  When the EPOXI spacecraft flew by Comet 103P/Hartley 2, it observed large
particles floating around the comet nucleus. These particles are likely
low-density, centimeter- to decimeter-sized clumps of ice and dust. While the
origin of these objects remains somewhat mysterious, it is possible that they
are giving us important information about the earliest stages of our Solar
System's formation. Recent advancements in planet formation theory suggest that
planetesimals (or cometestimals) may grow directly from the gravitational
collapse of aerodynamically concentrated small particles, often referred to as
"pebbles." Here we show that the particles observed in the coma of 103P are
consistent with the sizes of pebbles expected to efficiently form planetesimals
in the region that this comet likely formed, while smaller pebbles are may be
expected in the majority of comets, whose chemistry is often indicative of
formation in the colder, outer regions of the protoplanetary disk.

Earth and Planetary Astrophysics