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New planet is as light as cotton candy, but bigger than Jupiter 

A team of astronomers led by the Massachusetts Institute of Technology (MIT) and the University of Liège in Belgium has discovered a unique, low-density planet orbiting a distant star within our Milky Way galaxy. This finding could provide significant insights into the formation of such massive, lightweight planets.

The newly identified gas giant, WASP-193b, is notably larger than Jupiter, yet much less dense. Researchers determined that the planet is 50 percent larger than Jupiter but only a tenth as dense, with a density comparable to cotton candy.

Giant planet with incredibly low density 

WASP-193b is the second least dense planet discovered so far, following the smaller, Neptune-like Kepler 51d. Its combination of large size and extremely low density makes it an outlier among the more than 5,400 known planets.

“To find these giant objects with such a small density is really, really rare,” said lead author Khalid Barkaoui, a postdoctoral fellow in astronomy at MIT. “There’s a class of planets called puffy Jupiters, and it’s been a mystery for 15 years now as to what they are. And this is an extreme case of that class.”

No explanation for how the planet formed

“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,” added co-author Francisco Pozuelos, a senior researcher at the Institute of Astrophysics of Andalucia

“We cannot explain how this planet was formed, based on classical evolution models. Looking more closely at its atmosphere will allow us to obtain an evolutionary path of this planet.”

Discovery of the “cotton candy planet”

The planet was first detected by the Wide Angle Search for Planets (WASP), an international collaboration operating two robotic observatories. These observatories use wide-angle cameras to monitor the brightness of thousands of stars across the sky.

Between 2006 and 2008, and again from 2011 to 2012, the WASP-South observatory recorded periodic dips in light from WASP-193, a nearby sun-like star 1,232 light years away. These dips indicated a planet blocking the star‘s light every 6.25 days. The amount of light blocked during each transit helped estimate the planet’s super-Jupiter size.

To determine the planet’s mass, which would reveal its density and composition, astronomers used radial velocity techniques. This involves analyzing a star’s light spectrum as a planet orbits it. The gravitational pull of an orbiting planet shifts the star’s spectrum, providing clues about the planet’s mass.

Difficult to detect due to low density

For WASP-193b, astronomers collected high-resolution spectra but struggled to detect the planet’s mass due to its low density.

“Typically, big planets are pretty easy to detect because they are usually massive, and lead to a big pull on their star,” explained co-author Julien de Wit, an assistant professor of planetary sciences at MIT.

“But what was tricky about this planet was, even though it’s big – huge – its mass and density are so low that it was actually very difficult to detect with just the radial velocity technique. It was an interesting twist.”

“WASP-193b is so very light that it took four years to gather data and show that there is a mass signal, but it’s really, really tiny,” Barkaoui added.

“We were initially getting extremely low densities, which were very difficult to believe in the beginning,” Pozuelos said. “We repeated the process of all the data analysis several times to make sure this was the real density of the planet because this was super rare.”

Super fluffy cotton candy planet 

The team confirmed that the planet is indeed extremely light, with a mass approximately 0.14 times that of Jupiter and a density of about 0.059 grams per cubic centimeter. 

For comparison, Jupiter’s density is 1.33 grams per cubic centimeter, and Earth’s is 5.51 grams per cubic centimeter. WASP-193b’s density is similar to that of cotton candy, which is about 0.05 grams per cubic centimeter.

“The planet is so light that it’s difficult to think of an analogous, solid-state material,” Barkaoui explained. “The reason why it’s close to cotton candy is because both are mostly made of light gasses rather than solids. The planet is basically super fluffy.”

Intriguing mystery of WASP-193b

The researchers believe WASP-193b is primarily composed of hydrogen and helium, like other gas giants. Its inflated atmosphere likely extends tens of thousands of kilometers further than Jupiter’s atmosphere. 

How such an atmosphere can expand so much while remaining so light remains unexplained by current planetary formation theories.

To better understand this fluffy planet, the team plans to use a technique developed by de Wit to analyze its atmosphere’s temperature, composition, and pressure at various depths. 

These characteristics will help accurately determine the planet’s mass. For now, WASP-193b is considered an excellent candidate for further study by observatories like the James Webb Space Telescope.

“The bigger a planet’s atmosphere, the more light can go through. 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 try and resolve the mystery of puffy Jupiters,” de Wit concluded.

The study is published in the journal Nature.


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