How Close are Compact Multiplanet Systems to the Stability Limit?

Abstract Transit surveys have revealed a significant population of compact multiplanet systems, containing several sub-Neptune–mass planets on close-in, tightly-packed orbits. These systems are thought to formed through final phase giant impacts, which would tend leave close the edge stability. Here, we assess this hypothesis, comparing observed eccentricities in exhibiting transit-timing variations versus maximum compatible with long-term We use machine-learning classifier SPOCK (Tamayo et al.) rapidly classify stability numerous initial configurations and hence determine these limits. While previous studies argued that often maximally packed, sense they could not host any additional planets, find existing measured below limits allowed by factor 2–10. compare results against predictions from giant-impact theory planet formation, derived both N -body integrations theoretical expectations that, absence dissipation, orbits such should be distributed uniformly throughout space volume systematically lower than scenario predicts, median eccentricity about four times predicted. This suggests if then some dissipation must occur damp their eccentricities. may interactions natal gas disk or leftover planetesimals, over longer timescales coupling tidal secular processes.