Astronomers discover a mind-boggling ‘cosmic mystery’: a giant planet as fluffy as cotton candy

Fluffy exoplanet art concept

WASP-193b, a new exoplanet, is 50% larger than Jupiter, but seven times less massive, with an extremely low density comparable to cotton candy. Discovered by WASP and confirmed by observatories in Chile, the formation challenges current planetary theories and requires further research. Credit:

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 EXOTIC Laboratory of the University of Liège, in collaboration with MIT and the Astrophysics Institute in Andalusia have just discovered WASP-193b, an extremely low-density giant planet orbiting a distant Sun-like star.

A newly discovered planet, located about 1,200 light-years from Earth, is 50% larger than Jupiter but seven times less massive. This results in it having an extremely low density, similar to that of cotton candy.

“WASP-193b is the second least dense planet discovered so far, after Kepler-51d, which is much smaller,” explains Khalid Barkaoui, postdoctoral researcher at the EXOTIC Laboratory of the ULiège and first author of the paper published in Nature Astronomy. Its extremely low density makes it a real anomaly among the more than five thousand exoplanets discovered so far. This extremely low density cannot be reproduced by standard models of irradiated gas giants, even under the unrealistic assumption of a coreless structure.”

First discoveries and observations

The new planet was initially 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 South. Each observatory used an array of wide-angle cameras to measure the brightness of thousands of individual stars across the sky.

In data collected between 2006 and 2008, and again from 2011 to 2012, the WASP-South Observatory detected periodic transits, or dips in light, of the star WASP-193. Astronomers determined that the star’s periodic brightness dips were consistent with a planet passing in front of the star every 6.25 days. The scientists measured the amount of light blocking the planet with each pass, giving them an estimate of the planet’s size.

WASP-193b vs. cotton candy

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 in different wavelengths and determine the planetary nature of the stars to validate. obscuring object. Finally, they also used spectroscopic observations collected by the HARPS and CORALIE Spectrographs – also based in Chile (ESO) – to measure the mass of the planet.

Much to their surprise, the measurements collected revealed an extremely low density for the planet. Its mass and size, they calculated, were about 0.14 and 1.5, respectively, that of Jupiter. The resulting density was about 0.059 grams per cubic centimeter.

In contrast, Jupiter’s density is about 1.33 grams per cubic centimeter; and the earth is substantial 5.51 grams per cubic centimeter. One of the materials closest in density to the new, puffy planet is cotton candy, with 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 imagine a similar solid material,” says Julien de Wit, professor at the Massachusetts Institute of Technology (MIT) and co-author. “The reason why it is close to cotton candy is because both are virtually air. The planet is actually super fluffy.”

The researchers suspect that the new planet consists largely of hydrogen and helium, just like most other gas giants in the Milky Way. For WASP-193b, these gases likely form a massively inflated atmosphere that extends tens of thousands of kilometers further than Jupiter’s own atmosphere. How exactly a planet can blow up so much is a question that no existing theory of planetary formation can yet answer. It certainly requires a significant amount of energy deep in the planet’s interior, but the details of the mechanism are not yet clear.

Future research and challenges

“We don’t know where to place this planet in all the formation theories we have now, because it is an outlier among them all. We cannot explain how this planet was formed. A closer look at its atmosphere will help us determine an evolutionary path of this planet, says Francisco Pozuelos, astronomer at the Instituto de Astrofisica de Andalucia (IAA-CSIC, Granada, Spain).”

“WASP-193b is a cosmic mystery. To resolve this will require some more observational and theoretical work, in particular to measure its atmospheric properties with the JWST space telescope and to confront them with different theoretical mechanisms that might lead to such extreme inflation,” concludes Khalid Barkaoui.

For more information about this discovery, see Super Fluffy “Cotton Candy” Exoplanet Discovery Shocks Scientists.

Reference: “An extended low-density atmosphere around the Jupiter-sized planet WASP-193 b” by Khalid Barkaoui, Francisco J. Pozuelos, Coel Hellier, Barry Smalley, Louise D. Nielsen, Prajwal Niraula, Michaël Gillon, Julien de Wit , Simon Müller, Caroline Dorn, Ravit Helled, Emmanuel Jehin, Brice-Olivier Demory, Valerie Van Grootel, Abderahmane Soubkiou, Mourad Ghachoui, 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, Stephane Udry, Mathilde Timmermans, Amaury HMJ Triaud and Richard G. West, May 14, 2024, Nature Astronomy.
DOI: 10.1038/s41550-024-02259-y

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