Astronomers have discovered two Jupiter-sized exoplanets, TOI-791 b and c, with densities lower than cotton candy. The findings, published in the *Monthly Notices of the Royal Astronomical Society*, reveal the planets’ unusual characteristics and raise questions about planetary formation.
A Cosmic Contrast: Density Compared to Candy Floss
TOI-791 b and c are among the rarest celestial objects known. Their densities—0.038 and 0.047 grams per cubic centimeter, respectively—are lower than that of cotton candy, which typically measures around 0.05 grams per cubic centimeter. We liken it in density to something like candy floss or shaving foam,
Dransfield explained, emphasizing how their “pillowy” nature defies typical planetary expectations. For comparison, Earth’s density is 5.5 grams per cubic centimeter, making these planets 117 to 144 times less dense.
The planets were identified using NASA’s TESS space telescope, which detected their transits—periodic dips in their star’s light as they passed in front of it. However, determining their masses required a unique method. We were able to measure the masses of the planets by how much they kick each other’s orbits,
Dransfield said, referring to the gravitational interactions between the two planets. This fun little dance
allowed researchers to calculate their masses as 9.5 and 18.6 times Earth’s, respectively.
The Role of the Antarctic Telescope
Key to the discovery was the Antarctic Search for Transiting ExoPlanets (ASTEP) telescope, which observed the planets’ 12-hour transits uninterrupted. Without this telescope, this discovery would not have happened,
Dransfield said. Antarctica’s three months of continuous darkness each year provided the ideal conditions to monitor the planets’ orbital dance, a feat impossible for most other telescopes. The ASTEP data, combined with TESS observations, confirmed the planets’ extreme low densities.
The findings align with theories about super-puff planets, which are thought to form far from their stars and migrate inward, their atmospheres heated and expanded. However, the presence of two such planets in the same system is exceptionally rare. Only a handful of these super-puffy planets are known, and it is even rarer to find two in the same system,
Dransfield noted. The host star, TOI-791, is spinning quickly, which could be a sign that it’s relatively young, potentially suggesting the planets might still be cooling off and could contract as they get older.
Citizen Scientists and the Path to Discovery
The planets were initially flagged by the Planet Hunters citizen science project, which uses public volunteers to analyze NASA data. TOI-791 b was identified in 2019, and TOI-791 c in 2023. The project’s role highlights the growing impact of collaborative science in uncovering cosmic mysteries.
What’s Next for Super-Puff Research?
Future studies aim to use the James Webb Space Telescope to probe the planets’ atmospheres, offering insights into their composition and formation. Finding things like superpuffs, which are incredibly rare, means we can learn more about how planets form, how planets evolve and what outcomes are possible,
Dransfield said. The discovery also underscores the importance of gravitational resonance in planetary systems, as the two planets are locked in a rare mean-motion resonance
where the outer planet completes roughly three orbits for every five orbits completed by the inner planet.
For now, the puffiest planets remain a cosmic enigma, their fluffy exteriors hiding the secrets of their formation. As Dransfield put it, We might be catching these planets at an intermediate stage,
offering a glimpse into the dynamic processes shaping distant worlds.
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