Two Giant Planets Lighter Than Cotton Candy Are Rewriting What We Know About Planet Formation

Worlds That Defy Expectations

Astronomers have uncovered a pair of giant planets that are lighter than cotton candy — super-puffs the size of Jupiter orbiting a star 1,110 light-years away. That makes them the lightest known planets of their size, according to the University of Oxford's George Dransfield, who described their densities as "comparable to a nice blob of shaving foam, fresh from the can."

Although both planets are roughly the size of Jupiter, they are extraordinarily diffuse: TOI-791 b has a density of just 0.038 grams per cubic centimeter, while TOI-791 c has a density of 0.047 grams per cubic centimeter — around 28 to 35 times less dense than Jupiter. Their densities are even lower than candy floss, which typically has a density of about 0.05 grams per cubic centimeter. TOI-791 b is nearly the same size as Jupiter but contains just 3.0 percent of Jupiter's mass, while TOI-791 c is even larger than Jupiter but contains just 5.9 percent of its mass.

"The main reason these planets are interesting to study is that we didn't expect to see them at all," said Jon Jenkins, the science lead for the Science Processing Operations Center 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."

A Rare Discovery — And an Even Rarer Pair

TOI-791 b and TOI-791 c were flagged as candidate planets about four years apart by volunteers combing through data collected by NASA's Transiting Exoplanet Survey Satellite (TESS), as part of the Planet Hunters TESS citizen-scientist project. The team says TOI-791 is only the ninth known system with more than one transiting giant planet. "Only a handful of these super-puffy planets are known, and it is even rarer to find two in the same system," said Oxford's Dr. George Dransfield. "Their extremely low densities make them fascinating targets for understanding how planetary systems form and evolve."

The newly found super-puffs have unusually long orbits, with TOI-791 b taking 139 days and TOI-791 c taking 232 days to circle the host star. Such long-orbit planets are rare to find, needing long durations of telescope observation to capture and confirm their attributes. From its vantage point in high Earth orbit, TESS gathered 1,122 days of data on this planetary system over the course of seven years.

The discovery also relied on eight years of observations, including data collected by the Antarctic Search for Transiting ExoPlanets (ASTEP) telescope at Concordia Station in Antarctica. Antarctica's months-long winter darkness allowed researchers to observe the planets' unusually long transits in a single uninterrupted session — each transit lasted more than 11 hours, making them the longest continuous planetary transits ever observed in their entirety from the ground.

A Gravitational Dance That Unlocked Their Secrets

The two planets are locked in a rare gravitational relationship known as a 5:3 mean-motion resonance, meaning that for every five orbits completed by the inner planet, the outer planet completes almost exactly three. This gravitational interaction causes the planets to repeatedly tug on one another, producing measurable shifts in the timing of their transits. By carefully measuring those timing variations — a technique known as transit timing variation, or TTV — scientists were able to calculate both planets' masses with enough confidence to confirm their status as super-puff planets.

Astronomers still aren't quite sure how super-puff planets came into existence. One prominent theory hypothesizes they contain huge atmospheres rich in hydrogen and helium that formed while the planets orbited far from their host star inside colder areas of a protoplanetary disc. The planets are "siblings," believed to have formed together from the same disk of gas and dust surrounding their young star.

What Comes Next

"Large planet formation is believed to drive the evolution of a planetary system, so further study of these Jupiter-size, but far less than Jupiter-mass, planets is of high value," said Steve Howell, a NASA Ames research scientist involved in the study. Scientists hope to learn more about the chemical makeup of the planets' atmospheres, how their spin affects their shape, and how the tilt of their host star compares to their orbits. Deeper investigation could provide new insight into how TOI-791 b and TOI-791 c migrated through the planetary system during their development, whether their orbits were shaped by interactions with other planets, and how low-density super-puff planets form.

Amaury Triaud of the University of Birmingham called the