A substellar flyby that shaped the orbits of the giant planets (2412.04583v1)

Abstract: The modestly eccentric and non-coplanar orbits of the giant planets pose a challenge to solar system formation theories which generally indicate that the giant planets emerged from the protoplanetary disk in nearly perfectly circular and coplanar orbits. We demonstrate that a single encounter with a 2-50 Jupiter-mass object, passing through the solar system at a perihelion distance less than 20 AU and a hyperbolic excess velocity less than 6 km/s, can excite the giant planets' eccentricities and mutual inclinations to values comparable to those observed. We estimate that there is about a 1-in-100 chance that such a flyby produces a dynamical architecture similar to that of the solar system. We describe a metric to evaluate how closely a simulated system matches the eccentricity and inclination secular modes of the solar system. The scenario of a close encounter with a substellar object offers a plausible explanation for the origin of the moderate eccentricities and inclinations and the secular architecture of the planets.

• The paper investigates the plausibility of a substellar object flyby accounting for the observed eccentricities and inclinations of the solar system's giant planets.
• The study uses Monte Carlo simulations, finding that about 1% of substellar flybys within 20 AU can reproduce the solar system's orbital architecture.
• This research suggests external stellar encounters in young clusters can significantly shape planetary systems, potentially more than internal interactions alone.

Overview of "A Substellar Flyby that Shaped the Orbits of the Giant Planets"

The paper authored by Brown, Malhotra, and Rein discusses a novel mechanism for the orbital architecture of the giant planets in the solar system. It investigates the plausibility of a substellar object flyby influencing their orbits. This research is premised on the slight eccentricities and inclinations of the giant planets' orbits, which are inconsistent with traditional models of solar system formation that predict nearly circular and coplanar orbits post-formation from a protoplanetary disk.

Hypothesis and Methodology

The hypothesis posits that an encounter with a substellar object—between 2 and 50 Jupiter masses—within 20 AU, moving at hyperbolic excess velocities beneath 6 km/s, could align the current orbital characteristics of the giant planets. The paper utilizes numerical simulations through Monte Carlo methods to explore a sample of 50,000 such flyby scenarios to account for this influence statistically.

Main Findings

The research identifies that approximately 1% of the simulated flybys yield a dynamical configuration similar to the solar system. Such flybys successfully stimulated the eccentricities and inclinations to observed levels. One notable encounter involved an 8.27 Jupiter-mass object passing at a perihelion of 1.69 AU with a velocity of 2.69 km/s. Interestingly, this suggests a 1-in-100,000 chance of such a history in a typical open star cluster.

Numerical Analysis and Statistical Validation

The paper uniquely develops a metric for comparing the post-flyby orbits with the current secular modes in the solar system. This metric considers complex eccentricities and inclinations over time, thus systematically evaluating the secular architecture between simulated and actual planetary systems.

Implications and Future Work

The paper challenges the dominant narrative attributing the solar system's moderate eccentricities and inclinations purely to internal interactions such as planet-planet scattering or resonance crossings. Instead, it places significant weight on the potential of external perturbations from stellar encounters in youthful star clusters. The findings imply that such external perturbations can play a critical role in shaping planetary systems, inviting further examination of similar phenomena in extrasolar settings.

Future investigations could fruitfully extend this work by incorporating variable masses and semi-major axes for initial conditions, intersecting this model with known findings on minor solar system bodies like the Kuiper Belt or the Oort Cloud, and considering the dynamics of the terrestrial planets under similar flyby conditions.

In essence, this paper provides a comprehensive paper of a previously underexplored dynamical mechanism, offering a divergent perspective on solar system evolution and the role of primordial stellar interactions.