jacob benjamin gyllenhaal (born december 19, 1980) is an academy award-nominated american actor. he has appeared in diverse roles since his first lead role in 1999's "october sky", followed by the 2001 cult hit "donnie darko", in which he played a psychologically troubled teen and onscreen brother to his real-life sister, actress maggie gyllenhaal. in the 2004 blockbuster "the day after tomorrow" he portrayed a student caught in a cataclysmic global cooling event, alongside dennis quaid as his father. he then played against type as a frustrated marine in "jarhead" (2005). the same year, he won critical acclaim portraying a role that entered popular shorthand as a "gay cowboy" in the controversial but highly acclaimed film "brokeback mountain".