Leading astronomers declared last Thursday that Pluto is no longer a planet under new guidelines that dictate that a planet is "a celestial body(天体)that is in orbit around the sun, has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a ... nearly round shape, and has cleared the neighborhood around its orbit." Pluto, disqualified because its orbit overlaps with Neptune's, will be reclassified as a "dwarf planet."