Early Stellar Flybys are Unlikely: Improved Constraints from Sednoids and Large-q Trans-Neptunian Objects

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,

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

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.

Unlike the Solar System planets, thousands of smaller bodies beyond Neptune orbit the Sun on eccentric (e > 0.1 and i > 3°) orbits. While migration of the giant planets during the early stages of Solar System evolution could have induced substantial scattering of trans-Neptunian objects (TNOs), this process cannot account for the small number of distant TNOs (rp > 60 au) outside the planets’ reach. The alternative scenario of the close flyby of another star can instead produce all these TNO features simultaneously, but the possible parameter space for such an encounter is vast. Here we compare observed TNO properties with thousands of flyby simulations to determine the specific properties of a flyby that reproduces all the different dynamical TNO populations, their locations and their relative abundances, and find that a 0.8−0.1+0.1M⊙\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0.{8}_{-0.1}^{+0.1}\\,{M}_{\\odot }$$\\end{document} star passing at a distance of rp = 110 ± 10 au, inclined by i = 70°+5−10\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\,}_{-10}^{+5}$$\\end{document}, gives a near-perfect match. This flyby also replicates the retrograde TNO population, which has proved difficult to explain. Such a flyby is reasonably frequent; at least 140 million solar-type stars in the Milky Way are likely to have experienced a similar one. In light of these results, we predict that the upcoming Vera Rubin telescope will reveal that distant and retrograde TNOs are relatively common.

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.

Most stars form in clusters where relatively close encounters with other stars are common and can leave imprints on the orbital architecture of planetary systems. In this paper, we investigate the inclination excitation of debris disk particles due to such stellar encounters. We derive an analytical expression that describes inclination excitation in the hierarchical limit where the stellar flyby is distant. We then obtain numerical results for the corresponding particle inclination distribution in the nonhierarchical regime using a large ensemble of N-body simulations. For encounters with expected parameters, we find that the bulk inclination of the disk particles remains low. However, a distinct high-inclination population is produced by prograde stellar encounters for particles with final pericenter distances above 50 au. The maximum extent i t of the inclination distribution scales with the inclination of the encounter for massive star flybys with low incoming velocity. The inclination distribution of observed trans-Neptunian objects places constraints on the dynamical history of our solar system. For example, these results imply an upper limit on product of the number density n of the solar birth cluster and the Sun’s residence time τ of the form nτ ≲ 8 × 104 Myr pc−3. Stronger constraints can be derived with future observational surveys of the outer solar system.

Asymmetric debris discs have been found around stars other than the Sun; asymmetries are sometimes attributed to perturbations induced by unseen planets. The presence or absence of asymmetries in our own trans-Neptunian belt remains controversial. The study of sensitive tracers in a sample of objects relatively free from the perturbations exerted by the four known giant planets and most stellar flybys may put an end to this debate. The analysis of the distribution of the mutual nodal distances of the known extreme trans-Neptunian objects (ETNOs) that measure how close two orbits may get to each other could be such a game changer. Here, we use a sample of 51 ETNOs together with random shufflings of this sample and two unbiased scattered-disc orbital models to confirm a statistically significant (62σ) asymmetry between the shortest mutual ascending and descending nodal distances as well as the existence of multiple highly improbably (p < 0.0002) correlated pairs of orbits with mutual nodal distances as low as 0.2 au at 152 au from the Solar system’s barycentre or 1.3 au at 339 au. We conclude that these findings fit best with the notion that trans-Plutonian planets exist.

Trans-Neptunian Object Colors as Evidence for a Past Close Stellar Flyby to the Solar System