For decades, our solar system was thought to have nine planets, with Pluto considered the smallest and farthest. But in 2006, Pluto was officially reclassified as a dwarf planet by the International ...

A recent research paper suggests that a planet may exist far beyond Neptune — less than 20 years after the previous ninth planet, Pluto, was demoted. That research paper, accepted last month for ...

Space has a knack for bending our sense of scale. The Moon feels close enough to touch—after all, we see its craters with nothing but our eyes. Yet, that 238,855-mile gap between Earth and its lunar ...