Astronomers have found a sibling pair that looks enormous but weighs very little. TOI-791 b and TOI-791 c orbit an F7 dwarf about 1,110 light-years away; Oxford reports densities of 0.038 and 0.047 grams per cubic centimetre, far below Jupiter’s. [1][2] They are among the lowest-density giant planets measured, but “super-puff” is a description of structure, not a claim that the planets are made of candy floss.
Timing turns a transit into a scale
The candidates were first flagged by volunteers in the Planet Hunters TESS citizen-science project in 2019 and 2023. Researchers then combined transit depths, radial-velocity and timing data from telescopes around the world, including the ASTEP telescope in Antarctica. The inner planet completes five orbits while the outer completes about three, a 5:3 mean-motion resonance. Their roughly 11-hour transits and mutual gravitational tugs make the masses measurable. [1][2][3]
Formation remains an open question
A leading scenario places the planets in a cold, gas-rich part of their young system, where small cores could accumulate thick hydrogen and helium envelopes before migrating. But the data do not yet distinguish that route from alternatives such as unusual heating or atmospheric evolution. NASA says future observations should test their chemistry, spin and orbital history with space telescopes. [2][3]
The reusable lesson is to separate what a transit measures from what a formation model explains. Radius gives scale, timing gives mass, and atmosphere measurements can test the story. An exotic pair is valuable because it makes hidden assumptions about planet formation visible.
The pair is valuable because each measurement answers a different question. Transit depth estimates radius, radial velocity and timing constrain mass, and atmospheric spectra can test whether a thick envelope survived migration. Until those observations arrive, the 5:3 resonance is evidence about dynamics, not proof of one formation story. The linked James Webb course offers a wider frame for reading how instruments turn faint signals into physical models.
