TOI-791 b and c Confirmed as Jupiter-Sized Planets with Densities of 0.038 and 0.047 g/cm³

{"The planets were first flagged by Planet Hunters TESS volunteers in 2019 and 2023. Follow-up photometry from eight years of observations, including uninterrupted 11-hour transits captured at Concordia Station, yielded radii near 1 R_Jup. Radial-velocity and TTV mass constraints produced the record-low densities, 28–35 times lower than Jupiter.","Only four other multi-super-puff systems are known. Formation models must now explain how two siblings accreted massive H/He envelopes yet retained them at orbital distances that should drive rapid photoevaporation. The 5:3 resonance supplies an additional constraint: convergent migration and disk damping must have occurred before the gas disk dispersed.","Atmospheric retrievals on similar objects (e.g., Kepler-51d) show extended, low-metallicity envelopes whose scale heights challenge both core-accretion and pebble-accretion thresholds. TOI-791 therefore supplies a benchmark for testing whether high-altitude hazes or reduced interior entropy can inflate radii without requiring unrealistically low core masses.","JWST Cycle 3 transmission spectroscopy of TOI-791 b is scheduled for 2026; detection of a muted water feature or Rayleigh slope steeper than 5 scale heights would falsify pure H/He envelope models and force revision of the mass–radius–metallicity relation for low-density giants."}