All joking aside, I would guess that it is not designed to land on a planet. You may recall that Voyager has landed on planets a few times, being an Intrepid class starship, and this was quite a feat.
"One of the few Starfleet starship classes capable of atmospheric entry and planetary landing, the Intrepid-class starship was equipped with antigravity generators as well as impulse and lifters strategically placed at the mass and stress points on the bottom portion of the secondary hull." link
I haven't seen anything about the Defiant being equipped like that. And seeing as how the Defiant was designed primarily as a warship and is minimally equipped in other areas (no science labs, small infirmary, etc), I would imagine planetary landing would not be a priority to the designers.
However, I would bet good ol' O'Brian could find a way to modify it to land on a planet.
How LARGE would the PLANT be? The size of a moon? A planet? What classification of PLANET would it be? In STAR TREK continutiy, the USS Defiant enters the atmosphere of a class J planet successfuly, but obviously cannot land. Would a simple touchdown on some semblance of soil count as a "landing?" Please provide more specifics to your inquiry and resubmit.