A Planet the Size of Jupiter Has the Density of Cotton Candy. Scientists Just Found Two. They Shouldn't Exist.

Something is wrong with these planets.

They're bigger than Jupiter. They orbit a star. They have atmospheres. By every measure, they should be perfectly ordinary gas giants.

Except their density is comparable to cotton candy.

This week, astronomers using data from NASA's TESS space telescope announced the discovery of two new "super-puff" planets — the least dense worlds ever confirmed. If you could find a bathtub large enough, these planets wouldn't just float. They'd bob like foam.

~0.01 g/cm³

Estimated density of a super-puff planet — roughly the same as cotton candy spun at a fairground

For comparison: Earth has a density of 5.5 grams per cubic centimeter. Jupiter — the solar system's undisputed gas giant, a planet 1,300 times the volume of Earth — has a density of 1.3 g/cm³. These new worlds clock in at a fraction of that. Scientists are still working out what exactly is going on inside them. The measurements have been checked. They're not wrong.

~0.01 g/cm³Super-puff density

1.3 g/cm³Jupiter's density

5.5 g/cm³Earth's density

The Bathtub Test

Planetary scientists have a classic demonstration: drop an object in water and it floats if its density is below 1 g/cm³. Saturn already passes this test — it's the only planet in our solar system that would float in a sufficiently large ocean, which is a genuinely strange fact about our cosmic neighborhood.

These new planets? They don't just float. Their density is so vanishingly small that astronomers initially questioned the measurements. Gas giant planets shouldn't be this light. Something is inflating them to extraordinary sizes without giving them the mass to match — and scientists don't fully agree on what.

Key takeaway: These are the officially confirmed "puffiest" worlds ever found — giant planets with atmospheres so bloated they carry barely any mass relative to their enormous size. They force a rewrite of what a gas planet can actually be.

How TESS Finds Planets Nobody Can See

The Transiting Exoplanet Survey Satellite — TESS — has been watching the sky since 2018. It doesn't look at planets directly. It watches for shadows.

Think of it like watching a lamp flicker behind a window. You can't see the object passing in front. But the shadow tells you something is there — how large it is, how fast it moves, how long it takes to come back around.

How TESS detects a planet

Star at full brightnessPlanet crossingStarlight dips briefly

In the case of these two super-puffs, the dimming was detected, the orbital period was tracked, and the planet's size was calculated from how much starlight it blocked. Then ground-based telescopes measured the planet's mass through gravitational wobble.

The numbers came back strange. The planet was huge. Its mass was... unremarkable. Divide mass by volume, and you get density. The answer came back somewhere near cotton candy. The team ran the calculations again.

Same answer.

2018TESS launched

7,000+Exoplanet candidates found by TESS

2New super-puffs confirmed, June 2026

The Mystery Nobody Can Fully Explain

Super-puff planets aren't entirely new. Astronomers have found a handful before — worlds with inexplicably low densities that sit uncomfortably outside standard planetary models. But these two are the most extreme yet. The puffiest ever confirmed.

The leading explanation involves heat. These planets orbit close to their stars. Intense radiation heats their upper atmospheres to thousands of degrees. That heat drives atmospheric gas outward, dramatically inflating the planet's radius while barely affecting its mass — like a balloon expanding without gaining weight.

But there's a problem with that explanation. And it's a serious one.

If the atmosphere is that hot and that extended, the planet should be losing it. Stellar radiation strips atmospheric gas away, molecule by molecule, over millions of years. So why do these planets have anything left? Why haven't they been shaved down to bare rocky cores by now?

The uncomfortable question: Are we catching these planets at a rare, fleeting moment — slowly losing their atmospheres in geological slow motion? Or is something actively replenishing the gas? Scientists don't have a consensus answer. Both possibilities are unsettling in different ways.

What It Means for How Planets Form

Super-puff planets don't just challenge individual models. They challenge the entire framework of how gas giants are supposed to assemble themselves.

The standard picture: gas and dust swirl around a young star. Rocky material clumps together. A core forms. If that core gets massive enough, fast enough, its gravity pulls in surrounding hydrogen and helium — and a gas giant is born. The rocky core is the scaffolding. The gas envelope is the building that goes up around it.

Super-puff planets seem to have skipped that part. Their cores appear to be small. Their gas envelopes are enormous. They collected vast amounts of atmosphere without the massive rocky foundation that's supposed to anchor it all.

Jupiter × 1

Approximate size of a super-puff planet — but with a fraction of Jupiter's mass. Same box. Almost nothing inside.

One leading hypothesis: these planets didn't form where we're finding them. They formed further out from their stars, in colder, denser regions of the protoplanetary disk where conditions allowed them to accumulate gas more easily. Then they migrated inward — dragged by gravitational interactions with other planets or the disk itself — bringing their bloated, bizarre bodies with them into the hot inner system.

If that's true, it tells us something profound about every solar system we've ever mapped. The orbital architecture of a planetary system today might look nothing like it did a billion years ago. Planets move. A lot. The address doesn't tell you where something was born.

What Happens Next

These two planets are now near the top of several high-priority observing lists. The James Webb Space Telescope can analyze the chemical fingerprint of an exoplanet's atmosphere as it transits its star — reading the absorption lines in the starlight to identify molecules. Water vapor. Carbon dioxide. Methane. Things that shouldn't be there. Things that raise questions.

If these super-puffs have the extended, puffed-out atmospheres scientists think they do, JWST should be able to detect the signal clearly. The atmospheric composition might explain the inflation. Or it might raise entirely new questions that don't fit any current model.

That's the pattern in planetary science. Every answer is also a key that opens another locked door.

1,000°C+Estimated upper atmosphere temperature

MillionsYears before atmosphere may be fully stripped

JWSTNext instrument to peer inside these worlds

In the meantime, TESS keeps watching hundreds of thousands of stars. More super-puff planets are almost certainly already hiding in the archived data — waiting for someone to calculate their density and realize something is deeply, fascinatingly wrong.

Bottom line: These aren't just peculiar data points. They're stress tests for our entire model of how solar systems assemble. If a planet can be Jupiter-sized and cotton-candy-light, the physics of planet formation needs rewriting — and we're only just getting started on figuring out how.

Somewhere in the galaxy, a planet the size of Jupiter is drifting through space, lighter than the air above a carnival. Its atmosphere is slowly bleeding away into the void. And we've only just learned it exists.

Want to keep exploring? More stories like this on the SkyLens blog — or pull up the live satellite tracker to see what's actually in orbit above you right now.

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SkyLens editorial — live CelesTrak + NASA/JPL data (15821 objects)