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

Red dwarf stars, such as TRAPPIST-1, are the smallest and coolest stellar bodies populating our Milky Way galaxy. Comprising more than 75% of all stars, they have rapidly become focal points in the search for habitable exoplanets due to the prevalence of Earth-like planets in their orbit. However, the intrinsic characteristics of red dwarfs—primarily their intense magnetic activity and stellar flares—cast doubt on whether planets in their systems can sustain atmospheres dense enough to foster life. The TRAPPIST-1 system, with its seven rocky planets closely packed in tight orbits, serves as a natural laboratory, providing an extraordinary opportunity to investigate these tantalizing questions on planetary evolution and habitability under such extreme conditions.

This year marks the tenth anniversary since the discovery of the TRAPPIST-1 system, a milestone celebrated through a dedicated observational campaign deploying the JWST’s unprecedented infrared capabilities. Specifically, researchers targeted TRAPPIST-1b and TRAPPIST-1c, the two planets closest to the star, which would logically be the most susceptible to erosive stellar effects. These continuous 60-hour observations aimed to detect thermal phase curves—a method monitoring the variation in infrared brightness as planets orbit their star—to determine the presence or absence of atmospheres by assessing surface temperature contrasts between their day and night sides.

The results were striking: both TRAPPIST-1b and TRAPPIST-1c exhibited extreme temperature disparities, with daytime surface temperatures exceeding 200°C on TRAPPIST-1b and nearing 100°C on TRAPPIST-1c, while their nights plunged to temperatures below -200°C. Such a colossal diurnal temperature gradient strongly indicates the absence of a thick atmosphere capable of redistributing heat around the planet, a function seen in planetary bodies within our own solar system, including Earth and Venus. If these planets ever harbored atmospheres, they have evidently been stripped entirely away by relentless stellar radiation and energetic particle bombardment.

Contextualizing these observations requires understanding the dynamic environment red dwarf planets inhabit. Tidally locked due to their close proximities—meaning one hemisphere perpetually faces the star while the other remains in stygian night—these planets depend heavily on atmospheric presence to moderate temperature extremes through atmospheric circulation. Without an atmosphere, the designated dayside is scorched while the nightside freezes in darkness, producing hostile conditions for any prospective biospheres. Moreover, red dwarfs unleash intense ultraviolet radiation and coronal mass ejections that erode planetary atmospheres over time, magnifying the challenge for planetary habitability.

The implications of these findings extend well beyond the peculiarities of TRAPPIST-1b and c. They fundamentally reshape our understanding of atmospheric retention on rocky exoplanets orbiting red dwarfs. Where previously the existence of Earth-sized planets within habitable zones engendered optimism about life’s potential elsewhere, these new results underscore the fragility of atmospheres under harsh stellar influence. The paradigm shifts toward recognizing that only planets orbiting at sufficient distances, potentially shielded by magnetic fields or geological mechanisms, might sustain atmospheres conducive to life.

This ongoing line of inquiry is exemplified by the JWST’s current attention to TRAPPIST-1e, a planet residing comfortably within the star’s habitable zone—the range where temperatures could allow liquid water to exist on the surface. The hope is that unlike its inner siblings, TRAPPIST-1e may have preserved an atmosphere, possibly offering a more clement environment. The parallel drawn to our solar system—with Mercury stripped bare of atmosphere close to the Sun, while Earth and Venus retain theirs—provides a compelling comparative framework in this quest.

Dr. Emeline Bolmont, associate professor at the University of Geneva and a co-author of the study, emphasizes the value of the TRAPPIST-1 system as a premier natural laboratory for comparative planetology. “The diversity of planetary conditions in this system allows us to test and refine our models of planet formation, atmospheric loss, and habitability, particularly in environments so disparate from our own,” she notes. Her enthusiasm reflects the broader scientific community’s anticipation as further JWST observations hope to unlock the mysteries of other TRAPPIST worlds.

Prof. Brice-Oliver Demory from the University of Bern, also a co-author, highlights the instrumental role JWST has played, stating, “Detecting the presence or absence of an atmosphere on tidally locked planets around red dwarf stars is a critical first step for understanding their climate dynamics and potential habitability. The TRAPPIST-1 system’s proximity and richness make it an extraordinary case study.” Their meticulous measurements of thermal phase curves not only illuminate the current state of these planets but also contribute invaluable data for simulations predicting their atmospheric evolution under extreme stellar conditions.

Technically, these observations represent a major advancement in exoplanet atmospheric science. The JWST’s Near-Infrared Camera (NIRCam) captured continuous light curves over full planetary orbits with exquisite precision, enabling scientists to discern subtle changes attributable to surface temperatures. This approach marks a leap forward from previous methods reliant on transit spectroscopy, which often struggled to separate planetary signals from host-star noise, especially for small terrestrial planets. The success of this thermal phase curve technique heralds a new era in characterizing exoplanet climates directly.

This study, while decisive for the inner TRAPPIST-1 planets, raises further intriguing questions about atmospheric variability across the system. The outer planets, subject to weaker stellar fluxes and possibly better shielded, may retain atmospheres, or even tenuous envelopes of volatile substances, sustaining more hospitable climates. Continuous observations and improved modeling efforts aim to constrain these possibilities, with forthcoming JWST campaigns expected to provide richer datasets enabling unprecedented interplanetary comparisons.

In conclusion, the comprehensive climate mapping of TRAPPIST-1b and TRAPPIST-1c fundamentally establishes that dense atmospheres are unlikely on these worlds, reshaping how scientists assess habitability in red dwarf systems. The broader implication is clear: habitability around such stars is complex, contingent not merely on location within a habitable zone but also on a fragile equilibrium between stellar activity and planetary atmospheric retention. As we probe deeper into this nearby planetary system, each discovery refines our search for life beyond Earth, underscoring the vital role cutting-edge observatories like JWST play in unraveling the universe’s greatest mysteries.

Subject of Research: Not applicable

Article Title: No thick atmosphere around TRAPPIST-1 b and c from JWST thermal phase curves

News Publication Date: 3-Apr-2026

Web References: DOI: 10.1038/s41550-026-02806-9

Keywords

Exoplanets, TRAPPIST-1, James Webb Space Telescope, Red dwarf stars, Atmospheric stripping, Thermal phase curves, Planetary habitability, Tidal locking, Rocky planets, Climate mapping, Planetary atmospheres