DDA 2015 – The Formation of Terrestrial Planets from the Direct Accretion of Pebbles

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.

Hal Levison (SwRI)


Building the terrestrial planets has been a challenge for planeVormation models. In particular, classical theories have been unable to reproduce the small mass of Mars and instead predict that a planet near 1.5 AU should roughly be the same mass as the Earth (Chambers 2001, icarus 152,205). Recently, a new model, known as ‘slow pebble accretion’, has been developed that can explain the formation of the gas giants (Levison+ 2015, Nature submitted). This model envisions that the cores of the giant planets formed from 100 to 1000 km bodies that directly accreted a population of pebbles (Lambrechts & Johansen 2012, A&A 544, A32) – centimeter- to meter-sized objects that slowly grew in the protoplanetary disk. Here we apply this model to the terrestrial planet region and find that it can reproduce the basic structure of the inner Solar System, including a small Mars and a low-mass asteroid belt. In particular, our models show that for an initial population of planetesimals with sizes similar to those of the main belt asteroids, slow pebble accretion becomes inefficient beyond ~1.5 AU. As a result, Mars’s growth is stunted and nothing large in the asteroid belt can accumulate.


  • Standard view:
    • disk forms, dust settles to midplanet
    • dust accumulates, ~1-10 km
    • runaway growth
    • oligarchic growth
    • late-stage
      • violent endgame for terrestrial planets
    • main problem: Mars is way to small
  • possible solution: pebble accretion
    • dust
    • settling dust creates turbulence
    • ~10 mm – 1 m pebbles
    • large planetesimals can accrete pebbles very effectively
      • strong gas drag $\rightarrow$ huge collision cross section (~Hill sphere)
  • Can this explain the low mass of Mars?
    • low-pebble-mass exponential cutoff
      • encounter time too short
    • A Ceres can grow if $r \lt \sim 1$ AU, but it can’t grow if $r \gt \sim 1$ AU.
    • $\rightarrow$ leaves ~20 planets inside of ~1 AU
    • subsequently very unstable and < 1 AU largely clears out
    • leaves behind essentially the Solar System architecture

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