  The processes that led to the formation of the planetary bodies in the Solar
System are still not fully understood. Using the results obtained with the
comprehensive suite of instruments on-board ESA's Rosetta mission, we present
evidence that comet 67P/Churyumov-Gerasimenko likely formed through the gentle
gravitational collapse of a bound clump of mm-sized dust aggregates
("pebbles"), intermixed with microscopic ice particles. This formation scenario
leads to a cometary make-up that is simultaneously compatible with the global
porosity, homogeneity, tensile strength, thermal inertia, vertical temperature
profiles, sizes and porosities of emitted dust, and the steep increase in
water-vapour production rate with decreasing heliocentric distance, measured by
the instruments on-board the Rosetta spacecraft and the Philae lander. Our
findings suggest that the pebbles observed to be abundant in protoplanetary
discs around young stars provide the building material for comets and other
minor bodies.
