Complex sentence: Thus, comets are the residuals of the volatile-rich planetesimals that remained in the planetary system without having been being incorporated into the planets; they are considered as fossilized bodies witnessing the physical and chemical conditions prevailing at the time of planet formation.
Split into simpler sentences: Thus, comets are the residuals of the volatile-rich planetesimals that remained in the planetary system without having been incorporated into the planets. They are considered fossil bodies that have seen the physical and chemical conditions prevailing at the time of planet formation.