Exoplanets & Exomoons

On The Degree of Dynamical Packing In The Kepler Multi-planet Systems

By Keith Cowing
Status Report
June 27, 2023
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On The Degree of Dynamical Packing In The Kepler Multi-planet Systems
Example probability distributions of being unstable within 109 orbits for four different sets of adjacent pairs. An additional planet was inserted between the adjacent pair and the system was sampled 5000 times (using Rayleigh distributions with πœŽπ‘’ = 0.01, πœŽπ‘– = 0.5 β—¦ ). Each panel is a 2D histogram on a 25 by 25 grid showing the mean probability of all sampled configurations with an inserted planet whose mass and period fall within the bounds of each grid cell (white spaces have no inserted planets in that region). The white X shows the inserted planet’s mass and period for the adjacent pair’s FM13-style test. Each panel’s title shows its KIC, the placement of the inserted planet (0 is between the innermost planet and the second planet), the mean probability (pπ‘šπ‘’π‘Žπ‘›), the most stable configuration probability (p𝑀𝑆𝐢), and the probability of the pair’s FM13-style test (p𝐹𝑀13). Inserted planet periods are distributed uniformly in period and the masses according to Figure 1 (the vertical limits correspond to 95% of masses and 68% of masses fall within the region marked by the horizontal dashed lines). — astro-ph.EP

Current planet formation theories rely on initially compact orbital configurations undergoing a (possibly extended) phase of giant impacts following the dispersal of the dissipative protoplanetary disk.

The orbital architectures of observed mature exoplanet systems have likely been strongly sculpted by chaotic dynamics, instabilities, and giant impacts. One possible signature of systems continually reshaped by instabilities and mergers is their dynamical packing.

Early Kepler data showed that many multi-planet systems are maximally packed – placing an additional planet between an observed pair would make the system unstable. However, this result relied on placing the inserted planet in the most optimistic configuration for stability (e.g., circular orbits). While this would be appropriate in an ordered and dissipative picture of planet formation (i.e. planets dampen into their most stable configurations), we argue that this best-case scenario for stability is rarely realized due to the strongly chaotic nature of planet formation.

Consequently, the degree of dynamical packing in multi-planet systems under a realistic formation model is likely significantly higher than previously realized. We examine the full Kepler multi planet sample through this new lens, showing that ~60-95% of Kepler multi-planet systems are strongly packed and that dynamical packing increases with multiplicity. This may be a signature of dynamical sculpting or of undetected planets, showing that dynamical packing is an important metric that can be incorporated into planet formation modelling or when searching for unseen planets.

Alysa Obertas, Daniel Tamayo, Norm Murray

Comments: 15 pages, 4 figures. Accepted for publication in MNRAS
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2306.12967 [astro-ph.EP] (or arXiv:2306.12967v1 [astro-ph.EP] for this version)
Submission history
From: Alysa Obertas
[v1] Thu, 22 Jun 2023 15:25:32 UTC (591 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) πŸ––πŸ»