Outer solar system possibly shaped by a stellar fly-by (1807.02960v1)

Abstract: 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.

• The paper demonstrates that a stellar fly-by can reduce mass density beyond 30 AU and excite trans-Neptunian objects into eccentric, inclined orbits.
• Simulations indicate that a 0.5-1 solar mass star passing at about 100 AU is consistent with observed features, including the distribution of Sednoids.
• The study challenges existing models by proposing that frequent early stellar encounters played a critical role in sculpting the outer solar system.

An Analysis of Stellar Fly-by as a Mechanism for Structuring the Outer Solar System

The diaphanous balance governing the architecture of our solar system, especially beyond Neptune, has prompted myriad inquiries into the forces that might have influenced its current state. The paper "Outer solar system possibly shaped by a stellar fly-by" explores an intriguing hypothesis, suggesting a stellar fly-by is a potent mechanism that could account for the complex dynamical features exhibited by trans-Neptunian objects (TNOs). This discussion elucidates the findings, key numerical results, and implications of the paper.

Overview of Research Findings

The solar system formed from a primordial gas-dust disk, evolving to form planets and smaller bodies. Yet, some attributes of the outer solar system challenge this formation narrative. Notably, the unexpectedly low mass density of TNOs beyond Neptune and their non-coplanar, eccentric orbits raise questions. These features suggest an external, sudden perturbation might have played a role in reshaping the outer solar domains.

The paper, through meticulous numerical simulations, demonstrates that a close fly-by of a neighboring star could simultaneously reduce the cumulative mass density beyond 30 Astronomical Units (AU) and excite TNOs into inclined, eccentric orbits—a condition consistent with observations of TNOs, including Sednoids. Intriguingly, the research also finds that such interactions could justify why Neptune possesses a larger mass than Uranus, contradicting mass distribution trends among the other giants.

Numerical Results and Bold Assertions

The investigation defines a parameter space for the fly-by scenario, investigating various stellar masses, periastron distances, and orbital geometries. A scenario involving a 0.5 to 1 solar mass star passing at approximately 100 AU consistently resonates with TNO distribution characteristics. It is particularly compelling that this stellar archaeology potentially predicts more undiscovered Sednoids and posits the likely existence of large bodies in high-inclination orbits, correspondingly hypothesized as potential planet X analogues.

Moreover, the paper posits a remarkable assertion about the likelihood of such events. Historical consensus estimated stellar fly-bys to be rare during crucial planetary formation stages. The research challenges this, citing simulations that indicate these encounters might have been more frequent, particularly during the formative eons of star clusters akin to the Orion Nebula Cluster.

Implications for Future Research

The implications of a stellar fly-by extend beyond our solar shores. Should this model withstand rigorous scrutiny, it suggests our solar system's peculiarities are not unique but could be a common evolutionary branch in stellar systems. This insight profoundly influences our interpretations of other star systems' observed architectures, especially those exhibiting atypical planetary distributions.

Furthermore, this research encourages the scientific community to re-evaluate assumptions regarding planet formation timelines and the dynamism of nascent planetary systems post-formation. Such insights could harmonize with existing models or prompt the genesis of hybrid models—integrating aspects of the Nice model with external perturbative influences.

Paths Forward

While theories proliferate concerning the nuances of our celestial neighborhood's history, the notion of a stellar fly-by offers a unifying, simple explanation that accounts for several perplexing features in concert. Future studies might narrow focus on exploring different cluster densities and formation environments to refine likelihood estimations further. Additionally, as observational technology ventures deeper into space, potential confirmations of planet X or similar bodies could serve as a testament to the predictive power encoded in this model.

In conclusion, this research broadens the horizon of possibilities, suggesting that the seemingly abrupt and dramatic celestial dance of the early solar system might have been orchestrated, in part, by the ephemeral, yet potent, encounters with cosmic neighbors.