"Stars move around all the time, so if one wanders into the danger zone and stays for too long, it will probably never be able to form planets," said Zoltan Balog of the University of Arizona, Tucson, lead author of the new report, appearing May 20 in the Astrophysical Journal ("Astronomers," Par. 3).So, for a planet around a star in a nebula, small planets may be boiled away before they could finish forming. On planets that did form despite being in the nebula (not all areas of the nebula would be detrimental to planetary formation), life may have a bit of a struggle to hold on - and not only to hold on, but to be able to develop into sentient beings. If their star wanders into a "hot zone" in the nebula (too close to a super hot O type star), it will be blasted by dangerous solar winds and radiation that could, if it stayed too long, strip a planet of its atmosphere, or worst.
Some scientists think our sun was born in such an environment, but was able to keep its planets and move out into calmer regions of space.
"Astronomers Map Out Planetary Danger Zone." Spitzer - NASA. 18 March 2006. 22 November 2007. <http://www.nasa.gov/mission_pages/spitzer/news/spitzer-20070418.html>.