NASA’s Transiting Exoplanet Survey Satellite (TESS) has identified two of the lowest-density giant planets ever detected, orbiting a star more than 1,100 light-years from Earth.
The planets, roughly the size of Jupiter but with densities lower than those of cotton candy or shaving foam, represent an extreme example of the rare “super-puff” worlds, per NASA.
An international team led by researchers at the University of Oxford announced the findings on June 24-25, 2026. The planets, designated TOI-791 b and TOI-791 c, orbit an F7-type dwarf star in the southern constellation Volans. Their discovery adds to the small catalog of fewer than 40 confirmed super-puff planets among more than 6,000 known exoplanets.
The planets’ densities measure approximately 0.038 grams per cubic centimeter for TOI-791 b and 0.047 grams per cubic centimeter for TOI-791 c. For comparison, Jupiter’s density is about 1.33 grams per cubic centimeter—up to 35 times greater. Cotton candy has a density around 0.05-0.1 grams per cubic centimeter, making these worlds among the puffiest and lightest giant planets known.
“These are the puffiest planets we’ve ever found,” George Dransfield of the University of Oxford, who led the research, stated in reports on the discovery. The team published its peer-reviewed findings in the Monthly Notices of the Royal Astronomical Society.
NASA’s TESS (Transiting Exoplanet Survey Satellite) detected the planets through repeated dips in the host star’s brightness as the planets transited, or passed in front of it, from Earth’s perspective. Ground-based follow-up observations, including those from the ASTEP telescope in Antarctica, confirmed the signals and helped characterize the system.
The star TOI-791 lies approximately 1,110 light-years away. Both planets are long-period giants, with TOI-791 b completing an orbit in about 20-30 days and TOI-791 c taking longer, placing them in a regime that challenges standard models of planet formation and evolution. Their extremely low densities suggest thick, extended atmospheres of hydrogen and helium, possibly with minimal rocky cores or mechanisms that prevent atmospheric collapse.
Super-puff planets remain poorly understood. Astronomers speculate they may form in environments with high gas content but limited solid material, or that they retain puffy atmospheres due to specific migration histories or internal heating. The rarity of finding two such planets in the same system offers a valuable laboratory for testing these ideas.
“This discovery highlights the importance of continued international collaboration in astronomy. Bringing together observations from Antarctica, space telescopes and observatories across several continents was essential to revealing the true nature of these extraordinary planets,” stated Professor Tristan Guillot, Université Côte d’Azur / Observatoire de la Côte d’Azur, Principal Investigator of the ASTEP telescope and co-author of the study.
Future observations with the James Webb Space Telescope (JWST) are planned to probe the planets’ atmospheres in detail. JWST’s infrared capabilities could reveal the composition of their extended envelopes, including potential molecules and cloud structures, providing clues to how such low-density worlds persist.
The mission continues to uncover diverse planetary systems, expanding our understanding of how planets form beyond our solar system.
While gas giants like Jupiter and Saturn formed in our solar system with denser structures, super-puffs illustrate the diversity possible under different stellar and disk conditions. Researchers emphasize that these planets likely formed farther out in their system before any inward migration.
Data from the study were collected through collaboration among the University of Oxford, Université Côte d’Azur/Observatoire de la Côte d’Azur, and the University of Birmingham. No immediate public data releases beyond the paper were detailed, but the open-access nature of much TESS data allows broader community follow-up.
For now, TOI-791 b and c stand as record holders for size among the puffiest known exoplanets, offering a fresh puzzle for models of atmospheric retention and planetary interiors.
