Astronomers have confirmed two rare “super-puff” exoplanets orbiting the star TOI-791, about 1,110 light-years from Earth in the constellation Volans. The planets, designated TOI-791 b and TOI-791 c, are each roughly Jupiter-sized but far lighter for their volume, with measured densities of 0.038 and 0.047 grams per cubic centimeter, respectively. The findings were published in Monthly Notices of the Royal Astronomical Society, and researchers described the pair as among the least dense giant planets yet identified.
The two planets stand out not only for their unusually diffuse structure but also for their orbital relationship. They are linked in a 5:3 mean-motion resonance, meaning the inner planet completes about five orbits for every three by the outer one. Astronomers said the planets’ gravitational pull on each other produces small variations in the timing of their transits across the host star, helping researchers estimate their masses. Scientists believe the pair likely formed from the same disk of gas and dust around the young star, making them planetary siblings.
The discovery drew on eight years of observations from multiple instruments, including NASA’s Transiting Exoplanet Survey Satellite and the ASTEP telescope at Concordia Station in Antarctica. Volunteers with the Planet Hunters TESS citizen-science project first flagged TOI-791 b in 2019 and TOI-791 c in 2023 as possible planets. Researchers then combined transit measurements with timing variations caused by the planets’ mutual gravitational interactions to determine their size and mass. Antarctica’s long winter darkness was especially useful because each transit lasted more than 11 hours, allowing uninterrupted ground-based observations that researchers said are the longest continuous planetary transits ever fully observed from Earth.
Researchers say the system could help address a broader unanswered question: how super-puff planets form. One leading explanation is that such worlds developed large hydrogen- and helium-rich atmospheres after forming farther from their star in colder regions of a protoplanetary disk, where gas could accumulate rapidly around a solid core. Scientists emphasized that this remains a working theory rather than a settled conclusion, and they said future observations could test competing ideas. Some team members have proposed using the James Webb Space Telescope to examine whether the planets’ atmospheres contain carbon-, nitrogen- and oxygen-bearing species, which they argue could clarify how these unusual worlds originated.
The system is also considered rare in a broader exoplanet context. Researchers said only four other known planetary systems contain multiple super-puff planets, making TOI-791 an uncommon opportunity to study this class of world in detail. Supporters of the study describe it as a valuable laboratory for understanding planetary evolution and a demonstration of the role of international and citizen-science collaboration, while the main scientific caution in the findings is that the origin of super-puff planets remains unresolved and will require further observation.