Volume 570, October 2014
|Number of page(s)||10|
|Section||Planets and planetary systems|
|Published online||15 October 2014|
Formation of pebble-pile planetesimals
Lund ObservatoryDepartment of Astronomy and Theoretical Physics, Lund
Accepted: 1 September 2014
Asteroids and Kuiper belt objects are remnant planetesimals from the epoch of planet formation. The first stage of planet formation is the accumulation of dust and ice grains into mm- and cm-sized pebbles. These pebbles can clump together through the streaming instability and form gravitationally bound pebble clouds. Pebbles inside such a cloud will undergo mutual collisions, dissipating energy into heat. As the cloud loses energy, it gradually contracts towards solid density. We model this process and investigate two important properties of the collapse: (i) the collapse timescale and (ii) the temporal evolution of the pebble size distribution. Our numerical model of the pebble cloud is zero-dimensional and treats collisions with a statistical method. We find that planetesimals with radii larger than ~100 km collapse on the free-fall timescale of ~25 years. Lower-mass clouds have longer pebble collision timescales and collapse much more slowly, with collapse times of a few hundred years for 10 km scale planetesimals and a few thousand years for 1 km scale planetesimals. The mass of the pebble cloud also determines the interior structure of the resulting planetesimal. The pebble collision speeds in low-mass clouds are below the threshold for fragmentation, forming pebble-pile planetesimals consisting of the primordial pebbles from the protoplanetary disk. Planetesimals above 100 km in radius, on the other hand, consist of mixtures of dust (pebble fragments) and pebbles which have undergone substantial collisions with dust and other pebbles. The Rosetta mission to the comet 67P/Churyumov-Gerasimenko and the New Horizons mission to Pluto will provide valuable information about the structure of planetesimals in the solar system. Our model predicts that 67P is a pebble-pile planetesimal consisting of primordial pebbles from the solar nebula, while the pebbles in the cloud which contracted to form Pluto must have been ground down substantially during the collapse.
Key words: methods: analytical / methods: numerical / minor planets, asteroids: general / planets and satellites: formation / comets: general
© ESO, 2014
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