Mercury (planet) at a steep angle. Mercury is 10° above the horizon when the planet appears directly above the Sun (i.e. its orbit appears vertical) and is at maximum elongation from the Sun (28°) and also when the Sun is 18° below the horizon, so the sky is just completely dark. This angle is the maximum altitude at which Mercury is visible in a completely dark sky. At middle latitudes, Mercury is more often and easily visible from the Southern Hemisphere than from the Northern. This is because Mercury's maximum western elongation occurs only during early autumn in the Southern Hemisphere, whereas
Planet Nine X, Planet Next, or Giant Planet Five." Planet Nine Planet Nine is a hypothetical planet in the outer region of the Solar System. Its gravity could explain the unlikely clustering of orbits for a group of extreme trans-Neptunian objects (eTNOs) that orbit beyond Neptune with average distances greater than 250 AU. These eTNOs tend to have their points of closest approach to the Sun clustered in one direction, and many of their orbits have a similar tilt. This evidence suggests that the gravity from a large undiscovered object is influencing the orbits of the most distant known Solar System objects.