Vol. 557
In section 1. Letter to the Editor

Fluffy dust forms icy planetesimals by static compression

It has been a long-standing puzzle that millimeter- to meter-sized grains tend to drift very rapidly towards their parent star, particularly with observational evidence that these grains are present in circumstellar disks more or less independently of their age. The fact that these grains may be porous and not compact and would thus drift much more slowly has been advocated as a possible solution. Nevertheless, the puzzle has remained unsolved because asteroids and comets with a very low porosity (down to a density of 0.00001 g/cm^3!) have never been observed. The authors propose a way around this and show that grains should first grow by becoming extremely porous before being compacted by gas drag and then by self-gravity. This model can account for the observed population of comets and their comparatively high densities (about 0.1 to 1 g/cm3) by assuming that they are the remnants of planetesimals of sizes of order 10 km and beyond.