Terra incognita refer to the hypothesized continent Terra Australis Incognita ("The unknown land of the South"), as seen in the Theatrum Orbis Terrarum map by Abraham Ortelius (1570). During the 19th century "terra incognita" disappeared from maps of the earth, since both the coastlines and the inner parts of the continents had been fully explored. But the bottoms of the seas remain mostly un-mapped, as do many other land surfaces in the solar system. The phrase is now also used metaphorically by various researchers to describe any unexplored subject or field of research. For more history of the terms, see Terra Australis,
Planet Nine the disk had a distant inner edge, 100–200 AU, a planet encountering Neptune would have a 20% chance of being captured in an orbit similar to that proposed for Planet Nine. The observed clustering is more likely if the inner edge is at 200 AU. Unlike the gas nebula the planetesimal disk is likely to be long lived, potentially allowing a later capture. Planet Nine could have been captured from beyond the Solar System during a close encounter between the Sun and another star in its birth cluster. Three-body interactions during these encounters can perturb the path of planets on