In December 1993, the orbiting Space Telescope aimed its optics at curious disks of dust and debris surrounding distant stars. In these mixes of material, the primeval ingredients of planets -- protoplanets -- are believed to exist.
Why do astronomers look to find protoplanetary disks instead of actual planets?
Distant planets are impossible to view. They are astronomically small. Protoplanetary disks, however, are large clouds of debris usually 7.5 times the diameter of our own solar system. While planets emit no light, in many cases nearby stars shower the protoplanetary disks with radiation causing the clouds of dust to glow.
Photo. A protoplanetary disk near the Orion Nebula -- with a large, green cloud surrounding it. Courtesy C.R. O'Dell/Rice University, NASA.
Unable to observe actual planets, protoplanets offer astronomers the only opportunity to study the probabilities that other solar systems might exist. With increased knowledge of other solar systems, better theories on the possibilities of extraterrestrial life can be formulated. Protoplanets also enable astronomers to observe how our own solar system might have formed millions of years ago.
Before the Space Telescope, scientists had speculated about the origin of our solar system. They generally accepted the idea that the solar system had accreted from a cloud of hydrogen and helium. This cloud, named the solar nebula, also contained ice and dust particles. While this loose material was gravitationally drawn together, the cloud began orbiting around a dense center. As the center of the nebula gradually pulled in more material, it became a protosun.
Circling the embryotic star, the debris in the cloud eventually accreted into protoplanets. The cloud of dust that once orbited the protosun became a protoplanetary disk, a proplyd. From this point, the protoplanets would continue to accrete material until only a handful of planets remained. The protosun, however, would undergo a much more dramatic change. Temperatures and pressures at the center of the nebula increased, and at a certain point became high enough to ignite a thermonuclear reaction -- the birth of a star.
Photo. Proplyds in the Orion Nebula; notice debris surrounding these five young stars that remained after formation. Courtesy C.R. O'Dell/Rice University, NASA.