WASP-193b, a new exoplanet, is 50% larger than Jupiter but seven times less massive, with an extremely low density similar to cotton candy. Discovered by WASP and confirmed by observatories in Chile, its formation challenges current planetary theories and warrants further investigation. Credit: SciTechDaily.com
This exoplanet is larger but seven times less massive than Jupiter and is the second least dense planet discovered to date.
An international team led by researchers from the University of Liège’s EXOTIC Laboratory, in collaboration with MIT and the Institute of Astrophysics in Andalusia has just discovered WASP-193b, an extremely low-density giant planet orbiting a distant Sun-like star.
A newly discovered planet located approximately 1,200 light-years from Earth is 50% larger than Jupiter but seven times less massive. This results in an extremely low density comparable to that of cotton candy.
“WASP-193b is the second lowest-density planet discovered to date, after Kepler-51d, which is much smaller,” explains Khalid Barkaoui, a postdoctoral researcher at ULiège’s EXOTIC Laboratory and first author of the paper published in Natural astronomy. Its extremely low density makes it a true anomaly among the more than five thousand exoplanets discovered to date. This extremely low density cannot be reproduced by standard models of irradiated gas giants, even under the unrealistic assumption of a coreless structure.
Initial discovery and observations
The new planet was first spotted by the Wide Angle Search for Planets (WASP), an international collaboration of academic institutions that jointly operated 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 individual stars across the sky.
In data taken between 2006 and 2008 and again from 2011 to 2012, the WASP-South observatory detected periodic transits or dips in light from the star WASP-193. Astronomers found that the star’s periodic dips in brightness are consistent with a planet passing in front of the star every 6.25 days. Scientists measure the amount of light the planet blocks at each pass, which gives them an estimate of the planet’s size.
Artist’s impression of the density of WASP-193b compared to cotton candy. Credit: University of Liège
Detailed measurements and surprising density
The team then used the TRAPPIST-South and SPECULOOS-South observatories—led by Michaël Gillon, FNRS research director and astrophysicist at ULiège—located in the Atacama Desert in Chile to measure the planetary signal at different wavelengths and confirm the planetary nature of shading object. Finally, they also used spectroscopic observations collected by HARPS and CORALIE spectrographs – also located in Chile (ESO)- to measure the mass of the planet.
Much to their surprise, the accumulated measurements revealed an extremely low density on the planet. Its mass and size, calculated by them, are about 0.14 and 1.5 of Jupiter’s, respectively. The resulting density came out to be about 0.059 grams per cubic centimeter.
Jupiter’s density, by contrast, is about 1.33 grams per cubic centimeter; and Earth’s is a more substantial 5.51 grams per cubic centimeter. One of the materials closest in density to the new, puffy planet is cotton candy, which has a density of about 0.05 grams per cubic centimeter.
The mystery of the composition of WASP-193b
“The planet is so light that it is difficult to think of an analogous material in the solid state,” says Julien de Wit, a professor at the Massachusetts Institute of Technology (MIT) and co-author. “The reason it’s close to cotton candy is that they’re both almost air. 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 is a question that no existing theory of planet formation can yet answer. This certainly requires significant energy deposition deep in the planet’s interior, but the details of the mechanism are not yet understood.
Future research and challenges
“We don’t know where to put this planet in all the formation theories we have right now, because it’s outside of all of them. We cannot explain how this planet formed. A closer look at its atmosphere will allow us to constrain the evolutionary path of this planet, adds Francisco Posuelos, astronomer at the Instituto de Astrofisica de Andalucia (IAA-CSIC, Granada, Spain).”
“WASP-193b is a space mystery. Resolving it will require a bit more observational and theoretical work, in particular to measure its atmospheric properties with the JWST space telescope and to confront the various theoretical mechanisms that possibly lead to such extreme inflation,” concludes Khalid Barkawi.
For more on this discovery, see Super Fluffy ‘Cotton Candy’ Exoplanet Discovery Shocks Scientists.
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, Prajwal Niraula, Michael Guillon, Julien de Wit, Simon Muller, Caroline Dorn, Ravit Heled, Emmanuel Jehin, Brice-Olivier Demory, Valerie Van Grotel, Abderrahman Soubkiu, Murad Ghashoui, 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
