Scientists discovered an extraordinarily low-density giant planet

Its density similar to that of cotton candy.


Using the TRAPPIST-South and SPECULOOS-South observatories located in the Atacama Desert in Chile, 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, has just discovered WASP-193b, an extraordinarily low-density giant planet orbiting a distant Sun-like star.

Located 1,200 light-years from Earth, this new planet is 50% larger than Jupiter but seven times less massive. It is the second least dense planet discovered to date.

It stands out among the more than 5,000 exoplanets found so far due to its incredibly low density. Even with the unrealistic assumption of a coreless structure, current models of irradiated gas giants cannot reproduce this extraordinarily low density.

The Wide Angle Search for Planets (WASP), an international collaboration of academic institutions that operated two robotic observatories, one in the northern hemisphere and the other in the south- has initially spotted the planet.

The WASP-South observatory identified periodic transits, or dips in brightness, from the star WASP-193 data collected between 2006 and 2008 and 2011 and 2012. The star’s sporadic brightness dips were consistent with a planet passing in front of the star once every 6.25 days by astronomers. With each transit, the scientists assessed how much light the planet blocked, which allowed them to calculate its approximate size.

puffy cotton-candy world
An artist’s impression of a puffy cotton-candy world. (NASA/ESA/STScI)

Scientists in this study examined the planetary signal at several wavelengths to confirm that the eclipsing object is a planet. Lastly, they employed spectroscopic data from the CORALIE and HARPS spectrographs in Chile (ESO) to determine the planet’s mass. They were shocked to learn that the planet’s density was incredibly low based on the collection of readings. They calculated that its mass and size were around 0.14 and 1.5 times that of Jupiter, respectively.

The density obtained was approximately 0.059 grams per cubic centimeter. Earth’s density is a more substantial 5.51 grams per cubic centimeter, while Jupiter’s is approximately 1.33 grams per cubic centimeter. Cotton candy, with a density of roughly 0.05 grams per cubic centimeter, is among the materials whose densities are closest to those of the new, fluffy planet.

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

According to a study, most other gas giants in the galaxy, including the new planet, are thought to be composed primarily of hydrogen and helium. These gasses most likely create a massively inflated atmosphere for WASP-193b, stretching tens of thousands of kilometers beyond Jupiter’s atmosphere. No planetary formation hypothesis can explain precisely how a planet can inflate to such a great extent. It undoubtedly necessitates a substantial energy deposit well into the planet’s interior, but the specifics of the method are still unknown.

Francisco Pozuelos, an astronomer at the Instituto de Astrofisica de Andalucia, said, “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. Looking more closely at its atmosphere will allow us to constrain an evolutionary path of this planet.”

Khalid Barkaoui said“WASP-193b is a cosmic mystery. Solving it will require some more observational and theoretical work, notably to measure its atmospheric properties with the JWST space telescope and to confront them to different theoretical mechanisms that possibly result in such an extreme inflation.”

Journal Reference:

  1. Barkaoui, K., Pozuelos, F.J., Hellier, C. et al. An extended low-density atmosphere around the Jupiter-sized planet WASP-193 b. Nat Astron (2024). DOI: 10.1038/s41550-024-02259-y


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