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Astronomers have discovered a massive, low-density planet called WASP-193b that is 50% larger than Jupiter but has a density like cotton candy. This discovery challenges current theories of planet formation, as scientists cannot explain how such a planet could form.

Astronomers have discovered a huge, fluffy oddball of a planet orbiting a distant star in our own Milky Way galaxy. The discovery, reported May 14 in the journal Natural astronomy by researchers from at MITat the University of Liege in Belgium and elsewhere, is a promising key to the mystery of how such giant, ultralight planets form.

The new planet, called WASP-193b, appears to dwarf Jupiter in size, but is a fraction of its density. Scientists found the gas giant to be 50 percent larger than Jupiter and about a tenth as dense — an extremely low density comparable to that of cotton candy.

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

“Finding these giant objects with such a low density is really, really rare,” says lead study author and MIT postdoc Khalid Barkawi. “There’s a class of planets called puffy Jupiters, and it’s been a mystery for 15 years what they are. And this is an extreme case of this class.

“We don’t know where to place this planet in all the theories of formation that we currently have, because it is an outlier from all of them,” added one of the lead authors, Francisco Pozuelos, a senior researcher at the Institute of Astrophysics of Andalusia, in Spain. “We cannot explain how this planet formed based on classical evolutionary models. A closer look at its atmosphere will allow us to obtain the evolutionary path of this planet.

MIT co-authors of the study include Julien de Wit, assistant professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences, and MIT postdoctoral researcher Artem Burdanov, along with collaborators from multiple institutions across Europe.

Artist’s impression of the WASP-193b system. Credit: University of Liège

“Interesting Twist”

The new planet was first spotted by the Wide Angle Search for Planets, or WASP, an international collaboration of academic institutions that together operate two robotic observatories, one in the northern hemisphere and the other in the southern. Each observatory uses an array of wide-angle cameras to measure the brightness of thousands of individual stars across the sky.

In surveys conducted between 2006 and 2008 and again from 2011 to 2012, the WASP-South Observatory detected periodic transits or dips in light from WASP-193, a bright, nearby Sun-like star located 1,232 light-years from The Earth. Astronomers found that the star’s periodic dips in brightness are consistent with a planet orbiting the star and blocking its light every 6.25 days. Scientists measure the total amount of light the planet blocks with each transit, which gives them an estimate of the giant size of the super-Jupiter planet.

Astronomers then tried to determine the planet’s mass, a measure that would then reveal its density and potentially also clues to its composition. To get an estimate of mass, astronomers typically use radial velocity, a technique in which scientists analyze a star’s spectrum, or different wavelengths of light, as the planet orbits the star. A star’s spectrum can be shifted in specific ways depending on what is attracting the star, such as an orbiting planet. The more massive a planet is and the closer it is to its star, the more its spectrum can shift, 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 tried to use the radial velocity to calculate the planet’s mass. But they kept coming up empty — precisely because, as it turned out, the planet was too faint to have any noticeable pull on its star.

“Generally, large planets are quite easy to detect because they are usually massive and exert a large pull on their star,” explains de Wit. “But what was complicated about this planet was that even though it was big – huge – its mass and density were so low that it was actually very difficult to detect with just the radial velocity technique. It was an interesting twist.”

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

“Initially we were getting extremely low densities that were very hard to believe at first,” adds Pozuelos. “We repeated the process of analyzing all the data several times to make sure this was the true density of the planet, because it was super rare.”

An inflated world

Ultimately, the team confirmed that the planet was indeed extremely light. Its mass, they calculated, is about 0.14 that of Jupiter. And its density, derived from its mass, came out to be about 0.059 grams per cubic centimeter. Jupiter, by contrast, is about 1.33 grams per cubic centimeter; and Earth’s is a more substantial 5.51 grams per cubic centimeter. Perhaps the material closest in density to the new, puffy planet is cotton candy, which has a density of about 0.05 grams per cubic centimeter.

“The planet is so light that it’s hard to think of an analogous material in the solid state,” says Barkawi. “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.

Researchers suspect that the new planet is made mostly of hydrogen and helium, like most other gas giants in the galaxy. For WASP-193b, these gases likely form a highly inflated atmosphere that extends tens of thousands of kilometers beyond Jupiter’s own atmosphere. How exactly a planet can inflate so much while maintaining an ultralight density is a question that no existing theory of planet formation can yet answer.

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

“The larger the atmosphere of a planet, the more light can pass through it,” says de Wit. “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.

Reference: “Extended low-density atmosphere around the Jupiter-sized planet WASP-193 b” by Khalid Barkawi, Francisco J. Pozuelos, Coel Hellier, Barry Smalley, Louise D. Nielsen, Prajval Niraula, Michael Guillon, Julien de Wit, Simon Muller, Carolyn Dorn, Ravit Heled, Emmanuel Jehin, Brice-Olivier Demory, Valerie Van Grotel, Abderrahman Soubkiu, Murad Ghashui, David. R. Anderson, Zouhair Benkhaldoun, Francois Bouchy, Artem Burdanov, Laetitia Delrez, Elsa Ducrot, Lionel Garcia, Abdelhadi Jabiri, Monika Lendl, Pierre FL Maxted, Catriona A. Murray, Peter Pihlmann Pedersen, Didier Queloz, Daniel Sebastian, Oliver Turner, Stefan Udry, Mathilde Timmermans, Amory HMJ Triaud, and Richard G. West, 14 May 2024, Natural astronomy.
DOI: 10.1038/s41550-024-02259-y

This research was funded in part by Consortium Universities and the UK Science and Technology Facilities Council for WASP; European Research Council; Walloon-Brussels Federation; and the Heising-Simons Foundation, Colin and Leslie Masson, and Peter A. Gilman, supporting Artemis and the other SPECULOOS telescopes.