Scientists have discovered two remarkable planets that defy conventional expectations about planetary density. Despite being approximately the size of Jupiter, these newly identified worlds are extraordinarily diffuse—lighter even than cotton candy.
The planets, designated TOI-791b and TOI-791c, orbit a dwarf star located approximately 1,110 light years from Earth in the southern constellation of Volans. They rank among the lowest-density giant planets ever detected by astronomers, according to research published in the journal Monthly Notices of the Royal Astronomical Society.
The study was led by scientists from the University of Oxford, working in collaboration with researchers from the University of Birmingham and the Université Côte d'Azur in France.
The density measurements reveal the planets' extraordinary nature. Jupiter possesses a density of 1.33 grams per cubic centimeter, while TOI-791b measures just 0.038 grams per cubic centimeter and TOI-791c registers 0.047 grams per cubic centimeter. These figures make the planets 28 to 35 times lighter than Jupiter. For comparison, cotton candy typically has a density of approximately 0.05 grams per cubic centimeter, while Earth's density is 5.5 grams per cubic centimeter.
The two planets are believed to be siblings that formed together from the same disc of gas and dust surrounding their young host star. They are 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 exert force on one another, producing measurable shifts in the timing of their transits across the host star.
The rarity of such discoveries cannot be overstated. Very few of these super-puff planets have been identified, and finding two within the same planetary system is exceptionally uncommon. Their extremely low densities present valuable opportunities for understanding planetary system formation and evolution.
The initial identification came from an unexpected source: citizen-science volunteers participating in the Planet Hunters TESS project, which analyzes NASA data to search for possible new worlds. Professional astronomers then confirmed the discovery and measured the planets' properties using telescopes positioned around the globe.
A telescope in Antarctica played a critical role in the research. The ASTEP facility—Antarctic Search for Transiting ExoPlanets—took advantage of the Antarctic winter's months of continuous darkness to capture the planets' exceptionally long transits in single, uninterrupted observations. Each transit lasted more than 11 hours, representing the longest continuous planetary transits ever observed in their entirety from the ground.
When a planet passes in front of its host star during a transit, it slightly dims the star's light. The amount of dimming reveals the planet's size. The research team detected subtle variations in the timing of these transits, caused by the two planets gravitationally tugging on one another as they orbit. By analyzing these timing shifts, scientists were able to estimate the planets' masses and calculate their remarkably low densities.
"Bringing together observations from Antarctica, space telescopes, and observatories across several continents was essential to revealing the true nature of these extraordinary planets," said co-author Professor Tristan Guillot, from the Université Côte d'Azur.
The formation mechanisms of super-puff planets remain a subject of ongoing scientific debate. The research team plans to conduct additional investigations to better understand how these unusual worlds came into existence and to evaluate competing theoretical explanations.
Future observations will utilize the James Webb Space Telescope to analyze the planets' atmospheres for carbon-, nitrogen-, and oxygen-bearing species. These measurements could provide crucial insights into the formation processes of these enigmatic celestial bodies, according to study co-author Professor Amaury Triaud from the University of Birmingham.
The discovery demonstrates the value of collaborative research efforts that combine professional astronomical facilities with citizen-science initiatives, enabling groundbreaking discoveries that expand our understanding of planetary diversity throughout the galaxy.
