Two Jupiter-Sized Planets Found with Density 97% Lower Than Jupiter

For the space exploration community, the detection of TOI-791 b and c marks a pivotal moment for exoplanet characterization. These planets, lighter than shaving foam, stress-test formation models and highlight the critical role of sustained, multi-platform observation — from TESS surveys to Antarctic photometry — in uncovering the cosmos’s most extreme worlds. With JWST now tasked to peer into their atmospheres, the findings will directly inform mission planning and instrument utilization for years to come.

A team led by University of Oxford astronomer George Dransfield has unveiled two extraordinary exoplanets — TOI-791 b and TOI-791 c — that redefine the lower bounds of planetary density. Reported in the Monthly Notices of the Royal Astronomical Society and publicized via Phys.org, these gas giants each span roughly Jupiter’s physical diameter but pack densities of only 0.038 and 0.047 grams per cubic centimetre. Put in terrestrial terms, they are lighter than cotton candy and fluffier than shaving foam, with a density up to 35 times less than Jupiter’s 1.33 g/cm³. Located about 1,110 light-years away in the faint southern constellation Volans, the pair are the largest and most tenuous “super-puff” planets known, and finding two such objects in one system is exceptionally rare.

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A team led by University of Oxford astronomer George Dransfield has unveiled two extraordinary exoplanets — TOI-791 b and TOI-791 c — that redefine the lower bounds of planetary density.

The discovery relied on a marathon of observations. NASA’s Transiting Exoplanet Survey Satellite (TESS) first spotted the periodic dimmings of the host star. Because the planets’ transit durations exceed 11 hours — extremely long for close-in worlds — ground-based follow-up demanded unbroken monitoring. The team leveraged observations from Antarctica, where the continuous winter darkness permitted multi-hour runs without daytime gaps. Over nearly eight years, these transit-timing variations (TTV) allowed the researchers to back out the planetary masses and confirm their gossamer densities.

Super-puff planets occupy an extreme corner of exoplanet demographics. With fewer than 40 confirmed among the nearly 6,300 exoplanets catalogued to date, they challenge standard core-accretion models. A Jupiter-sized ball of hydrogen and helium with such low gravity should either contract as it radiates energy or lose its envelope quickly to stellar irradiation. The survival of TOI-791 b and c suggests either very recent formation, ongoing mass loss that is continuously replenishing the atmospheres, or a composition that includes heavier elements in a form that keeps the scale height inflated. Their 5:3 mean-motion resonance — for every five orbits of the inner planet, the outer completes three — points to a dynamically stable configuration that may have protected them from destructive scattering.

The immediate next step is already scheduled: NASA’s James Webb Space Telescope will conduct transmission spectroscopy to determine whether the planets are indeed hydrogen-helium dominated and to search for haze, clouds, or water vapour. Dransfield colourfully described their likely appearance as “a nice blob of shaving foam, fresh from the can,” with hues ranging from white to pale blue depending on cloud cover. Such atmospheric characterization could distinguish between a puffy primordial atmosphere and a dusty, metal-rich envelope that mimics low density.

Beyond the pure curiosity of worlds lighter than foam, the finding carries implications for how we sample the galaxy’s planetary diversity. TESS’s all-sky survey, combined with ultra-precise timing from long-baseline ground arrays, is turning up outliers that were invisible to earlier, narrower searches. The coexistence of two super-puffs in TOI-791 hints that these objects might form in groups when conditions are right — perhaps in disks with unusually low solid-to-gas ratios or around stars with quiet radiation histories. As JWST and next-generation observatories come online, the peculiar physics governing these puffballs will become a laboratory for atmospheric escape, radiative cooling, and magnetospheric interactions under extreme conditions.