Astronomers found two rare super puff planets lighter than cotton candy

Astronomers found two rare super puff planets lighter than cotton candy

An international research team has identified two giant planets with densities so low they are lighter than cotton candy. The "super-puff" siblings, designated TOI-791 b and TOI-791 c, orbit an F7-type dwarf star located approximately 1,110 light years from Earth in the southern constellation Volans, also known as the flying fish.

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, cotton candy typically has a density of about 0.05 grams per cubic centimeter. Jupiter is significantly denser at 1.33 grams per cubic centimeter, making it roughly 28 to 35 times denser than these new worlds. Earth's average density is much higher still at 5.5 grams per cubic centimeter.

Dr. George Dransfield of the University of Oxford, who led the study, described the planets as having densities comparable to a nice blob of shaving foam, fresh from the can.

A Rare Orbital Dance

The two planets are locked in a 5:3 mean-motion resonance. In this arrangement, the inner planet completes five orbits for every three orbits completed by the outer planet. As they orbit the star, their gravity pulls on one another, creating measurable changes in the timing of their transits.

The orbital periods are unusually long, with one planet taking 139 days and the other taking 232 days to circle the host star, according to NASA. The transits themselves are among the longest ever fully observed from the ground, each lasting more than 11 hours.

This discovery was made possible by eight years of observations, including data from the ASTEP (Antarctic Search for Transiting ExoPlanets) telescope at Concordia Station in Antarctica. The region's long winter nights provided months of uninterrupted darkness, which was essential for tracking the long transits.

The Mystery of Super-Puffs

Super-puffs are exceptionally rare. Out of nearly 6,300 confirmed exoplanets cataloged by NASA, Dransfield states that fewer than 40 are super-puffs. Finding two in a single system is even rarer, as only four other known planetary systems contain multiple super-puff planets.

Scientists believe these planets formed together from the same disc of gas and dust surrounding their young star. One leading theory suggests they formed in cold, distant regions of the protoplanetary disc, allowing thick atmospheres of hydrogen and helium to accumulate rapidly around solid cores. Some believe they form where there is more gas than dust and then shed material over time.

Dransfield suspects the planets appear white or blue depending on cloud cover, rather than the pink of cotton candy. Jon Jenkins, science lead for the Science Processing Operations Center at NASA's Ames Research Center, noted that these worlds represent a puzzle regarding how giant planets form because astronomers didn't expect to see them at all.

Discovery and Future Research

The planets were first flagged as candidates by volunteers in the Planet Hunters TESS citizen-science project, which analyzes data from NASA's Transiting Exoplanet Survey Satellite (TESS). TOI-791 b was flagged in 2019, followed by TOI-791 c in 2023.

To determine the planets' mass and density, researchers combined TESS data with measurements from telescopes across several continents. By analyzing the dip in starlight during transits and the timing variations caused by gravitational interactions, the team calculated the planets' sizes and masses.

The findings were published June 25, 2026, in Monthly Notices of the Royal Astronomical Society. The research involved collaborations between the University of Oxford, the University of Birmingham, and Université Côte d'Azur/Observatoire de la Côte d'Azur.

Researchers now plan to use the James Webb Space Telescope for space-based observations. Professor Amaury Triaud of the University of Birmingham stated the goal is to assess whether the atmospheres contain oxygen-, nitrogen-, and carbon-bearing species to gain further insight into how these unusual planets formed.