Astronomers have confirmed two rare super-puff planets that are less dense than cotton candy, even though each is roughly the size of Jupiter. Named TOI-791 b and TOI-791 c, the pair orbit an F7-type dwarf star about 1,110 light years from Earth in the southern constellation Volans, and their extraordinarily low densities make them prime targets for studying how the galaxy's strangest worlds form.
Lighter than candy floss
The numbers are striking. The international team, led by the University of Oxford, measured densities far below anything in our own solar system:
- TOI-791 b: 0.038 grams per cubic centimeter
- TOI-791 c: 0.047 grams per cubic centimeter
- Candy floss, for comparison: about 0.05 grams per cubic centimeter
- Jupiter: 1.33 grams per cubic centimeter, making it 28 to 35 times denser
Earth, at about 5.5 grams per cubic centimeter, is denser still. To put it plainly, these planets are so diffuse that they would float in a bathtub if you could find one large enough. The findings were published in Monthly Notices of the Royal Astronomical Society. For more discoveries like this, follow our science coverage.
A rare gravitational dance
Scientists believe the two planets formed together from the same disc of gas and dust, making them planetary siblings. They are locked in an unusual 5:3 mean-motion resonance, meaning the inner planet completes five orbits for almost every three of the outer planet.
Their mutual gravity produces tiny, measurable shifts in the timing of each transit, which helped researchers pin down the masses with confidence. Only four other systems are known to host multiple super-puff planets, which makes TOI-791 exceptionally valuable as a natural laboratory for planet formation.
How the super-puff planets were found
Citizen scientists in the Planet Hunters TESS project first flagged the candidates, spotting TOI-791 b in 2019 and TOI-791 c in 2023 in data from NASA's Transiting Exoplanet Survey Satellite. Researchers then combined observations from telescopes worldwide, including eight years of data and the ASTEP telescope at Concordia Station in Antarctica.
The continent's long winter nights allowed astronomers to capture the planets' unusually long transits, each lasting more than 11 hours, described as the longest continuous planetary transits ever fully observed from the ground. It is a reminder of how much modern exoplanet science now leans on a global network of instruments rather than any single observatory.
Why super-puffs matter
One leading theory holds that super-puff planets carry enormous hydrogen and helium atmospheres that account for a large fraction of their mass, possibly accumulated when the planets sat far from their star in colder regions of the protoplanetary disc where gas could rapidly build up around a solid core.
Lead author Dr George Dransfield, of the University of Oxford, said only a handful of these super-puffy planets are known, and it is even rarer to find two in the same system, calling their extremely low densities fascinating for understanding how planetary systems form and evolve.
What comes next
The team has proposed James Webb Space Telescope observations to assess whether the puffy atmospheres contain carbon, nitrogen and oxygen-bearing species, which could reveal fresh insight into how these unusual worlds came to be. As co-author Professor Tristan Guillot noted, the discovery relied on bringing together observations from Antarctica, space telescopes and observatories across several continents.
Frequently asked questions
How can a planet be lighter than cotton candy?
The two planets are thought to hold vast hydrogen and helium atmospheres that make up much of their volume but very little of their mass, leaving them with densities below 0.05 grams per cubic centimeter — less than candy floss.
How far away are TOI-791 b and c?
The pair orbit an F7-type dwarf star roughly 1,110 light years from Earth, in the southern constellation Volans.
Why is finding two super-puffs in one system important?
Only four other systems are known to host multiple super-puff planets, so TOI-791 offers a rare chance to study sibling worlds that formed from the same disc and to test theories of how such low-density planets arise.
