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Giant oddball planet has the density of fluffy cotton candy, study finds

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What is big, with a fluffy, cotton candy-like composition? Turns out, a planet.

An international coalition of astronomers has newly discovered an unusual planet, dubbed WASP-193b, that’s about 50% bigger than Jupiter and somehow still the second lightest planet ever found.

But WASP-193b, located beyond our solar system about 1,200 light-years from Earth, isn’t just a scientific oddity. The exoplanet could also be key to future research investigating atypical planetary formation, according to a study describing the find that published Tuesday in the journal Nature Astronomy.

This cotton candy planet isn’t alone; there are other similar planets belonging to a class scientists facetiously call “puffy Jupiters.” The lightest planet ever discovered is the superpuffy Kepler 51d, which is nearly the size of Jupiter but a hundred times lighter than the gas giant.

Puffy Jupiters have largely been a mystery for 15 years, said lead study author Khalid Barkaoui. But WASP-193b, because of its size, is an ideal candidate for further analysis by the James Webb Space Telescope and other observatories.

“The planet is so light that it’s difficult to think of an analogous, solid-state material,” said Barkaoui, a postdoctoral researcher of Earth, atmospheric and planetary sciences at the Massachusetts Institute of Technology, in a news release. “The reason why it’s close to cotton candy is because both are mostly made of light gases rather than solids. The planet is basically super fluffy.”

Low-density planet presents big challenge

WASP-193b, which researchers think is made up of mostly hydrogen and helium, was a huge puzzle for researchers to piece together. Because the exoplanet’s density is so light for its size, calculating its mass became a challenge.

Usually, scientists determine mass using a technique called radial velocity, in which researchers analyze how a star’s spectrum, a graph that indicates the intensity of light emissions in wavelengths, shifts as a planet orbits it. The bigger the planet, the more the star’s spectrum can shift — but this didn’t work for WASP-193b, which is so light, it didn’t make any pull on the star that the team could detect.

Because of how small the mass signal was, it took the team four years to gather data and calculate WASP-193b’s mass, Barkaoui explained. Because the extremely low numbers they found were so rare, the researchers completed multiple trials of data analysis, just to be sure.

“We were initially getting extremely low densities, which were very difficult to believe in the beginning,” said co-lead author Francisco Pozuelos, a senior researcher at Spain’s Institute of Astrophysics of Andalucia, in a news release.

Eventually the team discovered the planet’s mass is a measly 14% that of Jupiter, despite being so much bigger.

But a bigger size means a bigger “extended atmosphere,” said study coauthor Julien de Wit, an associate professor of planetary science at MIT. That means WASP-193b provides an especially useful window into these puffy planets’ formation.

But it’s also not clear how WASP-193b even formed, Barkaoui said. The “classical evolution models” of gas giants don’t quite explain the phenomenon.

“WASP-193b is an outlier of all planets discovered to date,” he said.

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