Rare Pair of Improbably Light 'Super-Puff' Planets Is Discovered

[Celerity]

They’re less dense than cotton candy, and they will help astrophysicists better understand the most unusual ways giant planets can form.

https://www.nytimes.com/2026/07/17/science/space/two-super-puff-planets-discovered.html?unlocked_article_code=1.yVA.Zbb2.W1S227UlqXx_&smid=url-share


An artist’s concept of the sunlike star TOI-791 and two giant planets, TOI-791b and TOI-791c, that are roughly the size of Jupiter but much less dense.Credit...Daniel Rutter/NASA

Astronomers have found a pair of the lightest giant planets known in the universe in a star system about 1,100 light-years away, according to a new study. The discovery of these two gas giants, which are less dense than cotton candy, will help astrophysicists better understand the most extreme and unusual ways planets can form. “We want to understand the full story of planet formation and evolution,” said George Dransfield, an astrophysicist at Oxford University who led the study. “The challenge with super-puff planets is that they don’t fit neatly into our models.”

A super-puff planet almost shouldn’t exist. It’s effectively a gas giant with an impossibly tiny core, one that should be too small to gravitationally pull in the vast volume of gases that scientists have found in their atmospheres. A gas giant’s core typically has a mass at least 10 times the mass of Earth. But many super-puffs have total masses — the core and atmosphere together — that are less than that. “So how does a core one or two or five Earth masses accrete this large amount of gas?” said Jessica Libby-Roberts, an astronomer at the University of Tampa who was not involved in the study.

It makes little physical sense, and it’s a puzzle scientists have been working on for about a decade. The first potential super-puff planets were identified in 2014, three of them orbiting a star called Kepler-51. That was the first time scientists had come across planets with astonishingly low densities, and they didn’t know what to make of them. “It just seemed impossible,” Dr. Libby-Roberts said. “Do we have to scratch everything we understand about planet formation and start over?”

Two years later, the term “super-puff” came onto the astronomy scene, coined by Eve J. Lee, an astrophysicist at the University of California, San Diego. Dr. Lee published her work on the possible ways that super-puffs could come to be. Her conclusion was that they formed in the right place under just the right conditions: It had to be cold enough that even an undersized core could pull in gases, which then wouldn’t have enough energy to escape. There couldn’t be too much dust, so gases could quickly accumulate. An unexpected atmosphere could build.

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