How do planetary systems form?
Our Sun belongs to the generation of stars created 4.6 billion years ago, when our galaxy was roughly half its present age. A cloud of interstellar gas, dust and ices, containing several generations of material, collapsed to form the nebula from which the Sun and the rest of our solar system grew. This collapse may have been triggered by a nearby supernova. Cosmologists believe that because the material in the nebula was rotating to some degree, not all of the nebular material fell directly into the central mass that would become the Sun. Instead, some of the material was confined to a flat, spinning disk, called a protoplanetary disk, around the young Sun. As time went on, the grains and ices in the disk bumped into and stuck to one another. 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.
Is planetary formation a natural end result of star formation, and therefore do most stars have planetary systems? The image on this page of the Orion Nebula was taken with the NASA's Hubble Space Telescope. 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. Will planetary systems evolve from these disks as one evolved in our solar system? What exactly are the processes that result in the birth of planets? What about the flurry of planet detection since 1994? Are the planets discovered so far only a sample of the thousands yet to be discovered? 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.