Gas giant exoplanets around bright nearby stars provide crucial insights into planetary system formation and evolution mechanisms. Most of these planets show certain average characteristics, serving as benchmarks for our understanding of planetary systems. However, outliers like WASP-193b offer unique opportunities to explore unconventional formation and evolution processes. This exoplanet has a mass of 0.14 Jupiter masses and a radius of 1.5 Jupiter radii, translating into an extremely low density of 0.059 g/cm3, at least one order of magnitude less than standard gas giants like Jupiter.
An artist’s impression of a gas-giant exoplanet. Image credit: Sci.News.
“WASP-193b is the second least dense planet discovered to date, after Kepler-51d, which is much smaller,” said Dr. Khalid Barkaoui, an astronomer at the Université de Liège.
“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.”
WASP-193b orbits an F9-type main-sequence star called WASP-193 once every 6.25 days.
Also known as TIC 49043968, this system is located 1,200 light-years away from Earth in the constellation of Hydra.
WASP-193b was initially detected in the archival data from the Wide Angle Search for Planets (WASP).
Dr. Barkaoui and colleagues then used the TRAPPIST-South and SPECULOOS-South observatories to measure the planetary signal in different wavelengths and to validate the planetary nature of the eclipsing object.
Finally, they used spectroscopic observations collected by the HARPS and CORALIE spectrographs to measure the mass of the planet.
“The measurements revealed an extremely low density for the planet. Its mass and its size, they calculated, were about 0.14 and 1.5 that of Jupiter, respectively,” they said.
“The resulting density came out to about 0.059 g/cm3. Jupiter’s density, in contrast, is about 1.33 g/cm3; and Earth is a more substantial 5.51 g/cm3.”
“One of the materials closest in density to the new, puffy planet, is cotton candy, which has a density of about 0.05 g/cm3.”
“The planet is so light that it’s difficult to think of an analogous, solid-state material,” said MIT Professor Julien de Wit.
“The reason why it’s close to cotton candy is because both are pretty much air. The planet is basically super fluffy.”
The astronomers suggest that WASP-193b is made mostly from hydrogen and helium, like most other gas giants in our Milky Way Galaxy.
These gases likely form a hugely inflated atmosphere that extends tens of thousands of km farther than Jupiter’s own atmosphere.
Exactly how a planet can inflate so much is a question that no existing theory of planetary formation can yet answer.
It certainly requires a significant deposit of energy deep into the planet’s interior, but the details of the mechanism are not yet understood.
“We don’t know where to put this planet in all the formation theories we have right now, because it’s an outlier of all of them. We cannot explain how this planet was formed,” said Dr. Francisco Pozuelos, an astronomer at the Instituto de Astrofisica de Andalucia.
“Looking more closely at its atmosphere will allow us to constrain an evolutionary path of this planet.”
“WASP-193b is a cosmic mystery. Solving it will require some more observational and theoretical work, notably to measure its atmospheric properties with Webb and to confront them to different theoretical mechanisms that possibly result in such an extreme inflation,” Dr. Barkaoui said.
The discovery is reported in a paper in the journal Nature Astronomy.
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K. Barkaoui et al. An extended low-density atmosphere around the Jupiter-sized planet WASP-193b. Nat Astron, published online March 24, 2024; doi: 10.1038/s41550-024-02259-y
