The newly confirmed planets, TOI-791 b and TOI-791 c, orbit an F7-type dwarf star about 1,110 light years from Earth in the southern constellation Volans. Although each planet is about the size of Jupiter, both are remarkably lightweight for their size.

TOI-791 b has a density of just 0.038 grams per cubic centimeter, while TOI-791 c measures 0.047 grams per cubic centimeter. Jupiter, by comparison, has an average density of 1.33 grams per cubic centimeter, making it roughly 28 to 35 times denser than these newly discovered worlds.

The comparison becomes even more striking when measured against candy floss, which has a typical density of about 0.05 grams per cubic centimeter. Earth is much denser still, averaging 5.5 grams per cubic centimeter.

Rare Planetary Twins Locked in a Gravitational Dance

Scientists believe the two planets formed together from the same disc of gas and dust surrounding their young star, making them planetary "siblings."

They are also linked by an unusual orbital arrangement called a 5:3 mean-motion resonance. For every five orbits completed by the inner planet, the outer planet finishes almost exactly three. As they circle their star, their gravity repeatedly pulls on one another, creating small but measurable changes in the timing of each planet's transit.

Only four other planetary systems are known to contain multiple super-puff planets, making TOI-791 an exceptionally rare opportunity to investigate how these unusual worlds originate and evolve.

Lead author Dr. George Dransfield (she/her) (Department of Physics, University of Oxford and a presenter for BBC Sky at Night) said:

"Only a handful of these super-puffy planets are known, and it is even rarer to find two in the same system. Their extremely low densities make them fascinating targets for understanding how planetary systems form and evolve."

Citizen Scientists Helped Find the Planets

Volunteers participating in the Planet Hunters TESS citizen-science project first flagged TOI-791 b in 2019 and TOI-791 c in 2023 as possible planets. The project searches observations collected by NASA's Transiting Exoplanet Survey Satellite (TESS) for signs of previously unknown worlds.

Researchers then combined measurements from telescopes around the world to determine the planets' sizes and masses, allowing them to calculate their exceptionally low densities.

When a planet crosses in front of its star during a "transit," it blocks a small amount of starlight. That dip in brightness reveals the planet's size. In the TOI-791 system, astronomers also detected tiny changes in the timing of the transits caused by the planets' gravitational interactions. Analyzing those timing variations allowed the team to estimate each planet's mass.

Antarctica Played a Key Role

The discovery was built on eight years of observations, including data from the ASTEP (Antarctic Search for Transiting ExoPlanets) telescope at Concordia Station in Antarctica. The telescope is jointly operated by researchers from Université Côte d'Azur/Observatoire de la Côte d'Azur and international collaborators.

Antarctica's long winter nights gave astronomers a major advantage. Months of uninterrupted darkness made it possible to observe the planets' unusually long transits, each lasting more than 11 hours, without interruption. According to the researchers, these are the longest continuous planetary transits ever fully observed from the ground.

How Do Super-Puff Planets Form?

Scientists are still trying to understand how super-puff planets develop.

One leading explanation is that these worlds possess enormous atmospheres rich in hydrogen and helium that account for a large fraction of their total mass. Researchers think these thick gaseous envelopes may have formed when the planets were much farther from their star, in colder regions of the protoplanetary disc where gas could rapidly accumulate around a solid planetary core.

Future observations are planned to better understand the origins of these unusual planets and test competing theories.

Professor Amaury Triaud (University of Birmingham), the UK Principal Investigator of ASTEP and co-author of the study, said:

"This system offers a unique laboratory for understanding how super-puff planets form and evolve. We propose to carry out space-based observations using the James Webb Space Telescope to assess if the puffy atmosphere contains carbon-, nitrogen-, and oxygen-bearing species, revealing new insight into how these unusual planets formed."

Professor Tristan Guillot (Université Côte d'Azur), Principal Investigator of ASTEP and co-author of the study, added:

"These multi-planetary systems are complex, with gravitational interactions between the planets that evolve over very long periods, tens of years or more. This discovery highlights the importance of continued international collaboration in astronomy. Bringing together observations from Antarctica, space telescopes and observatories across several continents was essential to revealing the true nature of these extraordinary planets."