In a study, an international team of astronomers discovered another low-density, massive planet, named WASP-193b. This “fluffy” gas giant orbits a Sun-like star located 1,232 light-years away from Earth. Although WASP-193b is about one and a half times the size of Jupiter, it is only around 14% as massive, making it the second-lightest exoplanet observed to date.
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WASP-193b measures approximately 1.5 times the width of Jupiter. It possesses just over a tenth of Jupiter’s mass making it exceptionally light for its size.
The planet has low density of about 0.059 grams per cubic centimeter. For comparison, Earth has a density of 5.5 grams per cubic centimeter and Jupiter’s density is around 1.3 grams per cubic centimeter.
This low density is why WASP-193b is likened to cotton candy which has a similar density of roughly 0.05 grams per cubic centimeter.
Located 1,200 light-years from Earth, WASP-193 b orbits its star, WASP-193, at a distance of about 6.3 million miles (0.07 AU). It completes an orbit in just 6.2 Earth days.
The planet was detected using the Wide Angle Search for Planets (WASP) system which consists of two robotic observatories with telescope arrays positioned in the northern and southern hemispheres.
WASP-193b was identified through the transit method which involves detecting tiny dips in the starlight as the planet crosses the face of its star. This method allowed researchers to determine the planet’s size and orbital period.
Determining the planet’s mass presented a huge challenge. Typically, a planet’s mass is measured by observing the gravitational pull it exerts on its star, causing a “wobble” detectable through shifts in the star’s light spectrum.
Due to WASP-193b’s extremely low mass, this wobble was difficult to discern. After four years of observations, a faint but detectable signal was finally observed confirming the planet’s mass.
The exploration of extrasolar planets has unearthed numerous candidates particularly those known as “Hot Jupiters.” These are gas giants comparable in size to Jupiter but orbit very closely to their stars.
Among these some possess extraordinarily low densities. Kepler 51 system, for instance hosts three “super puff” planets similar in size to Jupiter but approximately one hundred times less dense.
The new planet, WASP-193b was initially detected by the Wide Angle Search for Planets (WASP) project. This international collaboration operates two observatories, SuperWASP-North and WASP-South, that search for exoplanets using the Transit Method, also known as Transit Photometry.
Between 2006 and 2008, and again in 2011/2012, the WASP-South observatory recorded periodic dips in WASP-193’s brightness.
These dips were consistent with the presence of an exoplanet with an orbital period of 6.25 days, allowing researchers to estimate the planet’s size.
To obtain estimates of WASP-193b’s mass and density, astronomers used high-resolution spectra via the Radial Velocity Method from ground-based telescopes.
These attempts failed to yield precise information as the planet was too light to have an effect on its star. The analysis by Khalid Barkaoui, an MIT postdoctoral student and the study’s lead author and his team allowed them to constrain the planet’s mass and estimate its density at approximately 0.059 grams per cubic centimeter.
The extraordinarily low density of WASP-193b places it in a rare category of exoplanets often referred to as “cotton candy” giants.
As Barkaoui explained, “To find these giant objects with such a small density is really, really rare. 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.”
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Francisco Pozuelos, a senior researcher at the Institute of Astrophysics of Andalusia and co-lead author of the study, addressed the uniqueness of this discovery.
He stated, “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, based on classical evolution models. Looking more closely at its atmosphere will allow us to obtain an evolutionary path of this planet. We were initially getting extremely low densities, which were very difficult to believe in the beginning. 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.”
Researchers believe that WASP-193b is primarily composed of hydrogen and helium similar to other gas giants.
These elements likely form an inflated atmosphere that extends tens of thousands of kilometers farther than Jupiter’s atmosphere.
Due to its unusual characteristics WASP-193b is an ideal candidate for follow-up observations. The research team hopes to use the James Webb Space Telescope (JWST) for further studies.
MIT assistant professor Julien de Wit has developed a technique to measure the temperature, composition, and pressure of an exoplanet’s atmosphere at various depths, which can be used to determine the planet’s mass more precisely.
As de Wit noted, “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.”
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The discovery of WASP-193b contributes to our understanding of “super puff” exoplanets, a class of planets with cotton candy-like densities that has baffled astronomers for years.
NASA has noted that nothing similar exists within our solar system. The only known planet with a lower density than WASP-193b is Kepler-51d, a Neptune-like planet discovered a decade ago.
The discovery places WASP-193b in the class of “puffy Jupiters,” a group of planets characterized by their large sizes and low densities.
This class has puzzled scientists for 15 years and WASP-193b represents an extreme example within this category.
The planet is believed to be composed mainly of hydrogen and helium. These gases likely form an overinflated atmosphere much wider than that of Jupiter.
To better understand the composition and formation of WASP-193b, researchers plan to conduct detailed studies of its atmosphere.
This will involve analyzing the light passing through the planet’s atmosphere during transits to determine its temperature, composition, and pressure at various depths.
WASP-193b is considered an ideal target for the JWST. The planet’s extensive atmosphere allows more light to pass through providing a valuable opportunity to study atmospheric effects and gather data that could help resolve the mysteries of puffy Jupiters.
Khalid Barkaoui said, “To find these giant objects with such a small density is really, really rare. 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.”
Francisco Pozuelos stated, “We were initially getting extremely low densities, which were very difficult to believe in the beginning. 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 based on classical evolution models.”
Julien de Wit stated, “Typically, big planets are pretty easy to detect because they are usually massive and lead to a big pull on their star. 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.”
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