The best candidate yet for the elusive Planet Nine has been spotted in two deep infrared surveys taken 23 years apart. If this mystery object really is Planet Nine, it would have a mass greater than Neptune, and currently be about 700 times farther from the sun than Earth is.
The possibility of additional planets in our solar system has been proposed many times, going under names such as "Planet X," because experts thought the concept of an extra planet in our vicinity of the cosmos could explain a perceived regularity in mass extinctions on Earth. Perhaps, they said, the periodic influx of comets that impact Earth are pushed our way by an unseen planet. However, the supposed periodicity in mass extinctions has not held up to scrutiny, and so the need for that particular Planet X has gone away. This brings us to Planet Nine.
