'Super-puff' planets lighter than candy floss discovered by international team
An international collaboration has discovered two of the lowest-density giant planets ever detected: rare "super-puff" planets with densities lower than candy floss. The study—led by the University of Oxford, in collaboration with Université Côte d'Azur/Observatoire de la Côte d'Azur and the University of Birmingham—has been published in Monthly Notices of the Royal Astronomical Society.
Overview of Super-Puff Planets
The two planets, named TOI-791 b and TOI-791 c, orbit an F7-type dwarf star located around 1,110 light-years from Earth in the southern constellation Volans. 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.
By comparison, Jupiter's average density is 1.33 grams per cubic centimeter, around 28 to 35 times greater. To provide context, their densities are even lower than candy floss, which typically has a density of about 0.05 grams per cubic centimeter. In contrast, Earth's density is 5.5 grams per cubic centimeter.
A Unique Gravitational Relationship
The planets are "siblings," believed to have formed together from the same disk of gas and dust surrounding their young star. They are also 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 across the host star.
Citizen Science in Discovery
Only four other systems are known to contain multiple super-puff planets. This makes TOI-791 an exceptionally rare laboratory for studying how these planets form and evolve.
Lead author Dr. George Dransfield (Department of Physics, University of Oxford) stated, "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."
Observation and Measurement Techniques
TOI-791 b and TOI-791 c were first identified as candidate planets in 2019 and 2023, respectively, by volunteers participating in the Planet Hunters TESS citizen-science project. This project searches data from NASA's Transiting Exoplanet Survey Satellite (TESS) for possible new worlds. Researchers then measured the planets' densities by combining observations of their sizes and masses using telescopes around the world.
When a planet passes in front of its host star—an event known as a "transit"—it slightly dims the star's light. The amount of dimming reveals the planet's size.
Gravitational Effects on Transit Timing
In this system, the researchers also detected subtle variations in the timing of the transits, caused by the two planets gravitationally tugging on one another as they orbit the star. By analyzing these timing shifts, the team was able to estimate the planets' masses and calculate their remarkably low densities.
Antarctica's Unique Advantages for Astronomy
The discovery relied on eight years of observations, including from the ASTEP (Antarctic Search for Transiting ExoPlanets) telescope at Concordia Station in Antarctica, jointly operated by researchers from Université Côte d'Azur/Observatoire de la Côte d'Azur and international collaborators.
The Antarctic winter provided a unique advantage: months of continuous darkness enabled astronomers to capture the planets' exceptionally long transits, each lasting more than 11 hours, in a single uninterrupted observation. These are the longest continuous planetary transits ever observed in their entirety from the ground.
Theoretical Implications of Super-Puff Formation
Astronomers are still debating how super-puff planets form. One leading theory suggests that they possess enormous hydrogen- and helium-rich atmospheres that make up a significant fraction of their total mass. These giant gaseous envelopes may have accumulated when the planets formed far from their host star in cold regions of the protoplanetary disk, where gas could cool and gather rapidly around a solid core.
The researchers intend to carry out follow-up investigations to understand more about how these planets formed and to rule out some of the leading super-puff explanations.
Future Studies and Observations
Professor Amaury Triaud (University of Birmingham) stated, "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 whether 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) 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."
Conclusion
The discovery of the "super-puff" planets TOI-791 b and TOI-791 c expands our understanding of planetary bodies with extraordinarily low densities. As more studies are conducted using advanced observational techniques, we hope to learn more about the formation, evolution, and characteristics of these intriguing planets.
