Some discoveries in astronomy arrive with a shrug. A slightly warmer star here, a slightly odd orbit there. And then once in a while, a team of researchers looks at their numbers, checks them, checks them again, and has to admit that the universe has built something that does not fit the textbook.
That is the story of WASP-193b, a giant planet about 1,200 light-years away that has been nicknamed the cotton candy planet. The nickname is not marketing fluff. It comes straight from the physics: this world is roughly 50 percent wider than Jupiter, yet its average density is close to that of the spun sugar you buy at a fairground. Nothing in our standard picture of how gas giants form and behave explains it comfortably.
If you have seen the headlines and wondered what is actually going on, this guide walks through what was found, how astronomers know it, why it is so hard to explain, and what might finally crack the puzzle.
What astronomers actually found
WASP-193b was reported in the journal Nature Astronomy in May 2024. The international team was led by researchers at the EXOTIC Laboratory of the University of Liège in Belgium, working with colleagues at MIT and the Instituto de Astrofísica de Andalucía in Spain. According to the MIT summary of the work, the planet orbits a Sun-like star and sits in the Milky Way roughly 1,200 light-years from us.
The numbers are what make people stop scrolling. The planet is about 1.5 times the radius of Jupiter, but its mass is only around 0.14 times Jupiter’s. Put those together and you get a density of roughly 0.059 grams per cubic centimeter. Cotton candy comes in at about 0.05. For comparison, Jupiter is around 1.33 and Earth is around 5.51, as Cosmos Magazine laid out in its coverage.
The planet circles its star every 6.25 days, which means it is a very close-in world, the kind astronomers call a hot Jupiter. Heat matters for this story, and we will come back to it.
Khalid Barkaoui, the study’s first author, summed up the oddity this way: WASP-193b is the second least dense planet found so far, behind only Kepler-51d, which is a much smaller planet. In other words, among a catalogue that now runs well past five thousand confirmed exoplanets, this one is a genuine outlier. Barkaoui called it “a cosmic mystery,” and having read the paper, that does not feel like exaggeration.
How do you weigh a planet 1,200 light-years away?
It is fair to be skeptical. Nobody has put a scale under this planet, and nobody has seen it as anything more than a faint dip in starlight. So how can scientists say it has the density of cotton candy?
The answer is two separate measurements, each telling you something different.
First, the size. The planet was spotted by the Wide Angle Search for Planets, or WASP, a long-running survey that uses robotic telescopes to watch the brightness of huge numbers of stars. When a planet crosses in front of its star, the star dims by a tiny, repeating amount. The depth of that dip tells you how big the planet is compared with the star. The WASP-South observatory in the southern hemisphere picked up these periodic dips from the star WASP-193 in data taken between 2006 and 2008 and again in 2011 and 2012, as the ScienceDaily release describes. Follow-up observations from the TRAPPIST-South and SPECULOOS-South telescopes in Chile helped confirm the signal in different wavelengths and rule out impostors, according to Popular Science.
Second, the mass. This is the hard part. To weigh a planet, astronomers measure how much it tugs its star back and forth, which shows up as a very slight shift in the star’s light. This is called the radial velocity method. A heavy planet gives a big, obvious wobble. A feather-light planet gives almost nothing.
WASP-193b gives almost nothing. Barkaoui has said the planet is so light that it took four years of data to confirm that a mass signal was there at all, and even then the signal was tiny. That patience is part of why the result is trustworthy. A team does not spend four years chasing a whisper unless they are determined to be sure it is real.
Size from the transit, mass from the wobble, and density falls out of simple arithmetic. The result was so low that the researchers have said they were surprised by their own data.
Not the first fluffy world, but the biggest surprise
WASP-193b did not appear from nowhere. Astronomers have been collecting strangely airy planets for more than a decade, and they even have a name for them: super-puffs.
The best-known examples orbit a young star called Kepler-51, which was observed by NASA’s Kepler space telescope. Several planets there are roughly Saturn-sized or larger in diameter, yet their masses are only a few times that of Earth. That makes them astonishingly airy. NASA’s exoplanet program describes this unusual class of low-density worlds, and Kepler-51d is still the record holder for lowest density.
So why is WASP-193b considered such a big deal if super-puffs already existed? Two reasons.
The first is scale. Kepler-51d is a small, Neptune-like planet, and low-mass planets can be puffy for reasons that are relatively easy to imagine. They do not have enough gravity to hold their atmospheres tightly, so even a modest gas envelope swells up. WASP-193b is a different animal. It has roughly the mass of Saturn, hugely more than a super-puff of the Kepler-51 type, and that much gravity should squeeze a gas envelope considerably.
The second reason is the host star. Kepler-51 is young, and young planets are often still puffed up from their fiery births, with time ahead of them to cool and shrink. WASP-193 is a mature Sun-like star. There is no easy excuse of youth to lean on here.
Why the usual explanations fall short
Astronomers have a toolbox for explaining puffy planets. For WASP-193b, most of the tools come up short.
Stellar heating. Hot Jupiters orbit close to their stars, and the radiation they receive can warm and inflate them. That is the first suspect for any swollen giant. But the research team found that WASP-193b’s density cannot be reproduced by standard models of irradiated gas giants, and that holds even under the unrealistic assumption that the planet has no solid core at all. Remove the core, make the planet as light as physically sensible, and the models still produce something denser than what we observe.
Tidal heating. If a planet follows an elongated orbit, the star’s gravity kneads it as it swings closer and farther, generating internal heat. It is a real effect and it explains some inflated planets. But it generally needs an eccentric orbit, and that is not an obvious fit here.
Exotic internal heating. Other proposals involve electrical currents driven in the atmosphere, or heat being pushed deep into the interior by winds. These ideas are active areas of research, but they usually add modest inflation, not enough to turn a Saturn-mass planet into one with the density of sugar floss.
Formation history. Some researchers think the answer lies in how the planet was assembled. One idea, put forward in earlier work on another puffy world, WASP-107b, is that such planets may have formed far from their stars, where the gas in the disk is cold enough for the planet to gather gas quickly. Eve Lee of McGill University has argued this is the most plausible scenario for that planet, and then migrated inward. It is a good hypothesis, but it does not by itself explain why the planet would stay so inflated after arriving.
An atmosphere that is deceptively huge. There is one more subtlety worth understanding. When astronomers say a planet has a certain radius, they mean the point where the atmosphere becomes opaque enough to block starlight. If a planet has a very extended, tenuous upper atmosphere, or high-altitude haze, it can look bigger than the bulk of the planet really is. The WASP-193b paper is titled around exactly this idea: an extended low-density atmosphere. The researchers believe the planet is mostly hydrogen and helium, with an atmosphere that stretches tens of thousands of kilometers beyond what Jupiter’s would.
It may be a combination of several of these. Planets are messy, and a single clean explanation is rarely the end of the story.
Is it really like cotton candy?
The comparison is fun, and it does what a good analogy should: it makes an unimaginable number feel tangible. But it is worth being clear about where it works and where it does not.
It works on density. Cotton candy is mostly air with a thin lattice of sugar threads, and WASP-193b is mostly very thin gas. Both are fluffy in the same arithmetic sense, as the researchers noted: both are basically made of very little stuff spread across a lot of space.
It does not work on almost anything else. WASP-193b is not sweet, sticky or pink, and it would not survive a bite. The planet is very hot, bathed in radiation from its star, and made of hydrogen and helium rather than sugar. You cannot land on it, because there is no surface to land on. If you descended through the atmosphere, the pressure would climb gradually rather than hit solid ground. And it would not hold together in your hands, since the whole object is held up by its own gravity and heat.
A cleaner way to picture it: if you could somehow place WASP-193b in an enormous bathtub, it would float. That is true of Saturn, too, which is the classic example. But WASP-193b would float with room to spare.
What the James Webb Space Telescope could tell us
The researchers have been clear about the next step. They consider WASP-193b an excellent target for follow-up with observatories such as NASA’s James Webb Space Telescope.
There is a good reason for the excitement. Webb can split a planet’s light into a detailed spectrum as it passes in front of its star, revealing which molecules are in its atmosphere and how high they extend. For a planet like this one, that could answer several questions at once:
- Is there a layer of haze or cloud high in the atmosphere making the planet look larger than its bulk?
- How much heavy material, such as water, carbon dioxide or methane, is mixed into the hydrogen and helium?
- Does the chemical makeup point to formation far from the star, or somewhere closer?
Those clues matter because they connect directly to the competing theories. If Webb finds an atmosphere rich in heavy elements, that points to one origin story. If it finds a very clean, hydrogen-dominated atmosphere with a huge scale height, that points to another.
Webb has already been rewriting what we know about puffy planets. Observations of WASP-107b, another low-density world, have revealed surprising chemistry that challenged earlier ideas about its interior. It would be a mistake to assume WASP-193b will behave the same way, but it is a reminder that a single good spectrum can overturn assumptions that stood for years.
A note on timing: the 2024 discovery paper remains the foundation of what we know publicly. If you are reading this later, check the latest papers and the NASA exoplanet archive for newer measurements before quoting specific numbers.
Why one strange planet matters to everyone
It is tempting to file WASP-193b under curiosities, a weird data point for specialists to argue about. That undersells it.
Planet formation theory is, at its heart, a story we tell about how our own solar system came to be, then check against thousands of other systems. For years that story handled hot Jupiters reasonably well. Then planets like WASP-193b show up and quietly point out that the theory has gaps. Every outlier is a test: the model either stretches to include it or it breaks and has to be rebuilt.
There is also a practical angle. Scientists use planetary models to interpret everything from the first atmospheric spectra of distant worlds to the search for habitable planets. If our models of how giant planets inflate or cool are incomplete, then our readings of those spectra might be incomplete too. A fluffy giant 1,200 light-years away is a stress test for the tools we will rely on when we look at smaller, more Earth-like worlds.
And there is something to be said for plain wonder. Fifty years ago, we knew of nine planets, all in one system. Today we have catalogued well over five thousand, and a good share of them look nothing like what we expected. The universe keeps producing things that nobody thought to predict.
Frequently asked questions
What is the cotton candy planet? It is the common nickname for WASP-193b, a gas giant about 1,200 light-years away that is larger than Jupiter but has a density of only about 0.059 grams per cubic centimeter, close to cotton candy’s.
Is the cotton candy planet made of sugar? No. It is made mostly of hydrogen and helium. The name refers only to how little matter is packed into its volume.
Is there a less dense planet than WASP-193b? According to the discovery team, the only known planet with a lower density is Kepler-51d, which is much smaller.
Could humans ever visit it? Not realistically. It is far beyond our reach, extremely hot, and has no solid surface.
Why can’t scientists explain it yet? Standard models of irradiated gas giants, even with a coreless planet, cannot reproduce the measured density. Researchers are still weighing ideas about formation history, internal heating and atmospheric structure.
WASP-193b is a reminder that nature is under no obligation to follow our models. A planet larger than Jupiter, as airy as a fairground treat, has shown up around a perfectly ordinary Sun-like star, and our best theories have no tidy explanation for it.
That is not a failure of science. It is how science is supposed to work. A measurement that does not fit is the most useful kind, because it tells us exactly where to look next. With Webb and other observatories now able to read the atmospheres of distant worlds, there is a decent chance we will understand this cosmic fluffball far better in the coming years.
Until then, it remains what its discoverers called it: a mystery, and a wonderful one.
Sources and further reading
- MIT EAPS: Astronomers spot a giant planet that is as light as cotton candy
- ScienceDaily: WASP-193b, a giant planet with a density similar to that of cotton candy
- Cosmos Magazine: New exoplanets with cotton candy density can’t be explained
- Popular Science: A massive cotton candy-like exoplanet stumps astronomers
- ZME Science: Cotton candy exoplanet
- NASA Exoplanet Exploration
- Nature Astronomy
