Two planets orbiting TOI 791, 1,113 light-years away, measure 0.993 and 1.155 Jupiter radii but only 9.5 and 18.6 Earth masses, making them the lightest Jupiter-sized worlds yet found.
Their densities—0.038 and 0.047 grams per cubic centimeter, around cotton-candy levels—put them among the puffiest known planets, in the rare class called superpuffs.
TESS first detected the planets by transit, and astronomers then used years of ASTEP observations in Antarctica to track the worlds’ mutual gravitational tugs and calculate their masses.
Researchers say giant ring systems are unlikely to explain both planets, and a fast-spinning host star may indicate the system is young enough that the planets are still cooling and could later contract.
James Webb Space Telescope observations are the next step, with astronomers hoping atmospheric composition data will clarify how superpuffs form and evolve.