Scientists have discovered two of the lightest giant planets ever detected, dubbed ‘super-puffs’, orbiting a Sun-like star 1,110 light-years from Earth.
The rare worlds, TOI-791 b and TOI-791 c, are about the size of Jupiter but far less dense than cotton candy, challenging current ideas of how gas giants form.
Meet TOI-791 b and TOI-791 c: The ‘Puffiest’ Planets Yet
Published June 25, 2026 in _Monthly Notices of the Royal Astronomical Society, the study led by the University of Oxford with the University of Birmingham and Université Côte d’Azur confirms the two planets orbit an F7-type dwarf star in the southern constellation Volans.
Density breakdown:
TOI-791 b: 0.038 g/cm³
TOI-791 c: 0.047 g/cm³
For context, cotton candy is ∼0.05 g/cm³ and Jupiter is 1.33 g/cm³ — 28 to 35 times denser.
TOI-791 b is ∼1,400 times Earth’s volume but only 9.5 times heavier. TOI-791 c is ∼2,100 times Earth’s volume but only 18.6 times heavier.
Why ‘Super-Puffs’ Are So Rare
Fewer than 40 super-puff planets are confirmed out of nearly 6,300 known exoplanets, and it’s extremely rare to find two in the same system.
Researchers think they are mostly hydrogen and helium with enormous, puffed-up atmospheres. One theory: they formed far from their star in cold regions where gas could gather rapidly around a core.
The planets are planetary “siblings” formed from the same gas and dust disc, locked in a 5:3 orbital resonance where the inner planet orbits 5 times for every 3 orbits of the outer planet.
How They Were Found
NASA’s TESS space telescope spotted the planets using the transit method. Scientists then used 8 years of ground observations, including from the ASTEP telescope in Antarctica, to measure their masses via gravitational tug effects on transit timing.
“Only a handful of these super-puffy planets are known,” said lead author Dr. George Dransfield, University of Oxford. Prof. Amaury Triaud, University of Birmingham, called the system “a unique laboratory for understanding how super-puff planets form and evolve”.
