Rare Pair of Improbably Light ‘Super-Puff’ Planets Is Discovered

Astronomers Discover Pair of Improbably Light ‘Super-Puff’ Planets

An international team of astronomers has identified two of the fluffiest giant planets ever discovered, with densities so low they are less dense than cotton candy. The findings, published in the Monthly Notices of the Royal Astronomical Society, detail the discovery of the “super-puff” planets TOI-791 b and TOI-791 c, which orbit an F7-type dwarf star located approximately 1,110 to 1,113 light years from Earth in the southern constellation Volans.

While both planets are roughly the size of Jupiter, they are remarkably lightweight. TOI-791 b has a density of 0.038 grams per cubic centimeter, and TOI-791 c measures 0.047 grams per cubic centimeter. For comparison, Jupiter’s average density is 1.33 grams per cubic centimeter, making it 28 to 35 times denser than these newly discovered worlds. For further context, cotton candy typically has a density of about 0.05 grams per cubic centimeter, and Earth averages 5.5 grams per cubic centimeter.

Gravitational Dance and Orbital Resonance

Scientists believe the two planets formed together from the same disc of gas and dust surrounding their young star, designating them as planetary “siblings.” The pair is locked in an unusual orbital arrangement known as a 5:3 mean-motion resonance; for every five orbits completed by the inner planet, the outer planet completes almost exactly three.

As they circle their host star, the planets exert gravitational pulls on one another, resulting in measurable changes in the timing of each planet’s transit. These timing variations allowed researchers to estimate the mass of each planet. TOI-791 b contains approximately 3.0 percent of Jupiter’s mass, while the larger TOI-791 c contains about 5.9 percent. The orbital periods for the planets are 139 days for TOI-791 b and 232 days for TOI-791 c.

The Role of Citizen Science and Antarctic Observation

The discovery was made possible through a combination of citizen science and long-term telescope observations. Volunteers participating in the Planet Hunters TESS project first flagged TOI-791 b in 2019 and TOI-791 c in 2023. The project monitors observations from NASA’s Transiting Exoplanet Survey Satellite (TESS) to identify previously unknown worlds.

Data collection was bolstered by the ASTEP (Antarctic Search for Transiting ExoPlanets) telescope at Concordia Station in Antarctica. The telescope, jointly operated by researchers from Université Côte d’Azur/Observatoire de la Côte d’Azur and international collaborators, benefited from Antarctica’s long winter nights. This provided months of uninterrupted darkness, allowing astronomers to observe the planets’ transits—which last more than 11 hours—without interruption. Researchers noted these are the longest continuous planetary transits ever fully observed from the ground.

Understanding Planetary Evolution

The existence of these planets presents a challenge to current models of planetary formation. Lead author Dr. George Dransfield of the University of Oxford noted that finding two such planets in the same system is exceptionally rare, with only four other known systems containing multiple super-puff planets.

One leading theory suggests these worlds possess massive atmospheres rich in hydrogen and helium. Researchers hypothesize that these thick gaseous envelopes may have formed when the planets were located much farther from their star in the colder regions of the protoplanetary disc, where gas could rapidly accumulate around a solid core.

Future research aims to refine these theories. Professor Amaury Triaud of the University of Birmingham stated that the team plans to use the James Webb Space Telescope to assess whether the atmospheres contain carbon, nitrogen, or oxygen-bearing species. Such data could reveal new insights into how these planets formed and how they migrated through their system during their development.

Summary of Planetary Characteristics

As NASA Ames research scientist Steve Howell noted, because large planet formation is believed to drive the evolution of a planetary system, the study of these Jupiter-sized, low-mass worlds remains a high priority for understanding the origins of planetary systems.