A groundbreaking discovery in the field of exoplanetary science has revealed two of the lowest-density giant planets ever observed, orbiting a Sun-like star known as TOI-791. These planets, intriguingly dubbed “super-puffs,” possess densities far below that of any gas giant previously known, challenging current theories of planet formation and atmospheric composition. This exceptional finding was the result of an international collaborative effort led by researchers at the University of Oxford, working closely with teams from Université Côte d’Azur/Observatoire de la Côte d’Azur and the University of Birmingham. The full study detailing the discovery was published recently in the Monthly Notices of the Royal Astronomical Society.
TOI-791 is classified as an F7-type dwarf star situated approximately 1,110 light years from Earth, nestled in the southern constellation of Volans. The two exoplanets, TOI-791 b and TOI-791 c, both rival Jupiter in size but possess a mass only a tiny fraction of Jupiter’s, leading to extraordinarily sparse densities. TOI-791 b exhibits an anomalously low density of 0.038 grams per cubic centimeter, whereas TOI-791 c’s density is measured at 0.047 grams per cubic centimeter. For context, Jupiter’s mean density is 1.33 grams per cubic centimeter, a stark contrast that underscores the extreme puffiness of these newly discovered worlds.
Remarkably, the densities of the TOI-791 planets are even lower than that of candy floss, a substance often used as a metaphor for low-density matter, which averages about 0.05 grams per cubic centimeter. This comparison paints a vivid picture of just how diffuse these celestial bodies truly are. Earth, by way of further contrast, has a density of approximately 5.5 grams per cubic centimeter, highlighting the tenuous and fragile state of these super-puffs compared to terrestrial planets in our own solar system.
The discovery adds a rare example to the very limited list of multiple super-puff planets found in a single planetary system. The planets are believed to have formed together in a circumstellar disc of gas and dust, emerging from the protoplanetary material surrounding their host star. The orbital dynamics of TOI-791 b and c are equally compelling, as they are locked in a 5:3 mean-motion resonance. This resonance means that the inner planet completes five orbits in almost the exact time it takes the outer planet to complete three, creating periodic gravitational tugs that affect their transit timings.
The gravitational interactions causing shifts in the timing of transits were indispensable to determining the planetary masses. When these planets transit their star — a phenomenon detected when the star’s brightness dims slightly as the planet passes in front of it — minor variations in the timing occur due to their mutual gravitational influence. By meticulously analyzing these transit timing variations (TTVs), astronomers could estimate their masses and, in combination with their radii obtained from transit depths, calculate the planets’ remarkably low densities.
This breakthrough would not have been possible without data that spanned nearly a decade, incorporating observations from instruments around the world. Notably, the ASTEP (Antarctic Search for Transiting ExoPlanets) telescope, located at Concordia Station in Antarctica, played a vital role. The Antarctic winter, characterized by months of continuous darkness, enabled uninterrupted monitoring of these planets’ unusually long transit events, with durations extending over 11 hours. These are among the longest continuous planetary transits ever observed from the ground, highlighting the unique and invaluable contribution of polar-based observatories in exoplanet research.
Despite the precise measurements of size and mass, the formation mechanism of super-puff planets remains an active area of debate among astronomers. One dominant hypothesis suggests that these planetary giants have accumulated massive, hydrogen- and helium-dominated atmospheres, which constitute a substantial proportion of their overall mass. Such atmospheres might have formed in the cold outer regions of the planet-forming disc, where temperatures were sufficiently low to allow rapid gas accretion onto a solid core.
The unusual properties of TOI-791 b and c thus present a valuable natural laboratory to test models of planetary evolution, atmospheric physics, and system dynamics. The density and resonance features observed challenge existing frameworks and call for updated simulations that incorporate complex gas interactions and orbital mechanics. Investigating the atmospheric composition through future missions may yield key insights into chemical abundances and thermal structures.
Looking forward, the research team plans follow-up studies to further unravel the mysteries of these rare cosmic phenomena. In particular, they intend to employ the James Webb Space Telescope (JWST) for space-based observations, which will facilitate sensitive spectroscopic analysis of the super-puff atmospheres. Detecting molecules containing elements such as carbon, nitrogen, and oxygen could provide a definitive understanding of how these atmospheres originated and evolved, potentially identifying pathways distinct from typical gas giant formation.
The discovery epitomizes the pivotal role of international scientific cooperation and multi-modal observational strategies. Bringing together data from ground-based polar telescopes, space observatories like TESS (Transiting Exoplanet Survey Satellite), and global observatories spanning continents proved essential in characterizing these elusive planets. This cooperative model underscores how modern astronomy relies on an interconnected global network to probe the universe’s most challenging and intriguing phenomena.
As Dr. George Dransfield from the University of Oxford eloquently stated, the rarity of finding two super-puff planets in the same system is extraordinary and provides a unique opportunity to deepen our understanding of planetary formation theories. Their findings not only illuminate anomalies in exoplanet densities but also push the boundaries of detecting and understanding diverse planetary systems beyond our own solar neighborhood, potentially influencing the search for habitable worlds and the general comprehension of planetary system architectures.
In conclusion, TOI-791 b and TOI-791 c represent a striking paradigm shift in exoplanet science, demonstrating the astounding diversity of planetary characteristics in our galaxy. Their incredibly low densities challenge preexisting models and open new avenues for research. The ongoing and future study of these super-puff worlds will undoubtedly enrich the broader narrative of planetary astrophysics and contribute valuable knowledge that could redefine how we perceive the formation and evolution of planets on a cosmic scale.
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Subject of Research: Discovery and characterization of two super-puff exoplanets, TOI-791 b and TOI-791 c, with extremely low densities orbiting the Sun-like star TOI-791.
Article Title: ASTEP confirmation of a pair of long-period Jupiter-sized planets with extremely low densities transiting TOI-791
News Publication Date: 25 June 2026
Web References:
DOI: 10.1093/mnras/stag864
Image Credits: NASA/Daniel Rutter
Keywords
Space sciences, Astronomy, Celestial bodies, Observational astronomy, Cosmology, Planetary science, Space research, Exoplanetary science, Exoplanets, Planets, Telescopes
