A celestial body not part of our solar system, but on an elliptical orbit around the sun of our solar system, passing between the earth and the sun. This would cause the earth to become dark for a period of 30-70 hours.
When Planet X passes between earth and the sun the earth will be dark for a longer period than a solar eclipse, making this the first interstellar eclipse in our lifetime
by Abercrombie March 8, 2016