This illustration depicts the Sun-like star TOI-791 and two giant planets that NASA’s TESS space telescope discovered in its orbit. These planets, designated TOI-791 b and TOI-791 c, are roughly the size of Jupiter but a tiny fraction of its mass, meaning they have an extraordinarily low density. Credit: NASA/ Daniel Rutter.
Two of the lowest-density giant planets ever detected, rare ‘super-puff’ planets with densities lower than candy floss, have been discovered.
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 centimetre, while TOI-791 c has a density of 0.047 grams per cubic centimetre.
By comparison, Jupiter’s average density is 1.33 grams per cubic centimetre, around 28 to 35 times greater.
Their densities are even lower than candy floss, which typically has a density of about 0.05 grams per cubic centimetre. In contrast, Earth’s density is 5.5 grams per cubic centimetre.
The study was led by the University of Oxford, in collaboration with Université Côte d’Azur/ Observatoire de la Côte d’Azur and the University of Birmingham.
Comparison of the exoplanets in the TOI-791 system with planets in our solar system. Credit: NASA/ Daniel Rutter.
Siblings
The planets are ‘siblings’, believed to have formed together from the same disc 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.
Lead author Dr George Dransfield of the Department of Physics, University of Oxford and a presenter for BBC Sky at Night, said: “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.”
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.
Dr George Dransfield and colleagues with the Antarctic telescope, ASTEP (Antarctic Search for Transiting ExoPlanets), during a summer service mission in 2021/2022. Credit: Karim Agabi/ IPEV/ PNRA.
Transit
This searches data from NASA’s Transiting Exoplanet Survey Satellite (TESS) for possible new worlds.
The 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.
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 analysing these timing shifts, the team was able to estimate the planets’ masses and calculate their remarkably low densities.
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.
Super-puffs
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 disc, 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.
The University of Birmingham’s Professor Amaury Triaud, the UK principal investigator of ASTEP and co-author of the study, said: “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 if the puffy atmosphere contains carbon-, nitrogen-, and oxygen-bearing species, revealing new insight into how these unusual planets formed.”
Study co-author Professor Tristan Guillot of the Université Côte d’Azur and principal investigator of ASTEP, 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.”
The study is published in Monthly Notices of the Royal Astronomical Society.
