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 require a relatively close encounter with Planet Nine, however, which becomes less likely at large distances from the Sun. Planet Nine does not have an official name and will not receive one unless its existence is confirmed, typically through optical imaging. Once confirmed, the International Astronomical Union will certify a name, with priority usually given to a name proposed by its discoverers. It is likely to be a name chosen from Roman or Greek mythology. In their original article, Batygin and Brown simply referred to the object as "perturber", and only in later press releases did they use "Planet Nine".