Protoplanetary disks are common.

Protoplanetary disks are common

Evidence for planetary systems around stars other than thesun continues to accumulate. Most recently, Bruce Campbell and co-workers report evidence that several stars may be accompanied by planet-sized objects (SN: 6/27/87, p.405), and Stephen E. Strom of the University of Massachusetts at Amherst reports evidence that may neatly complement that finding: Protoplanetary disks, the material out of which planets may form, seem to be fairly common accompaniments of young stars.

Strom worked with Susan Edwards of the University ofMassachusetts and Immo Appenzeller of the University of Heidelberg, West Germany, to determine that 20 young stars in the Taurus molecular cloud have such disks. From this they deduced that about 15 percent of young stars should be accompanied by protoplanetary disks at their formation.

Present telescopes cannot resolve the disks. The observingtechnique used spectroscopy of the stars' outflowing stellar winds to detect the presence of disks of dust. Stars, including our sun, give off gas--stellar wind--that flows away from them in all directions. As terrestrial observers view this wind, they should see equal amounts coming toward them and going away from them--unless a disk of preplanetary dust is present. The disk cuts off some of the wind from the observers' sight, and they see an unbalanced distribution of velocities. Of the 20 stars surveyed, "every single object gave evidence only for approaching gas,' Strom says.

There is a surprise here for the accepted theory of planetformation. From the ages of these stars it appears that some of these disks last as long as 3 million years. Theory, Strom says, would have predicted that planets would finish forming in a tenth of that time.