FYI -- Plutos Portrait From New Horizons: Ice Mountains and No Craters 'A second surprise was that the dwarf planets surface was unmarred by craters. We have not yet found a single impact crater in this image, Dr. Spencer said.' http://www.nytimes.com/2015/07/16/science/pluto-flyby-photos-reveal-mountain... I don't see the problem. Most of the chaotic behavior during planet formation occurs in the inner Solar system -- in particular, inside of the large planets -- e.g., Jupiter, Saturn, Uranus, Neptune. Outside of these planets, the density is far, far smaller, and the opportunity for collisions is also far, far smaller. Time -- at least measured by number of impact craters -- runs far, far slower in these outer regions. Not only is the "year" for Pluto much longer, but the density of material to run into is much smaller -- even if you integrated it all the way around its orbit.