These planets are 1,000 times bigger than Earth, but lighter than cotton candy: study

About 1,113 light years away from Earth, locked in orbit around a single star, there are two planets that appear to be the size of Jupiter, but have the density of cotton candy. 

These newly discovered exoplanets are what’s known as “super-puffs” — rare planets characterized by their extremely low density. 

University of Oxford astrophysicist George Dransfield, lead author of a study describing these puffs, says they’re “comparable to a nice blob of shaving foam, fresh from the can.”

So how does a puffy space blob become a planet 1,000 times bigger than Earth?

“We don’t have the answer yet,” co-author Antoine Petit, a mathematician at France’s Centre National de la Recherche Scientifique, told As It Happens host Nil Koksal. 

“There’s a lot of theoretical work that needs to be done to try to understand how these planets could keep being that big.”

The findings were published Thursday in the Monthly Notices of the Royal Astronomical Society. 

What is a super-puff?

Super-puffs are believed to be quite rare in the cosmos. NASA’s tally of planets outside our solar system currently stands at nearly 6,300 confirmed, and fewer than 40 are super-puffs.

“The fact that there’s two around the same star, I think, makes it particularly compelling of a discovery,” said Lisa Dang, an astrophysicist at the University of Waterloo in Ontario.

They’re also something of a mystery. Planets with a lot of mass tend to be denser, not necessarily bigger. 

“Essentially, the more mass they get, the more gravity they have to kind of contract the planet,” said Dang, who is not involved in the study.

“These super-puff planets are exciting, because it probably means that there must be some kind of internal mechanism to kind of puff them up.”

There are two main theories about what allows these giant puffballs to exist, says Nicolas Cowan, a professor of physics and planetary sciences at McGill University in Montreal — either they’re very young, or they’re very hot.

“They’re a ball of gas, and gas is compressible. And so what determines the size of the planet has a lot to do with its temperature, basically,” said Cowan, who was not involved in the discovery.

“So you can have a planet made of the same stuff as Jupiter, and if you were to make the inside of it hotter, then the entire planet would be more puffed up and bigger, or vice versa.”

Young super-puffs, he says, “literally haven’t had enough time for their innards to cool down and compress.”

The older ones, meanwhile, may have a constant source of heat, possibly from a process called tidal heating, the friction generated from its gravitational interactions with other celestial bodies.

But if these new planets are as big as they seem to be, Cowan says they will upend any theoretical models scientists have to explain how super-puffs form and cool. 

“To be that puffy when you’re that huge is hard,” he said. 

Potentially the puffiest of puffs

Detected by NASA’s Tess satellite over the past decade, these two new planets orbit a star in the southern constellation Volans, known as the flying fish. 

The researchers suspect they are made of hydrogen and helium, although it will take follow-up observations by NASA’s Webb Space Telescope to confirm their chemical makeup.

According to NASA, one of them is nearly the same size as Jupiter, the largest planet in our solar system, but contains just three percent of Jupiter’s mass. The other is even larger than Jupiter but contains just 5.9 percent of Jupiter’s mass.

Petit says they are sibling planets, locked in a long orbital pattern that allows them to tug on each other, gravitationally.

“They were born together and are still living together, looking alike,” he said. “It’s quite poetic that they stayed in this configuration.”

Petit and his colleagues calculated the planets’ mass by measuring their gravitational pull on each other, and say further research will be needed to confirm their calculations.

Cowen cautioned that further research could disprove the study’s conclusions about the planets’ size and mass. But he says hopes — “for the sake of science” — that they turn out to be exactly what they seem.

“I’ll teach us something really profound about planet evolution,” he said.