WASP-193b is a gas giant roughly the size of Jupiter but with barely a tenth of its mass, giving it a density so low that researchers have compared it to cotton candy. At about 0.06 grams per cubic centimeter, the planet is less dense than any other known Jupiter-sized world and lighter than Styrofoam, which makes its very existence difficult to explain with standard models of how giant planets form and evolve. Discovered by the Wide Angle Search for Planets survey and confirmed through ground-based follow-up observations, WASP-193b has become a test case for competing theories about what can make a planet look impossibly fluffy.
A Planet That Should Not Be This Light
The numbers behind WASP-193b are what make it so striking. Its radius is about 1.3 times that of Jupiter, so it takes up a similar amount of space in the sky during a transit. But its mass is only around 0.11 Jupiter masses, meaning it weighs roughly a ninth of what Jupiter does while being physically larger. That combination yields a bulk density of about 0.06 grams per cubic centimeter, with some measurement uncertainty around that figure.1The Astronomical Journal. The Super-puff WASP-193 b is on a Well-aligned Orbit For perspective, water has a density of one gram per cubic centimeter, and Saturn, the least dense planet in our solar system, sits around 0.69. WASP-193b is about eleven times less dense than Saturn.
The planet orbits a late F-type star, which is somewhat hotter and more massive than our Sun. Its orbit is tightly aligned with its host star’s equator, with a measured misalignment of only about 16 degrees.2The Astronomical Journal. The Super-puff WASP-193 b is on a Well-aligned Orbit That alignment tells astronomers something about the planet’s history. When a giant planet’s orbit is wildly tilted relative to its star’s spin, it often points to a violent past involving gravitational scattering by another planet. A well-aligned orbit, by contrast, suggests a calmer migration inward through the protoplanetary disk or at least a history that did not involve being flung around.
Interior Models and the Reinflation Idea
When researchers tried to model what WASP-193b might actually be made of, they found that its bulk properties can be reproduced if the planet consists of roughly equal parts metals and hydrogen-helium gas, with a metal fraction of about 0.42.2The Astronomical Journal. The Super-puff WASP-193 b is on a Well-aligned Orbit That is a surprisingly high metal content for a planet this puffy. In standard planet-formation theory, a metal-rich planet tends to be denser, not less dense, because heavier elements pack more mass into a given volume. Something has to be counteracting that tendency, and the leading explanation involves the star itself.
WASP-193b’s host star appears to be evolving off the main sequence, meaning it is aging and its luminosity is increasing. As a star brightens, it dumps more energy onto any nearby planet. For a gas giant on a tight orbit, that extra energy can heat the deep atmosphere and cause the planet to physically swell. This process, sometimes called reinflation, is thought to be actively underway at WASP-193b. The planet is also likely losing mass as its atmosphere escapes into space, driven by the intense irradiation from the nearby star.2The Astronomical Journal. The Super-puff WASP-193 b is on a Well-aligned Orbit
Reinflation is a well-accepted mechanism for explaining “hot Jupiters” that are somewhat larger than expected, and it works well for planets that are, say, 10 to 20 percent puffier than standard models predict. But WASP-193b is not just a little inflated. It is so dramatically underdense that reinflation alone strains credulity for some researchers, which is why alternative and supplementary explanations have attracted serious attention.
Could High-Altitude Hazes Make a Planet Look Bigger Than It Is?
One possibility is that WASP-193b is not quite as physically large as its transit measurements suggest, because something in its atmosphere is tricking our instruments. When astronomers measure a planet’s size, they do it by watching it pass in front of its star and noting how much starlight it blocks. That method measures the apparent size of whatever is opaque to starlight, which is usually the planet’s atmosphere at a certain depth. If there is a layer of opaque material high above the actual “surface” of the atmosphere, the planet looks bigger than it really is.
Photochemical hazes can do exactly that. These are tiny particles formed when ultraviolet light from the host star breaks apart molecules in a planet’s upper atmosphere, and the fragments recombine into complex, often sooty compounds. Research has shown that when these hazes form at very high altitudes, at pressures lower than about a hundred-thousandth of Earth’s sea-level pressure, they can block enough starlight to inflate a planet’s apparent transit radius dramatically.3The Astrophysical Journal. Grain Growth in Escaping Atmospheres: Implications for the Radius Inflation of Super-Puffs In some modeled scenarios, the radius enhancement reaches close to 200 percent for planets with moderate atmospheric outflows, large atmospheric scale heights, and haze particles lofted to very low pressures without clumping together too quickly.4The Astrophysical Journal. Deflating Super-puffs: Impact of Photochemical Hazes on the Observed Mass–Radius Relationship of Low-mass Planets
A related idea involves dusty outflows rather than photochemical hazes. In this scenario, the planet’s atmosphere is not static but actively escaping, and it carries extremely fine dust grains, just a nanometer or so in size, up to high altitudes. Those tiny particles, even in small quantities, can form an opaque shell around the planet and flatten its transmission spectrum, making it look both bigger and featureless to spectroscopic observations.5The Astrophysical Journal Letters. Dusty Outflows in Planetary Atmospheres: Understanding “Super-puffs” and Transmission Spectra of Sub-Neptunes The distinction between hazes and dusty outflows is partly about origin: hazes form in place from photochemistry, while dust can be carried upward from deeper layers. Both produce similar observational signatures, which makes them hard to distinguish without high-quality spectral data.
These haze and dust explanations were originally developed for smaller super-puff planets, mostly sub-Neptunes in the Kepler catalog. Whether they scale up to explain a planet the size of WASP-193b is an open question. The underlying physics works better for planets with large atmospheric scale heights relative to their solid cores, and WASP-193b’s extreme puffiness means its scale height is enormous. That could make it either a perfect candidate for the haze explanation or an object that has blown past what hazes alone can account for.
The Ring Hypothesis
A more exotic explanation asks whether some super-puffs might not be puffy at all but instead surrounded by rings. If a planet has a broad, opaque ring system oriented roughly face-on toward Earth, the rings would block additional starlight during a transit. An observer measuring the transit depth would then overestimate the planet’s radius, because the light curve would look the same as a much larger, ringless planet. The inferred density would plummet as a result, since the calculated volume would be too large for the actual mass.6The Astronomical Journal. Exploring Whether Super-puffs can be Explained as Ringed Exoplanets
Rings would also neatly explain another puzzle about super-puffs: their transmission spectra tend to be flat and featureless. If you are looking at the spectrum of a giant atmosphere, you expect to see absorption features from water vapor, methane, or other molecules. But if you are actually looking at starlight filtered through a ring of solid particles, those molecular signatures would be absent. Flat spectra are exactly what has been observed for several known super-puffs, and rings predict that naturally.
The ring hypothesis has not been easy to confirm or rule out. For another super-puff, Kepler-51d, James Webb Space Telescope observations placed some constraints on this idea. The data showed no detectable oblateness in the planet’s transit shape, which limited the planet’s rotation speed. But for the ring scenario specifically, the results required that any ring would have to be oriented within about 30 degrees of face-on and have an inner edge very close to the planet’s surface, within about 1.2 times the planet’s radius.7The Astrophysical Journal Letters. Slow Rotation for the Super-puff Planet Kepler-51d Those are tight conditions, but they do not outright kill the hypothesis. The same JWST observations also constrained the dusty outflow model for Kepler-51d, since the light curve showed no anomalies during ingress and egress that the outflow model would predict.
For WASP-193b specifically, the ring scenario faces additional scrutiny. The planet orbits very close to its star, and intense stellar radiation and tidal forces could destabilize a ring system over time. Rings around hot Jupiters are theoretically possible, but maintaining them long-term is harder than around a cold, distant planet like Saturn. Future spectroscopic and photometric observations will likely be needed before rings can be convincingly invoked or dismissed for WASP-193b.
What Other Low-Density Planets Have Taught Us
WASP-193b is not the first planet to challenge density expectations, and insights from studying other unusually light worlds help set the stage for understanding it. WASP-107b, a warm Neptune-mass planet with a similarly low density, was recently observed in detail by JWST across a wide range of wavelengths. Those observations detected water, methane, carbon monoxide, carbon dioxide, sulfur dioxide, and ammonia in its atmosphere, and the chemical fingerprints pointed to an internal temperature significantly higher than what the planet’s distance from its star would predict, above 345 Kelvin.8Nature. A high internal heat flux and large core in a warm Neptune exoplanet
That excess internal heat turns out to be a critical clue. For WASP-107b, the best explanation is tidal heating: the planet’s orbit is slightly eccentric, and gravitational interactions with its star continuously flex the planet’s interior, generating heat the way bending a paperclip warms the metal. That tidal energy acts on a Neptune-like internal structure and can naturally explain why the planet is bloated without requiring exotic atmospheric effects. Its atmospheric metal enrichment was measured at roughly 10 to 18 times solar levels, consistent with a planet that has a substantial core but an atmosphere being puffed up from the inside.8Nature. A high internal heat flux and large core in a warm Neptune exoplanet
Whether tidal heating plays a similar role at WASP-193b is uncertain. The alignment between the planet’s orbit and its star’s equator suggests a relatively circular orbit, which would produce less tidal flexing. But orbital eccentricity can be subtle, and even a small residual eccentricity pumped by an unseen companion planet could contribute meaningful internal heating. More radial velocity measurements over time would help pin down the orbit’s shape and determine whether tidal effects are relevant.
Why Super-Puffs Matter Beyond the Novelty
It is tempting to treat WASP-193b as a cosmic curiosity, a planet with a fun nickname and a weird density. But super-puffs as a class are important to planetary science for reasons that go beyond their headline-grabbing properties. They probe the boundaries of planet-formation models. Standard core-accretion theory, which explains most known gas giants reasonably well, struggles with planets this underdense. Either the theory needs modifications to account for extreme inflation mechanisms, or the observational techniques need corrections to account for hazes, rings, or other effects that bias radius measurements.
Super-puffs also serve as natural laboratories for studying atmospheric escape. A planet with gravity this weak cannot hold on to its atmosphere as effectively as a denser planet. WASP-193b is likely shedding mass right now, and how fast it does so depends on the composition and structure of its upper atmosphere. If mass loss is rapid enough, the planet could shrink substantially over its remaining lifetime, eventually ending up as a much smaller and denser world. Studying the current state of the atmosphere can help constrain how long such planets persist and how many might have already evaporated into smaller remnants that we see in other exoplanet populations.
There is also a measurement problem that super-puffs highlight. Transit surveys measure radius, and radial velocity measurements give mass. Both have uncertainties, and for a planet where the mass is low and difficult to pin down against the noise of the star’s own variability, the density calculation carries large error bars. WASP-193b’s density of 0.06 grams per cubic centimeter has an uncertainty of about 0.02 in either direction, meaning the true density could be anywhere from about 0.04 to 0.08.1The Astronomical Journal. The Super-puff WASP-193 b is on a Well-aligned Orbit Even at the upper end of that range, the planet is still remarkably light, but the distinction matters for choosing between models. More precise mass measurements would narrow the field of viable explanations.
What JWST Could Reveal
The James Webb Space Telescope is the tool most likely to crack the mystery of WASP-193b. Its infrared instruments can observe the planet’s atmosphere in far greater detail than any ground-based telescope, potentially detecting molecular absorption features or confirming the flat, featureless spectrum that hazes and rings predict. If JWST sees clear molecular signatures, it would argue against thick hazes and rings as the primary explanation for the planet’s apparent size, and push the answer toward genuine inflation of the gas envelope. If the spectrum is flat, it would strengthen the case for an opaque high-altitude layer, though distinguishing between photochemical hazes and dust would require careful modeling.
Thermal emission measurements during secondary eclipses, when the planet passes behind its star, could constrain the planet’s internal temperature. As the WASP-107b observations demonstrated, an unexpectedly high internal temperature can reshape the entire interpretation of a low-density planet, turning a mystery about missing mass into a story about excess heat. If WASP-193b also has a higher-than-expected internal temperature, it could indicate that some combination of stellar irradiation, tidal heating, or other energy sources is responsible for the extreme bloating, independent of any atmospheric optical illusion.
The planet’s mass-loss rate is another observable target. Ultraviolet observations of the stellar hydrogen and helium lines during transit can detect gas escaping from the planet’s atmosphere. For a planet this weakly bound by gravity, the escape rate could be substantial, and measuring it would tell us whether WASP-193b is a long-lived oddity or a planet caught in the act of disappearing. The Kepler-51 system’s super-puffs have already been targets for this kind of work, and extending it to WASP-193b would provide a comparison point at a very different mass and irradiation level.
Why “Cotton Candy” Undersells the Strangeness
The cotton candy comparison has become the shorthand for WASP-193b in popular coverage, and in terms of density it is roughly accurate. Cotton candy sits at about 0.05 grams per cubic centimeter depending on how tightly it is packed, which is in the same ballpark as WASP-193b’s measured value. But the comparison can be misleading in a couple of ways. Cotton candy is a solid, or at least a tangled web of spun sugar fibers with air trapped between them. WASP-193b is a hot gas ball. There is no solid structure holding it together, just gravity, and not very much of it relative to the planet’s volume.
The more telling comparison might be to Earth’s own atmosphere. At sea level, air has a density of about 1.2 kilograms per cubic meter, or 0.0012 grams per cubic centimeter. WASP-193b’s average density is roughly 50 times higher than that, so it is not literally “made of air,” but it is far closer to an atmosphere than to anything you could stand on. The entire planet is essentially a vast, hot, diffuse gas envelope surrounding whatever small dense core might exist at its center. For a planet larger than Jupiter, that is genuinely strange and remains one of the more confounding discoveries in the still-young field of exoplanet science.