Astronomers are closing in on what looks like a rare prize: a world almost the size of Earth, circling a Sun-like star at a distance that gives it a familiar, year-long orbit. The candidate, known as ...

Astronomers believe they have uncovered evidence of a previously unknown ninth planet lurking in the not-too-distant reaches of the solar system. This hypothetical world, dubbed "Planet Y," could be ...

Ah, Earth. We may not think about our planet on a day-to-day basis, but the world we live in is a pretty interesting place. The third rock from the Sun—and to this day, the only one with known ...