If we could find a scale big enough to weigh Jupiter would measure about 1.9 × 10²⁷ kilograms. It is the giant of the solar system, a sphere of hydrogen and helium so dense that, although it is made up almost entirely of gases, its average density reaches 1.33 grams per cubic centimeterfour times less than Earth, because we inhabit a rocky planet. Now let’s imagine a planet practically Same size as Jupiter… but thirty times lighter than it and 120 times less dense than Earth.
That’s exactly what an international team of astronomers has just discovered: two giant worlds whose density is so low that scientists compare them to cotton candy. The results, published in Monthly Notices of the Royal Astronomical Societydescribe one of the strangest planetary systems found to date.
The protagonists are TOI-791 by TOI-791 c, two gas giants orbiting a star located about 1,110 light years from Earthin the constellation of Volans. Both have a diameter very similar to that of Jupiter, but their mass is only a small fraction of that of the giant of our solar system. As a consequence, they have densities of only 0.038 and 0.047 grams per cubic centimeterlower even than those of cotton candy.
The question is logical, how can such a planet exist? At first glance it seems impossible. If a planet is the size of Jupiter, it should contain enough matter for its own gravity to compress the gases and increase their density. However, these two worlds defy that intuition.
The authors of the study, led by Georgina Dransfield, from the University of Oxford, They classify them within a rare family known as “super-puff planets” or super-spongy planets: giants enveloped by enormous atmospheres of hydrogen and helium. They occupy a huge volume, but contain very little mass. It’s like inflating a gigantic balloon with just a few breaths of air.
“The main reason these planets are interesting to study is that we didn’t expect to see them at all – explains Jon Jenkins, chief scientist of the Scientific Processing Operations Center at NASA Ames Research Center in Silicon Valley, California -. “They represent an enigma that we must solve about how giant planets like Jupiter and superplanets form.”
Until now, some planets of this type were already known, such as WASP-193b, discovered in 2024, whose density was also reminiscent of cotton candy. but find two ultralight giants orbiting the same star It is something exceptional. As if that were not enough, both interact gravitationally with each other. Every time they complete one revolution around their star, They exert small mutual pulls that slightly modify the exact moment in which they pass in front of it. These tiny variations allowed scientists to calculate their actual masses with great precision.
The big question remains how they came to be like this. One of the hypotheses suggests that these planets were born very far from their star, where low temperatures facilitated the accumulation of enormous amounts of hydrogen and helium. Later they would have migrated towards the interior of the system, preserving those extraordinarily inflated atmospheres. Another possibility is that some internal mechanism keeps its upper layers warm, preventing gravity from compressing them.
For now, no explanation is able to completely reproduce its properties. The Dransfield team has already requested observations with the James Webb Space Telescope. By analyzing the light that passes through their atmospheres, they hope to detect molecules such as water, methane, carbon dioxide or compounds of carbon, nitrogen and oxygen. This data could reveal where these worlds were born and why they ended up becoming true giants… extraordinarily light.