there giant planets too close to the habitable zone, so that an Earth-sized planet would be thrown out into interstellar space by the gravity of the giants?Also, what would be the parameters of an HZ around a binary or trinary star systems? Right now, there is not enough data, nor enough computer modeling done, to answer those kinds of questions.
"Modeling Other Earths." Astrobiology Magazine. 1 February 2007. 22 November 2007. <http://www.astrobio.net/news/article2225.html>.