New Hubble Image Cuts the '10th Planet' Down to Size

摘要

Los altos Hills, California—Confounding previous estimates, the so-called 10th planet is Pluto's near-twin in size, according to a new image from the Hubble Space Telescope. The object is just a "smidge" bigger than Pluto, not 25% to 50% bigger, an astronomer reported here last week, and unusually reflective. The downsizing illustrates the quandary facing scientists as they try to define whether large residents of the frigid Kuiper belt are bona fide planets.