Alien Skies: Visibility of Planetary Retrograde Motion from Rocky Habitable-Zone Worlds

Abstract

We investigate whether observers on rocky habitable-zone (HZ) planets in known multi-planet systems could recognise the apparent retrograde motion of neighbouring planets. Using the NASA Exoplanet Archive data, we compute apparent trajectories and reflected-light brightness for ordered observer-target pairs, retaining usable reported phase information and available sky inclinations. Our sample comprises 40 optimistic and 19 conservative rocky-HZ pairs; 36 and 15, respectively, have evaluable orbital inputs. Every evaluated HZ configuration exhibits geometric retrograde motion. We classify events into three nested visibility classes (easy naked-eye, strict naked-eye, and telescopic) using brightness, angular excursion, and host-star separation. For the reference orbital model with a geometric albedo of Ag = 0.30, the mean fractions of configurations with a qualifying telescopic event are 90.4 and 80.0 per cent for the optimistic and conservative samples, respectively. The corresponding fractions for the strict naked-eye and easy naked-eye criteria are 75.0 and 19.0 per cent in the optimistic sample and 73.5 and 17.0 per cent in the conservative sample. These values describe sampled orbital configurations under the adopted model, rather than fractions of observing time. All six outward-looking optimistic pairs satisfy the easy naked-eye criteria in every reference realisation. Host-star separation restricts interior-target visibility, whereas HZ class fractions remain unchanged across the tested albedo grid. Observer-target geometry therefore strongly affects retrograde visibility in this sample. The classes provide comparative bolometric criteria; predictions for individual pairs remain limited by uncertain orbital configurations and omitted effects, including gravitational interactions and observing conditions.