An international collaboration of scientists has discovered two of the smaller-density giant planets ever detected, the rare ‘supersponsive’ planets with densities below that of cotton candy.
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25/06/2026
by
Lusa
O study, led by the University of Oxford (United Kingdom), in collaboration with Université Côte d’Azur/Observatoire de la Côte d’Azur (France) and the University of Birmingham (United Kingdom), was published in the magazine ‘Monthly News of the Royal Astronomical Society’, reported on Thursday the Europa Press agency.
The two planets, designated TOI-791 b and TOI-791 c, orbit an F7 dwarf star located approximately 1,110 light-years from Earth in the southern constellation of Volans.
Although both planets are similar in size to Jupiter, they are very diffuse and the TOI-791 b has a density of only 0.038 grams per cubic centimeter, while the TOI-791 c has a density of 0.047 grams per cubic centimeter.
In comparison, the average density of Jupiter is 1.33 grams per cubic centimeter, between 28 and 35 times higher.
Its density is still lower than that of cotton candy, which typically has a density of about 0.05 grams per cubic centimeter. In contrast, the earth's density is 5.5 grams per cubic centimeter.
It is thought that these planets are ‘brothers’ and formed together from the same gas and dust disk that surrounded their young star.
In addition, they are linked by a unique gravitational relationship known as average motion resonance 5:3, which means that for every five orbits the inner planet completes, the outer planet completes almost exactly three.
This gravitational interaction causes planets to repeatedly attract, producing remarkable changes in the time of their transit through the host star.
Only four other systems containing multiple ‘supersponsive’ planets are known. This makes the TOI-791 an exceptionally unique laboratory to study how these planets form and evolve.
The main author of the study, George Dransfield (Department of Physics of the University of Oxford and presenter of the BBC Sky at Night), pointed out that “only some of these ‘supersponsive’ planets are known, and it is even rarer to find two in the same system.”.
“ Their extremely low densities make us fascinating targets to understand how planetary systems form and evolve,” he said.
Astronomers still debate how ‘supersponsive’ planets are formed. One of the main theories suggests that they have enormous atmospheres, rich in hydrogen and helium, which constitute a significant fraction of their total mass.
These gigantic gas casings may have accumulated when the planets formed away from their host star, in cold regions of the protoplanetary disk, where the gas could cool and agglomerate rapidly around a solid core.
Researchers plan to carry out further investigations to better understand how these planets formed and to discard some of the main explanations for their massive expansion.