Astronomers Discover the Second-Lightest “Cotton Candy” Exoplanet to Date.

The hunt for extrasolar planets has revealed some truly interesting candidates, not the least of which are planets known as “Hot Jupiters.” This refers to a particular class of gas giants comparable in size to Jupiter but which orbit very closely to their suns. Strangely, there are some gas giants out there that have very low densities, raising questions about their formation and evolution. This is certainly true of the Kepler 51 system, which contains no less than three “super puff” planets similar in size to Jupiter but is about one hundred times less dense.

These planets also go by the moniker “cotton candy” giants because their density is comparable to this staple confection. In a recent study, an international team of astronomers spotted another massive planet, WASP-193b, a fluffy gas giant orbiting a Sun-like star 1,232 light-years away. While this planet is roughly one and a half times the size of Jupiter, it is only about 14% as massive. This makes WASP-193b the second-lightest exoplanet observed to date. Studying this and other “cotton candy” exoplanets could provide valuable insight into how these mysterious giants form.

The research team consisted of astronomers from the Astrobiology Research Unit and the Space Sciences, Technologies, and Astrophysics Research (STAR) Institute at the Université de Liège, the Oukaimeden Observatory at Cadi Ayyad University, the Massachusetts Institute of Technology (MIT), the Instituto de Astrofísica de Andalucía (IAA-CSIC), the European Southern Observatory (ESO), the Center for Space and Habitability at the University of Bern, the Center for Computational Astrophysics, the Cavendish Laboratory, and the British aerospace company Space Forge. The paper that describes their findings recently appeared in the journal Nature Astronomy.

The new planet was initially spotted by the Wide Angle Search for Planets (WASP), an international collaboration that operates two observatories (SuperWASP-North and WASP-South) and searches for exoplanets using the Transit Method (aka. Transit Photometry). Between 2006 and 2008, and again in 2011/2012, the WASP-South observatory detected periodic dips in WASP-193’s brightness. These dips were consistent with an exoplanet with an orbital period of 6.25 days and provided estimates of the planet’s size.

As Khalid Barkaoui, an MIT postdoctoral student and the study’s lead author, explained in an MIT News statement, “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… [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.”

To obtain estimates of the planet’s mass and density, astronomers relied on high-resolution spectra (aka. the Radial Velocity Method) from ground-based telescopes. Unfortunately, these attempts failed to yield accurate information because the planet was far too light to have any detectable effect on its star. In the end, Barkaoui and his team’s analysis allowed them to constrain its mass, which allowed them to estimate its density at about 0.059 grams per cubic centimeter. This is a far cry from Jupiter, which has a density of about 1.33 grams per cubic centimeter.

Said Francisco Pozuelos, a senior researcher at the Institute of Astrophysics of Andalucia and the co-lead author of the study:

“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.”

The researchers suspect that WASP-193b is composed mostly of hydrogen and helium, like all gas giants, and that these form a hugely inflated atmosphere that extends tens of thousands of kilometers farther than Jupiter’s atmosphere. These findings cannot be explained by conventional theories of planet formation and evolution, which makes WASP-193b an ideal candidate for follow-up observations. In the near future, the team hopes to conduct follow-up studies using the James Webb Space Telescope (JWST) and a technique developed by MIT assistant professor Julien de Wit.

This technique allows astronomers to measure the temperature, composition, and pressure of an exoplanet’s atmosphere to various depths, which can be used to precisely determine the planet’s mass. “The bigger a planet’s atmosphere, the more light can go through,” de Wit says. “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.”

Further Reading: MIT, Nature Astronomy