DDA 2015 – On the Robust Production of Super Earths and Suppressed Emergence of Gas Giants in Dynamically Evolving Protostellar Disks

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.

Session: Planet Formation I

Doug Lin (UC Santa Cruz) (Brouwer award winner)


Radial velocity and transit surveys indicate the presence of super Earth around half of the main sequence stars regardless of their mass and metallicity. In contrast, the frequency of gas giants is much lower and increases with stellar mass and metallicity. I will show how the emergence of super-Earth is a robust process whereas the formation of gas giant planets is a threshold phenomena. The topics to be discussed include physical barriers in the planet building process, the role of migration in their evolving natal disks, planets’ interaction with each other and with their host stars. I will also discuss some key observations which may provide quantitative tests for planet formation theories.


  • Observed properties of exoplanets: Howard 2013 (Science)
  • Showstoppers:
    • disk formation
    • grain growth: the “meter barrier”
      • Trapping of refractory grains beyond the magnetospheric cavity
      • Tends to pile up at boundary
    • grain growth: the “kilometer barrier”
      • collisional fragmentation vs. grav.
      • oligarchic barrier: isolation mass
        • typically very small
    • embryo retention barrier — Type I migration
      • planet-disk tidal interaction
      • get to high mass $\rightarrow$ migrate outward
      • resonant sweeping $\rightarrow 2^{nd}$ generation
    • core barrier: embryo resonant trapping
      • bypass the resonant barrier
        • inner scattered outward, outer scattered inward $\rightarrow$ collisions $\rightarrow$ impacts of super Earths
    • gas accretion barrier
      • Is there a threshold mass for gas accretion?
      • runaway accretion
        • Why didn’t this happen for observed super Earths?
      • plenty of material left over: why didn’t they turn into gas giants?
      • Measured disk accretion rate…?
      • metal rich stars: no observed dependence, despite theory
        • But metallicity of star and disk need not be the same.
    • Rapid growth of proto gas giants
    • grand design barrier: dynamical instability
      • How did gas giants acquire their eccentricities?
      • Type II migration
        • provides constraint on growth process
      • Why did hot Jupiters stop their inward migration?
  • Close in planets
    • e.g. Kepler-78
      • 8-hour period
      • Star is magnetic
        • ~15 g
        • analogous to Jupiter-Io
        • induced EMF (unipolar induction) $\rightarrow$ energy dissipation at expense of planet’s orbit
        • Planet surface cannot be iron; must be silicates.
        • Flux tube footprints on star should move at period of planetary orbit, not stellar rotation.
  • Many other issues!

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