New exoplanet’s cotton candy-density can’t be explained

Astronomers continue to be puzzled about the formation of the so-called cotton candy planets, and now they’ve found another one that has the least density of any exoplanet yet found.

An international team of astronomers say the new planet, WASP-193b, is about 1,200 light-years from Earth. The gas giant is 50% larger than Jupiter but 7 times less massive.

According to Khalid Barkaoui, a postdcotral researcher at the University of Liège in Belgium and first author of the article published in Nature Astronomy, this extremely-low-density cannot be reproduced by standard models of irradiated gas giants.

“WASP-193b is the second least dense planet discovered to date, after Kepler-51d, which is much smaller,” says Barkaoui.

“Its extremely low density makes it a real anomaly among the more than five thousand exoplanets discovered to date.”

WASP-193b density is about 0.059 grams per cubic centimetre, which is closer to cotton candy’s (0.05 g/cm3) than to Jupiter’s (1.33 g/cm3) and Earth’s (5.51 g/cm3).

“The planet is so light that it’s difficult to think of an analogous, solid-state material,” says co-author Julien de Wit, associate professor of planetary science at Massachusetts Institute of Technology (MIT) in the US.

“The reason why it’s close to cotton candy is because both are pretty much air. The planet is basically super fluffy.”

Previously researchers have struggled to come to grips with how these planets form with a different cotton candy planet WASP-107b.

“The most plausible scenario is that [WASP-107b] formed far away from the star, where the gas in the disc is cold enough that gas accretion can occur very quickly,” said Eve Lee, from McGill University in Canada.

The planet was later able to migrate to its current position, either through interactions with the disc or with other planets in the system.

“Another explanation is that It could be the result of tidal interactions with the star, as a result of the planet having a slightly non-circular orbit.”

The team examining WASP-193b suspect the exoplanet is made mostly from hydrogen and helium, like most other gas giants in the galaxy. These gases likely form a hugely inflated atmosphere, which their models indicate cannot be explained by any existing theory of planetary formation.

“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,” says astronomer Francisco Pozuelos of the Instituto de Astrofisica de Andalucia in Spain, who co-authored the paper.

“Looking more closely at its atmosphere will allow us to constrain an evolutionary path of this planet.”

The researchers think its formation would have required a significant deposit of energy deep into the planet’s interior, but the details of the mechanism are not yet understood.

“WASP-193b is a cosmic mystery,” adds Barkaoui.

“Solving it will require some more observational and theoretical work, notably to measure its atmospheric properties with the James Webb Space Telescope (JWST) and to confront them to different theoretical mechanisms that possibly result in such an extreme inflation.”

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