Are There Planets Less Dense Than Cotton Candy?

Are There Planets Less Dense Than Cotton Candy?

Yes — astronomers have confirmed the existence of planets so low in density that they are lighter than cotton candy, and would theoretically float on water. These extraordinary worlds, known as “super-puff” planets, include the recently discovered TOI-791 b and TOI-791 c, two of the most extreme examples ever found.

What Are Super-Puff Planets?

Super-puff planets are a rare class of exoplanet that share one defining characteristic: they are roughly the size of Jupiter but have a fraction of its mass. This combination produces densities so low they defy intuition. Cotton candy has a density of roughly 0.05 grams per cubic centimeter. Some super-puff planets come in even lower than that. For comparison, Earth’s density is about 5.5 g/cm³, and even Saturn — the least dense planet in our solar system — clocks in at around 0.69 g/cm³. Super-puffs blow past even that record.

TOI-791 b and c: A Rare Planetary System

TOI-791 b and TOI-791 c orbit the same star and represent one of the rarest planetary system configurations ever catalogued. Both planets are Jupiter-sized in diameter but possess masses so low that their mean densities place them firmly in super-puff territory. Scientists estimate their densities are comparable to — or even lower than — cotton candy, meaning that if you could find an ocean large enough, these planets would float.

What makes this system especially valuable to researchers is the fact that it contains two super-puff planets orbiting the same host star. Finding one super-puff is uncommon. Finding two in a single system gives astronomers a unique opportunity to study the conditions that produce such bizarre worlds.

Why Are These Planets So Light?

Scientists are still working to fully explain super-puff planets, and they remain one of the open puzzles in planetary science. Leading hypotheses include:

• Massive extended atmospheres: These planets may have retained enormous envelopes of hydrogen and helium gas, inflating their size without adding much mass.
• Rings mistaken for bulk size: Some researchers have proposed that what appears to be planetary volume is actually a system of rings, making the planet look larger — and therefore less dense — than it truly is.
• Atmospheric escape and inflation: Proximity to the host star can heat and puff out a planet’s atmosphere over billions of years, dramatically increasing its apparent radius.

No single explanation fits every super-puff planet discovered so far, which is part of what makes them so scientifically compelling.

Why Does This Challenge Our Understanding of Planet Formation?

Standard planetary formation models predict that gas giants accumulate heavy cores before pulling in gas. Super-puff planets challenge this picture because they seem to have skipped the “heavy” part entirely. Their existence suggests that planet formation is far more varied and unpredictable than models built from observing our own solar system would suggest.

Every new super-puff discovery forces scientists to revisit the assumptions baked into their simulations and models, pushing the field toward a more complete — and more surprising — understanding of how planets are born.

How Do Scientists Detect Super-Puff Planets?

Most super-puff planets are discovered using the transit method, where a space telescope like NASA’s TESS (Transiting Exoplanet Survey Satellite) detects the slight dimming of a star’s light as a planet passes in front of it. By combining the planet’s measured radius (from transit depth) with its mass (from radial velocity measurements), scientists can calculate density. When that number comes back astonishingly low, a super-puff planet enters the record books.

The Universe Is Stranger Than We Expected

TOI-791 b and c are a reminder that the universe does not feel obligated to match our expectations. Planets lighter than cotton candy are not science fiction — they are documented, peer-reviewed reality. As telescope technology improves and surveys like TESS continue scanning the sky, more of these improbable worlds are likely waiting to be found.