Planet Nine Could Be Big Yet Hidden in a Deep Cosmic Dimness

Planet Nine, if it exists, could have several Earth masses and be over twice Earth’s diameter, but at hundreds of AU from the Sun it would reflect so little light that its brightness would be extraordinarily faint and often vanish into the background. The light that does reach Earth must travel outward and back, so the overall reflected flux drops roughly with the inverse fourth power of distance, making even a relatively large planet extremely hard to spot with current surveys. Ongoing searches, including the Rubin Observatory’s LSST, aim to detect it or tighten where it could lie; however, non-detection so far does not rule out a distant, faint world.