The planetary boundary layer (PBL) mediates the exchange of heat, momentum, and mass between a planetary surface and its overlying atmosphere, where turbulent processes govern vertical energy transport. Given the stark disparities in atmospheric properties across Venus, Earth, Mars, and Jupiter, it remains an open question whether the conventional Reynolds number (Re) remains a valid metric for cross-planetary comparisons of boundary layer turbulence. Using published mission datasets, atmospheric profiles, and peer-reviewed literature, we compile key dynamical parameters—atmospheric density, dynamic viscosity, characteristic wind speed, and characteristic length scale—to derive representative Re values for these four planetary bodies. These estimates are then benchmarked against observed and numerically simulated turbulence regimes. The calculated Re values follow the descending order: Jupiter (10 10 ) > Venus (10 9 ) > Earth (10 8 ) > Mars (10 7 ). While values for Jupiter and Earth align well with documented turbulent behaviors, Venus exhibits a high Re alongside weak surface winds and subdued turbulence. Conversely, Mars yields the lowest Re despite hosting vigorous convective boundary layer turbulence. These divergences indicate that Re does not scale linearly with actual turbulence intensity across distinct planetary regimes. Consequently, while Re provides a useful quantitative baseline for cross-planetary scaling, it cannot serve as a standalone criterion for assessing boundary-layer turbulence strength. This work does not attempt to derive a universal scaling law. Instead, using publicly available data, it systematically delineates the conditions under which a single Reynolds-number criterion holds and where it breaks down across planetary scales.