Astronomers have discovered a massive, low-density planet called WASP-193b, which is 50% larger than Jupiter but has the density of cotton candy. This discovery casts doubt on existing theories of planet formation, as scientists cannot explain how such a planet could have formed.

Astronomers have discovered a massive, fuzzy planet orbiting a distant star in our Milky Way galaxy. The discovery, reported in the journal May 14 Nature Astronomy Work by researchers from the Massachusetts Institute of Technology, the University of Liege in Belgium and other cities offers a tantalizing clue into the mystery of how such giant ultralight planets form.

The new planet, called WASP-193b, appears larger than Jupiter, but only a fraction of its density. Scientists found that the gas giant is 50 percent larger than Jupiter and about one-tenth less dense than Jupiter; This is an extremely low density, comparable to cotton candy.

WASP-193b is the second lightest planet discovered to date, after the smaller Neptune-like planet Kepler 51d. The new planet’s much larger size and ultralight density make WASP-193b an oddball among the more than 5,400 planets discovered to date.

“It’s really rare to find such massive objects with such low density,” says Khalid Barkawi, the study’s lead author and an MIT postdoctoral researcher. “There is a class of planets called Chubby Jupiters, and what they are has remained a mystery for 15 years. And this is an extreme example of this class.”

“We don’t know where to place this planet among all the formation theories we have, because it goes beyond all of them,” adds one of the lead authors, Francisco Posuelos, a senior researcher at the Andalusian Institute of Astrophysics. In Spain. “We cannot explain how this planet was formed according to classical evolutionary models. A closer examination of its atmosphere will allow us to obtain the evolutionary path of this planet.”

Co-authors of the MIT study include Julien de Wit, an associate professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences, and MIT postdoctoral researcher Artem Burdanova, as well as collaborators from many institutions across Europe.

“An interesting twist”

The new planet was first detected by the Wide Angle Search for Planets, or WASP, an international collaboration of academic institutions that jointly operate two robotic observatories, one in the Northern Hemisphere and the other in the Southern Hemisphere. Each observatory uses an array of wide-angle cameras to measure the brightness of thousands of stars in the sky.

During surveys conducted between 2006 and 2008 and 2011 and 2012, the WASP-Southern Observatory detected periodic transits, or dips, of light from WASP-193, a bright, nearby, sun-like star located 1,232 light-years from Earth. Astronomers determined that the star’s periodic dimming corresponds to the planet orbiting the star and blocking its light every 6.25 days. Scientists measured the total amount of light the planet blocked with each pass, giving them an estimate of the planet’s massive size, a super-Jupiter.

Astronomers then tried to determine the mass of the planet; This measurement would then reveal its density and potentially give a clue to its composition. To make an estimate of mass, astronomers often use radial velocity, a technique in which scientists analyze a star’s spectrum, or the different wavelengths of light, as a planet orbits a star. The spectrum of a star can vary in certain ways depending on what is pulling on the star, such as an orbiting planet. The larger a planet is and the closer it is to its star, the more its spectrum can vary; This is a distortion that can give scientists an idea of ​​the planet’s mass.

For WASP-193 b, astronomers obtained additional high-resolution spectra of the star taken by various ground-based telescopes and attempted to use radial velocity to calculate the mass of the planet. But they kept coming up empty; This was because, as it turned out, the planet was too light to have a noticeable impact on its star.

“In general, giant planets are quite easy to detect because they are usually very massive and have a strong pull on their star,” De Wit explains. “But the most difficult thing about this planet was that although it was large, its mass and density were so low that it was actually very difficult to detect using radial velocity techniques alone. It was an interesting change.”

“[WASP-193b] “It’s so light that it took four years to collect data and show that it’s a very big signal, but it’s really very small,” Barkawi says.

“We were getting extremely low density in the beginning, which was very hard to believe at first,” adds Pozuelos. “We went through the process of analyzing all the data several times to make sure this was the true density of the planet because it was extremely rare.”

inflated world

Ultimately, the team confirmed that the planet is indeed extremely light. According to their calculations, its mass was about 0.14 that of Jupiter. Its density, derived from its mass, was approximately 0.059 grams per cubic centimeter. Jupiter, by contrast, weighs about 1.33 grams per cubic centimeter; and the Earth has a value of more than 5.51 grams per cubic centimeter. Perhaps the material closest in density to a new fluffy planet is cotton candy, which has a density of about 0.05 grams per cubic centimeter.

“The planet is so light that it is difficult to imagine a similar solid-state material,” Barkawi says. “The reason it is close to cotton candy is that both are mostly light gases, not solids. “The planet is actually extremely puffy.”

Researchers suspect that the new planet consists mainly of hydrogen and helium, like most of the other gas giants in the galaxy. For WASP-193b, these gases form an extremely inflated atmosphere that likely extends tens of thousands of kilometers further than Jupiter’s own atmosphere. Exactly how a planet can swell so much while maintaining its ultralight density is a question that no current theory of planet formation has yet answered.

To get a better picture of the new puffy world, the team plans to use the previously developed de Wit technique to first obtain certain properties of the planet’s atmosphere, such as its temperature, composition, and pressure at different depths. These properties can then be used to accurately determine the mass of the planet. For now, the team sees WASP-193b as an ideal candidate for further study by observatories such as the James Webb Space Telescope.

“The larger the planet’s atmosphere, the more light can pass through it,” says De Wit. “So it’s clear that this planet is one of the best targets for studying atmospheric effects. It will be the Rosetta Stone that will try to solve the mystery of the chubby Jupiters.”