An international collaboration has discovered two of the lowest-density giant planets ever detected: rare ‘super-puff’ planets with densities lower than candy floss. The study -led by the University of Oxford, in collaboration with Université Côte d’Azur/Observatoire de la Côte d’Azur and the University of Birmingham - has been published today (25 June) in Monthly Notices of the Royal Astronomical Society .
The two planets, named TOI-791 b and TOI-791 c, orbit an F7-type dwarf star located around 1,110 light years from Earth in the southern constellation Volans. Although both planets are roughly the size of Jupiter, they are extraordinarily diffuse: TOI-791 b has a density of just 0.038 grams per cubic centimetre, while TOI-791 c has a density of 0.047 grams per cubic centimetre. By comparison, Jupiter’s average density is 1.33 grams per cubic centimetre , around 28 to 35 times greater.
Their densities are even lower than candy floss, which typically has a density of about 0.05 grams per cubic centimetre. In contrast, Earth’s density is 5.5 grams per cubic centimetre .
The planets are “siblings”, believed to have formed together from the same disc of gas and dust surrounding their young star. They are also locked in a rare gravitational relationship known as a 5:3 mean-motion resonance, meaning that for every five orbits completed by the inner planet, the outer planet completes almost exactly three. This gravitational interaction causes the planets to repeatedly tug on one another, producing measurable shifts in the timing of their transits across the host star.
Lead author Dr George Dransfield (she/her) (Department of Physics, University of Oxford and a presenter for BBC Sky at Night) said: “Only a handful of these super-puffy planets are known, and it is even rarer to find two in the same system. Their extremely low densities make them fascinating targets for understanding how planetary systems form and evolve.”
TOI-791 b and TOI-791 c were first identified as candidate planets in 2019 and 2023 respectively, by volunteers participating in the Planet Hunters TESS citizen-science project. This searches data from NASA’s Transiting Exoplanet Survey Satellite (TESS) for possible new worlds. The researchers then measured the planets’ densities by combining observations of their sizes and masses using telescopes around the world.
When a planet passes in front of its host star - an event known as a ‘transit’ - it slightly dims the star’s light. The amount of dimming reveals the planet’s size. In this system, the researchers also detected subtle variations in the timing of the transits, caused by the two planets gravitationally tugging on one another as they orbit the star. By analysing these timing shifts, the team was able to estimate the planets’ masses and calculate their remarkably low densities.
The discovery relied on eight years of observations, including from the ASTEP (Antarctic Search for Transiting ExoPlanets) telescope at Concordia Station in Antarctica , jointly operated by researchers from Université Côte d’Azur/Observatoire de la Côte d’Azur and international collaborators. The Antarctic winter provided a unique advantage: months of continuous darkness enabled astronomers to capture the planets’ exceptionally long transits, each lasting more than 11 hours, in a single uninterrupted observation. These are the longest continuous planetary transits ever observed in their entirety from the ground.
Astronomers are still debating how super-puff planets form. One leading theory suggests that they possess enormous hydrogen- and helium-rich atmospheres that make up a significant fraction of their total mass. These giant gaseous envelopes may have accumulated when the planets formed far from their host star in cold regions of the protoplanetary disc, where gas could cool and gather rapidly around a solid core.
The researchers intend to carry out follow-up investigations to understand more about how these planets formed, and to rule out some of the leading super-puff explanations.
Professor Amaury Triaud (University of Birmingham), the UK Principal Investigator of ASTEP and co-author of the study, said: “This system offers a unique laboratory for understanding how super-puff planets form and evolve. We propose to carry out space-based observations using the James Webb Space Telescope to assess if the puffy atmosphere contains carbon-, nitrogen-, and oxygen-bearing species, revealing new insight into how these unusual planets formed.”
Professor Tristan Guillot (Université Côte d’Azur), Principal Investigator of ASTEP and co-author of the study, added: “These multi-planetary systems are complex, with gravitational interactions between the planets that evolve over very long periods, tens of years or more. 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.”
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For media enquiries and interview requests, contact Caroline Wood: caroline.wood@admin.ox.ac.uk
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The study ‘ASTEP confirmation of a pair of long-period Jupiter-sized planets with extremely low densities transiting TOI-791’ will be published in Monthly Notices of the Royal Astronomical Society on 02:01 BST Thursday 25 June / 21:01 ET Wednesday 24 June 2026 at https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stag864 . The link will become active after the embargo lifts. To view a copy of the study before this under embargo, contact Caroline Wood: caroline.wood@admin.ox.ac.uk
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Monthly Notices of the Royal Astronomical Society
ASTEP confirmation of a pair of long-period Jupiter-sized planets with extremely low densities transiting TOI-791