DDA 2015 – Consolidating and Crushing Exoplanet Systems

This is one of a series of notes taken during the 2015 meeting of the AAS Division on Dynamical Astronomy, 3-7 May, at CalTech. An index to this series (all the papers presented at the meeting) is here.

Kathryn Volk (U. British Columbia)


Kepler revealed the common existence of tightly-packed planetary systems around solar-type stars, existing entirely on orbits with periods shorter than ~200 days. Those systems must have survived for the ages of their host stars (~5 Gyr), so their formation mechanism must provide inter-planet spacings that permit long-term stability. If one postulates that most planetary systems form with tightly-packed inner planets, their current absence in some systems could be explained by the collisional destruction of the inner system after a period of meta-stability. The signatures of such intense collisional environments may have been observed around stars in the form of rapidly varying debris disks; in these observed disks, collisional products are being disposed of via drag down onto the star or grinding to the nearly instantaneous dust blow-out limit. We use the orbital spacings and planet masses of the observed Kepler multi-planet systems to investigate the stability and long-term behavior of the systems. We find that many of our Kepler system analogs are unstable on 100 Myr timescales, even for initially small eccentricities (0-0.05); the instability timescales in these systems are distributed such that equal fractions of the systems experience planetary collisions in each decade in time. We discuss the likely outcomes of collisions in these systems based on the typical collision speeds from our numerical integrations and what implications this has for interpreting the observed Kepler multi-planet systems. The possible implications for our Solar System are discussed in a companion abstract (Gladman and Volk).


  • Architectures of close-in (closely packed) planetary systems (from Kepler)
  • Fabrycky 2014
  • ~5-10% ofFGK field stars
    • These systems must be stable on Gyr timescales
  • Are all stars formed tightly packed?
  • Modeled 13 such Kepler systems
    • Preserved $a$ and masses, orbital angles randomized
    • Allowed $e_0$ to vary $0 < e_0 < 0.05$
    • Sudden onset of instability in 11 of these 13 after tens to ~100 Myr
      • [why is she surprised?]
    • These eccentricities are in range of observed values
    • Decay rates consistent with e.g. Holman & Wisdom (1992 AJ)
  • Why sudden onset?
    • History is very sensitive to ICs [duh]
    • Consolidation (low-speed collisions) vs. Destruction (high-speed collisions)
    • First collision is often near the accretion/erosion boundary — i.e., low-speed
    • Masses in 4-5 planet systems tend to be lower, while individual masses in ~3-planet systems are higher: mergers?
    • Tracked collision speeds during integrations.
    • Second collision often goes into erosion regime (i.e., high-speed)
      • Observing debris should be rare (but see Meng et al. 2012)
    • Ergodicity allows large variety of outcomes
  • $\Rightarrow$ tightly packed systems could be ubiquitous initially
    • Young stars should show higher fraction
  • The remaining ~95% should be 0-2 planet systems

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