Illustration planets TOI-791b and TOI-791c with host star TOI-791 Credit: NASA/Daniel Rutter
Two ‘super-puff’ planets lighter than candy candy have been discovered by scientists—and they’re bigger than Jupiter.
They are among the lowest-density giant planets ever detected, according to a new study.
The two planets orbit a dwarf star located around 1,110 light years from Earth in the southern constellation of Volans.
Although both planets—named TOI-791b and TOI-791c—are around the size of Jupiter, researchers say they are “extraordinarily” diffuse.
Jupiter’s density is 1.33 grams per cubic centimeter, while TOI-791b has a density of just 0.038 grams per cubic cm, and TOI-791c’s density is just 0.047—that’s 28 to 35 times lighter.
The new planets’ densities are even lower than cotton candy, 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, according to the study published in the journal Monthly Notices of the Royal Astronomical Society, led by University of Oxford scientists working with researchers from the University of Birmingham and the Université Côte d’Azur in France.
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.
The research team says that gravitational interaction causes the planets to repeatedly “tug” on one another, producing measurable shifts in the timing of their transits across the host star.
“Only a handful of these super-puffy planets are known, and it is even rarer to find two in the same system,” said lead author Dr. George Dransfield, from Oxford.
“Their extremely low densities make them fascinating targets for understanding how planetary systems form and evolve.”
They were found by citizen scientists – and an Antarctica telescope was critical
They were first identified as possible planets by citizen-science volunteers participating in the Planet Hunters TESS project, which searches NASA data for possible new worlds.
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Scientists then measured the planets’ densities by combining observations of their sizes and masses using telescopes located around the world, including Antarctica (where the ASTEP is located—Antarctic Search for Transiting ExoPlanets).
Illustration of the Antarctic ASTEP and other telescopes observing the TOI-791 system – via SWNS
The Antarctic winter provided a unique advantage for one of their telescopes: months of continuous darkness enabled astronomers to capture the planets’ exceptionally long transits, each lasting more than 11 hours, in a single uninterrupted observation—the longest continuous planetary transits ever observed in their entirety from the ground.
Transits are crucial for the research because when a planet passes in front of its host star the view slightly dims the star’s light, and the amount of dimming reveals the planet’s size.
The researchers 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 the timing shifts, the team was able to estimate the planets’ masses and calculate their remarkably low densities.
“Bringing together observations from Antarctica, space telescopes, and observatories across several continents was essential to revealing the true nature of these extraordinary planets,” said co-author Professor Tristan Guillot, from the Université Côte d’Azur
Astronomers are still debating how super-puff planets form.
The researchers intend to carry out further investigations to understand more about how the puff-planets formed, and to rule out some of the explanations.
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“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,” explained study co-author Professor Amaury Triaud, from the University of Birmingham.
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