On the Orbital Spacing Pattern of Kepler Multiple Planet Systems

TL;DR

This study revisits the orbital spacing patterns of Kepler multiple planet systems, revealing a dichotomy in period ratio correlations linked to mean motion resonances, influenced by observational biases and planetary migration.

Contribution

It demonstrates that orbital spacing correlations are confined to tightly packed systems and are related to mean motion resonances, refining previous uniformity assumptions.

Findings

Significant correlation only in tightly packed systems

Transition boundary at mean period ratio ~1.5-1.7

Orbital spacing dichotomy linked to MMR and planetary evolution

Abstract

The Kepler space mission has detected a large number of exoplanets in multiple transiting planet systems. Previous studies found that these Kepler multiple planet systems exhibit an intra-system uniformity, namely planets in the same system have similar sizes and correlated orbital spacings. However, it is important to consider the possible role of selection effects due to observational biases. In this paper, we revisit the orbital spacing aspect of the pattern after taking observational biases into account using a forward modeling method. We find that orbital spacings, in terms of period ratios, of Kepler multiple planet systems are significantly correlated only for those tightly packed systems, and the transition from correlation to non-correlation is abrupt with a boundary at mean period ratio $\overline{PR}$ $\sim 1.5 - 1.7$. In this regard, the pattern of orbital spacing is more like a dichotomy rather than a global correlation. Furthermore, we find that such an apparent orbital spacing dichotomy could be essentially a projection of a dichotomy that related to mean motion resonance (MMR), which we dub as MMR dichotomy, and itself could be a natural result of planet migration and dynamical evolution.