Super fluffy ‘cotton candy’ discovery of exoplanets shocks scientists – ‘We can’t explain how this planet formed’

Fluffy exoplanet concept

Astronomers have discovered a huge, low-density planet called WASP-193b, which is 50% larger than Jupiter but has a cotton candy-like density. This finding challenges existing theories of planet formation. (Artist’s concept.) Credit:

Astronomers have discovered a huge, low-density planet called WASP-193b, which is 50% larger than Jupiter but has a cotton candy-like density. This finding challenges current planetary formation theories, because scientists cannot explain how such a planet could form.

Astronomers have discovered a huge, fluffy eccentric planet orbiting a distant star in our planet Milky Way universe. The discovery will be reported in the journal on May 14 Nature Astronomy by researchers from at MITthe University of Liège in Belgium, and elsewhere, holds a promising key to the mystery of how such giant, super-light planets form.

The new exoplanet, called WASP-193b, appears smaller in size than Jupiter, but is still a fraction of its density. The scientists found that the gas giant is 50 percent larger than Jupiter, and about one-tenth the density – an extremely low density, comparable to that of cotton candy.

WASP-193b is the second-lightest planet discovered so far, after the smaller, Neptune-like world, Kepler 51d. The much larger size of the new planet, combined with its super light density, makes WASP-193b the odd one out among the more than 5,400 planets discovered so far.

“Finding these gigantic objects with such a small density is really very rare,” said study lead author and MIT postdoc Khalid Barkaoui. “There is a class of planets called puffy Jupiters, and it has been a mystery for fifteen years what they are. And this is an extreme case of that class.”

“We don’t know where to place this planet in all the formation theories we have now, because it is an outlier among all theories,” said co-lead author Francisco Pozuelos, a senior researcher at the Institute of Astrophysics of Andalusia. in Spain. “We cannot explain how this planet formed based on classical evolutionary models. If we take a closer look at the atmosphere, we can determine an evolutionary path for this planet.”

MIT co-authors include Julien de Wit, assistant professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences, and MIT postdoc Artem Burdanov, along with collaborators from multiple institutions across Europe.

WASP-193b system

Artist’s impression of the WASP-193b system. Credit: University of Liège

“An interesting twist”

The new planet was initially spotted by the Wide Angle Search for Planets, or WASP – an international collaboration of academic institutions that jointly operate two robotic observatories, one in the Northern Hemisphere and the other in the South. Each observatory uses an array of wide-angle cameras to measure the brightness of thousands of individual stars across the sky.

In surveys conducted between 2006 and 2008, and again from 2011 to 2012, the WASP-South observatory detected periodic transits, or dips, of WASP-193 – a bright, nearby, Sun-like star located 1,232 light-years from Earth. Astronomers determined that the star’s periodic dips in brightness were consistent with a planet orbiting the star and blocking its light every 6.25 days. The scientists measured the total amount of light the planet blocked on each pass, giving them an estimate of the planet’s gigantic super-Jupiter size.

The astronomers then tried to determine the planet’s mass – a measure that would then reveal its density and possibly clues to its composition. To get an estimate of mass, astronomers typically use radial velocity, a technique in which scientists analyze a star’s spectrum, or different wavelengths of light, as a planet orbits the star. A star’s spectrum can shift in specific ways depending on what’s pulling on the star, such as a planet orbiting the star. The more massive a planet is, and the closer it is to its star, the more its spectrum can shift – a distortion that can give scientists an idea of ​​a planet’s mass.

For WASP-193 b, astronomers obtained additional high-resolution spectra of the star, captured by several ground-based telescopes, and attempted to use its radial velocity to calculate the planet’s mass. But they continued to appear empty – precisely because, as it turned out, the planet was far too luminous to have any detectable pull on its star.

“Normally, large planets are quite easy to detect, because they are usually enormous and exert a strong gravitational pull on their star,” De Wit explains. “But the tricky thing about this planet was that even though it’s big – huge – its mass and density are so low that it was actually very difficult to detect using just the radial velocity technique. It was an interesting twist.”

“[WASP-193b] is so light that it took four years to collect data and show that there is a mass signal, but it is really very small,” says Barkaoui.

“Initially we had extremely low densities, which was very hard to believe at first,” Pozuelos adds. “We repeated the process of all the data analysis several times to make sure this was the true density of the planet, because this was super rare.”

An inflated world

Ultimately, the team confirmed that the planet was indeed extremely light. Its mass, they calculated, was about 0.14 that of Jupiter. And the density, derived from mass, came to about 0.059 grams per cubic centimeter. Jupiter, on the other hand, weighs about 1.33 grams per cubic centimeter; and the earth is substantial 5.51 grams per cubic centimeter. Perhaps the closest material in density to the new, puffy planet is cotton candy, which has a density of about 0.05 grams per cubic centimeter.

“The planet is so light that it is difficult to imagine a similar solid material,” says Barkaoui. “The reason why it is close to cotton candy is because both are composed largely of light gases rather than solids. 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 while maintaining super light density is a question that no existing theory of planetary formation can yet answer.

To get a better picture of the new fluffy world, the team plans to use a technique that De Wit previously developed, to first infer certain properties of the planet’s atmosphere, such as its temperature, composition and pressure at different depths. These features can then be used to accurately calculate the planet’s mass. For now, the team sees WASP-193b as an ideal candidate for further study by observatories such as the James Webb Space Telescope.

“The larger a planet’s atmosphere, the more light can pass through it,” says De Wit. “So it’s clear that this planet is one of the best targets we have for studying atmospheric effects. It will be a Rosetta stone to solve the mystery of the swollen Jupiters.”

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

This research was partly funded by consortium universities and the UK Science and Technology Facilities Council for WASP; the European Research Council; the Wallonia-Brussels Federation; and the Heising-Simons Foundation, Colin and Leslie Masson, and Peter A. Gilman, in support of Artemis and the other SPECULOOS telescopes.

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