As they grew larger, their gravitational forces increased, attracting more matter from the disk and gradually building up kilometer-sized bodies called planetesimals, some of which in turn formed the nuclei of the planets as we know them today. Other planetesimals became either comets or asteroids. This protoplanetary disk was thus the transition state between our stellar ancestors and the planetary objects that evolved from it. The inset shows a single protoplanetary disk silhouetted against the glowing gas of the nebula. These Hubble images support recent findings that disks are common in clusters of young stars.
Many questions remain.
Understanding star and planet formation, detecting other planetary systems, and searching for signs of life on such planets are key objectives of NASA's Origins Program. The Origins Education Forum is the central node for the education and outreach activities of the Origins Theme of NASA's Office of Space Science . The Origins Education Forum is managed for NASA by the Space Telescope Science Institute .