arXiv subjects

Nathan A. Kaib

Publications and source records attributed to Nathan A. Kaib.

At least 19 recordsLinked to original sources

arXiv2026

Capture of interstellar objects during stellar encounters

As they orbit within the Galaxy, stars swim through a vast population of interstellar objects (ISOs). In this paper, we use N-body simulations to show that a fraction of ISOs within $\sim$1 pc of the Sun (its tidal radius) may be captured during the flyby of another star -- a mechanism that requires no planets. Capture is most efficient when the impulse imparted by the flyby is comparable to the escape speed at the widest stable orbit, which is roughly 0.1 km/s for the Sun. ISO capture is dominated by the few highest-impulse stellar flybys, typically involving relatively slow encounters with massive stars. Most ISOs are captured in the outer parts of the Oort cloud, with semimajor axes greater than $\sim$50,000 au. Using Monte Carlo simulations, we show that the Sun underwent only a small number of ISO-capturing flybys in its history (median [mean] of 1 [1.7]). Using the {Ō}tautahi-Oxford population model, we estimate that a few times $\sim$$10^{4}$ `Oumuamua-sized ISOs were likely captured by the Sun. This only represents a $\sim$$10^{-8\pm1}$ contribution to the total Oort cloud population, yet it contains roughly as many present-day captured ISOs as Jupiter-assisted capture provides. Given that flybys are unavoidable in the Galactic field, most stars should host sparse Oort clouds populated with ISOs captured during stellar flybys. Massive stars are both the main drivers of capture when they fly by a given star, and more efficient at capturing ISOs around themselves than low-mass stars.

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arXiv2026

A Potential Signature of HD 7977's Passage Among Observed Long-Period Comet Orbits

It is generally presumed that the tidal field of the Milky Way's disk is the main perturbation that has driven observed long-period comets (LPCs) from the Oort cloud into the inner solar system. The tide's influence on the Oort cloud should produce a distinct anisotropy in the arguments of perihelion ($ω$) of dynamically new LPCs with semimajor axes ($a$) over 10$^4$ au. Simulating LPC production dominated by the Galactic tide, we find that observed dynamically new LPCs are more isotropic than expected. Meanwhile, our simulation exhibits much better agreement between simulated and observed ``returning'' LPCs that have made a handful of passages through the inner solar system prior to discovery. The isotropy of new LPCs can be explained if the Oort cloud is much less centrally concentrated than the conventional Oort cloud formation model predicts. However, a second possibility also exists. Additional simulations we perform show that the observed $ω$ distributions of new and returning LPCs can both be well-replicated if the star HD 7977 passed within $\sim$6000--10000 au of the Sun $\sim$2.5 Myrs ago. In such a scenario, our solar system is still undergoing the latter stages of a comet shower. These simulations imply the modern observed LPC flux is $\sim$twice as high as the longer-term (tide-dominated) rate. This also implies that estimates of the Oort cloud's population should be revised downward by a factor of $\sim$2. Our LPC analysis predicts the upcoming Gaia data release will favor an HD 7977 impact parameter of $\sim$6000--10000 au.

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arXiv2026

The fragility of the Uranian moons during the giant planet instability

It is thought that, sometime after their formation, the solar system's giant planets experienced a dynamical instability that caused their orbits to excite, diverge, and ejected one or more objects with masses comparable to the ice giants. A key feature of this model is that the planets experience encounters with other planetary bodies, and these encounters facilitate the capture of nearby small bodies as irregular satellites. Instability simulations indicate that planet-planet encounter distances can typically fall below 0.1 au, which is only roughly an order of magnitude larger than the radial extent of the modern planets' regular satellite systems. In this paper we model the effects of these encounters on the dynamical stability of the regular moons of Jupiter and Uranus. We tested encounter histories from 122 plausible outer solar system dynamical histories. We find that the survival probability for the Jovian and Uranian moon systems are both less than 15%. Moreover, we only identify one case where both Uranus and Jupiter's large satellites consistently survive the same instability. Interestingly, Jupiter's moons are most likely to survive in instabilities initialized with two smaller extra ice giants, and cases with one larger additional planet provide more favorable conditions for Uranian system survival. In either case, if Uranus encounters another ice giant at D<0.02 au, or one of the gas giants at D<0.1 au, satellite system destruction is effectively guaranteed. Wider encounters can also affect the system, particularly when they occur successively. Since the Laplace resonance likely would not be in place today if Jupiter's moons experienced an instability that led to collisions, our results indicate that Uranus' moons were likely perturbed to the point of collisions at least twice: as a result of both the impact that tilted the planet and the giant planet instability.

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arXiv2025

Was the Solar System's dynamical instability triggered by a (sub)stellar flyby?

An instability among the giant planets' orbits can match many aspects of the Solar System's current orbital architecture. We explore the possibility that this dynamical instability was triggered by the close passage of a star or substellar object during the Sun's embedded cluster phase. We run N-body simulations starting with the giant planets in a resonant chain and an outer planetesimal disk, with a wide-enough planet-disk separation to preserve the planets' orbital stability for $>$100 Myr. We subject the system to a single flyby, testing a wide range in flyby mass, velocity and closest approach distance. We find a variety of outcomes, from flybys that over-excite the system (or strip the planets entirely) to flybys too weak to perturb the planets at all. An intermediate range of flybys triggers a dynamical instability that matches the present-day Solar System. Successful simulations -- that match the giant planets' orbits without over-exciting the cold classical Kuiper belt -- are characterized by the flyby of a substellar object ($3-30 M_{Jup}$) passing within 20 au of the Sun. We performed Monte Carlo simulations of the Sun's birth cluster phase, parameterized by the product of the stellar density $η$ and the cluster lifetime $T$. The balance between under- and over-excitation of the young Solar System is at $ηT \approx 5 \times 10^4$~Myr pc$^{-3}$, in a range consistent with previous work. We find a probability of $\sim$1% that the Solar System's dynamical instability was triggered by a substellar flyby. The probability increases to $\sim$5% if the occurrence rate of free-floating planets and low-mass brown dwarfs is modestly higher than predicted by standard stellar initial mass functions.

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arXiv2025

Evaluating an Embryo Origin for Detached TNOs within Full Kuiper Belt Formation Models

With perihelia well beyond Neptune, but semimajor axes and eccentricities indicative of substantial perturbation, the origins of detached trans-Neptunian objects (TNOs) remain a dynamical puzzle. In particular, detached TNOs with orbital inclinations below ~25 degrees are not easily generated from any known mechanism currently in the modern solar system. One notable hypothesis for the origins of detached TNOs is that a ~Mars- to Earth-mass planetary embryo detached the perihelia of these objects from Neptune during the process of Kuiper belt formation before the embryo itself was ejected. We numerically model this scenario via simulations of Kuiper belt formation from a primordial planetesimal belt that is dispersed through the migration of the giant planets. In addition to ~100,000 Kuiper belt objects, each of our simulations contains a hypothetical population of embryos in the primordial belt. We find that our embryos are unlikely to reach the high-perihelion, large semimajor axis orbit necessary to efficiently detach TNO perihelia from Neptune's influence. Moreover, embryos will typically take at least 100 Myrs to reach these unlikely orbits, at which point most of the primordial belt will have already been ejected by the planets, limiting the available population that can be detached. Finally, the TNOs that our embryos do detach consistently have a semimajor axis distribution that is more biased toward small values than observed detached TNOs have. Thus, we conclude that planetary embryos in the primordial Kuiper belt are not likely to have been the primary mechanism for the origin of detached TNOs.

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arXiv2025

Very-wide-orbit planets from dynamical instabilities during the stellar birth cluster phase

Gas giant planets have been detected on eccentric orbits several hundreds of astronomical units in size around other stars. It has been proposed that even the Sun hosts a wide-orbit planet of 5-10 Earth masses, often called Planet Nine, which influences the dynamics of distant Trans-Neptunian objects. However, the formation mechanism of such planets remains uncertain. Here we use numerical simulations to show that very wide-orbit planets are a natural byproduct of dynamical instabilities that occur in planetary systems while their host stars are still embedded in natal stellar clusters. A planet is first brought to an eccentric orbit with an apoastron of several hundred au by repeated gravitational scattering by other planets, then perturbations from nearby stellar flybys stabilise the orbit by decoupling the planet from the interaction with the inner system. In our Solar System, the two main events likely conducive to planetary scattering were the growth of Uranus and Neptune, and the giant planets instability. We estimate a 5-10% likelihood of creating a very wide-orbit planet if either happened while the Sun was still in its birth cluster, rising to 40% if both were. In our simulated exoplanetary systems, the trapping efficiency is 1-5\%. Our results imply that planets on wide, eccentric orbits occur at least $10^{-3}$ per star.

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arXiv2025

The Influence of Passing Field Stars on the Solar System's Dynamical Future

The long-term dynamical future of the Sun's planets has been simulated and statistically analyzed in great detail, but most prior work considers the solar system as completely isolated, neglecting the potential influence of field star passages. To understand the dynamical significance of field star encounters, we simulate several thousand realizations of the modern solar system in the presence of passing field stars for 5 Gyrs. We find that the impulse gradient of the strongest stellar encounter largely determines the net dynamical effect of field stars. Because the expected strength of such an encounter is uncertain by multiple orders of magnitude, the possible significance of field stars can be large. Our simulations indicate that isolated models of the solar system can underestimate the degree of our giant planets' future secular orbital changes by over an order of magnitude. In addition, our planets and Pluto are significantly less stable than previously thought. Field stars transform Pluto from a completely stable object over 5 Gyrs to one with a ~5% instability probability. Furthermore, field stars increase the odds of Mercury's instability by ~50-80%. We also find a ~0.3% chance that Mars will be lost through collision or ejection and a ~0.2% probability that Earth will be involved in a planetary collision or ejected. Compared to previously studied instabilities in isolated solar systems models, those induced by field stars are much more likely to involve the loss of multiple planets. In addition, they typically happen sooner in our solar system's future, making field star passages the most likely cause of instability for the next 4-4.5 Gyrs.

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arXiv2024

A Non-Primordial Origin for the Widest Binaries in the Kuiper Belt

Nearly one-third of objects occupying the most circular, coplanar Kuiper belt orbits (the cold classical belt) are binary, and several percent of them are "ultra-wide" binaries (UWBs): 100-km-sized companions spaced by tens of thousands of km. UWBs are dynamically fragile, and their existence is thought to constrain early Solar System processes and conditions. However, we demonstrate that UWBs can instead attain their wide architectures well after the Solar System's earliest epochs, when Neptune's orbital migration implants the modern non-cold, or "dynamic", Kuiper belt population. During this implantation, cold classical belt binaries are likely to have close encounters with many planetesimals scattered across the region, which can efficiently dissociate any existing UWBs and widen a small fraction of tighter binaries into UWB-like arrangements. Thus, today's UWBs may not be primordial and cannot be used to constrain the early Solar System as directly as previously surmised.

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arXiv2024

More Realistic Planetesimal Masses Alter Kuiper Belt Formation Models and Add Stochasticity

We perform simulations here that include the gravitational effects of the primordial planetesimal belt consisting of ~10^5 massive bodies. In our simulations, Neptune unlocks from resonance with the other giant planets and begins to migrate outward due to interactions with planetesimals before a planetary orbital instability is triggered, and afterward, residual Neptunian migration completes the formation of the modern Kuiper belt. Our present work exhibits a number of notable differences from prior work. First, Neptune's planetary resonance unlocking requires the Neptunian 3:2 mean motion resonance to sweep much of the primordial disk interior to 30 au prior to the giant planet instability. The pre-instability population of planetesimals is consequently lower in semimajor axis, eccentricity, and inclination, and this effect persists after the instability. Second, direct scattering between Pluto-mass bodies and other small bodies removes material from Neptunian resonances more efficiently than resonant dropout resulting from small changes in Neptune's semimajor axis during scattering between Pluto-mass bodies and Neptune. Thus, the primordial population of Pluto-mass bodies may be as few as ~200 objects. Finally, our simulation end states display a wide variety of orbital distributions, and clear relationships between final bulk Kuiper belt properties and Neptune's migration or initial planetesimal properties largely elude us. In particular, we find that the rapid, stochastic planetary orbital evolution occurring during the giant planet instability can significantly alter final Kuiper belt properties such as its inclination dispersion and the prominence of resonant populations. This complicates using modern Kuiper belt properties to confidently constrain early solar system events and conditions, including planetary orbital migration and the primordial Kuiper belt's characteristics.

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arXiv2024

Passing Stars as an Important Driver of Paleoclimate and the Solar System's Orbital Evolution

Reconstructions of the paleoclimate indicate that ancient climatic fluctuations on Earth are often correlated with variations in its orbital elements. However, the chaos inherent in the solar system's orbital evolution prevents numerical simulations from confidently predicting Earth's past orbital evolution beyond 50-100 Myrs. Gravitational interactions among the Sun's planets and asteroids are believed to set this limiting time horizon, but most prior works approximate the solar system as an isolated system and neglect our surrounding Galaxy. Here we present simulations that include the Sun's nearby stellar population, and we find that close-passing field stars alter our entire planetary system's orbital evolution via their gravitational perturbations on the giant planets. This shortens the timespan over which Earth's orbital evolution can be definitively known by a further ~10%. In particular, in simulations that include an exceptionally close passage of the Sun-like star HD 7977 2.8 Myrs ago, new sequences of Earth's orbital evolution become possible in epochs before ~50 Myrs ago, which includes the Paleocene-Eocene Thermal Maximum. Thus, simulations predicting Earth's past orbital evolution before ~50 Myrs ago must consider the additional uncertainty from passing stars, which can open new regimes of past orbital evolution not seen in previous modeling efforts.

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arXiv2023

The Disk Orientations of Perseus Protostellar Multiples at 8 au Resolution

We present a statistical characterization of circumstellar disk orientations toward 12 protostellar multiple systems in the Perseus molecular cloud using the Atacama Large Millimeter/submillimeter Array at Band 6 (1.3 mm) with a resolution of 25 mas (8 au). This exquisite resolution enabled us to resolve the compact inner disk structures surrounding the components of each multiple system and to determine the projected 3-D orientation of the disks (position angle and inclination) to high precision. We performed a statistical analysis on the relative alignment of disk pairs to determine whether the disks are preferentially aligned or randomly distributed. We considered three subsamples of the observations selected by the companion separations, a <100 au, a >500 au, and a < 10,000 au. We found for the compact (< 100 au) subsample, the distribution of orientation angles is best described by an underlying distribution of preferentially aligned sources (within 30deg) but does not rule out distributions with 40% misaligned sources. The wide companion (>500 au) subsample appears to be consistent with a distribution of 40%-80% preferentially aligned sources. Similarly, the full sample of systems with companions (a< 10, 000 au) is most consistent with a fractional ratio of at most 80% preferentially aligned source and rules out purely randomly aligned distributions. Thus our results imply the compact sources (<100 au) and the wide companions (>500 au) are statistically different.

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arXiv2023

Future trajectories of the Solar System: dynamical simulations of stellar encounters within 100 au

Given the inexorable increase in the Sun's luminosity, Earth will exit the habitable zone in ~1 Gyr. There is a negligible chance that Earth's orbit will change during that time through internal Solar System dynamics. However, there is a ~1% chance per Gyr that a star will pass within 100 au of the Sun. Here, we use N-body simulations to evaluate the possible evolutionary pathways of the planets under the perturbation from a close stellar passage. We find a ~92% chance that all eight planets will survive on orbits similar to their current ones if a star passes within 100 au of the Sun. Yet a passing star may disrupt the Solar System, by directly perturbing the planets' orbits or by triggering a dynamical instability. Mercury is the most fragile, with a destruction rate (usually via collision with the Sun) higher than that of the four giant planets combined. The most probable destructive pathways for Earth are to undergo a giant impact (with the Moon or Venus) or to collide with the Sun. Each planet may find itself on a very different orbit than its present-day one, in some cases with high eccentricities or inclinations. There is a small chance that Earth could end up on a more distant (colder) orbit, through re-shuffling of the system's orbital architecture, ejection into interstellar space (or into the Oort cloud), or capture by the passing star. We quantify plausible outcomes for the post-flyby Solar System.

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arXiv2023

Oort cloud (exo)planets

Dynamical instabilities among giant planets are thought to be nearly ubiquitous, and culminate in the ejection of one or more planets into interstellar space. Here we perform N-body simulations of dynamical instabilities while accounting for torques from the galactic tidal field. We find that a fraction of planets that would otherwise have been ejected are instead trapped on very wide orbits analogous to those of Oort cloud comets. The fraction of ejected planets that are trapped ranges from 1-10%, depending on the initial planetary mass distribution. The local galactic density has a modest effect on the trapping efficiency and the orbital radii of trapped planets. The majority of Oort cloud planets survive for Gyr timescales. Taking into account the demographics of exoplanets, we estimate that one in every 200-3000 stars could host an Oort cloud planet. This value is likely an overestimate, as we do not account for instabilities that take place at early enough times to be affected by their host stars' birth cluster, or planet stripping from passing stars. If the Solar System's dynamical instability happened after birth cluster dissolution, there is a ~7% chance that an ice giant was captured in the Sun's Oort cloud.

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arXiv2023

Comparisons of the core and mantle compositions of earth analogs from different terrestrial planet formation scenarios

The chemical compositions of Earth's core and mantle provide insight into the processes that led to their formation. N-body simulations, on the other hand, generally do not contain chemical information, and seek to only reproduce the masses and orbits of the terrestrial planets. These simulations can be grouped into four potentially viable scenarios of Solar System formation (Classical, Annulus, Grand Tack, and Early Instability) for which we compile a total of 433 N-body simulations. We relate the outputs of these simulations to the chemistry of Earth's core and mantle using a melt-scaling law combined with a multi-stage model of core formation. We find the compositions of Earth analogs to be largely governed by the fraction of equilibrating embryo cores and the initial embryo masses in N-body simulations. Simulation type may be important when considering magma ocean lifetimes, where Grand Tack simulations have the largest amounts of material accreted after the last giant impact. However, we cannot rule out any accretion scenarios or initial embryo masses due to the sensitivity of Earth's mantle composition to different parameters and the stochastic nature of N-body simulations. Comparing the last embryo impacts experienced by Earth analogs to specific Moon-forming scenarios, we find the characteristics of the Moon-forming impact are dependent on the initial conditions in N-body simulations where larger initial embryo masses promote larger and slower Moon-forming impactors. Mars-sized initial embryos are most consistent with the canonical hit-and-run scenario onto a solid mantle. Our results suggest that constraining the fraction of equilibrating impactor core and the initial embryo masses in N-body simulations could be significant for understanding both Earth's accretion history and characteristics of the Moon-forming impact.

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arXiv2023

Mercury's formation within the Early Instability Scenario

The inner solar system's modern orbital architecture provides inferences into the epoch of terrestrial planet formation; a ~100 Myr time period of planet growth via collisions with planetesimals and other proto-planets. While classic numerical simulations of this scenario adequately reproduced the correct number of terrestrial worlds, their semi-major axes and approximate formation timescales, they struggled to replicate the Earth-Mars and Venus-Mercury mass ratios. In a series of past independent investigations, we demonstrated that Mars' mass is possibly the result of Jupiter and Saturn's early orbital evolution, while Mercury's diminutive size might be the consequence of a primordial mass deficit in the region. Here, we combine these ideas in a single modeled scenario designed to simultaneously reproduce the formation of all four terrestrial planets and the modern orbits of the giant planets in broad strokes. By evaluating our Mercury analogs' core mass fractions, masses, and orbital offsets from Venus, we favor a scenario where Mercury forms through a series of violent erosive collisions between a number of ~Mercury-mass embryos in the inner part of the terrestrial disk. We also compare cases where the gas giants begin the simulation locked in a compact 3:2 resonant configuration to a more relaxed 2:1 orientation and find the former to be more successful. In 2:1 cases, the entire Mercury-forming region is often depleted due to strong sweeping secular resonances that also tend to overly excite the orbits of Earth and Venus as they grow. While our model is quite successful at replicating Mercury's massive core and dynamically isolated orbit, the planets' low mass remains extremely challenging to match. Finally, we discuss the merits and drawbacks of alternative evolutionary scenarios and initial disk conditions.

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arXiv2022

Close TNO Passages as a Driver of the Origin and Evolution of Ultra-Wide Kuiper Belt Binaries

Within the dynamically cold low inclination portion of the Classical Kuiper Belt, there exists a population of weakly bound binary systems with a number of unusual properties; most notable of which is their extremely wide orbital separations; beyond 7% of their Hill radii. The stability and evolution of these Ultra-Wide Trans-Neptunian Binaries (TNBs) have, in the past, been studied extensively under the assumption that the primary evolving mechanisms are interactions between the binary components and impacting Trans-Neptunian Objects (TNOs). Here, we instead study their evolution as driven by the gravitational perturbations of close passing but non-impacting TNOs. By simulating these passages, we show that the aggregate effects of encounters over billions of years have a significant effect on Kuiper Belt binary evolution. Such processes can lead to tight binaries widening significantly over time, approaching and sometimes surpassing the separation of the widest known TNBs. We also find that the eccentricity and inclination distributions of observed Ultra-Wide TNBs can be sampled from such widened binaries. While we are unable to produce enough wide binaries to explain their abundance, the orbital properties of ones we do produce are consistent with known wide binaries.

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arXiv2022

Dynamical Fates of S-Type Planetary Systems in Embedded Cluster Environments

The majority of binary star systems that host exoplanets will spend the first portion of their lives within a star-forming cluster that may drive dynamical evolution of the binary-planet system. We perform numerical simulations of S-type planets, with masses and orbital architecture analogous to the solar system's 4 gas giants, orbiting within the influence of a 0.5 solar-mass binary companion. The binary-planet system is integrated simultaneously with an embedded stellar cluster environment. ~10% of our planetary systems are destabilized when perturbations from our cluster environment drive the binary periastron toward the planets. This destabilization occurs despite all of our systems being initialized with binary orbits that would allow stable planets in the absence of the cluster. The planet-planet scattering triggered in our systems typically results in the loss of lower mass planets and the excitement of the eccentricities of surviving higher mass planets. Many of our planetary systems that go unstable also lose their binary companions prior to cluster dispersal and can therefore masquerade as hosts of eccentric exoplanets that have spent their entire histories as isolated stars. The cluster-driven binary orbital evolution in our simulations can also generate planetary systems with misaligned spin-orbit angles. This is typically done as the planetary system precesses as a rigid disk under the influence of an inclined binary, and those systems with the highest spin-orbit angles should often retain their binary companion and possess multiple surviving planets.

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arXiv2022

Dynamical Population of Comet Reservoirs

The Oort cloud and the scattered disk are the two primary reservoirs for long-period and short-period comets, respectively. In this review, we assess the known observational constraints on these reservoirs' properties and their formation. In addition, we discuss how the early orbital evolution of the giant planets generated the modern scattered disk from the early, massive planetesimal disk and how $\sim$5\% of this material was captured into the Oort cloud. We review how the Sun's birth environment and dynamical history within the Milky Way alters the formation and modern structure of the Oort cloud. Finally, we assess how the coming decade's anticipated observing campaigns may provide new insights into the formation and properties of the Oort cloud and scattered disk.

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