Astronomers have discovered WASP-193 b, a giant planet with a density comparable to cotton candy. This extraordinary planet, located about 1,200 light-years away, is 50% larger than Jupiter — the largest planet in the solar system — but with one seventh of its mass.

The international team of astronomers led by the EXOTIC Laboratory at the University of Liège, in collaboration with MIT and the Astrophysics Institute in Andalusia, was stunned by the discovery. Current planetary formation models cannot account for such ‘fluffy’ planets, but they nevertheless exist.

“WASP-193 b is the second least dense planet discovered to date, after Kepler-51 d, which is much smaller,” said Khalid Barkaoui, a Postdoctoral Researcher at ULiège’s EXOTIC Laboratory and first author of the new study.

“Its extremely low density makes it a real 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.

A Celestial Fluffball

The discovery began with the Wide Angle Search for Planets (WASP), a collaboration of academic institutions using robotic observatories that track stellar brightness across the sky. The WASP-South observatory detected periodic dips in the star WASP-193’s light, indicating a planet passing in front of the star every 6.25 days.

Further observations were made using the TRAPPIST-South and SPECULOOS-South observatories in Chile to confirm the planetary nature of the object. Spectroscopic data from the HARPS and CORALIE spectrographs provided the planet’s mass.

To their surprise, the measurements revealed a density of about 0.059 grams per cubic centimeter. This was much lower than Jupiter’s 1.33 grams per cubic centimeter and Earth’s 5.51 grams per cubic centimeter. For reference, cotton candy has a density of 0.05 grams per cubic centimeter.

Although it’s a very large planet, the exoplanet’s thin atmosphere and great distance from our observatories made estimating its mass a huge challenge. The astronomers had to crunch data over four years of observation to detect the minute gravitational influence in the wobble of its parent star.

The Mysteries of WASP-193 b

The extremely low density of WASP-193 b suggests a planet mostly composed of hydrogen and helium, forming an inflated atmosphere tens of thousands of kilometers thick.

“The planet is so light that it’s difficult to think of an analogous, solid-state material,” says Julien de Wit, professor at Massachusetts Institute of Technology (MIT) and co-author. “The reason why it’s close to cotton candy is because both are pretty much air. The planet is basically super fluffy.”

Francisco Pozuelos from the Instituto de Astrofísica de Andalucía highlighted the need for more observations to understand how such a planet could form. The extremely low density is likely due to the close proximity to its parent star, but this speculative for now. Future studies, especially with the James Webb space telescope, will be crucial in uncovering the secrets of its atmosphere and formation.

“WASP-193b is a cosmic mystery,” concluded Barkaoui. Solving it will require further observational and theoretical work to measure its atmospheric properties and understand the mechanisms behind its extreme inflation.

The findings appeared in the journal Nature Astronomy.