Astronomers de-fog exoplanet atmospheres 100-fold

Johns Hopkins and University of California, Santa Cruz researchers reported on May 21 that they used James Webb Space Telescope data to separate the cloudy and clear sides of the hot Jupiter WASP-94A b, a gas giant nearly 700 light-years from Earth. The work, published in *Science*, identified a repeating cloud cycle in which mineral clouds build on the planet’s morning side and clear by evening. (hub.jhu.edu) The team said that split view let them measure the atmosphere without blending both sides together, a problem that has complicated exoplanet readings for years. (science.org) ### How did the researchers “de-fog” an exoplanet atmosphere? (science.org) The James Webb Space Telescope provided separate measurements of WASP-94A b’s leading and trailing edges as the planet crossed in front of its star, according to Johns Hopkins and Exeter summaries of the study. The leading edge corresponds to morning on the tidally locked planet, while the trailing edge corresponds to evening. Sagnick Mukherjee, the study’s lead author, and co-authors used that limb-resolved view to avoid averaging the two sides together. The *Science* paper said the morning limb is cooler and cloudy, while the evening limb is hotter and shows gaseous water absorption features. (hub.jhu.edu) ### What exactly did Webb see on WASP-94A b? WASP-94A b shows “cloudy mornings and clear evenings,” the *Science* paper said. Johns Hopkins said the morning side is filled with clouds made of magnesium silicate, a mineral common in rocks, while the evening side has clear skies. University of Exeter said the team thinks one of two processes may explain the pattern. (hub.jhu.edu) Powerful winds may lift clouds on the cooler side and then drive them downward into the hotter dayside, or the clouds may evaporate in a process the researchers compared to morning fog burning off on Earth, but at much higher temperatures. (news.ucsc.edu) ### Why does that change older atmospheric readings? The *Science* paper said atmospheric asymmetry can “severely bias” chemical abundance measurements unless limb-resolved spectroscopy is available. (science.org) UC Santa Cruz said older observations tended to blur the cloudy and clear sides together, while the new method looks through the clearer evening skies. David Sing, a Bloomberg Distinguished Professor of Earth and Planetary Sciences at Johns Hopkins and a co-author, said clouds have long been “a thorn in our side” because they act like a foggy window. Johns Hopkins said isolating the clouds gave researchers one of the clearest pictures yet of the planet’s composition. (news.exeter.ac.uk) ### Where does the “100-fold” claim come from? UC Santa Cruz said the clearer evening-side view showed WASP-94A b’s chemical makeup is much closer to Jupiter’s than earlier interpretations suggested. The *Science* paper did not use the phrase “100-fold” in the abstract returned by search, but it did say cloud-driven asymmetry can strongly distort abundance estimates when both limbs are combined. (science.org) That means the 100-fold figure appears to be a press-summary characterization of how large those distortions can be, rather than a number stated in the abstract itself. Nature’s news coverage and university releases also described the result as evidence that cloud cycles, rather than a permanent photochemical haze, dominate the aerosols on this planet. (hub.jhu.edu) That distinction matters because the source of the aerosols changes how researchers infer composition and atmospheric history. ### Who did the work, and what comes next? Sagnick Mukherjee led the study while at UC Santa Cruz and is now a postdoctoral fellow at Arizona State University supported by a 51 Pegasi b Fellowship, according to UC Santa Cruz. David Sing was the program principal investigator and a co-author, and the author list in *Science* includes researchers from Johns Hopkins, Exeter and other institutions. (news.ucsc.edu) The paper is now in *Science*, and the next step described by the researchers is applying limb-resolved spectroscopy to other hot Jupiters to test whether similar cloud cycles are affecting atmospheric measurements there as well. The WASP-94A b observations provide one of the first such cases with JWST data. (news.ucsc.edu) (science.org)