Astronomers have found two of the lightest worlds ever discovered, a pair of giant planets so wispy that, gram for gram, they are less dense than candy floss. Each is roughly the size of Jupiter yet holds almost nothing inside and the two circle the same distant star as siblings, locked in a gravitational dance that sees them tug one another off schedule as they orbit. It was observations from the depths of the Antarctic winter, that let astronomers weigh them and uncover just how astonishingly insubstantial they are. Now they want to know just how a planet ends up barely heavier than air at all.