NASA's TESS Discovers the 'Puffiest' Planets Ever: Cotton-Candy Worlds TOI-791 b and c

NASA's TESS (Transiting Exoplanet Survey Satellite) mission has identified two of the lowest-density giant planets ever detected — "super-puff" worlds so light that their density is comparable to cotton candy . Peer-reviewed findings published in Monthly Notices of the Royal Astronomical Society describe these planets as the "puffiest" worlds ever found .

The Two Super-Puff Exoplanets: TOI-791 b and TOI-791 c

Both planets orbit the F7-type dwarf star TOI-791, located about 1,110 light-years from Earth in the southern constellation Volans .

Key Characteristics

The two planets — TOI-791 b (the inner planet) and TOI-791 c (the outer planet) — are roughly Jupiter-sized but with extremely low masses . Lead author Dr. George Dransfield of the University of Oxford described their densities as comparable to "a nice blob of shaving foam, fresh from the can" .

Parameter	TOI-791 b (inner)	TOI-791 c (outer)
Size	Roughly Jupiter-sized	Roughly Jupiter-sized
Density	0.038 g/cm³	0.047 g/cm³
Mass	Extremely low, a tiny fraction of Jupiter's	Extremely low, a tiny fraction of Jupiter's
Density vs. Jupiter	~35× less dense than Jupiter	~28× less dense than Jupiter
Orbital ratio	~5 orbits for every 3 of partner	~3 orbits for every 5 of partner

For context, cotton candy has a density of about 0.05 g/cm³, making these planets even lighter than that sugary treat .

Rare 5:3 Mean-Motion Resonance

The two planets are locked in a rare 5:3 mean-motion resonance — for every five orbits completed by TOI-791 b (the inner planet), TOI-791 c (the outer planet) completes almost exactly three orbits . This gravitational interaction produces measurable shifts in the timing of their transits, which allowed the research team to estimate their masses .

Discovery and Confirmation

Citizen science origins: Both planets were first identified as candidate planets in 2019 and 2023 by volunteers participating in the Planet Hunters TESS citizen-science project, which enlists the public to search NASA's TESS data for possible new worlds .

Ground-based confirmation: The team combined transit observations from multiple telescopes around the world to measure sizes and masses . This included the ASTEP telescope (Antarctic Search for Transiting ExoPlanets) at Concordia Station in Antarctica . The Antarctic winter provided months of continuous darkness, allowing astronomers to capture the planets' exceptionally long transits — each lasting over 11 hours — in a single uninterrupted observation. These are the longest continuous planetary transits reported as fully observed from the ground .

Future JWST Observations

Researchers expect follow-up observations with the James Webb Space Telescope to reveal these planets' chemical makeup . Dransfield noted the planets are probably mostly hydrogen and helium, but JWST will be needed to confirm their atmospheric composition .