Astronomers have confirmed two Jupiter-sized planets that are simultaneously the largest and the least dense exoplanets ever found, with densities lower than cotton candy. The discovery, led by the University of Oxford in collaboration with Université Côte d’Azur/Observatoire de la Côte d’Azur and the University of Birmingham, was published on June 25, 2026 in Monthly Notices of the Royal Astronomical Society. The planets were detected by NASA’s Transiting Exoplanet Survey Satellite, TESS.
What makes these planets extraordinary
The two planets, designated TOI-791 b and TOI-791 c, orbit an F7-type dwarf star located approximately 1,113 light-years from Earth in the southern constellation Volans, the flying fish. Both are roughly the size of Jupiter, but that is where the comparison ends. TOI-791 b contains just 3.0 percent of Jupiter’s mass; TOI-791 c, slightly larger than Jupiter in size, contains just 5.9 percent. The result is a density that strains credulity: 0.038 and 0.047 grams per cubic centimetre respectively. Cotton candy typically has a density of about 0.05 grams per cubic centimetre. Jupiter’s is 1.33 — roughly 28 to 35 times greater than either of these worlds.
«These two planets have densities comparable to a nice blob of shaving foam, fresh from the can,» said lead author Dr. George Dransfield of Oxford University’s Department of Physics.
How they were found — and why it took years
TOI-791 b was first flagged as a candidate in 2019 and TOI-791 c in 2023, both by volunteers participating in Planet Hunters TESS, a citizen-science project that searches TESS data for candidate worlds. The planets’ unusually long orbital periods — 139 days for the inner planet and 232 days for the outer — meant that TESS needed to observe the system over multiple years to capture and confirm each transit. In total, the satellite accumulated 1,122 days of data on this system across seven years.
The team confirmed the planets’ masses through a gravitational effect: the two worlds are locked in a 5:3 mean-motion resonance, meaning that for every five orbits completed by the inner planet, the outer planet completes almost exactly three. That rhythmic gravitational interplay produces measurable shifts in the timing of their transits, and those shifts allowed the team to calculate the masses precisely.
What they tell us about planet formation
TOI-791 b and TOI-791 c are considered siblings — formed together from the same protoplanetary disc of gas and dust around their young star, then migrated outward to their current positions. Only four other planetary systems are known to contain multiple super-puff planets, making this system an exceptionally rare laboratory. Fewer than 40 super-puffs are known among roughly 6,300 confirmed exoplanets catalogued to date.
«The main reason these planets are interesting to study is that we didn’t expect to see them at all,» said Jon Jenkins, science lead for the Science Processing Operations Center at NASA’s Ames Research Center. «They represent a puzzle for us to solve about how giant planets like Jupiter and the super-puffs form.»
The planets are probably composed primarily of hydrogen and helium, though their atmospheric chemistry remains to be confirmed. Observations with the James Webb Space Telescope are expected to provide that detail and deepen our understanding of how worlds this extreme come to exist.
© 2026 SKYCR.ORG | Homer Dávila Gutiérrez, FRAS. All rights reserved. Reproduction in whole or in part is prohibited without express authorization. Original source: Dransfield, G. et al. ASTEP confirmation of a pair of long-period Jupiter-sized planets with extremely low densities transiting TOI-791. Monthly Notices of the Royal Astronomical Society (2026). DOI: 10.1093/mnras/stag864
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