Astronomers have confirmed two Jupiter-sized exoplanets with densities so low they are comparable to shaving foam, making them the lightest known planets of their size. The discovery, reported in the Monthly Notices of the Royal Astronomical Society, adds a rare pair of “super-puffs” to a short list of fewer than 40 such objects among nearly 6,300 confirmed exoplanets. Detected by NASA’s TESS satellite and verified with ground-based telescopes, the planets orbit the star TOI-791 in the southern constellation Volans, roughly 1,110 light-years from Earth.

The planets, designated TOI-791 b and c, have radii similar to Jupiter but contain just 3% and 5.9% of its mass, respectively. Their extremely low densities suggest they are composed mostly of hydrogen and helium, likely shrouded in thick atmospheres that give them a fluffy, possibly white or blue appearance. Researchers believe such worlds form in the gas-rich disks around newborn stars, where they accrete voluminous envelopes that later dissipate, leaving behind these puffed-up remnants. The pair’s unusually long orbital periods—139 days and 232 days—and their mutual gravitational resonance allowed scientists to precisely measure their masses through transit-timing variations, a technique that reveals how the planets tug on each other.

The team, led by George Dransfield of the University of Oxford, notes that the planets’ existence was unexpected. “We didn’t expect to see them at all,” said Jon Jenkins of NASA’s Ames Research Center, underscoring the challenge they pose to standard models of giant planet formation. Viewed from major research centres in Europe and the United States, the findings force a re-examination of how gas giants evolve, particularly in systems where the protoplanetary disk is gas-dominated. The discovery also highlights the growing capability of transit surveys to identify not just planets, but their dynamical interactions, even at distances of over a thousand light-years.

Follow-up observations with the James Webb Space Telescope are planned to determine the chemical makeup of these worlds, which could clarify whether they are indeed hydrogen-helium dominated or contain other volatiles. In a separate line of inquiry, scientists studying Titan and Pluto have reported new evidence of prebiotic chemistry on icy surfaces, where sunlight-driven reactions produce complex organic molecules. While unrelated, both lines of research expand the catalogue of exotic environments that inform our understanding of planetary formation and the potential pathways to life. The next milestone for the super-puff system will be the Webb spectroscopic analysis, expected to reveal atmospheric composition and further constrain formation scenarios.