K. Ivanov
An extremely low-density planet called WASP-193b is larger than Jupiter but a fraction of its mass.
Sign up for CNN’s Wonder Theory science newsletter. Explore the universe with news of fascinating discoveries, scientific breakthroughs and more.
CNN
—
What’s big, fluffy, like cotton candy? It turns out, a planet.
An international coalition of astronomers recently discovered an unusual planet called WASP-193b, which is about 50% larger than Jupiter and somehow still the second lightest planet ever discovered.
But WASP-193b, located outside our solar system about 1,200 light-years from Earth, is not just a scientific oddity. The exoplanet could also be key to future research investigating atypical planetary formation, according to a study describing the discovery published Tuesday in the journal Nature Astronomy.
This cotton candy planet is not alone; there are other similar planets, belonging to a class that scientists playfully call “bloated Jupiters”. The lightest planet ever discovered is the super fluffy Kepler 51d, which is almost the size of Jupiter but a hundred times lighter than the gas giant.
Puffy Jupiters have largely been a mystery for 15 years, said the study’s lead author, Khalid Barkawi. 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 hard to think of an analogous material in the solid state,” Barkawi, a postdoctoral fellow in Earth, Atmospheric and Planetary Sciences at MIT, said in a news release. “The reason it’s close to cotton candy is that both are made mostly of light gases, not solids. The planet is actually super fluffy.
WASP-193b, which researchers believe is composed mostly of hydrogen and helium, was a huge puzzle for researchers to piece together. Because the exoplanet’s density is so low for its size, calculating its mass has become a challenge.
Scientists typically determine mass using a technique called radial velocity, in which researchers analyze how a star’s spectrum, a graph that shows the intensity of light emissions in wavelengths, shifts as the planet orbits it. The larger the planet, the more the star’s spectrum can shift—but that didn’t work for WASP-193b, which is so light that it exerted no 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, Barkawi explained. Because the extremely low numbers they found were so rare, the researchers completed multiple attempts to analyze the data just to be sure.
“Initially, we were getting extremely low densities that were very hard to believe at first,” co-author Francisco Pozuelos, a senior researcher at Spain’s Institute of Astrophysics of Andalusia, said in a news release.
Ultimately, the team found that the planet’s mass was a paltry 14% of that of Jupiter, even though it was much larger.
But the larger size means a larger “extended atmosphere,” said study co-author Julien de Wit, associate professor of planetary science at MIT. This means that WASP-193b provides a particularly useful window into the formation of these fluffy planets.
“The bigger the atmosphere of a planet, the more light can get through,” de Wit told CNN. “So it’s clear that this planet is one of the best targets we have for studying atmospheric effects. It will be a Rosetta stone to try to solve the mystery of the bloated Jupiters.
But it’s also unclear how WASP-193b formed in the first place, Barkawi said. “Classical evolution models” of gas giants do not fully explain the phenomenon.
“WASP-193b is an outlier among all planets discovered to date,” he said.
