A robust well trajectory planning and drilling or completion planning system that integrates well trajectory optimization and well development planning optimization so that optimized solutions are generated simultaneously. The optimization model can consider unknown parameters having uncertainties directly within the optimization model. The model can systematically address uncertain data and well trajectory, for example, comprehensively or even taking all uncertain data into account. Accordingly, the optimization model can provide flexible optimization solutions that remain feasible over an uncertainty space. Once the well trajectory and drilling or completion plan are optimized, final development plans may be generated. Additionally, the optimization model may generate and implement modified well development planning and modified well trajectory in real-time.