Two Jupiter-sized worlds, lighter than candy, dancing in gravitational resonance
Roughly 1,110 light-years away, two Jupiter-sized planets have been confirmed that are so extraordinarily diffuse they rival cotton candy in density — a configuration so rare that finding two such worlds in the same system challenges prevailing assumptions about how planets are born. TOI-791 b and TOI-791 c orbit their star in a precise gravitational resonance, suggesting a shared origin and offering astronomers an uncommon window into the stranger possibilities of planetary formation. Their discovery, stitched together over eight years by citizen scientists and professional researchers alike, is a quiet reminder that the universe routinely exceeds the boundaries of earthly intuition.
- Two planets lighter than spun sugar yet larger than Jupiter have been confirmed, densities so extreme they belong to a class of worlds with fewer than a handful of known examples.
- Finding both in the same star system is exceptional — only four other systems are known to host even one super-puff planet, making TOI-791 a statistical anomaly that demands explanation.
- Their locked 5:3 orbital resonance implies they formed together from the same primordial disk, and their gravitational tugging on each other leaves measurable fingerprints in transit timing data.
- Confirming them required Antarctica's unbroken winter darkness, where the ASTEP telescope captured transits exceeding 11 hours — the longest continuous planetary transits ever fully observed from the ground.
- The leading theory — bloated hydrogen-helium atmospheres accumulated in the cold outer disk and retained through inward migration — awaits a decisive test from the James Webb Space Telescope.
Somewhere in the southern sky, about 1,110 light-years away, two planets orbit a star in a way that defies intuition. TOI-791 b and TOI-791 c are each roughly Jupiter-sized, yet so insubstantial they would float through cotton candy. Their densities — 0.038 and 0.047 grams per cubic centimeter respectively — make them 28 to 35 times less dense than Jupiter, and lighter even than that carnival confection. Earth, by comparison, weighs in at 5.5. Size and density, it turns out, do not always move together.
What deepens the discovery is the orbital choreography. The two planets are locked in a 5:3 mean-motion resonance: for every five orbits the inner planet completes, the outer one finishes almost exactly three. This is no coincidence — it points to a shared origin in the same disk of gas and dust that surrounded their young star. As they orbit, their mutual gravity creates measurable shifts in transit timing, the moments when each planet dims its star by passing in front of it. Only four other systems are known to host multiple super-puff planets, making this system an extraordinarily rare laboratory.
The path to confirmation was long and collaborative. Citizen scientists with the Planet Hunters TESS project first flagged TOI-791 b in 2019 and its companion in 2023, sifting through data from NASA's TESS satellite. Professional astronomers then spent eight years building the observational case, including critical data from the ASTEP telescope at Concordia Station in Antarctica. That remote outpost proved essential: its months of uninterrupted winter darkness allowed researchers to track transits lasting more than 11 hours without interruption — the longest continuous planetary transits ever fully observed from the ground.
How such impossibly light worlds come to exist remains partially mysterious. The leading hypothesis holds that they accumulated vast envelopes of hydrogen and helium in the cold outer regions of their protoplanetary disk, then migrated inward while retaining those bloated atmospheres. Researchers have already proposed James Webb Space Telescope observations to test this idea, searching the planets' atmospheres for carbon-, nitrogen-, and oxygen-bearing molecules that would reveal how these strange worlds assembled themselves. What this discovery ultimately offers is a reminder that the universe contains configurations of matter that Earth-bound intuition struggles to accommodate — and that it is still willing to teach us something fundamental about how planets form.
Somewhere in the southern sky, about 1,110 light-years away, two planets are orbiting a star in a way that defies intuition. They are enormous—each one roughly the size of Jupiter—yet so insubstantial that they would float through cotton candy like a stone through air. TOI-791 b and TOI-791 c represent a discovery so rare that only a handful of such worlds have ever been confirmed, and finding two in the same system is exceptional enough to reshape how astronomers think about planetary birth.
The numbers alone convey the strangeness. TOI-791 b has a density of 0.038 grams per cubic centimeter; its sibling, TOI-791 c, measures 0.047. Jupiter, by comparison, packs 1.33 grams into the same space—making it roughly 28 to 35 times denser than these newcomers. Cotton candy, that carnival confection, sits at about 0.05 grams per cubic centimeter. Earth, solid and familiar, weighs in at 5.5. These two distant worlds are lighter than spun sugar, yet they contain enough mass to dwarf our own planet many times over. The contradiction is the point: size and density, it turns out, do not always move together.
What makes the discovery even more compelling is the orbital choreography. The two planets are locked in what astronomers call a 5:3 mean-motion resonance. For every five times the inner planet circles its star, the outer one completes almost exactly three orbits. This is no accident. Scientists believe both worlds formed from the same disk of gas and dust that surrounded their young star billions of years ago, making them planetary siblings born from the same nursery. As they orbit, their gravity reaches across the void and tugs at each other, creating measurable shifts in the timing of their transits—the moments when they pass in front of their star and dim its light. Only four other planetary systems are known to harbor multiple super-puff planets, making TOI-791 an extraordinarily rare laboratory for understanding how such anomalies come into being.
The discovery itself is a product of patient, distributed effort. Citizen scientists working with the Planet Hunters TESS project—volunteers sifting through data from NASA's Transiting Exoplanet Survey Satellite—first spotted TOI-791 b in 2019 and its companion in 2023. What they flagged as candidates, professional astronomers then confirmed through eight years of accumulated observations, including crucial data from the ASTEP telescope stationed at Concordia Station in Antarctica. That remote location proved essential. Antarctica's months of uninterrupted winter darkness allowed researchers to track the planets' unusually long transits—each lasting more than 11 hours—without the interruption of sunrise. These are the longest continuous planetary transits ever fully observed from the ground, a record made possible only by the continent's extreme geography.
The mechanism that produces such impossibly light worlds remains partially mysterious, though leading theories point toward massive atmospheres. Scientists propose that these planets accumulated enormous envelopes of hydrogen and helium when they formed in the cold outer regions of the protoplanetary disk, where gas could accumulate rapidly around a solid core. As the system matured and the planets migrated inward, they retained these bloated atmospheres, creating worlds that are mostly air. Testing this hypothesis will require the next generation of observation. Researchers have already proposed using the James Webb Space Telescope to analyze the atmospheric composition of TOI-791 b and c, searching for carbon-, nitrogen-, and oxygen-bearing molecules that would reveal clues about how these strange planets assembled themselves.
What emerges from this discovery is a reminder that the universe contains configurations of matter that Earth-bound intuition struggles to accommodate. Two Jupiter-sized worlds, lighter than candy, dancing in gravitational resonance around a distant star—this is not the solar system we inhabit, but it is real, and it is waiting to teach us something fundamental about how planets form.
Citas Notables
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.— Dr. George Dransfield, University of Oxford
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.— Professor Amaury Triaud, University of Birmingham
La Conversación del Hearth Otra perspectiva de la historia
Why does it matter that we found two of these super-puff planets together rather than just one?
Because when you find two rare things in the same place, you stop thinking of them as flukes. It suggests there's a mechanism that produces them. One super-puff planet could be an accident. Two in the same system, locked in gravitational resonance, tells you something about how planetary systems actually work.
The cotton candy comparison—is that just for effect, or does it actually help us understand something?
It's both. Density is abstract. But when you say a Jupiter-sized world is lighter than spun sugar, suddenly the absurdity becomes concrete. It forces you to confront that size and mass are not the same thing. These planets are enormous but almost entirely atmosphere.
How did citizen scientists find these planets if they're so far away?
They didn't observe them directly. They looked at data from a space telescope—TESS—that measures tiny dips in starlight when planets pass in front of their stars. Millions of those dips exist in the data. Humans are still better at spotting patterns that algorithms might miss, so volunteers scanned the observations and flagged candidates. Professionals then spent years confirming what the volunteers found.
Antarctica seems like an odd place to study distant planets.
It's actually ideal for this work. You get months of continuous darkness, which means you can watch a single transit for 11 hours straight without the sun interrupting. That long, unbroken observation is what allowed researchers to measure the planets' masses precisely. You can't get that anywhere else on Earth.
What's the 5:3 resonance really telling us?
It's a gravitational fingerprint. When two planets form together and migrate through a disk, they can get locked into these orbital ratios. The fact that TOI-791 b and c are in 5:3 resonance suggests they've been together since birth, that they formed from the same material and have been gravitationally bound ever since.
So what's the next step?
James Webb will look at the atmosphere itself—what molecules are in it. That will tell us whether these planets formed close to their star or far away, and whether they migrated. The composition is the story of their origin.