  The Rosetta mission to comet 67P/Churyumov-Gerasimenko has provided new data
to better understand what comets are made of. The weak tensile strength of the
cometary surface materials suggests that the comet is a hierarchical dust
aggregate formed through gravitational collapse of a bound clump of small dust
aggregates so-called ``pebbles'' in the gaseous solar nebula. Since pebbles are
the building blocks of comets, which are the survivors of planetesimals in the
solar nebula, estimating the size of pebbles using a combination of thermal
observations and numerical calculations is of great importance to understand
the planet formation in the outer solar system. In this study, we calculated
the thermal inertias and thermal skin depths of the hierarchical aggregates of
pebbles, for both diurnal and orbital variations of the temperature. We found
that the thermal inertias of the comet 67P/Churyumov-Gerasimenko are consistent
with the hierarchical aggregate of cm- to dm-sized pebbles. Our findings
indicate that the icy planetesimals may have formed via accretion of cm- to
dm-sized pebbles in the solar nebula.
