Both Panasonic FX48 and Nikon D5000 have 12.0 MP resolution sensors but Panasonic FX48's sensor is 1/2.3-inch (6.08 x 4.56 mm ) and Nikon D5000's sensor is APS-C (23.6 x 15.8 mm ).
Since Nikon D5000's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Panasonic FX48.
Below you can see the FX48 and D5000 sensor size comparison.
As seen above, Nikon D5000 has a 13.5x Larger sensor area than Panasonic FX48. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.