When astronomers talk about exotic worlds beyond our solar system, they often describe gas giants that dwarf Jupiter, or rocky planets scorched by nearby stars. But a newly published study has added something altogether stranger to that catalog: two planets so impossibly fluffy that they are less dense than cotton candy.
The worlds, designated TOI-791 b and TOI-791 c, orbit an F7-type dwarf star roughly 1,110 light-years from Earth in the southern constellation Volans. They were first identified by citizen scientists participating in the Planet Hunters TESS program, then confirmed and characterized in a paper published in the Monthly Notices of the Royal Astronomical Society by a team led by Dr. George Dransfield at the University of Oxford.
What Makes These Planets Extraordinary
TOI-791 b and TOI-791 c are roughly the size of Jupiter — enormous by any reasonable measure. But unlike Jupiter, which has an average density of 1.33 grams per cubic centimeter, these two worlds are barely there.
TOI-791 b clocks in at just 0.038 grams per cubic centimeter. TOI-791 c is slightly denser at 0.047 grams per cubic centimeter. For comparison, cotton candy — the fairground staple that looks like it could float away on a breeze — has a density of approximately 0.05 grams per cubic centimeter.
In other words: these planets are less dense than the spun-sugar treat your child might carry through an amusement park. Jupiter is between 28 and 35 times denser. Earth is roughly 145 times denser.
"Only a handful of these super-puffy planets are known, and it is even rarer to find two in the same system," said Dr. Dransfield in a statement from the University of Oxford. "Their extremely low densities make them fascinating targets for understanding how planetary systems form and evolve."
A Gravitational Dance
What makes the TOI-791 system even more remarkable is the relationship between its two worlds. They orbit their star in what astronomers call a 5:3 mean-motion resonance: for every five orbits completed by the inner planet, the outer one completes almost exactly three.
This gravitational interplay causes the planets to tug at each other, producing measurable shifts in their transit timings — the moments when each planet passes in front of its star as seen from Earth. These timing variations were central to precisely determining the planets' masses and, from there, calculating their extraordinary densities.
The Mystery of Formation
How such enormous, diffuse worlds form remains one of the more fascinating open questions in planetary science. One leading hypothesis suggests they began life far from their host star, in the cold outer reaches of their planetary system, where hydrogen and helium could accumulate without evaporating. Over time, gravitational or dynamical forces may have moved them inward to warmer orbits.
The research team — which includes scientists from the Université Côte d'Azur and the University of Birmingham — is planning follow-up observations using the James Webb Space Telescope. By examining the chemical composition of the planets' atmospheres, specifically looking for carbon, nitrogen, and oxygen-bearing molecules, they hope to reconstruct the conditions under which these improbable worlds were born.
The Power of Citizen Science
TOI-791 b was first spotted in 2019 and TOI-791 c in 2023, both by volunteers participating in Planet Hunters TESS — a citizen science project that invites members of the public to help analyze data from NASA's Transiting Exoplanet Survey Satellite (TESS).
The fact that ordinary people, not professional astronomers, were among the first to notice these remarkable worlds says something meaningful about the changing landscape of space science. With the right tools, publicly accessible data, and genuine curiosity, the universe is something we can explore together.
Two planets lighter than candy floss, spotted by citizen scientists, orbiting a star over a thousand light-years away: it is hard to imagine a better illustration of just how extraordinary our moment in astronomical history truly is.
