A planet orbiting the Sun so that it was always on the other side of the Sun from Earth—with the same orbiting velocity and path as Earth—could, in theory, have such an orbit because it was the same distance from the Sun. Thus, what would make it undetectable from Earth would also make it possible to have a range of surface temperatures similar to Earth; it is also possible that it might be habitable for beings very similar to humans. (A similar gravity and atmospheric pressure to Earth's would require a similar mass/size as Earth.)