It looks like "Defying Gravity" is indeed getting canceled. During its September 13 episode, ABC announced it as the "Season Finale" and did not give any previews to the next episode which is traditionally done to other continuous series.
The series about eight astronauts in a space mission was not exactly a hit maker. It premiered to only 3.83 million viewers on August 2 and the number has constantly deteriorated since then. Last week, ABC has excluded listing "Gravity" on the schedule after the September 13 episode but did not give explanation to that.
The series was supposed to have at least two more episodes that would run through October but the Alphabet apparently has another option for its Sunday 10/9c slot. "Brothers & Sisters" meanwhile, will begin the fourth season on September 27.
ABC is expected to release the official statement about the cancellation soon. Meanwhile, it is unclear whether CTV which is also airing the show, will continue with the remaining episodes.
Update 9/14: A publicist of the show has confirmed that "Defying Gravity" has not been canceled but ABC still can't determine when to air the remaining episodes.