Though the number of planets in our solar system has fluctuated in the last 11 years, it never exceeded single digits—until now. Two planetary scientists at the University of Arizona in Tucson believe they’ve found a 10th planet (assuming the unconfirmed ninth planet discovered last year exists), orbiting beyond Neptune, New Scientist reports. The Mars-sized object is thought to exist in the Kuiper belt region at a distance that spans 30 and 55 AU (the distance between our Earth and sun). Still, many experts doubt that a planet of that size at that distance (the proposed ninth planet is 700 AU) would have gone unseen for so long.
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