Inflated Jupiter-sized planet has a uniquely uneven atmosphere, study finds

Imagine a planet the size of Neptune, itself about four times the size of our home planet. Instead of orbiting our Sun, this exoplanet revolves around a star in the Virgo constellation roughly 200 light-years from Earth. Perhaps most notably, it has a very low mass — 10% the mass of Jupiter despite being 80% its volume, with an atmosphere of water vapor, methane, carbon dioxide, carbon monoxide and ammonia. Which means it probably smells pretty gnarly.

Meet WASP-107 b, a planet that has long transfixed astronomers ever since it was discovered in 2017 with help from an array of telescopes known as the Wide Angle Search for Planets (WASP), which is also where its name comes from.

Now a recent study in the journal Nature Astronomy reveals something new and enigmatic about WASP-107 b. It has to do with all of those light gases that comprise the planet's atmosphere, or more specifically how they are composed within the atmosphere. Although most planets have relatively even atmospheres, WASP-107 b's is a few hundred meters taller on one side than on the other. This leads to more serious limb asymmetry, or the contrast between morning and evening conditions at a planet's opposite sides.

Dr. Matthew N. Murphy, lead author of the Nature Astronomy study and an astronomer at the University of Arizona's Steward Observatory, told Salon that the new findings help astronomers better understand not only this weird distant world, but our own home.

"There are a lot of strange and unexpected worlds out there beyond our own," Murphy said. "While they may not be like our own planet, every one of them teaches us a little more about how planets work, how they formed in the first place, and ultimately how special our own planet is. This knowledge is incredibly important — it pushes us closer to figuring out exactly how we got here, and the importance of our tiny planet in the (ever-growing) diversity of other planets out there."

To discover this, Murphy and his colleagues utilized the James Webb Space Telescope to examine the planet as it passed between Earth and its star. Analyzing the planet's terminators, or the moving line separating the daylight and nocturnal sides of a given world, the scientists realized that there was tremendous asymmetry. From there, the scientists further crunched their numbers and learned two things for sure that were not known before.

"First, it proves that exoplanetary atmospheres can be inhomogeneously mixed in regimes that we didn't expect," Murphy said. "We expected (and have since confirmed) that large temperature differences between evening and morning should exist on very hot exoplanets, but these were supposed to go away as the planet got colder because, at that point, it should be easier for winds and air currents to mix up and uniformly distribute heat and molecules around the entire planet. Our study proves that this isn't necessarily the case, and you can still have an extreme atmosphere at colder temperatures."

Additionally, the study demonstrates that transmission spectroscopy, or the technique of studying the atmospheres of distant planets through the sunlight it absorbs, can effectively separate the difference between what is happening in a day region of a planet and what is happening in its night region. The planet is tidally locked, meaning that one side is always facing its star, experiencing daytime, while the other is in perpetual night.

"With the new power of the James Webb Space Telescope and new analysis techniques, we can finally do so for the first time in this study," Murphy said. "This paves the way for a lot more exciting discoveries going forward."

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That said, even the current cutting edge technology is not entirely adequate to the task of teaching us about these distant planets. In the Nature Astronomy paper, the authors conclude by explaining that they have learned a great deal about how the planet's asymmetric atmosphere impacts the world itself.

"We found that the temperature has the strongest effect on both the relative shapes of the evening and morning spectra as well as their relative offset," the authors explain, adding that clouds also became more or less opaque as temperatures adjusted. Yet they could not observe more because of technological limitations.

"This precise behaviour is obviously subject to our parametrization of these clouds as a vertically uniform grey opacity source," the authors write. "For example, more detailed cloud or haze models, especially ones focused on specific species, may affect the shapes differently or even induce relative slopes across the spectra. Disentangling these details will require more observations over a wider wavelength coverage as well as much more complex multi-dimensional modelling. Nevertheless, our results show that recovering the separate properties of a transiting exoplanet’s morning and evening terminators is possible, and that WASP-107 b exhibits significant morning-to-evening asymmetry."

For his part, Murphy is optimistic that the scientists behind the latest set of discoveries are up to the challenge of cracking more of the mysteries behind WASP-107 b and other far away exoplanets.

"First, this research would not have been possible without the help of my amazing and supportive team of researchers from around the world," Murphy said. "I cannot thank them enough. It goes to show that the best work comes from collaborations."

He also pointed to the importance of ongoing government and private sector subsidies, saying that "none of this, and none of science, would be possible without the generous and noble funding from public and private institutions. As I touched on, astronomy has the power to unite the world while pushing us into the future. Recently, funding for science in general has generally been at risk, and there are so many other brilliant young scientists out there who deserve the support to do groundbreaking research, and contribute to a better world. We should all push for more support!"