observations. The $^{13}$CO-based reconstructions almost universally produce detectable outer rings, whereas the $^{12}$CO-based reconstructions show more variation. We further test delayed formation of the outer pressure bump and enhanced outer-disc turbulence as possible suppression mechanisms of ring formation. Enhanced turbulence weakens the predicted outer continuum emission most effectively, although the efficiency of the mechanism depends on the system explored and on where the turbulence transition is placed. Delayed bump formation generally does not prevent rapid outer dust accumulation.
The classical Roche limit is often used as a boundary between ring-dominated and satellite-dominated regions, although it is derived for a self-gravitating fluid satellite orbiting a spherical primary. This assumption is poorly suited to ring-bearing minor bodies, which are often triaxial. Here, we compute the equilibrium figures and disruption limits of inviscid fluid satellites orbiting triaxial primaries, modelling their gravity fields through J2 and C22. The primary's oblateness and equatorial asymmetry shifts the disruption limit outward relative to the classical Roche limit, with C22 dominating the correction at sufficiently high values, which require highly oblate primaries. The disruption location also depends on the primary's orientation, defining a radial disruption region rather than a single limit in non-double-synchronous systems. Increasing orbital inclination shifts the limit inward, significantly so only at high inclinations. Symbolic regression provides an empirical expression for the disruption limit around triaxial primaries, which we apply to ring-bearing minor bodies. The correction is most relevant for Haumea and Chariklo, whereas the rings of Chiron and Quaoar located beyond the classical Roche limit remain outside the disruption region for realistic particle densities. This indicates that treating the disruption limit as a strict boundary between ring-dominated and satellite-dominated regions is an oversimplification, since the long-term survival of a ring also depends on collisional evolution, particle properties, and resonant confinement driven by the primary's non-axisymmetric gravity field.
The ALMA survey to Resolve exoKuiper belt Substructures (ARKS) presents detailed constraints on structure and properties of a sample of 24 debris discs at millimetre wavelengths, including measurements of vertical thicknesses for a subsample of discs. We estimated sizes and total masses of large bodies that feed collisional cascades producing the observed millimetre dust in 11 ARKS discs with a well constrained vertical thickness. Separately, we estimated the sizes and total masses of large bodies that could be gravitationally stirring these discs, assuming that such stirring is the origin of the vertical thickness. We used a semi-analytic particle-in-a-box collisional model and a semi-analytic model of stirring through gravitational scattering. We accounted for the dependence of collisional damping efficiency on the average collision velocity. We find that planetesimals feeding the collisional cascades are less than 20 km in size, while the total masses in the planetesimals are lower than 200 M$_\oplus$. There is a good agreement between minimum disc masses inferred from millimetre and infrared observations. The hypothetical stirrers are always larger in size than the planetesimals, but they do not necessarily contain most of the disc mass. In agreement with previous work, the observed dust masses can be explained by steady-state collisional models with reasonable disc masses. The measured vertical scale-heights can be explained by gravitational stirring from embedded large bodies of reasonable properties. However, more advanced models are needed to address the observed non-Gaussian vertical profiles, along with the dependence of disc scale-height on particle size and stellocentric distance.
We investigate the potential of the Ariel space telescope to detect and characterise exocomet transits through their wavelength-dependent photometric signatures. Building on a radiative-transfer framework, we model exocomet transits for three representative dust morphologies, three grain-size distributions, and four homogeneous dust compositions using synthetic photometric bands covering the Ariel wavelength range. The modelled events typically last 1-2 days and produce photometric variations of a few millimagnitudes. We find that the wavelength dependence of the transit signal provides stronger diagnostics of the dust properties than transit morphology alone. In particular, colour variations relative to the VISPhot band and their wavelength-dependent slopes separate the considered compositions into two broad groups: carbonaceous dust and graphite, and olivine and pyroxene. While the compositions within each group show similar behaviour, the two groups remain distinguishable across the dust morphologies and grain sizes considered. We further assess the detectability of these signals using the $\beta$~Pictoris spectral energy distribution and ArielRad simulations. Integrating over broad synthetic photometric bands substantially improves the signal-to-noise ratio, allowing millimagnitude-level signals to be detected with short integration times for sufficiently bright targets. Ariel's simultaneous optical-to-infrared coverage therefore offers the possibility not only of detecting exocomet transits, but also of characterising their chromatic signatures and constraining broad classes of dust properties. We also discuss the limitations of the modelling assumptions and the potential of coordinated observations with long-baseline photometric missions.
A naturalization, in the sense of Deprit, of Vinti's dynamical system makes the construction of a first order analytical satellite perturbation solution without critical divisors possible. The process gives hints on how to recast Brouwer's classical solution in a form that is free from small divisors too. The functionality of classical perturbation solutions is in this way extended to the close vicinity of the critical inclination. The new analytical solution bears terms that may grow unbounded, but their negative effects do not disclose in time intervals in which this kind of solution may be useful in practice.
Meteoritic inclusions indicate that the early Solar System contained a substantial abundance of the short-lived radionuclide 10Be. Whether it was inherited from the parent molecular cloud or requires local irradiation or late injection remains debated. Here we present the first three-dimensional simulations of a Milky Way-like disc that follow the multi-phase interstellar medium, cosmic rays (CRs), and the production and transport of 10Be by galactic CRs (GCRs) and in supernova remnants (SNRs). Nearly all 10Be is produced by GCR interactions; SNRs contribute at most about ten percent of the 10Be inherited by newly formed stars. The 10Be/9Be ratio traces the CR energy density, declining smoothly with galactocentric radius, and because stars form where supernovae inject CRs, their natal gas is CR-enriched roughly fivefold relative to the mean interstellar medium, which one-zone estimates miss. Accounting for a possibly higher Galactic star formation rate 4.6 Gyr ago, inheritance of GCR-produced 10Be can then explain the early Solar System's 10Be/9Be in our models, without late injection or local irradiation. Even when GCR-induced 10Be production in dense gas is reduced tenfold, the meteoritic value is reached in the innermost disc and missed by only a factor of a few elsewhere, so the main conclusion is robust to uncertain low-energy GCR penetration. The inferred birth radius of the Sun depends strongly on the CR diffusivity, ranging between 3 and 8 kpc, suggesting that improved modelling of low-energy CR transport could sharpen constraints on the birthplace of the Solar System.
We analyze Fractional Orbital Bombardment Systems (FOBS) as a means of delivering nuclear weapons by placing warheads into partial orbits around the Earth, and show that they do not represent a game-changing advance in technology or capability. We consider potential uses of such a weapon, including evading missile defenses and delivering attacks with short warning time. FOBS could underfly current exoatmospheric defenses, but the required payload reduction may not justify its use if other countermeasures are deemed effective. If a FOBS attack could be successfully disguised as a satellite launch, the warning time could be short enough to raise fears of a decapitating strike. The development of such a capability could lead countries to shorten decision-making timelines in a crisis, which would be destabilizing. The US can take steps to ensure that very-short-warning attacks would not succeed, thereby enhancing deterrence during a crisis.
Lunar seismic records are characterized by long-lasting spindle-shaped coda, which has long been considered to be caused by strong velocity contrasts in a continuous-medium lunar crust; however, simulated waveforms could not well match the observation even with unrealistically strong velocity contrasts, demonstrating its mechanism still unsolved. Here we propose a novel model of coda wave generation with random fractures in the lunar crust, where vacuum fractures prevent seismic wave propagation as insulating boundaries, causing strong persistent reverberation in terms of total reflection among fractures. Our model can easily reproduce the typical characteristics of observed Apollo data even using a limited number of simplified fractures in homogeneous matrix, with envelope correlations over 0.9, much higher than previous studies. Numerical simulations suggest that fractures in the uppermost ~1 km of the crust may dominate lunar seismic scattering. This model enables us to investigate the shallow lunar fracture system using seismic data and to suppress seismic coda for detecting critical phases reflected from deep interiors.
Earth's surface and deep mantle are strongly depleted in carbon compared to the Sun. Sublimation of solid carbonaceous material in the inner midplanes of young disks could cause this depletion. We study such thermal processing in HOPS-68, a Class I protostar with a rich hydrocarbon spectrum. A companion paper has shown that it is recondensing refractory dust from a hot inner disk region, and that oxygen-bearing species are tracing a warm (300-500 K) molecular layer entrained in a disk-wind. In this work, we analyze the carbon species in the JWST spectrum of this source using local thermal equilibrium slab models. We detect warm (~400 K) HCN and C2H2 at the base of a disk wind, as well as colder (~40 K) C4H2, CH3, and CS in the outer disk or outflow cavity wall. Additionally, 13CCH2, CH4, and C6H2 are detected, while ALMA observations reveal C2H tracing the outflow cavity walls. We suggest that in a "refractory carbon destruction zone" in the inner disk, carbon-bearing species are released from the grains, possibly contributing to the observed carbon depletion of Earth. The disk wind then redistributes carbon carried by HCN and C2H2 from the inner disk to the colder outer disk, where UV-induced gas-phase reactions with C2H in the outflow cavity walls could produce the aliphatic carbon chains seen. This result implies that the Warm Carbon Chain Chemistry (WCCC, e.g. N. Sakai & S. Yamamoto 2013) might trace outflows rather than infall. The formation of aliphatic chains in the disk wind could promote the abundant aliphatic organics found in meteorites and asteroids.