Can planetary instability explain the Kepler dichotomy? (1206.6898v2)

Abstract: The planet candidates discovered by the Kepler mission provide a rich sample to constrain the architectures and relative inclinations of planetary systems within approximately 0.5 AU of their host stars. We use the triple-transit systems from the Kepler 16-months data as templates for physical triple-planet systems and perform synthetic transit observations. We find that all the Kepler triple-transit and double-transit systems can be produced from the triple-planet templates, given a low mutual inclination of around five degrees. Our analysis shows that the Kepler data contains a population of planets larger than four Earth radii in single-transit systems that can not arise from the triple-planet templates. We explore the hypothesis that high-mass counterparts of the triple-transit systems underwent dynamical instability to produce a population of massive double-planet systems of moderately high mutual inclination. We perform N-body simulations of mass-boosted triple-planet systems and observe how the systems heat up and lose planets, most frequently by planet-planet collisions, yielding transits in agreement with the large planets in the Kepler single-transit systems. The resulting population of massive double-planet systems can nevertheless not explain the additional excess of low-mass planets among the observed single-transit systems and the lack of gas-giant planets in double-transit and triple-transit systems. Planetary instability of systems of triple gas-giant planets can be behind part of the dichotomy between systems hosting one or more small planets and those hosting a single giant planet. The main part of the dichotomy, however, is more likely to have arisen already during planet formation when the formation, migration or scattering of a massive planet, triggered above a threshold metallicity, suppressed the formation of other planets in sub-AU orbits.

Analysis of Planetary Instability in Explaining the Kepler Dichotomy

The paper explores whether planetary instability can account for the observed dichotomy in the architectures of planetary systems discovered by the Kepler mission. By examining triple-transit systems from Kepler data, the researchers seek to understand the dynamics that lead to the current distribution of single, double, and triple-transit systems. The paper attempts to reconcile the differences in planetary size and orbital arrangements among these systems through detailed synthetic transit simulations and N-body simulations.

Methodology and Findings

The approach involved using Kepler's triple-transit systems as templates and varying internal inclination parameters to assess their impact on observed transit configurations. The core findings indicate that a low mutual inclination of around five degrees can reproduce both triple and double transits seen in Kepler data. However, the data also shows an excess of large planets within single-transit systems that cannot be explained by simple extension of triple-planet templates. This discrepancy leads to the hypothesis that massive, high-mass counterparts underwent significant dynamical instabilities.

N-body simulations were employed to investigate these suggestions. By simulating mass-boosted triple-planet systems, the paper observes how planets may undergo collisions or ejections, ultimately forming reduced double-planet systems. The results of these simulations provide evidence that instability in systems with initially high mass can produce the large planets seen in single-transit systems, though they do not account for an excess of low-mass planets found among single-transit systems.

The paper further conjectures that a significant part of the observed dichotomy might have originated during planet formation. Specifically, the formation or migration of a massive planet—triggered by crossing a minimum metallicity threshold—could suppress additional planet formation in sub-AU orbits.

Implications

The implications of these findings are multifold:

1. Exoplanetary Dynamics: This paper supports the assertion that dynamical processes significantly influence observed planetary system architectures. The potential for planet-planet interactions to initiate instability and reduce multiplicity presents a vital dynamic that could affect planetary formation theories and their predictions.
2. Planet Formation Theories: By suggesting a role for initial conditions such as metallicity and the consequent formation of massive planets, the research underscores the importance of these factors in shaping planetary systems.
3. Interpretation of Exoplanet Statistics: The work stresses a cautionary approach to interpreting observed Kepler data, recommending consideration of diverse outcomes from planetary instability processes and highlighting the challenges in drawing conclusions about planet occurrence rates without acknowledging potential evolution paths.

Speculations on Future Developments in AI

While the paper relates to astrophysics, its methods and implications align with potential developments in machine learning and AI involved in astronomical data analysis. The integration of AI techniques could significantly enhance the processing of large datasets, like those from Kepler, enabling more nuanced simulations and perhaps quicker resolutions to complex dynamic systems via advanced algorithms. Deep learning models could be trained to predict stability outcomes and better simulate the range of planetary architectures resulting from varied initial conditions.

In summary, this research provides significant insights into the role of dynamical instability in exoplanetary systems, offering explanations for observations inconsistent with high multiplicity assumptions. Moreover, it opens avenues for theoretical and practical advancements in understanding planetary formation and evolution. The involvement of AI in such complex simulations promises further refinement of these results, potentially offering deeper and more predictive insights into the architecture of exoplanetary systems.