Astronomers find giant cotton candy-like exoplanets in distant star system

TOI-791 b and TOI-791 c orbit a Sun-like star about 1,113 light-years from Earth in the constellation Volans. The two giant exoplanets are so diffuse they are being measured in the same breath as cotton candy.

TOI-791 b is nearly the size of Jupiter but has only 3.0% of Jupiter’s mass. TOI-791 c is even larger than Jupiter but contains just 5.9% of Jupiter’s mass. The University of Birmingham measured their densities at 0.038 and 0.047 grams per cubic centimeter, about 28 to 35 times less dense than Jupiter and even lower than candy floss. George Dransfield, an astrophysicist at the University of Oxford, described them as a “nice blob of shaving foam.”

The findings were published June 25, 2026 in Monthly Notices of the Royal Astronomical Society. Worlds this large and light strain core-accretion models, which have to explain how a planet can gather so much gas without collapsing into a denser structure or losing its atmosphere. Dransfield’s Oxford profile puts the exoplanet count above 5,800 planets around other stars.

NASA’s TESS mission first flagged the pair by watching the star dim as the planets crossed in front of it. Later analysis used transit timing variations, the gravitational tug one planet exerts on the other, to estimate their masses more precisely. TOI-791 b circles its star every 139.29931 days, while TOI-791 c takes 232.01570 days. Their orbits are linked closely enough that they sit within 0.07% of a second-order 5:3 period commensurability.

The system’s timing data were strong enough to track transit timing variations of up to 50 minutes. The transits lasted more than 11 hours, and Antarctic Search for Transiting ExoPlanets observations from Concordia Station in Antarctica captured full transits from the ground. Those were the longest-duration transits ever observed in their entirety from the ground. The University of Birmingham’s eight-year observing campaign found one of only four other known planetary systems with multiple super-puff planets, and TOI-791 b and TOI-791 c formed from the same disk of gas and dust around their young star.