In-Universe:
The auto pilot unexpectedly takes the U.S.S. Defiant through the energy barrier and safely away from the planet.
Production:
I believe this is a CGI model for these reasons: The opening scene shows the Defiant get pretty small as it get's closer to the planet. I'm not sure if they could pull the camera that far away on a motion controlled dolly. Also, at the very end of the show when the Defiant leaves the 8 circular plasma vents are pink/purple, not blue/white.
