This illustration shows the star TOI-791 (left) and its two planets. Image: ap / nasa / Daniel Rutter
Science News
June 27, 2026, 6:01 p.mJune 27, 2026, 6:02 p.m
Their average density is lower than that of cotton candy: the two planets of the star TOI-791, 1,100 light-years away, are as big as Jupiter but only have about a thirtieth of its mass.
This is reported by an international research team in the journal “Monthly Notices of the Royal Astronomical Society”. The scientists now hope to learn something about the formation and development of these mysterious objects through more precise observations.
“So far we only know a handful of such super fluffy planets,” explains lead author Georgina Dransfield from the English University of Oxford. “Finding two such planets in one system is an even greater rarity. Their extremely low density makes them fascinating touchstones for our understanding of the formation and evolution of planetary systems.”
Amateur researchers tracked down the two planets of TOI-791 in the archive data of the TESS (Transiting Exoplanet Survey Satellite) space telescope in 2019 and 2023. This has been monitoring the brightness of millions of stars since 2018 to search for planets. If a planet passes in front of its star on its orbit as seen from Earth, it regularly weakens its brightness slightly during these transits and in this way reveals its existence.
Average density of about 0.04 grams per cubic centimeter
Researchers can determine the size of the planets from the strength of the attenuation of the starlight: TOI-791’s two companions are about the size of Jupiter. For several years after the discovery, Dransfield and her colleagues tracked the planets’ transits and discovered slight irregularities. The planets interfere with each other with their gravitational pull, and from these disturbances the researchers were able to calculate what mass the planets contain.
The result was a surprise: both are much lighter than Jupiter. This results in an average density of around 0.04 grams per cubic centimeter – which is even slightly lower than the typical value of 0.05 for cotton candy. For comparison: the average density of Earth is 5.5, that of Jupiter is 1.3 grams per cubic centimeter. How can such a low value be explained?
Our TESS telescope helped find two new “super-puff” planets that are about the size of Jupiter but have the density of cotton candy. TOI-791 b and c might even be the puffiest planets ever found! Further study can help us learn about planetary evolution. https://t.co/KvecR1vKJM pic.twitter.com/0GDuBjctBE
— NASA Universe (@NASAUniverse) June 25, 2026
The new planets in size comparison.
Different approaches to explaining the phenomenon
Dransfield and her colleagues do not assume that the planets actually have a fluffy consistency like cotton candy – a celestial body made of such material would quickly disintegrate under the influence of gravity. Rather, the researchers suspect that the enormous size of the planets – and thus their low density – is only simulated by an extensive shell of gas or dust.
The planets could have a relatively small core made of rock and metals and a correspondingly high density, but which is enveloped in a huge atmosphere of hydrogen and helium that swallows up the starlight during transits. Computer simulations show that such planets can form in cool, dust-poor regions around a young star. An alternative explanation would be huge clouds of dust that could envelop a planet after asteroid impacts, for example, making it appear larger.
The team now wants to take a closer look at the two planets using the James Webb space telescope. With its high resolution, the telescope should be able to peer into the planets’ atmospheres and thus solve the mystery of their low density. (sda/dpa/jul)