Data from NASA’s TESS (Transiting Exoplanet Survey Satellite) mission has identified two new “super‑puff” planets—giant worlds whose densities approach that of cotton candy. These Jupiter‑sized planets, designated TOI‑791 b and TOI‑791 c, are the most extremely low‑density planets ever confirmed.
The planets orbit a Sun‑like star, TOI‑791, located roughly 1,113 light‑years from Earth. TESS detected them by measuring periodic dips in the star’s brightness indicative of planetary transits. Subsequent analysis confirmed the presence of two unusually large planets with atypical characteristics.
TOI‑791 b has a radius nearly identical to Jupiter’s but a mass of only 3.0 % of Jupiter’s. In contrast, TOI‑791 c is larger than Jupiter yet contains just 5.9 % of Jupiter’s mass.
“The primary intrigue of these planets lies in the fact that we did not anticipate their existence,” said Jon Jenkins, science lead for the Science Processing Operations Center at NASA’s Ames Research Center in California’s Silicon Valley, which supplied the processed TESS data used in this study. “They present a puzzle regarding the formation mechanisms of giant planets like Jupiter and super‑puffs.”
The newly identified super‑puffs also possess exceptionally long orbital periods, with TOI‑791 b completing a circuit in 139 days and TOI‑791 c in 232 days—orbits rarely observed and requiring extended observation campaigns for detection and confirmation. Over seven years, TESS collected 1,122 days of data on this system, providing a rich dataset for detailed study.
Further modeling revealed that TOI‑791 b and TOI‑791 c are locked in a gravitational resonance, causing them to tug on one another and modulate the timing of their transits. By measuring these timing variations, researchers derived the planets’ masses with high precision, confirming their status as low‑density super‑puffs.
“Only a few super‑puffy planets are known, and it is even rarer to find two within the same system,” noted lead author George Dansfield of the University of Oxford’s Department of Physics in England. “Their extraordinarily low densities make them prime targets for investigating planetary system formation and evolution.”
Future investigations aim to characterize the atmospheric composition of these planets, assess how their rotation influences their shape, and compare the tilt of their host star’s axis to their orbital planes. Such studies could illuminate how TOI‑791 b and TOI‑791 c migrated through their system, whether interactions with other bodies shaped their orbits, and the fundamental processes behind the formation of low‑density super‑puff worlds.
The research, published today in *Monthly Notices of the Royal Astronomical Society*, was led by the University of Oxford in collaboration with Université Côte d’Azur/Observatoire de la Côte d’Azur and the University of Birmingham.
