A study in Monthly Notices has confirmed a rare pair of giant planets orbiting the star TOI-791 and both worlds appear so swollen and light that astronomers classify them among the strangest known exoplanets. The work reports two long-period, Jupiter-sized planets with extremely low densities, a combination that places the system in the unusual family of super-puff planets.
Two such planets in one system make the finding especially valuable. Super-puff worlds are already scarce in the known exoplanet catalog. Finding a pair around the same star gives researchers a cleaner way to compare planets that likely grew in a shared environment.
The study was led by Georgina Dransfield and involved researchers connected with institutions including the University of Oxford and the University of Birmingham. The planets were confirmed using ASTEP observations, according to the journal paper’s title. Their unusual properties now make TOI-791 a strong target for future atmospheric studies.
A rare pair of super-puff planets
Super-puff planets are giant in outline and featherweight in density. They can have radii similar to gas giants while carrying far less mass than expected for worlds of that size. That combination means their atmospheres take up an enormous fraction of their apparent volume.
Dransfield described the rarity of the discovery in unusually direct terms. “Only a handful of these super-puffy planets are known and it is even rarer to find two in the same system,” she said. The comment captures why TOI-791 has drawn attention so quickly.
For astronomers, density is a clue to a planet’s interior and atmosphere. A dense planet may have a large rocky or icy core. A very low-density giant suggests a planet wrapped in an extended gas envelope. In the case of TOI-791, the study points to Jupiter-sized planets that are far more diffuse than familiar gas giants.
That airy structure is why the “cotton candy” comparison is so common in coverage of super-puff worlds. It gives readers a useful image, though the real objects are immense planets with atmospheres shaped by gravity, radiation, chemistry and time.
Why the TOI-791 system stands out
The surprise begins with the number two. A single super-puff planet can already stretch current models of planetary evolution. A pair in the same star system raises a sharper question. What happened around TOI-791 that allowed two giant worlds to remain so inflated?
The study title identifies the planets as long-period worlds transiting TOI-791. Transiting planets pass in front of their star from our viewpoint. During a transit, the planet blocks a tiny fraction of starlight. That dip lets astronomers estimate the planet’s size.
Mass comes from a different set of measurements. When both size and mass are known, researchers can calculate density. That simple relationship is powerful. It turns faint changes in starlight into physical clues about a planet’s structure.
In this system, the inferred densities are extremely low. Dransfield said, “Their extremely low densities make them fascinating targets for understanding how planetary systems form and evolve.” That makes TOI-791 more than a curiosity. It becomes a natural experiment in how planets grow and change.
The shared host star matters too. Two planets orbiting the same star have experienced related conditions, even if their individual histories differ. Comparing them may help scientists separate system-wide causes from planet-specific effects.
Planets with Jupiter size and extreme low density
Jupiter offers the familiar benchmark for gas giants. It is huge, massive and dense enough to compress its deep atmosphere under enormous pressure. The TOI-791 planets appear to occupy a more delicate part of the planetary family tree.
A planet’s low density can come from several broad ingredients. It may have a relatively small core. It may hold a large hydrogen-rich atmosphere. It may also receive or retain heat in a way that keeps its outer layers expanded. Each possibility changes the story of how the planet formed.
Extreme low density also affects what telescopes can see. When a planet has an extended atmosphere, starlight passing through that gas during transit can carry chemical fingerprints. Those fingerprints may reveal molecules and atoms in the planet’s outer layers.
The long-period nature of the TOI-791 planets adds more interest. Many inflated exoplanets orbit close to their stars, where intense radiation can heat their atmospheres. Longer-period super-puff planets give scientists a chance to test how inflation works under different orbital conditions.
That distinction matters because planetary swelling can have more than one cause. Some planets may be puffed up by heat from their stars. Others may retain internal heat, form with unusual compositions, or evolve through interactions with neighboring planets. TOI-791 gives researchers a place to test those ideas with two related worlds.
A clue to strange planet formation
Planet formation starts in a disk of gas and dust around a young star. Solid grains collide and grow. Larger bodies gather material. Some planets become big enough to pull in thick envelopes of gas before the disk fades.
Super-puff planets complicate that picture because their present-day sizes look surprisingly large for their masses. A planet with a weak gravitational grip can struggle to hold a huge atmosphere over long periods. Yet the TOI-791 planets appear to have retained expansive envelopes.
Several explanations may be explored by researchers. The planets could have formed with unusually light atmospheres. Their gases may include molecules that make the atmosphere appear larger during transit. Clouds or hazes could also affect the measured radius by blocking starlight high in the atmosphere.
TOI-791 is especially useful because the two planets can be studied together. If both worlds show similar atmospheric chemistry, that could point toward a shared formation pathway. If they differ strongly, their separate orbital histories may have shaped them after birth.
The system also has value for testing planetary migration. Many giant planets may form farther from their stars and later move inward. A pair of long-period giant planets with low densities may preserve clues about where they formed and how they reached their present orbits.
What Webb could reveal next
The next step is atmosphere hunting. A telescope such as Webb can watch a transit and split the star’s light into colors. When some wavelengths are absorbed more strongly than others, researchers can infer which gases may be present.
Amaury Triaud of the University of Birmingham framed the system as a special opportunity. “This system offers a unique laboratory for understanding how super-puff planets form and evolve,” he said. That laboratory is distant, but its transits can bring the planets within reach of modern instruments.
Triaud also pointed to the kind of follow-up that could sharpen the picture. “We propose to carry out space-based observations using the James Webb Space Telescope,” he said. Such observations could help test whether the puffy atmospheres contain carbon-, nitrogen- and oxygen-bearing species.
Those chemical clues would matter because they trace where and how a planet assembled. Different regions of a young planetary disk contain different mixtures of ice, rock and gas. If Webb detects key molecules in the planets’ atmospheres, researchers could connect today’s swollen worlds to the early disk that built them.
The TOI-791 system now joins a small group of exoplanet systems that challenge simple expectations. Its two cotton candy giants show how much variety exists beyond our solar system. With future observations, astronomers may learn how planets can grow so large, stay so light and survive as some of the most fragile-looking giants yet found.
