Early Stellar Flybys are Unlikely: Improved Constraints from Sednoids and Large-$q$ TNOs

TL;DR

The paper investigates whether close stellar flybys in the early Solar System can explain the Sednoid population while matching four observational constraints: implantation efficiency $\eta>5\%$, semimajor axis coverage around observed sednoids, a low-inclination proportion with $i<30^{\circ}$, and a primordial clustering of perihelion directions measured by the concentration parameter $\kappa$. Using extensive N-body simulations of 768 encounter configurations, it finds that only a small fraction ($\lesssim 5\%$) can satisfy all four constraints, requiring near-precise flyby geometry (e.g., very close periastron $q_\star=300$ au and specific orientations), and that many viable cases are dynamically unlikely given early-Solar-System encounter rates. The results strongly constrain the stellar-flyby hypothesis and, when combined with alternatives like rogue planets or Planet Nine, provide a framework to discriminate outer-Solar-System formation scenarios. The study emphasizes that future discoveries of additional sednoids with well-determined orbits are crucial to confirm the low-inclination trend and primordial alignment, thereby refining our understanding of the Solar System’s early dynamical evolution.

Abstract

Sedna-like objects (a.k.a. sednoids) are transneptunian objects (TNOs) characterized by large semimajor axes and exceptionally high perihelia. Their high-$q$ orbits are detached from the influence of the four giant planets and need extra perturbation to form. One hypothesis posits that close stellar flybys could have perturbed objects from the primordial scattering disk, generating the sednoid population. In this study, we run N-body simulations with different stellar encounter configurations to explore whether such a close stellar flyby can satisfy new constraints identified from sednoid (and detached extreme TNO) observation, including the low-inclination ($i<30^\circ$) profile and primordial orbital alignment. Our results suggest that flybys with field stars are unable to generate a sufficient population, whereas flybys within the birth cluster fail to produce the primordial orbital alignment. To meet the inclination constraint of detached extreme TNOs, flybys have to be either coplanar ($i_\star \sim 0^\circ$) or symmetric about the ecliptic plane ($ω_\star \sim 0^\circ, i_\star \sim 90^\circ$). After taking into account their occurrence rate at the early stage of the Solar System, we conclude that close-in stellar flybys ($q_\star \le 1000$~au) that satisfy all constraints are unlikely to happen ($\lesssim$5\%). Future discoveries of additional sednoids with precise orbital determinations are crucial to confirm the existence of the low-inclination tendency and the primordial alignment, and to further constrain the early dynamical evolution of the Solar System.

Figures (6)

• Figure 1: The $a$-$q$-$i$ distribution of all TNOs produced by four example stellar encounters for a flyby $M_\star=1M_\odot$. In the upper panels, the definitions of sednoids ($200<a<1500$ au, $q>60$ au) and detached extreme TNOs (eTNOs, $200<a<1500$ au, $q>40$ au) are emphasized by the green and blue shaded regions. Synthetic sednoids, eTNOs and other TNOs are marked as green, blue and gray small dots. The red square dots are three observed sednoids, and the orange circle ones are observed eTNOs excluding sednoids. In the lower panels, the black lines mark out $i=30^{\circ}$. The implantation efficiency of sednoids ($\eta$) and the fraction of $i<30\degree$ sednoids ($f_{i<30\degree}$) are annotated in the upper right corners for clarification.
• Figure 2: Distributions of sednoid semimajor axis from simulations for different $v_\infty$ and $q_\star$, with both $i_\star$ and $\omega_\star$ fixed to 0. The red short line indicates the median value of the distribution, and the black short lines show the 2.5% and 97.5% percentiles. The semimajor axes of three observed sednoids are marked out as gray lines.
• Figure 3: The implantation efficiency of sednoids for $M_\star = 0.3 M_{\odot}$ and $M_\star = 1 M_{\odot}$, projected onto the $(v_\infty, q_\star)$ space. The mean implantation efficiency across 24 possible sets of ($i_\star, \omega_\star$) for fixed $(v_\infty, q_\star)$ is marked above the parentheses and also represented by the grid color，while the minimum and maximum values are marked in the parentheses.
• Figure 4: The fraction of $i<30\degree$ sednoids for stellar encounters with $M_\star = 1 M_{\odot}$, projected onto the $(v_\infty, q_\star)$ (left panel) and $(i_\star, \omega_\star)$ (right panel) parameter spaces. The mean $i<30\degree$ fraction across the remaining stellar parameters is marked above the parentheses and also represented by the grid color, while the minimum and maximum values are marked in the parentheses, same as Figure \ref{['fig:implantation-rate']}.
• Figure 5: Inclination distributions of synthetic sednoids ($q>60$ au, upper panel) and detached eTNOs ($q>40$ au, lower panel) produced by stellar flybys with $i_\star=0^\circ$ (blue) and $i_\star=180^\circ$ (orange) for $v_\infty=1$ km/s, $q_\star = 300$ au, and $M_\star=1M_\odot$. The intrinsic distributions (which should not be compared to the real detections) are shown with dotted lines, and the real observed ones are shown with solid red lines. The synthetic simulated distributions (dashed lines) are the biased distributions produced by the simulated surveys with a typical coverage of $\pm30^\circ$ around the ecliptic plane. The $p$-values of the KS test between the simulated detections and the observed samples are given. Distributions for the four othr $i_\star$ cases are not plotted (for clarity), but their intrinsic and synthetic curves fall between blue and orange ones.