A novel image segmentation technique for extracting limbs and terminators of planetary bodies is proposed. Conventional edge-based histogramming approaches are employed to trace object boundaries. The limb and terminator bifurcation is achieved by locating the harmonized segment in the two equations representing the 2-D parametrized boundary curve. This technique is quite efficient and robust. Although only spherical (near-spherical) planetary bodies in full view are addressed in detail, the proposed methodology can be extended to accommodate partially viewed and/or arbitrary-shaped objects; formulations for these cases are also given. Real planetary images from Voyager 1 and 2 serve as representative test cases to verify the proposed methodology.