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

Trans-Neptunian Objects (TNOs) are remnants of the planetesimal population formed during the planet formation stage \citep{Gladman:2021}. Unlike the planets, most TNOs move on inclined eccentric orbits. Different models are proposed to explain these dynamics. However, another constraint comes from the TNOs' chemical composition, which may provide additional valuable insights into the Solar System's early history. Both dynamics and composition have to be explained simultaneously using the same model. This puts tight constraints on any early solar system model. We show that a stellar flyby can stand this rigorous test of explaining the TNOs dynamics and colour distribution simultaneously. Recently, we looked at a stellar flyby as an alternative to the planet instability model to explain the TNOs dynamics (Pfalzner et al. 2004). While migration of the giant planets during the early stages of Solar System evolution could have induced substantial scattering of planetesimals producing TNOs on inclined, eccentric orbits, this process cannot account for the small number of distant TNOs (p > 60 au) outside the planets' reach and retrograde TNOs. The alternative scenario of the close flyby of another star delivers all these TNO features simultaneously. We found that a 0.8 M⊙ star passing at a distance of p = 110 au, inclined by i = 70°, gives a near-perfect match. This flyby also reproduces the cold, hot, Sena-like, retrograde TNO populations. The next step is to test whether the same flyby can also account for the TNO's colour distribution. Detailed compositional data mainly exists for the largest TNOs. The smaller TNOs are often too faint for spectroscopic observations (e.g. Emery et al. 2024}. As a result, their surface composition is typically analysed using broadband photometry. This type of observation shows that the colour distribution of TNOs ranges from grey to very red {e.g. Barucci et al. 2020}. At greater distances from the Sun, temperatures drop significantly, which could have strongly influenced local chemistry during planetesimal formation. Therefore, one may expect that the colours of TNOs varied with their distance from the Sun. However, observations do not show such a straightforward correlation between TNO colour and heliocentric distance {Jewitt:2001}. Early studies already showed that TNOs with low inclination and low eccentricity are predominantly very red. In contrast, TNOs with higher inclinations (i.e., i > 5 degrees) and higher eccentricities, typical of the hot Kuiper Belt, exhibit a mix of red and grey colors. Recent studies, including the Outer Solar System Survey (OSSOS) (Schwamb et al. 2019) and the Dark Energy Survey (DES) (Bernardinelli:2023), along with observations from the James Webb Space Telescope (JWST) (Pinilla:2024) have confirmed these findings. We simulate the effect of a stellar flyby on a disc represented by massless test particles, which initially orbit the Sun on Keplerian trajectories. We assume that before the flyby, the model disc extended to 150 au and exhibited a colour gradient due to its TNO composition. We follow the trajectories of the test particles during the flyby and investigate their final properties using the REBOUND code. We find that the flyby explains the observed colour structures found in the OSSOS and DES survey. The complex colour distribution directly results from this transport in the spiral arms. It successfully links the colours to the dynamics of the TNOs. In particular, the flyby naturally produces the increased dominance of grey vs. very red TNOs for higher inclinations. This results in the scarcity of very red TNOs for inclinations >21°. It also leads to the observed lack of very red TNO among very eccentric (e > 0.42) TNOs. The lack of very red objects among the irregular moons can be explained as a direct result of originating in the outer regions ($r >$ 60 au) of the disc (Pfalzner & Govind 2004). We find now that they may originate from the same reservoir as the high-inclination TNOs. The combined reproduction of the TNO dynamics and colours significantly strengthens the argument for a stellar flyby being responsible for the intricate structure of the solar system beyond Neptune. Up-coming instruments, in particular LSST, hold the promise of detecting many thousands of new TNOs. References Gladman Gladman, B., & Volk, K. 2021, ARA&A, 59, 203, Pfalzner, S., Govind, A., & Portegies Zwart, S. (2024), Nature Astronomy, 8, 1380. Emery, J. P., Wong, I., Brunetto, R., et al. 2024, Icarus, 414, 116017 Barucci, M. A., & Merlin, F. 2020, in The Trans-Neptunian Solar System, ed. Prialnik, M. A. Barucci, & L. Young (Elsevier), 109–126 Jewitt, D. 2018, AJ, 155, 56 Schwamb, M. E., Fraser, W. C., Bannister, M. T., et al. 2019, ApJS, 243, 12 Bernardinelli, P. H., Bernstein, G. M., Jindal, N., et al. 2023, ApJS, 269, 18 Pinilla-Alonso, N., Brunetto, R., De Pra ́, M. N., et al. 2025, Nature Astronomy, 9, 230 Pfalzner, S., Govind, A., & Wagner, F. W. 2024, ApJL, 972, L21,

We review the role of stellar flybys and encounters in shaping planet-forming discs around young stars, based on the published literature on this topic in the last 30 years. Since most stars $$\le ~2$$ Myr old harbour protoplanetary discs, tidal perturbations affect planet formation. First, we examine the probability of experiencing flybys or encounters: More than 50% of stars with planet-forming discs in a typical star-forming environment should experience a close stellar encounter or flyby within 1000 au. Second, we detail the dynamical effects of flybys on planet-forming discs. Prograde, parabolic, disc-penetrating flybys are the most destructive. Grazing and penetrating flybys in particular lead to the capture of disc material by the secondary to form a highly misaligned circumsecondary disc with respect to the disc around the primary. One or both discs may undergo extreme accretion and outburst events, similar to the ones observed in FU Orionis-type stars. Warps and broken discs are distinct signatures of retrograde flybys. Third, we review some recently observed stellar systems with discs where a stellar flyby or an encounter is suspected—including UX Tau, RW Aur, AS 205, Z CMa, and FU Ori. Finally, we discuss the implications of stellar flybys for planet formation and exoplanet demographics, including possible imprints of a flyby in the Solar System in the orbits of trans-Neptunian objects and the Sun’s obliquity.

The architecture and evolution of planetary systems are shaped in part by stellar flybys. Within this context, we look at stellar encounters that are too weak to immediately destabilize a planetary system but are nevertheless strong enough to measurably perturb the system’s dynamical state. We estimate the strength of such perturbations on secularly evolving systems using a simple analytic model and confirm those estimates with direct N-body simulations. We then run long-term integrations and show that even small perturbations from stellar flybys can influence the stability of planetary systems over their lifetime. We find that small perturbations to the outer planets’ orbits are transferred between planets, increasing the likelihood that the inner planetary system will destabilize. Specifically, our results for the Solar system show that relative perturbations to Neptune’s semimajor axis of order 0.1 per cent are strong enough to increase the probability of destabilizing the Solar system within 5 Gyr by one order of magnitude.

<p><strong>Introduction</strong></p> <p>GAIA mission astrometric data for stars in Solar System neighbourhood allow us to find a probable past and future stellar flybys at distance lower than 3000 au (for example HD 7977 flyby 2.5 million years ago). However the distance and geometry of such flybys have large relative uncertainties and it is possible that flybys at a distance of several hundreds of astronomical units have happened in recent past. We performed the numerical analysis of very close stellar flyby investigating different flyby geometries and distances. In the analysis we simulated the dynamics of planets and Kuiper Belt minor bodies during the passage of a Sun-like star using the Rebound n-body simulator software. Using the past flyby of HD 7977 star as a reference we assumed the star with mass close to the mass of the Sun and relative velocity of tens of km/s.</p> <p>Before analysing the effect of star passage on minor bodies we decided to check the impact on the orbits of giant planets. We discovered that flybys at distance greater than 300 au change the eccentricity of giant planets by less than 0.01 and the semimajor axis by less than 0.1 au. This allows us to ensure that such close flyby can have small effect on planets and not disturb the stability of Solar System and therefore they could happened in the past. The probability of significant disturbance of planetary orbits grows when the minimal star-Sun distance is smaller, but even for extremely close distances (about 100 au) in most cases it is possible to find specific geometry of flyby where planetary orbits are not heavily changed.</p> <p><strong>Kuiper Belt and scattered disc simulation</strong></p> <p>In order to investigate the effect of stellar flyby on Kuiper Belt objects, we simulated the synthetic, randomly generated population of minor bodies with semimajor axis greater than 30 au. We checked the statistical properties properties of orbital elements of this population after the close flyby. The results shows that flyby at 3000 au will not create significant change in the orbital parameters of this objects. However the closer passages do affect the population and may create various statistical effects on the population.</p> <p>Last years the analysis of the orbital parameter of most distant Solar System objects shows the significant perihelion direction clustering. This leads to the formulation of the Planet Nine hypothesis by Batygin et al. In this model, the orbital characteristic of distant objects is explained by the existence of an undiscovered massive planet that orbits the Sun at a distance of about 1000 au. Our simulations also included scattered disc objects including distant bodies with the semimajor axis equal to several hundreds of astronomical units. We want to show if the very close star passage can be used as an alternative explanation of the orbital properties of distant bodies. We also checked what would be the possible effect of a star flyby on hypothetical Planet’s Nine orbit.</p> <p><strong>Conclusion</strong></p> <p>Our simulation shows that there is possibility of a close stellar flyby which do not disturb the planetary system affecting its stability, but such flyby can affect the minor body population, especially the distant part of the Kuiper Belt. This possibility should be taken into account in modelling the outer Solar System minor body population.</p>

We investigate the survivability of solar system-like planetary systems during close encounters in stellar associations using a suite of 1980 N-body simulations. Each system is based on one of the possible five-planet resonant configurations proposed to represent the initial solar system architecture and is systematically scaled in both planetary mass and orbital compactness to explore the parameter space of observed exoplanetary architectures. Simulations explore a range of stellar encounter scenarios drawn from four distinct cluster environments. Our results show that system survival depends critically on the interplay between planetary mass and orbital scale: compact configurations are more resistant to external perturbations, while increased planetary mass improves resilience only up to a threshold, beyond which internal instabilities dominate. No system whose planets are twice as massive as the ones in the solar system survives stellar encounters. Systems that are at least an order of magnitude more compact than the solar system remain stable under typical encounter conditions. These findings place strong constraints on the initial architectures of planetary systems that can endure stellar-dense birth environments.

Escalating observations of exo-minor planets and their destroyed remnants both passing through the Solar system and within white dwarf planetary systems motivate an understanding of the orbital history and fate of exo-Kuiper belts and planetesimal discs. Here, we explore how the structure of a 40–1000 au annulus of planetesimals orbiting inside of a Solar system analogue that is itself initially embedded within a stellar cluster environment varies as the star evolves through all of its stellar phases. We attempt this computationally challenging link in four parts: (1) by performing stellar cluster simulations lasting 100 Myr, (2) by making assumptions about the subsequent quiescent 11 Gyr main-sequence evolution, (3) by performing simulations throughout the giant branch phases of evolution, and (4) by making assumptions about the belt’s evolution during the white dwarf phase. Throughout these stages, we estimate the planetesimals’ gravitational responses to analogues of the four Solar system giant planets, as well as to collisional grinding, Galactic tides, stellar flybys, and stellar radiation. We find that the imprint of stellar cluster dynamics on the architecture of ≳100 km-sized exo-Kuiper belt planetesimals is retained throughout all phases of stellar evolution unless violent gravitational instabilities are triggered either (1) amongst the giant planets, or (2) due to a close (≪103 au) stellar flyby. In the absence of these instabilities, these minor planets simply double their semimajor axis while retaining their primordial post-cluster eccentricity and inclination distributions, with implications for the free-floating planetesimal population and metal-polluted white dwarfs.

Escalating observations of exo-minor planets and their destroyed remnants both passing through the Solar system and within white dwarf planetary systems motivate an understanding of the orbital history and fate of exo-Kuiper belts and scattered discs. Here, we explore how the structure of a 40-1000 au annulus of bodies (comets, planetesimals, asteroids) orbiting inside of a Solar system analogue that is itself initially embedded within a stellar cluster environment varies as the star evolves through all of its stellar phases. We attempt this computationally challenging link in four parts: (1) by performing stellar cluster simulations lasting 100 Myr, (2) by making assumptions about the subsequent quiescent 11 Gyr main-sequence evolution, (3) by performing simulations throughout the giant branch phases of evolution, and (4) by making assumptions about the belt's evolution during the white dwarf phase. Throughout these stages, we estimate the planetesimals' gravitational responses to analogues of the four Solar system giant planets, as well as to collisional grinding, Galactic tides, stellar flybys, and stellar radiation. We find that the imprint of stellar cluster dynamics on the architecture of ≳100 km-sized exo-Kuiper belt planetesimals is retained throughout all phases of stellar evolution unless violent gravitational instabilities are triggered either (1) amongst the giant planets, or (2) due to a close (≪1000 au) stellar flyby. In the absence of these instabilities, these minor planets simply double their semimajor axis while retaining their primordial post-cluster eccentricity and inclination distributions, with implications for metal-polluted white dwarfs and the free-floating planetesimal population.Caption: Cartoon describing how we modelled exo-Kuiper belts through all stages of evolution with different numerical codes and forces.

The objects beyond Neptune are thought to be the most pristine material remaining from the formation process of our solar system. Therefore, one of the most important tasks in planetary science is to understand the architecture of the outer solar system and explain the remarkable diversity in the physical properties and compositions of trans-Neptunian objects (TNOs), Oort cloud comets, and irregular satellites. The most widely accepted models, which successfully reproduce many observed features of the outer solar system, belong to the family of planetary instability models (see Nesvorny, 2018, for a review) derived from the original Nice model by Tsiganis et al. (2005). These models have withstood the test of time and have been successfully adapted to account for the growing body of observational evidence.Some features of the outer solar system populations, however, pose challenges to the planet-migration models. For example, the existence of Sedna-like TNOs on highly eccentric orbits and high-inclination TNOs are difficult to explain using planet instability model simulations on their own. These and other anomalies have opened avenues for exploring additional mechanisms to populate the trans-Neptunian region, notably the hypothesis for the existence of an undiscovered massive planet in the outer solar system (e.g. Batygin & Brown, 2016). Another scenario, which has recently shown promising results, is the stellar flyby hypothesis (see Pfalzner et al., 2024). In that framework, the outer solar system's architecture could be replicated by the flyby of a star several billion years ago. This event could have occurred either as an alternative or in addition to planet migration.The dynamical models proposed to explain the solar system's architecture serve as a backbone of planetary science research. They shape our understanding of early solar system evolution and are incorporated into the assumptions of almost every major research project focused on minor planets. It is therefore essential that these models are rigorously tested and continuously refined based on state-of-the-art observations. We are now at a pivotal moment for evaluating theoretical hypotheses against new observations. The last few years have brought an abundance of new observational evidence, some of which is challenging the existing models. Following the first two cycles of JWST, we now have an unprecedented window into the direct compositional evidence of TNOs and irregular satellites. Additionally, recent large TNO survey programs (e.g., DES, OSSOS) have significantly advanced our understanding of the orbital distribution and the range of surface properties and physical characteristics of TNOs. Last but not least, the in-situ experiments of space missions (Rosetta and New Horizons) have provided unprecedented details about the properties of comets and TNOs.In order to consolidate the community’s understanding of how recent observational evidence aligns with the different dynamical models, we are organizing a 3-day Forum on 3–5 September 2025, hosted by the International Space Science Institute (ISSI) in Bern, Switzerland. The forum will bring together around 25 key members of the community with transdisciplinary expertise, encompassing observations of the orbital, physical, chemical, and surface properties of TNOs, irregular satellites, and comets, as well as planetary instability and stellar flyby models. The primary focus will be to work toward consensus on the key observational tests of these dynamical models that should be prioritized in the coming years. We will aim to solidify agreement on the main priorities for making optimal use of recent and upcoming major observing facilities (including JWST and the ELTs), particularly in preparation for the Rubin Observatory’s LSST survey, launching in 2025. For example, LSST is expected to increase the number of observed TNOs from ~4,000 to more than 35,000 over the next five years. Given the volume of data expected from LSST and the limited resources for follow-up observations, it will be essential to identify the most pressing questions that need to be addressed in order to test the existing dynamical models and improve our understanding of the processes that shaped the early solar system. The forum’s main outcome will be a peer-reviewed publication summarizing the current level of agreement between models and observations, and outlining the diagnostic observational tests that should be prioritized in the near future. At EPSC/DPS, we will share the key outcomes of the forum, highlight the main insights, and open the discussion to the wider community. The EPSC/DPS presentation will offer an excellent opportunity to engage the wider community and to gather further input on how we can best test and refine current models for the formation and evolution of the outer solar system.References Batygin, K., & Brown, M. E. (2016), The Astronomical Journal, 151, 22 Nesvorný, D. (2018), Annual Review of Astronomy and Astrophysics, 56, 137 Tsiganis, K., Gomes, R., Morbidelli, A., & Levison, H. F. (2005), Nature, 435, 459 Pfalzner, S., Govind, A., & Portegies Zwart, S. (2024), Nature Astronomy, 8, 1380

\n Context. Most stars form in embedded clusters. Stellar flybys may affect the orbital architecture of the systems by exciting the eccentricity and causing dynamical instability. \n Aims. Since, incidentally, the timescale on which a cluster loses it gaseous component and begins to disperse is comparable to the circumstellar disk’s lifetime, we expect that closer and more perturbing stellar flybys occur when the planets are still embedded in their birth disk. We investigate the effects of the disk on the dynamics of planets after the stellar encounter to test whether it can damp the eccentricity and return the planetary system to a nonexcited state. \n Methods. We used the hydrodynamical code FARGO to study the disk+planet(s) system during and after the stellar encounter in the context of evolved disk models whose superficial density is 10 times lower than that of the minimum mass solar nebula. \n Results. The numerical simulations show that the planet’s eccentricity, excited during a close stellar flyby, is damped on a short timescale (~10 Kyr) in spite of the disk’s low initial density and subsequent tidal truncation. This damping is also effective for a system of 3 giant planets, and the effects of the dynamical instability induced by the passing star are quickly absorbed.\n Conclusions. If the circumstellar disk is still present around the star during a stellar flyby, a planet (or a planetary system) is returned to a nonexcited state on a short timescale. This does not mean that stellar encounters do not affect the evolution of planets, but they do it in a subtle way with a short period of agitated dynamical evolution. At the end of it, the system resumes a quiet evolution and the planetary orbits are circularized by the interaction with the disk. \n

Recent GAIA observations revealed that the K-type star Gliese 710 will cross the Oort cloud in a distance between approximately 4000 and 12000 au in about 1.3 Myrs. This occurrence motivated us to study the influence of a stellar encounter on comets in the outer region of the solar system. Even if the Oort cloud extends to 100000 au from the sun, we restrict our study to the region between 30 and 25000 au where 25 million objects are distributed randomly. Comets at larger distances are not taken into account as they hardly enter the observable region after a single stellar fly-by. An overview of all objects that are scattered towards the sun for the different fly-by distances at 4000, 8000 and 12000 au shows that only a handful of objects are moving towards the sun immediately after the stellar encounter.However, a subsequent long-term study of all objects that are moved into highly eccentric motion by the stellar fly-by shows a significant increase of comets crossing Jupiter’s orbit and entering into the observable region. In addition, our study shows the first comets crossing the orbit of Earth only about 2.5 Myrs after the stellar fly-by. Thus, the impact risk for the Earth increases only some million years after the stellar fly-by.

Most planetary systems—including our own—are born within stellar clusters, where interactions with neighboring stars can help shape the system architecture. This paper develops an orbit-averaged formalism to characterize the cluster’s mean-field effects, as well as the physics of long-period stellar encounters. Our secular approach allows for an analytic description of the dynamical consequences of the cluster environment on its constituent planetary systems. We analyze special cases of the resulting Hamiltonian, corresponding to eccentricity evolution driven by planar encounters, as well as hyperbolic perturbations upon dissipative disks. We subsequently apply our results to the early evolution of our solar system, where the cluster’s collective potential perturbs the solar system’s plane, and stellar encounters act to increase the velocity dispersion of the Kuiper Belt. Our results are twofold. First, we find that cluster effects can alter the mean plane of the solar system by ≲1° and are thus insufficient to explain the ψ ≈ 6° obliquity of the Sun. Second, we delineate the extent to which stellar flybys excite the orbital dispersion of the cold classical Kuiper Belt and show that while stellar flybys may grow the cold belt’s inclination by the observed amount, the resulting distribution is incompatible with the data. Correspondingly, our calculations place an upper limit on the product of the stellar number density and residence time of the Sun in its birth cluster, η τ ≲ 2 × 104 Myr pc−3.

Passing stars (also called stellar flybys) have notable effects on the solar system’s long-term dynamical evolution, injection of Oort cloud comets into the solar system, properties of trans-Neptunian objects, and more. Based on a simplified solar system model, omitting the Moon and the Sun’s quadrupole moment J 2, it has recently been suggested that passing stars are also an important driver of paleoclimate before ∼50 Myr ago, including a climate event called the Paleocene-Eocene Thermal Maximum (∼56 Myr ago). In contrast, using a state-of-the-art solar system model, including a lunar contribution and J 2, and random stellar parameters (>400 simulations), we find no influence of passing stars on paleoclimate reconstructions over the past 56 Myr. Even in an extreme flyby scenario in which the Sun-like star HD 7977 (m = 1.07M ⊙) would have passed within ∼3900 au about 2.8 Myr ago (with 5% likelihood), we detect no discernible change in Earth’s orbital evolution over the past 70 Myr, compared to our standard model. Our results indicate that a complete physics model is essential to accurately study the effects of stellar flybys on Earth’s orbital evolution.

The planets of our solar system formed from a gas-dust disk. However, there are some properties of the solar system that are peculiar in this context. First, the cumulative mass of all objects beyond Neptune (TNOs) is only a fraction of what one would expect. Second, unlike the planets themselves, the TNOs do not orbit on coplanar, circular orbits around the Sun, but move mostly on inclined, eccentric orbits and are distributed in a complex way. This implies that some process restructured the outer solar system after its formation. However, some of TNOs, referred to as Sednoids, move outside the zone of influence of the planets. Thus external forces must have played an important part in the restructuring of the outer solar system. The study presented here shows that a close fly-by of a neighbouring star can simultaneously lead to the observed lower mass density outside 30 AU and excite the TNOs onto eccentric, inclined orbits, including the family of Sednoids. In the past it was estimated that such close fly-bys are rare during the relevant development stage. However, our numerical simulations show that such a scenario is much more likely than previously anticipated. A fly-by also naturally explains the puzzling fact that Neptune has a higher mass than Uranus. Our simulations suggest that many additional Sednoids at high inclinations still await discovery, perhaps including bodies like the postulated planet X.

Gliese 710 is a K7V star located 19 pc from the Sun in the constellation of Serpens Cauda, which is headed straight for the solar system. Berski & Dybczynski (2016) used data from Gaia DR1 to show that this star will be 13366 AU from the Sun in 1.35 Myr from now. Here, we present an independent confirmation of this remarkable result using Gaia DR2. Our approach is first validated using as test case that of the closest known stellar flyby, by the binary WISE J072003.20-084651.2 or Scholz's star. Our results confirm, within errors, those in Berski & Dybczynski (2016), but suggest a somewhat closer, both in terms of distance and time, flyby of Gliese 710 to the solar system. Such an interaction might not significantly affect the region inside 40 au as the gravitational coupling among the known planets against external perturbation can absorb efficiently such a perturbation, but it may trigger a major comet shower that will affect the inner solar system.

Most stars form in dense stellar environments, where frequent close encounters can strongly perturb and reshape the early architecture of planetary systems. The Solar System, with its rich population of distant comets, provides a natural laboratory to study these processes. We performed detailed numerical simulations using the LonelyPlanets framework that combines NBODY6++GPU and REBOUND to explore the evolution of debris disks around Solar System analogues embedded in stellar clusters. Two initial configurations are considered, the Extended model and the Compact model, each containing four giant planets and either an extended or compact debris disk. We find that compact disks primarily form Kuiper belt and scattered disk-like populations through planet–disk interactions, while extended disks are more strongly shaped by stellar encounters, producing Oort cloud-like structures and interstellar comets with ejection velocities of 1–3 km s −1 . Stellar perturbations are most effective for encounter inclinations between 0° and 30°, giving rise to distinct dynamical populations, like Sednoids, and inner Oort cloud analogues, and a characteristic tail in semimajor axis-eccentricity space. In coplanar encounters, the disk remains largely flattened, whereas polar flybys redistribute angular momentum vertically, producing nearly isotropic outer populations that resemble an emerging Oort cloud. Our results suggest that cometary reservoirs and interstellar objects are natural byproducts of planet–disk interactions and stellar flybys in dense clusters, linking the architecture of outer planetary systems to their birth environments.