WASHINGTON, July 3 -- For decades, astronomers have known how stars die. They swell into red giants, shed their outer layers, and collapse into white dwarfs: dense stellar remnants no larger than Earth itself. What nobody could say with certainty was what happened to the planets left behind.

A new study from the James Webb Space Telescope has produced the first direct answer. WD 1856 b, a Jupiter-sized planet orbiting a dead star 80 light-years from Earth, not only survived its star's death but has retained an atmosphere. Webb detected a mixture of methane and small cloud particles as the planet crossed in front of the stellar remnant in a transit lasting just a few hours every 34 days.

The findings, published in Nature on July 1, repre...