DDA 2015 – Did our Solar System once have a STIP?

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.

Brett Gladman (CITA)


Continuing the established tradition in the field of speculative “fairy tales”, we postulate that our Solar System once had a set of several additional Earth-scale planets interior to the orbit of Venus. This would resolve a known issue that the energy and angular momentum of our inner-planet system is best explained by accreting the current terrestrial planets from a disk limited to 0.7-1.1 AU; in our picture the disk material closer to the Sun also formed planets, but they have since been destroyed. By studying the orbital stability of systems like the known Kepler systems, Volk and Gladman (companion abstract) demonstrate that orbital excitation and collisional destruction could be confined to just the inner parts of the system. In this scenario, our Mercury is the final remnant of the inner system’s destruction via a violent multi-collision (and/or hit-and-run disruption) process.This would provide a natural explanation for Mercury’s unusually high eccentricity and orbital inclination; it also fits into the general picture of long-timescale secular orbital instability, with Mercury’s current orbit being unstable on 5 Gyr time scales. The common decade spacing of instability time scales raises the intriguing possibility that this destruction occurred roughly 0.6 Gyr after the formation of our Solar System and that the lunar cataclysm is a preserved record of this apocalyptic event that began when slow secular chaos generated orbital instability in our former super-Earth system.


  • Motivation
    • inner edge of terrestrial planet zone
    • Mercury is weird.
    • Why don’t we have a STIP (system of tightly-packed inner planets)?
  • Mercury:
    • surfing the edge of secular chaos
    • not clear how it got to $e^2 + i^2 \sim (0.25)^2$
    • tough to strip mantle without it quickly falling right back
    • Ausphaug & Reiner (2014): Mercury is the end state of a sequence of collisions.
  • Why is there an inner edge?
    • Wetherill 1978 (Protostars & Planets): E and L of terrestrial planets requires an inner edge ~0.6 AU.
    • Historical way out: it’s too hot.
      • But modern studies indicate $T < 1500$K until much later.
  • If there is (collision) debris, where does it go?
    • radiation pressure: days
    • PR drag: kyr
    • meteoritic transfer: kyr-Myr
    • planetary interactions: ~10 Myr
    • $\rightarrow$ disappears quickly
    • if self-collisional, it will still disappear quickly
  • Secular architecture rearrangement
    • pump up to large $e$
    • fast collisions (~50 km/s)
      • vapor production
      • “bullet factory” — erosion of remnants
  • Meng et al. 2014 (Science)
    • spike of hot dust around young star
    • decay ~1 yr

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