These exoplanets are Jupiter-sized but as light as cotton candy

Introduction: gas giants that are almost weightless

TOI-791 b and c, two off-the-charts "super-puff" worlds

Did you know that somewhere in our galaxy there are exoplanets as large as Jupiter whose mass amounts to only a tiny fraction of that gas giant's own? That's exactly what NASA's TESS mission revealed when it identified two planets nicknamed "super-puffs": TOI-791 b and TOI-791 c. Their radius rivals Jupiter's, but their density is so low that astronomers, half-jokingly, compare them to cosmic cotton candy.

That cotton candy image is far from a throwaway line: it captures a measurable physical reality. A planet as voluminous as Jupiter but this light implies an extraordinarily low density, a parameter that has intrigued astrophysicists ever since the first exoplanets of this kind were discovered several years ago.

This story takes you on a journey through these balloon worlds, from how they were spotted to the questions still open about their origin, along the way highlighting the key role played by a telescope installed in one of the most extreme environments on our planet.

The star these two planets orbit, cataloged as TOI-791, has in just a few months become a reference point in specialized publications, since the system it hosts illustrates, almost like a caricature, the unexpected diversity of planetary worlds discovered in recent years.

Why the term "super-puff" is used in astronomy

The term "super-puff" designates a specific category of exoplanets characterized by a large radius but relatively low mass, giving them an extremely low density, sometimes lower than that of many common materials on Earth. This category remains rare among the thousands of exoplanets cataloged to date, which makes every new discovery of this kind especially valuable to researchers hoping to build a broader statistical picture of how common such extreme worlds really are.

In the case of TOI-791 b and TOI-791 c, this low density immediately caught the attention of scientific teams, because it raises a fundamental question: how can a planet reach a size comparable to Jupiter's without accumulating a proportional amount of matter?

This statistical rarity explains why the publication of these results generated particular interest within the community of researchers specializing in characterizing exoplanetary atmospheres, a field expanding rapidly thanks to new generations of space telescopes.

How the TESS mission tracked down these lightweight worlds

The principle of detection via the transit method

The TESS mission, short for Transiting Exoplanet Survey Satellite, is a NASA space telescope specifically designed to spot exoplanets using the so-called transit method. This principle involves measuring the slight dip in a star's brightness when a planet passes in front of it from our vantage point, a tiny but measurable phenomenon with sufficiently sensitive instruments.

By continuously monitoring thousands of stars, TESS was able to detect the characteristic signatures of TOI-791 b and TOI-791 c, two planets orbiting the same star. The regularity of these brightness dips allowed researchers to deduce the size of each planet, an essential first step before estimating their respective masses through further analysis.

This method, while indirect, has established itself as one of the most effective in modern astronomy for cataloging a large number of exoplanets in a relatively short time, compared to far more resource-intensive direct observation methods that would simply be impossible to scale to thousands of targets.

The TESS satellite was designed specifically to systematically sweep large portions of the sky, sector by sector, allowing it to continuously accumulate data on tens of thousands of different stars, considerably increasing the chances of detecting rare planetary configurations like the one at TOI-791.

Confirmation from the ASTEP telescope in Antarctica

Spotting a transit isn't always enough to definitively confirm a planet's existence and precisely characterize its properties. In the case of TOI-791 b and c, complementary observations were carried out using the ASTEP telescope, installed in one of the most extreme places on the planet: the Antarctic continent.

This choice of location isn't incidental. Antarctica offers exceptional observing conditions, with particularly long, stable nights during the austral winter, allowing continuous monitoring of certain stars over extended periods without interruptions from the usual day-night cycle, something few other places on Earth can offer. There's something poetic about the idea that a telescope planted in the middle of the polar ice is helping unravel the mysteries of distant worlds made of almost impalpable gas.

These complementary observations helped refine the mass and radius estimates of the two planets, solidifying their classification among the most striking super-puff exoplanets identified to date, and giving researchers a far more confident basis for future comparisons.

The collaboration between an orbiting satellite like TESS and a ground-based instrument installed in conditions as extreme as those in Antarctica illustrates well the complementarity needed between different observational approaches to confirm discoveries this delicate to characterize.

The mystery of their incredible lightness

A density that defies common intuition

To truly grasp the extent of this lightness, it helps to compare TOI-791 b and c to more familiar planets. Jupiter, already considered a relatively low-density gas giant compared to rocky planets like Earth, itself looks like a compact object next to these newcomers. The density difference is so extreme that the cotton candy comparison isn't just a fun image: it reflects a physical reality verifiable through calculation, once the numbers for radius and mass are plugged into a simple formula.

This characteristic raises a central question for astrophysicists: what mechanism could allow a planet to accumulate such a voluminous atmosphere without also accumulating a large mass of solid or compressed gaseous matter at its core?

Several avenues are currently being explored to answer this question, each relying on distinct physical processes that could, alone or combined, explain this highly unusual configuration, and none of them can yet be ruled out with confidence. This openly acknowledged uncertainty, rather than a ready-made explanation, is exactly what makes this story so alive to tell.

Three competing hypotheses to explain the phenomenon

The first hypothesis put forward by researchers is thermal inflation: the intense heat emitted by the host star could expand the planet's atmosphere, much like a gas expanding when heated, increasing its apparent volume without significantly changing its total mass.

The second hypothesis points to a past orbital migration: these planets could have formed farther from their star, in a colder environment, before migrating to a closer orbit, a shift that could have influenced their internal structure and current atmospheric composition.

Finally, a third hypothesis suggests that these planets are actively losing their atmosphere, gradually eroded by the intense radiation of their star, a process that could explain both their currently large radius and their low residual mass. What fascinates me about this kind of unresolved mystery is that none of the three explanations has been ruled out yet: the truth could very well lie at the crossroads of all three mechanisms.

What these planets teach us about cosmic diversity

Worlds that expand our catalog of references

The discovery of TOI-791 b and c considerably enriches the already impressive catalog of planet types identified beyond our Solar System. While our immediate neighborhood contains only rocky planets and relatively dense gas giants, the universe appears to be teeming with far more exotic configurations, of which these ultra-lightweight giants are just one example among many.

This diversity pushes scientists to regularly revise their theoretical models of planetary formation, initially built on the sole observation of our own system, which in reality represents only a tiny fraction of the possible configurations observed elsewhere in the galaxy. What once looked like a reasonably complete picture now looks more like a rough first draft, one that keeps getting rewritten as new instruments come online.

This finding also encourages space agencies to invest in ever more sensitive instruments, capable of detecting planetary configurations even more subtle than those already spotted around the star TOI-791, in hopes of more precisely mapping this still poorly understood planetary diversity.

Every newly confirmed super-puff exoplanet helps statistically refine how frequent this phenomenon really is, valuable data for understanding whether it's a rare curiosity or, instead, quite simply a common stage in the evolution of certain planetary systems across the galaxy. What I like about this patient statistical approach is that it turns a single isolated curiosity into genuine general knowledge about how planets form across the galaxy.

An invitation to keep exploring these extreme worlds

Faced with these still-open questions, several research teams are already planning new observation campaigns specifically targeting TOI-791 b and c, hoping to better characterize their exact atmospheric composition and settle the debate between the various hypotheses currently under study. Each additional data point should help narrow down which of the three competing scenarios best fits what is actually observed.

This quest for answers illustrates well the dynamic at the heart of modern astronomy: every discovery almost invariably opens up new questions, fueling a cycle of curiosity and research that keeps sharpening our understanding of the diversity of worlds populating our galaxy.

The coming years should therefore bring more precise answers about the exact nature of TOI-791 b and c, particularly thanks to instruments capable of directly analyzing the chemical composition of their stretched-out atmospheres.

Why this "cotton candy" planet story is so fascinating

A memorable image for a complex phenomenon

One of the great merits of the cotton candy comparison is that it makes an otherwise fairly abstract scientific concept accessible: extremely low planetary density. This image lets the general public intuitively grasp a physical reality that would otherwise require more complex technical explanations about the relationship between mass and volume.

This ability to make complex phenomena relatable through vivid imagery contributes greatly to renewed public interest in space news, making advanced astrophysics concepts understandable without sacrificing their scientific accuracy, and giving newcomers an easy entry point into a subject that can otherwise feel intimidating.

A symbol of our galaxy's still-unsuspected richness

Beyond the amusing anecdote, TOI-791 b and c symbolize well the unsuspected richness of our galaxy, where worlds as diverse as arid rocky planets, dense gas giants, and exoplanets as light as a cloud of spun sugar all coexist. This diversity continues to fuel the curiosity of researchers and the general public alike.

These discoveries remind us, once again, that modern space exploration still holds countless surprises, and that every new mission like TESS helps paint an ever more detailed and fascinating portrait of our cosmic environment. Much like a child marvels at cotton candy that grows huge without ever feeling heavy in the hand, astronomers keep marveling at these planets that seem to defy the usual laws of matter, all while remaining perfectly consistent with the fundamental principles of physics once properly understood. In the end, the strangest part of the story may simply be how ordinary the extraordinary is starting to feel in this new golden age of exoplanet discovery.

By Maxime Marquette, columnist

Sources

Primary sources

NASA Science — TESS mission — 2026

NASA Exoplanet Archive — Exoplanet news archives — 2026

NASA — TESS Transiting Exoplanet Survey Satellite — 2026

Secondary sources

The Automated Daily — Discovery of the most cotton-candy-like exoplanets — June 2026

Futura Sciences — Science section — 2026

Wikipedia — Category: Exoplanet discovered in 2026