Nasa's findings show the planetary pair circling a star more than 1,100 light years away
Astronomers have pinpointed a pair of extraordinarily lightweight "superpuff" giants orbiting the same distant star in a discovery of the least dense exoplanets ever recorded.
The two worlds, named TOI-791 b and TOI-791 c, possess densities even lower than candyfloss, despite it being much larger than Jupiter.
"Only a handful of these super-puffy planets are known, and it is even rarer to find two in the same system," said George Dransfield of Oxford University, who led the research team.
The findings, published in Monthly Notices of the Royal Astronomical Society, reveal the planets circling a star roughly 1,110 light-years from Earth.
Despite matching Jupiter's dimensions, these worlds contain only a fraction of its mass. TOI-791 b holds merely 3.0 per cent of Jupiter's mass, whilst its sibling TOI-791 c contains just 5.9 per cent.
"These two planets have densities comparable to a nice blob of shaving foam, fresh from the can," Mr Dransfield explained.
Jupiter is up to 35 times denser than these featherweight siblings.
The pair also demonstrate an unusual orbital relationship known as a 5:3 mean-motion resonance, where the inner planet completes five circuits around its host star for every three orbits made by the outer world.
Their transits last approximately 11 hours each, among the longest ever observed.
Nasa's Tess spacecraft initially spotted the planets by detecting subtle dips in starlight as they passed in front of their host star.
The Planet Hunters citizen science programme first flagged TOI-791 b as a potential world in 2019, with TOI-791 c identified in 2023.
Researchers subsequently confirmed the discovery using ground-based observatories worldwide, including the ASTEP telescope stationed at Concordia in Antarctica.
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"The main reason these planets are interesting to study is that we didn't expect to see them at all," said Jon Jenkins, science lead at Nasa's Ames Research Center.
"They represent a puzzle for us to solve about how giant planets like Jupiter and the super-puffs form."
Scientists believe super-puff planets likely emerge in the cold outer reaches of protoplanetary discs.
In such a situation, the planets accumulate around small rocky cores more readily than dust, eventually forming vast hydrogen and helium atmospheres.
Fewer than 40 such worlds have been confirmed among the nearly 6,300 known exoplanets.
"This system offers a unique laboratory for understanding how super-puff planets form and evolve," said Amaury Triaud of the University of Birmingham.
The team has proposed observations using the James Webb Space Telescope.
Such equipment will allow scientists to examine whether the planets' atmospheres contain carbon, nitrogen and oxygen compounds, all of which would provide crucial insights into their origins.