Cotton candy exoplanet is the 2nd lightest planet ever discovered

Astronomers have discovered a strange new planet outside the solar system that is as fluffy and light as cotton candy.

The extrasolar planet or “exoplanet” named WASP-193 b is about 1.5 times the width of Jupiter but has just over a tenth of the solar system’s gas giant mass. It is therefore the second lightest planet in the exoplanet catalogue, which contains over 5,400 entries. Only the Neptune-like world, Kepler 51 d, is lighter than WASP-193 b.

Located about 1,200 light years from Earth, WASP-193 orbits a star about 6.3 million miles, which is about 0.07 times the distance between Earth and the sun. That means it completes an orbit of its sun star, WASP-193, in 6.2 Earth days.

The discovery of WASP-193 b adds to the wide and fascinating variety of planets outside the solar system, which, in this case, could help scientists redefine models of how planets form.

Related: Some of the planets are ‘death spirals’ in their stars and scientists may now know why

“It’s really rare to find these massive objects at such a low density,” discovery team co-leader and Massachusetts Institute of Technology (MIT) postdoc Khalid Barkaoui said in a statement. “There’s a class of planets called puffy Jupiters, and it’s been a mystery for 15 years now what they are. And this is a very large case of that class.”

A fluffy world is an “outlier” on models of planet formation

The MIT team discovered WASP-193 b using the Wide Angle Search for Planets (WASP) system consisting of two robotic observatories and telescope arrays, one in the northern hemisphere and the other in the southern hemisphere.

WASP identified the puffy gas giant planet using tiny dips in starlight that occurred as it crossed or “transited” the face of the star. These transits also allowed researchers to determine the planet’s plus-Jupiter size and its orbital period (how long it takes to complete one trip around its star).

The mass of WASP-193 b was determined by looking at the gravitational tug it exerts on its star as it orbits. This creates a “swing” in the star’s motion, which shows up as a change in the wavelength of the light coming from it. The size of the distortion tells the scientists the mass of the planet. In this case, however, WASP-193 b is so faint that its star pull cannot be identified using the “radial velocity” method.

“Typically, large planets are easy to detect because they are massive and have a strong pull on their star,” said team member Julien de Wit, an assistant professor in MIT’s Department of Earth, Atmospheric and Planetary Sciences. “But the tricky thing about this planet is that even though it’s big – huge – it’s mass and density is so low that it was really hard to detect with just the radial velocity technique. It was an interesting twist.”

It took four years of WASP observations to detect a tiny detectable “wobble” signal from this star and to determine the mass of this orbiting exoplanet. This confirmed that WASP 193 b is so “fluffy” that it challenges concepts of planet formation.

“We were finding very low densities at first, which was very hard to believe at first,” said team co-leader Francisco Pozuelos, a senior researcher at the Andalusian Institute of Astrophysics. “We don’t know where to place this planet in all of our current formation theories because it is an outlier in all of them.

“We cannot explain how this planet was formed based on classical evolution models.”

Illustration of the puffy exoplanet WASP-193 b around its sun star

A depiction of the puffy exoplanet WASP-193 b around its sun star

To give you an idea of ​​how light and fluffy WASP-193 b is, Earth has a density of 5.5 grams per cubic centimeter, while solar system gas giant Jupiter has a density of about 1.3 grams per cubic centimeter. WASP-193 b has a density of just 0.059 grams per cubic centimeter. Cotton candy is roughly analogous because it has a density of about 0.05 grams per cubic centimeter.

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

The team thinks that WASP-193 b is composed mostly of hydrogen and helium like Jupiter and other gas giants. They theorize that these gases are creating a huge super-illuminated atmosphere that is thousands of miles wider than Jupiter’s atmosphere.

Although the puffed-out atmosphere is likely the result of WASP-193 being bombarded by radiation from its nearby star, what the team still doesn’t know is exactly how the super-quiet nature was maintained this at WASP-193 b, as planet formation models do not account for this.

“We will be able to find the evolutionary path of this planet by taking a closer look at its atmosphere,” said Pozuelos.

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To gather information about the atmosphere of WASP-193 b and hopefully reveal the secrets of its origin, the team will turn to a technique developed by de Wit. This technique also uses the dips in starlight experienced by a planet as it passes in front of its star to determine the characteristics of the atmosphere such as temperature, composition and pressure at different depths.

The team also thinks WASP-193 could be an ideal exoplanet target for the James Webb Space Telescope (JWST).

“The more atmosphere the planet has, the more light can pass through,” said de Wit. “So this planet is clearly one of our best targets for studying the effects of the atmosphere. It will be a Rosetta stone to try to solve the mystery of puffy Jupiter.”

The team’s research was published on Tuesday (May 14) in the journal Nature Astronomy.

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